Solid State Guitar Amp Forum | DIY Guitar Amplifiers

   Hi everyone. I have a hartke 3500 that is need of a component. I'm still learning so thank you for bearing with me.

   The area in question is the power amp I think. The part number from the board is Q 310 ,and I'm pretty sure its a transistor. Its totally destroyed and in need of replacement. The numbers off of the part itself are:          K
                                                                                                 A968A
                                                                                                 Y  241
  I haven't had much luck on the net. So I'm asking where I could go to get that part or its equivalent. Any help will be appreciated.

On all those asian type transistors, you add a "2S" to the number, so your part is a 2SA968A, just as the upper side one is a 2SC2238.  And 2SD2155 and so on.   The "2S" is understood.

So look for a 2SA968 or equivalent.

If Q310 is blown up, then for sure look at R321, and there is a real good probability that one or more of the V- side outputs are shorted, those being the 2SB1429 at Q314,16,18,20.  And if any of those are bad, check the 0.47 ohm 5W resistor associated with it.



Assuming I am looking at the same schematic that you are.

   Thanks Enzo. The other components seem fine. I'm using a DMM on continuity and on diode test and the readings for Q314,16,18,20 are comparable to Q313,15,17,19 respectively. I didnt want to power it up until I got that part for fear of doing more damage. I'm hoping that is all I need
 
    As for a schematic I don't have one, I'm simply reading the PCB for component designations. I've tried to look at different schemes but it doesn't help much,as I dont know how to read  them yet. Thank you for your guidance.

Let me ask if your going to use a light bulb limiter when you fire it up? If you already have one and know that you should use one, then great. If you don't have a light bulb limiter you are going to want to make one.

http://www.ssguitar.com/index.php?topic=2093.0

Edit: here is a schematic

   Hey Gonz. Yes I do have a limiter that I intend to use when the time comes. Finding the part in question, however, is proving to be a little overwhelming. I have a RFQ with American Micro semiconductor for an exact replacement ( 2SA968A-Y props to Enzo). We'll see how that pans out. I also have options with Toshiba, Ali express and Suburban electronics. Thanks for the link,also.


Edit: just ordered matched pair  from an eBay sight. Fingers crossed.

Those part search firms are expecting folks looking to buy quantity.  Sample quantities maybe not their favorite order.

Those drivers are 1.5A 160v 25w TO220s.   Just about anything that meets those specs will work.   I probably do have exactlty those or similar parts, but if I had to start from scratch, I think I would not hesitate to grab a pair from the MJE15033 (PNP) and MJE15032 (NPN) drawers.

   Point taken. It seems I was trying so hard to find the exact part that I overlooked the obvious. When searching data sheets I noticed some were close but one or two of the parameters were off. I didn't know which to hold and which weren't as critical for clean operation. Again, I still only searched a specific part,not on specs alone. I'll chalk this one up. So much to learn.

    Ok , so my transistors came and everything checks out. Got it installed on the board and now the moment of truth. The problem I have, is that the power amp has a relay switch as some sort of safety. When I use my limiter, the relay won't click over. I metered  the limiter and with a 40 watt bulb it is sending 65 v  ac . I then metered D401 and CN502 for B+ and B- voltages and got 63 and -63 respectively.( I think on the scheme D401 is D418). I stopped there as I found a shoddy regulater at IC401. What can I do to power up the power amp with low voltage to make sure everything's on the level? Jumper the relay and use a limiter?

+/-63V looks pretty reasonable.  The relay is to protect the speakers.

The key thing is to check that the half-rail (R334) is very close to zero volts (say +/-0.1V).  If not you still have a fault to find.

   Fired up the power amp and it didn't explode so I'm happy for that. I metered TP1 and TP2 and had 38 v. I also tested R334 and had the same. When I say I metered them, I put the pos. lead on TP1/2 and neg lead to ground. For R334, I did the same, one lead on the side and one to ground. My impression was that I should have +/- 15 v at the TPs, and I guess since I have 38 v at R334 that's the reason I don't? More digging.

Because you measured 38V to ground at R334 I'll assume the test points you were looking at are TP301 and TP302.  These test points are to assist in checking the output stage idle current, not the +/-15V preamp supply.


The mid-point of the main supply is connected to ground (the symbol between C332 and C402) which is also one side of the speakers.  The voltages on "B+" and "B-" should be roughly equal each side of ground (i.e. +/-63V which you previously reported; 'tho it may be +/- 30-something volts, the circuit doesn't give a value; on reconsideration +/-63V is possible, but a bit unlikely).

The 38 volts you are measuring on R334 to ground is a massive output offset which explains why the protection relay won't close, and indicates that you still have a fault somewhere, most likely dead silicon.

Do not connect the speakers until you are getting zero (+/-0.1V) between R334 and ground (or they may be fried).
Continue to use the limiting lamp - do not directly power the amp.

What I normally do at this point is remove (or otherwise isolate, depending on the physical arrangement) all of the output transistors Q313-Q320 and the driver transistors Q309 and Q310 and check them out of circuit.  Take careful note of where each of the transistors go, and which way around (macro pix are good before you pull them out).

If the drivers test okay (or you obtain replacements) restore them to their respective correct positions, and restore a single pair of the output transistors, Q313 and Q314 (keeping in mind that half of these driver and output transistors are NPN and the other half PNP, and they must go back in the correct places).

With a single pair of output transistors restored again check the voltage at R334 to ground and confirm that it is now 0+/-0.1V.  If so then fit another pair and repeat.  If at any point you find the voltage at R334 is not close to zero one of the transistors you have just replaced is dud.

This is a bit of a chore but don't be tempted to skip any steps because you will only have to backtrack and repeat anyway.


So first, remove all the output and driver transistors (taking careful note of what goes where) and test out of circuit.  Do not assume that the first faulty one you find is the only faulty one - you have to check them all.

See; Testing Semiconductors;
http://www.ozvalveamps.org/repairs/solidstateamprepair.htm (http://www.ozvalveamps.org/repairs/solidstateamprepair.htm)

The PNP ones have the "internal diodes" in reverse to the NPN's, but all should test open circuit between Collector and Emitter.

Let us know what you find.


As a side issue, how did you decide that regulator IC401 was faulty?

   Thanks Roly. This was a good exercise for a beginner like myself. I stripped down all transistors and checked them out of circuit. I found that Q309 was faulty. Upon replacement, I began reinstalling the output transistors as you described. In the end, I now have 4.6 MV at R334 which is within the 0.1V you specified,right? I also rechecked the B+/B- values at D418 and now have +/- 38V and at CN 502/501 (I think the supply for the power amp) I have +/- 42V. While I was poking around, I metered at R342 and R346 ( thinking I would find 15V) and found 38V. I'm trying to follow the schematic and test values.
     As for IC401, I noticed it was very loose. I took it out  and tested it. I wired two 9V batteries in series for roughly 17V  and wired to the input and ground respectively. Woo she get hot yes! Measured output was 15V so it is OK.
     Do these values sound acceptable and are there  more tests I can or should perform.

Quote from: js1970I found that Q309 was faulty. Upon replacement, I began reinstalling the output transistors as you described. In the end, I now have 4.6 MV at R334 which is within the 0.1V you specified,right?

So it looks like the B+/- supplies are around 40 volts which is more "normal" ('tho I fancy I had a Hartke in a while back that had +/-55V rails on hot heatsinks giving a total of 110V to be careful of).

The previous plus-38V on the half-rail (output) would be consistent with Q309 being damaged and pulling the output up to the +ve rail.

So now you have 4.6MegaVolts ... er ... 4.6milliVolts on the output which is a really nice value; the DC-coupled output stage is now re-balancing itself to very close to zero across the output (a major sign of good health) which is also a nice low offset (it is almost never exactly zero).


I have no idea what a "7317" (IC301) actually is, but it's powered from the B+/- rails via droppers, so its actual supply voltage could be anywhere between about +/-12V and +/-18V, or maybe more, so there is nothing there we can really hang our hat on, but with the offset now nice and low I would hope that the relay would click in after a few seconds (but may not with the limiting lamp in circuit).

This circuit has two functions, anti-thump delay to let the amp settle before connecting the speakers, and detecting any standing offset voltage, say due to a fault such as you just found, to prevent the speakers being burned out by high DC fault current.

Since you now have a good voltage on the half-rail you can try a higher wattage limiting lamp, or take the plunge and power it up directly.  If you are still getting a nice low output offset you can plug the speakers in and make a very loud noise (however I would give the speaker lead and connectors a very close scrute first in case there is a possible short that was the original cause of the transistor failure).


Three-pin regulators aren't indestructible, but they aren't far off it.  If it was loose on its heatsink it might get quite hot, but then it would self-protect by shutting down its output.  If you are getting 15v out then just make sure it is firmly clamped to its heatsink, maybe with a dab of heatsink goo, and it should be right.  {The tab is ground on the 78xx +ve version, the middle leg and tab being connected, but is the input on the 79xx -ve version, so will normally need to be insulated from its heatsink.}


So yes, it looks like you are now out of the woods and good to go, so give it burl and let us now how you get on.   :dbtu:

   Cool, the power amp section seems to be working as it should. With a 100 watt bulb in the limiter, the relay kicks over and it will play.
   However, I think I have a bit more to look over. With the amp at idle, no instrument and no load,my limiter glows a bit.
   Speaker load hooked up, bulb stays lit. No change.
  Speaker and instrument hooked up, bulb still lit. No change
With volume up, the blb will get brighter when I hit the strings
  When playing, there is a hum,and a slight distortion in the sound.
  The hum  increases with volume
  So that's it in a nutshell. With power amp disconnected, the bulb still glows. Ground wires checked OK. Unplugged the tube and no change. What do you think?  Is there a place you think is a likely culprit or should I just check all the components value out of circuit? As always, your guidance is appreciated.

A dim glow is to be expected, the amp draws some current when idle and will draw more when you try and drive it.

You should actually measure the offset voltage on R334, but the fact that the protection relay pulls in is a good indication that all is well.

Disconnect the speaker.  Measure the output offset.  If it's still zero+/-0.1V with a 100W globe, and the two supply rails are the same and around +/-40-odd volts, try connecting the amp directly to mains power.  If the relay clicks in, reconnect the speaker and signal test.

It's always a bit of a "hold your breath" moment going direct mains power, but a dim glow is a very hopeful sign and I would normally consider that healthy.

http://www.bucek.name/pdf/ta7317.pdf

an excellent anti thump / short / DC protector.

   Ok, so I plugged direct,no limiter. My offset voltage is around 20 mV and the supply is 87 V.
   I connected speakers and still have that hum.  I also noticed the offset increased with volume. I assume this is normal, as its driving the speakers?
  The hum is affected by ,and increased with, the master volume, and the contour knobs  ( low pass and high pass). To some degree the EQ will filter the hum by turning down the 64,125,250,500 Hz,and 1 KHz knobs.
   
    It would seem as this is a different issue unrelated to the blown transistor. If so let me know and I'll start another thread. Thanks to all who responded for your helpful guidance

You may have missed a ground connection somewhere while fixing the other problem,,,or unwittingly added an extra ground path, both can cause the dreaded hum.
Phil.

Quote from: js1970It would seem as this is a different issue unrelated to the blown transistor.

So it does.

Quote from: js1970The hum is affected by ,and increased with, the master volume, and the contour knobs  ( low pass and high pass). To some degree the EQ will filter the hum by turning down the 64,125,250,500 Hz,and 1 KHz knobs.

This suggests that it is originating before the graphic EQ in the signal chain.


Check all the inter-board connections, firmly plugged on or none broken if soldered.

Clean all four sockets, Send, Return, Active and Passive, with dry metho, isopropyl, or DeOxit, and work a plug in and out when you do (cleans the internal contacts).


Is the Compression switched OFF?

Is it in both Solid VR and Tube VR channels?

Does pulling the twin triode make any difference?

   Adjusting the solid or tube knobs makes no difference. Niether does the compression / EQ. All ground connections to the chassis showed good for continuity. I don't know what the twin triode is.
   I'll go ahead and give the board another good look for bad joints.
   Could a bad pre amp tube give me that hum?

The twin triode IS the preamp tube.

You just now said the Compression/EQ makes no difference, but earlier you posted

QuoteTo some degree the EQ will filter the hum by turning down the 64,125,250,500 Hz,and 1 KHz knobs.

So either the EQ controls affect the hum or they do not.  Which is it?

This amp, like any other, is a stream through a circuit.  The hum comes into the stream at SOME point.  We are trying to find what is before and after that point.

Signal Flow

(circuit ref: http://www.ssguitar.com/index.php?action=dlattach;topic=3592.0;attach=5068 (http://www.ssguitar.com/index.php?action=dlattach;topic=3592.0;attach=5068)

Start - top-left, "Passive" and "Active".

These go into two op-amp input stages with differing gain, IC101A and B.

These are then mixed together at IC102A, and the signal then split three different ways;

- 1. via the preamp twin-triode valve/tube, the "Tube VA" path,


- 2. via IC103A for the "Solid VA" path,


- 3. into the Compression control circuit (which boils down to a gain control voltage going to IC403 pin 11).


The first two are re-combined at IC109A, and fed into a gain control block IC403, for Compression control (pin 11).

From pin 14 the signal then goes to IC107A where it is lifted from -2dB to +4dB nominal, and sent (left) to the Fx Send/Return sub-assembly, IC501A and B.

IC107B and IC108A bandwidth limit before the "Contour" controls around IC201B and A.

If the EQ is engaged, SW101, then the signal goes via the rank of gyrators in the graphic EQ section IC205 and IC206.

IC202A is the final output buffer for the preamp section which drives the main power amp input.

Quote from: J M Fahey on October 29, 2014, 10:11:45 AM
http://www.bucek.name/pdf/ta7317.pdf

an excellent anti thump / short / DC protector.

Thanks.   :dbtu:

  I pulled the tube with limiter engaged and no speaker load before. There was no change in the bulb brightness. I didn't know I could pull the tube and run it through the speakers. I will do that this weekend for sure.
   As for the ICs you mentioned, again I see the +/-15 off of pins 4and 8. Is that how I would check that they were functioning correctly?

    The hum is present, whether the EQ is on or off. It makes no difference. However, with the EQ on, I can filter the hum down to some degree.

 The point about the eq was to try and narrow down the area.  If the eq can affect it, then it is happening before the eq.

It's funny how they cheat with the tube  :grr

After Ic102a , signal is sent to two parallel connected preamps; "Tube" and "SS" .

SS is unity gain and flat .

Tube is 50X gain and passes through a fixed tone control, roughly Fender style , set to, hear this:
*  Treble on 10
*  Bass on 10
*  Mid on 5

and even considering tone control attenuation, signal is still hotter by some 5 to 10 dB :o

Talk about cheating  :trouble

This perpetuates the myth that "Tubes are always better than SS" yadda yadda yadda , no surprise users praise the`"hot punchy Tube sound" over the "clean but unintreresting SS one".

To check whether that tube adds some magic on its own, I have bypassed the added EQ network and padded the tube output so both channels have exact same gain and level.

Guess what? ... nobody could tell both channels apart ;)

The Fendery fixed tone control is:
R112/113/115/116 plus C109/110/111/112 .

Somebody might simulate it and post the curve here ;)
Thanks in advance.

Quote from: js1970There was no change in the bulb brightness.

Now that the output half-rail is balancing to (near) zero volts correctly, have you tried powering the amp directly from the mains?  It could be that you hum is due to the voltage reduction of the lamp.

---

Quote from: J M FaheySomebody might simulate it and post the curve here ;)

8|

Now now Jaun!  Keep calm. Don't go spinning the wheels on your Falcon you might blow a tyre.  :lmao:

But yes I do see your point, So the valve gets tone while the SS does not.

I took the time to simulate the valve section.
The SS preamp is just the compressor so I'm not going to try to do that compressor and anyway the response will likely be flat.
(Roly Beat me to it but no matter)

In effect the Valve section uses one extra tone shaping system and as I discovered a long time back great things can be done with an extra tone stage. ;)

FWIW, Don't worry R118 is 150k which pulls ~5dB off the signal so they stuffed it anyway. ::)
I'm stunned that many big name gear fails to grasp the simple fact that those old tone circuits only work right when followed by a high Z input. I've added a second pic to show the loss.
Phil.

THANKS guys, what would I do without you  <3)

Quote from: J M FaheyTHANKS guys, what would I do without you  <3)

Have a life?   :lmao:

Quote from: phattI'm stunned that many big name gear fails to grasp the simple fact that those old tone circuits only work right when followed by a high Z input.

Early in my career I had to work under the direction of a young engineer who was, by parts, truly obnoxious, arrogant, stupid, and accident-prone.  He had a really serious case of "I'm a ginger-beer and you can't tell me ANYTHING" (even that I'm leaning on my soldering iron and burning the crap out of my jumper, shirt, and arm - true; "no sense, no feeling" as my mother would say).

The sight of a pool of his blood on the floor was a frequent, if not weekly occurrence.  Warmly loved by one and all, we had a sweep going to guess when he would be flattened by a falling piano, and planned a right booze-up when it finally happened.

The major product prototype he had been working on was finally an installed prototype on the clients' site, and instead of coming good in a week or two, months were ticking by with the clients' machine still sitting idle (and on the phone demanding to know what we were playing at).

Then he simply stopped turning up at all and it fell to me to sort out his mess.

In a nutshell he had managed to get the logic levels between the machine feedback and the new control board inverted, High should have been Low, then tried to compensate by tacking resistors on each end as pull-ups and pull-downs until the logic was dithering in the error band.  A bit of Vero with three logic inverting stages did the trick.

So I reported back to the boss what I'd found and what I'd done to move things along, but I had a big question; how could a qualified engineer, supposedly an "expert" in micrologic, make such a basic error, then go right up the beach rather than finding and fixing it as I (a lowly tech at that time) had done in a day or so?

{It's also a great object lesson in knowing when to stop and ask for help.}

Good question.

He had presented all his qualification at interview when he was employed, but of course nobody bothered to actually check any of them, and it wasn't until somebody rang checking his employment history after he had walked out that the whole story started to unravel - he had failed out of first semester Electronic Engineering, so he had simply forged his qualifications, so by the time he made 30 he had red lines through his engineering qualifications, and a conviction for professional fraud on his CV.  I know he persisted with is own electronics company for a couple of years after that, pirating clients and designs of his previous employers, but then vanished from the Melbourne electronics scene.  Being into electricity and scuba diving, and being accident-prone, I expect he selected himself out of the gene pool.

That's the sort of thing that goes on in the background.

It's not often that it actually surfaces (http://www.theaustralian.com.au/business/andrew-flanagan-myers-star-hire-now-fired-knew-how-to-fool-recruiters/story-e6frg8zx-1226969455521?nk=572437589181d66263b01cff21f508b1).

Quote from: Roly on November 03, 2014, 01:54:25 AM

Quote from: js1970There was no change in the bulb brightness.

Now that the output half-rail is balancing to (near) zero volts correctly, have you tried powering the amp directly from the mains?  It could be that you hum is due to the voltage reduction of the lamp.

    I did take the plunge and fire it up off of mains power. Everything functions as it should, but with that incessant him. I removed the tube, with no change. When you gave me the run down of signal flow I started to wonder. Hear me out on this. The flow was from the instrument inputs, through the compression,through the EQ, then out the master volume.
What got me thinking, was that tbe amp hums with no signal in. So I followed the power input. What I found was a resistor ,R406 inline with the EQ switch, that appeared discolored. I will check its value when I get a chance.
    Also, when using the light bulb limiter, the bulb glows steadily even after power up. It is my impression that that means there is a short somewhere. I ve run other amps through the limiter to see how it worked. They were SS guitar amps and upon initial start up the bulb lights and fades out. Is it because of the preamp tube the bulb stays lit or is it signifying a problem? It is my theory that the limiter bulb and the hum are connected. If I find the problem, the bulb will go out and the hum will cease.
    Does this have any merit,or am I looking at it the wrong way?

Then measure R value of R411 which should read 10 Ohms it's likely the ground lift from the chassis. (I think)
If it reads very low then somewhere on that board is a second grounding path which will likely be the culprit.

I was working on a board once and after refitting with only a couple of screws to test I noticed that if I pressed on a corner the hum came back.

As it turned out the circuit had 2 ground points and that corner standoff hole had a ground copper trace across the hole. (design flaw,, Never assume the factory got it right, Hint) :-X
I simply cut the track around the hole and not only had I fixed the initial problem I also fixed the bad hum.

Getting ones head around grounding can be harder than designing a circuit.
After dozens of builds I've learned to respect ground paths a lot more.
Phil.

Ed; R406 will run hot as it's the heater supply,,, says 10 Volts,,
Yikes :o should be 6.3V +/- 10% that Valve will have a short life :-[ :'( :duh

I have a rack compressor/limiter somebody else took apart, a "bucket" job (comes in a bucket), with just this sort of grounding problem I have never been able to totally overcome - very tight case and missing fiber washers. 

---

Quote from: js1970Also, when using the light bulb limiter, the bulb glows steadily even after power up. It is my impression that that means there is a short somewhere. I've run other amps through the limiter to see how it worked.

That depends.

Every amp draws some residual power when it is idle, and this may result in a residual glow depending on how much idle power the amp pulls, and the wattage of the globe used.  With valve amps the residual power is normally higher due to the tube heaters.

If it's running okay, half-rail balancing to very low output offset, and it ran for more than a second direct-on-mains with no smoke or distress from the power amplifier, you can put the limiting lamp to one side.  I'd call that power amp a "goer".


At one point however you got distracted by the preamp +15V supply regulator, and I can't help but wonder if your problem now may be related to that.

{as a sidebar, this is why we try to tackle one fault at a time.  Normally amps come in with only one fault, such as your blown output stage (which may be due to a faulty speaker lead/connectors - it may well have a proximate cause you should look for.  You don't always find a cause, but you should always look for one.}


If turning the Master volume right down to zero kills the hum then it must be getting into the preamp somehow, and now that the power amp seems to be going okay we can concentrate on the preamp alone as a sub-system.  (and since the Master Vol is an op-amp stage it suggests that the supply rails are clean)

Try injecting a clean signal at Fx Return.  If this is hum-free then we need to look back towards the front end, however we really should confirm somehow that the preamp supplies are the correct voltage (easy) and hum-free (somewhat more difficult without an oscilloscope, but possible).

The Rx Return is low hanging fruit, and depending on what you report of this test we can work forward or back, and there are several places where you can harmlessly shunt the signal path to ground with a clip lead o.n.o. to isolate the problem to before/after.  (if this test comes up clean it very strongly suggests that the supplies are okay)

We normally come at problems like this by first applying binary splits, is it before or after the Master volume? (because the Master volume normally is between the preamp and power amp.  In you case we now think the power amp is okay, so we use the Fx Send and Return to split the preamp.

We keep doing this until we have a bite sized chunk where we think the fault is, then we work methodically, node by node, either backwards or forwards through the suspect circuit block.

You should also try shorting both inputs, again with a couple of plugs and clipleads o.n.o.  If this shuts your hum up then one of the input shorting contacts in the socket isn't.  This should be an early test.


So I particularly want to know;

a) input a signal at Fx Return, clean or hummy?

b) both inputs shorted, silent or hummy?

   Roly, sorry but,I don't know what o.n.o means :o

Most likely means; Or Nearest Object.
In this case one which will conduct and bridge the switch.
, often a screwdriver.
Then again he could mean the nearest Observer. ;)
Phil.

Quote(design flaw,, Never assume the factory got it right, Hint) :-X

Getting ones head around grounding can be harder than designing a circuit.

After dozens of builds I've learned to respect ground paths a lot more.

Phil.

Print these 3 maxima with big bold letters and attach that paper to the shop wall, they will help you often.

Quote from: js1970 on November 04, 2014, 09:16:03 PM
   Roly, sorry but,I don't know what o.n.o means :o

o.n.o. "or near offer" (commercial), "roughly", "about", "something like".

   OK, so when I send a signal through the FX return, I get a fuzzy/ distorted sound,and the hum is still present. When I short the inputs it is still hummy.
   I also put the ohm meter on R411,like phatt suggested,and got 0.00.
With power off, I touched the leads to either side of the resistor while still in circuit

Quote from: js1970when I send a signal through the FX return, I get a fuzzy/ distorted sound,and the hum is still present

...and you eliminate the preamp.

This is now without the limiting lamp, right?  Then you still have a problem in your power amp or power supplies - somewhere.

So we return to basics...
Check that the voltage on the output half-rail with the speaker disconnected is zero +/-0.1V.
Main +ve voltage.
Main -ve voltage.
Preamp +/-ve supply voltages.

Please post five (exact) measurements.


Now you really need an oscilloscope, or at least that's what I would be reaching for at this point.

   Here are current supply voltages:
Half rail     23.1mV
Main +    IN  87.2V  / OUT  33.44V
Main --  IN  87.2V  / OUT  33.29V
Pre +   23.07V
Pre --   15.34V

  The B+ value on the pre amp is pretty high I see. I took that value from IC401( the voltage reg 7815). Is this the cause of my hum?
Earlier on I saw it was really loose. Could it have sustained damage to the board?
  I had tested it for output,and it checked out. When I resoldered it, I made sure to not run puddles together,so nothing is touching. As loose as it was though, could it have cracked,or otherwise stressed out, the trace in the board? Then, by me applying heat and solder, a bridge was created across the stressed areas.

uh huh.

The 7815 is a +15 volt regulator, and if you are getting more than that on the preamp +ve supply rail it certainly seems like the primary feed voltage (24V?) is somehow making it past the regulator; and yes, this could easily be the cause of excessive hum.

If you have a new one to hand (or it's easy to get) then I'd be inclined to just replace it, however while they are not totally indestructible they are pretty damn robust, so I suspect that the middle (ground) pin is not actually making ground for some reason (e.g. dry solder joint, cracked trace), so I would first check continuity from the middle leg (or tab) to a known ground point, and I think you'll find it's not zero ohms as it should be.


An alternative faultfinding pathway is that with the amp on, if the middle (ground) leg is open I'd expect about 23V on the output (right leg), a bit more, say 24V on the input (left leg), and perhaps 24-15=9V on the middle, supposedly grounded, leg, (but it could be just about any voltage, and it should be nothing).

If you have 24V in, 23V out and the middle pin is grounded, then the regulator is either stuffed or shorted somehow.

Can you post above and below macro pix of the 7815 area please?

And in case you are unaware, the 7815 and 7915 regulators have their inputs and outputs in different positions.

    Yup. I checked continuity to ground and got OL.

Excellent.  :dbtu:   Now all you've got to do is find the open and fix it.

The first place I would look is the middle leg solder joint, and if this looks okay, measure continuity between it and some other ground point.  What frequently happens is that the track fractures just where it meets the solder donut, and it can be finer than a human hair.  If so, just scratch a bit of the green overprint off the ground track just back from the solder joint and bridge with a short bit of wire.

  Cool, I'll get after it and let you know the results.

  Well guys, I'm happy to report, the B+ preamp supply voltage is clocking in at a cool 14.75V. The hum is no longer.
  I'd like to thank everyone who responded to my call. I couldn't have gotten this far without you. And a special thank you to Roly for your patience, and walking me through step by step.
  So after sorting out the supply voltage, I hooked up the speakers and fired her up for a victory lap. Plugged in my daughters bass and nothing. Had signal in, no sound out.
WHAT?!
Unfettered, and with a newfound electrical prowess, I began to poke and wiggle components on the board. When performing this technical troubleshooting, I found CN404 ( a plugged connection) was loose where it fastened to the board. Not sure how it ties in,but a closer look revealed another broken trace. I tightened her up and tried again.
   Excelsior! Self induced noise and no hum. That's one for the good guys.
  My victory was short lived,I regret. The sound was fuzzy,.particularly when playing the high strings. That's with the EQ on or off. I'm wondering if it might be the cab,but I haven't been able to try another yet. Thoughts?

Quote from: js1970And a special thank you to Roly for your patience, and walking me through step by step.

My pleasure as always.  None of us were born knowing any of this stuff.  Very well done on getting this far.   :dbtu:

Quote from: js1970I'm wondering if it might be the cab,but I haven't been able to try another yet. Thoughts?

Well you should try another cab if possible to eliminate that as a possibility.  If you get exactly the same result then we need to look back at the amp, otherwise at the cab and particularly the driver.  Alternatively if you have another amp you could try driving the cab with that and see if it sounds the same or clean.

   Hey guys. I finally got to try the 3500 on a known working cab.
Its still a little sick.
When I play, the sound is fuzzy/distorted. Otherwise, its silent, no buzz or hiss or hum. I ran a tone through the  FX return with the same result. What do you think?
Will I be able to fault find with DMM, or will I inevitably need a scope or some other test equipment?

If the Amp has plenty of power and all seems ok then maybe crossover distortion.
Easy to hear it with the volume low and with your ear near the speaker play one single note on your guitar and let it ring out. It sounds like a torn speaker a fizzy buzzing sound behind the signal. SVR302 may need adjustment, I'll leave better minds to add more.

Unlikely but there is always the outside chance of RL301 having failing contacts.
Phil.

Quote from: js1970Will I be able to fault find with DMM?

There's still a fair bit you can check with your trusty meter.   :dbtu:


With the volume controls at zero (no signal condition), measure the voltage between TP301 and TP302.  There is about 1 ohm between these two points and the idle current is flowing through them producing a voltage related to the idle current of one millivolt per milliamp.  I would expect an idle current somewhere around 25-50mA per transistor pair, so you should see a voltage of something like 25 to 50mV.

You would also be wise to check the individual voltages across R324 to R313 (all 0.47ohm 5watt) which should all be roughly equal (which indicates that all the output transistors are passing similar idle currents).  These might not be exactly identical, but should still be within about 20% of each other.

If phatt is right then you may find that the idle current is low or even zero.  The idle current is set by Q308 and its surrounding network including trimpot SVR302.  Normally we wouldn't touch SVR302 because it rarely needs resetting, but if you have low or no idle current you first need to investigate the Collector-Emitter voltage on Q308.

Do we still have a fault, i.e. Q308 shorted, or is it only that the new power transistors require a different bias setting?  Since it is uncommon to need to reset the bias control after transistor replacement I would proceed with caution and not touch SVR302 until we are sure that the bias setting transistor is healthy (excessive bias could be a snake that lands you right back at square one with a blown output stage - be careful here).

The voltage on the Collector of Q308 will be the sum of the voltage across the emitter resistors, plus the EB drop of the power transistors, plus the EB drop of the driver transistors, somewhere in the range;

Low
2 * ( 12 + 500 + 500 ) = 2024mV (~2 volts)

High
2 * ( 25 + 700 + 700 ) = 2850mV (~3 volts)

If phatt's guess is right then you might find only two volts across Q308, and perhaps a lot less if there is still a fault.

If you have sensible voltages you should try sticking your finger on thermistor TH302 to heat it a bit while observing the idle current, and as it warms the idle current should trend down a bit (indicating that the thermal negative feedback is working).


So firstly, run the amp at idle and measure the voltages between TP301 and TP302, and the voltage to ground from those two points, post them, and we'll see what they suggest.  (you should now be running without the Limiting Lamp).


(BTW trim pot SVR301 is for trimming the output idle output voltage to zero - it also should not be touched without good reason).

HTH

 I've seen it said there should be 25mV between TP301 and 302.  I've also seen it said 2.5mV, however the early versions were mosfet output devices and I believe the 2.5mV was for the mosfet version.
  Can anyone confirm or opine whether that sounds likely?

  When you say between the TPs, are you referring to the halfrail voltage?

No, between the test points, meter +ve to TP301 and meter -ve to TP302. 

  Here's the values you asked for:
TP 301 to ground- 23.2 mV
TP 302 to ground- 24.0 mV
TP 301 to TP 302- 0.00 mV
You guys nailed that one, eh. Does this mean only one half of the power amp is working? Or, that both halves work, just not together? Why the fuzz sound?

Wherever you have push-pull output stages the signal is split between the upper and lower signal halves, and it is important that the transition between one side of the output conducting and the other side conducting is a smooth hand-over, otherwise crossover distortion (http://en.wikipedia.org/wiki/Crossover_distortion) results.  That is what the bias setting in solid-state amps is for; to slightly turn on both parts so that each side starts to pick up the ball before the other side drops it.

With zero bias, as you seem to have, one side stops conducting before the other side starts, so there is a small dead zone in the middle of the waveform.  As it happens the human ear is quite sensitive to this crossover distortion.

(http://www.electronics-tutorials.ws/amplifier/amp26.gif)

What worries me here is that you don't seem to have any bias at all, so you need to carry out the measurements I mentioned above to see if the bias network transistor Q308 has some voltage across it, or may have been shorted by the original fault (or there is some other problem in this area).

 Cool, thanks for the info. I will get after Q308 asap. Happy Thanksgiving to you all

Hey guys. I metered Q308 under the diode setting, and I'm pretty sure its faulty. I couldn't get an OL reading with any combination of the leads. I didn't get an exact replacement of C3200, instead, I got a C2240 transistor. Its supposed to be comparable.
  While I was waiting for my tranny's to arrive. I tried to meter the components Roly had suggested. At least, how I comprehended Roly's suggestions.
  So , my voltage on the collector of Q308, with com. lead on ground and pos. lead on the collector, I read 1.7V.
  When I tried to measure across the emitter resistors, I get 00.0 mV. That was with com. on one leg of the resistor and pos. on the other leg of the same resistor.  However, when I held the com. on earth ground, I was able to read each side of those resistors. I'm unsure if the readings I got are what Roly had in mind,but I tried his equation anyway:
    Sum of voltage across emitter resistors-- 1.6 mV
+ emitter/ base drop of power tran.-- .4mV
+ emitter/base drop of driver tran.-- .3mV
If I'm reading the equation right 2( 1.6+ 400+300)=1403.2mV (~1.4V)
Assuming all readings are correct, does this mean the bias is simply running  cool as was suggested earlier?

    Finally got replacement transistor for the faulty Q308. I'll get to work asap.
  Happy holidays to you all!

   Finally got some free time to tinker with the hartke. I swapped the transistor at Q308 from the scheme. I must say, I was a little apprehensive about getting  the BEC configuration correct, but after some deliberation I went for it. When the tranny was soldered in,I did the tests that were suggested from previous posts.
No pops or smoke upon power up. I metered  TP 301&302 both to ground and btween. Here's what I found:
          TP301 to G - 65mV
          TP302 to G - 63mV
          Btw TP301and TP302 - 2.3 mV
         Voltage at collector of Q308 -  the DMM flashes 2.3V breifly then holds about
         1.2V
    I haven't fired amp up with instrument or load yet. With the voltages I posted, does it seem as though things are in order? Are there other things I should check before reassembly?   
       

Quote from: js1970TP301 to G - 65mV
          TP302 to G - 63mV
          Btw TP301and TP302 - 2.3 mV
         Voltage at collector of Q308 -  the DMM flashes 2.3V breifly then holds about
         1.2V

The actual half-rail offset voltage will be half way between these;

(63+((65-63)/2) = +64mV DC offset - good.

As a check;

(65-63) = 2 (should be 2.3V) - good agreement.

Idle current (and we have to resort to Ohms Law).

Vbtw = 2.3mV
Rbtw = 2 * 0.47 = 0.94 ohms

I = E/R
2.3/0.94 = 2.45mA - is something, but it's not much.  About one-tenth or less of expected 20-50mA.

In this context the voltage across bias transistor Q308 also seems a bit low at 1.2V.

At a minimum I'd expect four times Vbe plus some more for the emitter resistor voltage drop.

In (round) numbers;

4 * 0.6 = 2.4 + (0.02 * 0.94) = 2.42V

So at the very least it has to be 2 volts to even start current flow, and more like somewhere around 2.4V for correct idle.

However we adjust this by working out the voltage at the test points that will give us the desired idle current, then with the meter clipped on and holding our breath, we twist the tail of the tiger, SVR302.  {if you badly overshoot on this critical adjustment you can easily melt the output stage.  Initially you move the pot a little bit and observe what happens.  (also: keep a note of the pot initial position)}

Let's aim low first, 20mA idle.  At this the voltage between the Test Points will be;

E=I*R
0.02*0.94=0.02V or 20mV

So as they have arranged it, the voltage between Test Points is almost exactly one millivolt per milliamp - just hook up the meter on DC mV and read it as mA idle.

  I went to increase the idle as suggested. When I turned the dial, there was no change in the value. Its currently set about halfway, as from the factory, and I turned in both directions with no change as the result. Is there something I'm missing?  Should I run the amp to get good and warm before adjusting? I can't help but think there is still a component that is faulty. As I'm posting this, I'm thinking I will meter Q308 again. This time from base to emitter, rather than just collector. To be sure. Could the trimmer be bad,and showing high resistance? That is, if all other values are within reason.

Quote from: js1970When I turned the dial, there was no change in the value.

This is not right.  {I hope you are turning the correct trim pot, SVR302, and not the Offset Adjustment SVR301}

There might be a small difference between cold and warm but the action of the bias setting should be obvious at all temperatures.  (yes, bias should normally be set with the amp at warm idle, but seeing that you can't set it at all it's a rather moot point at the moment; but that's how you should finally tweek it when you get it working correctly.)

Quote from: js1970Is there something I'm missing?

Let us instead say that there is still some problem yet to be found.   8|

Quote from: js1970I'm thinking I will meter Q308 again. This time from base to emitter, rather than just collector.

Actually the voltage we are interested in the Q308 bias transistor Collector to Emitter voltage, the voltage across the transistor, not to ground.  You will see on the circuit that this sets the voltage between the driver Bases of Q309 and Q310 and should be variable from very roughly 1 volt up to a maximum of about 2.5/3.0 volts.


{This particular bias arrangement has a serious weakness; if the wiper of the trim pot should lose contact with its track (a not uncommon thing to happen) the bias transistor Q308 will lose its Base current injection and switch off.  This will cause the idle current setting to go to maximum current - not a healthy situation.  For this reason whenever you are messing with a s.s. bias adjustment you should have the amp on a limiting lamp to avoid destroying the output transistors.  There are some smarter arrangements where the wiper going open causes the current to go to minimum, a much safer arrangement.}


Try clipping your multimeter across Q308, Collector to Emitter (or the Base of Q308 to the Base of Q310 driver transistors, which is the same thing).  We are looking for around 2-and-a-bit volts, and this should change when trimmer SV302 is varied.  As it doesn't seem to be having any effect on the idle current it would seem that it isn't making the 2.4-odd volts required to start turning on the output stage.

The question now is, is it the bias circuit that isn't developing enough voltage, or is it something following there (e.g. a driver transistor) that is failing to transmit the change to the output transistors?

If the voltage across Q308 varies in sympathy with the pot then it's something downstream.  But if the voltage across Q308 doesn't change with the pot then it's something in the bias circuit, Q308 and surrounds.

Please check and report.

   I metered the voltage btw collecter and emitter of Q308 to be 1.9V. I metered btw Q309 and Q310 to be 1.8V.Then, I was going to meter btw bases of Q308 and Q310 to compare. When attempting this, I shorted across the legs of Q310 and popped the fuse. :-\ ( boy it didnt take much!) I didn't get the chance to try the trim pot while metering Q308. Hopefully, I didn't cause any damage when this happened. I'll try to acquire a fuse tomorrow, from the shack,and get back to the business at hand.

Oh dear.  A bit of a setback.

{In a previous life I serviced Variable Speed drives up to 100HP, rows of Silicon Controlled Rectifiers as big as beer cans directly connected to 415V three phase mains, and supposedly "protected" by expensive ultra-fast "solid-state fuses" specifically intended to blow before the SCR it was "protecting".  The trouble was that it was neck-and-neck which would melt first, and typically the SCR would short and then blow the fuse (so any old fuse would have done that job).  If you found a blown fuse it was more than likely that the "protected" SCR had shorted and had to be replace as well.

Generally I try to be hopeful, and if you had the limiting lamp in series you might get lucky, but you will need to take a step backward and treat the output stage as if it has just had a blow-up and bring it back up gently (lowest wattage limiting lamp) until you are satisfied all the transistors actually survived.

Try not to be too hard on yourself, this sort of accident happens to the best of us and is very annoying, but we just have to dust ourselves off and recover the situation.  This is why I said "clip the meter probes" - if you make your test connections using grip probes or clip leads before you power up for the test the chances of a slip are much less, but even then clips have been known to suddenly let go and float around.}

Anyway we've got something, 1.8-1.9V doesn't seem enough so this is suggesting that the problem is in the bias circuit around Q308.

Once you replace the fuse bring the amp up with the lowest wattage limiting lamp and check that the half-rail is very close to ground, and if it is you have got lucky, but if not you will have to reapply your cold resistance tests to the driver and output transistors until you find the one that has failed.  Fingers crossed.

    I tested the driver and output transistors out of circuit as was described on page one of this thread. I found one of the output transistors (Q313 to be exact) to be faulty. I was unable to get an OL display on the meter in any combo of the leads. I have an exact replacement on order. Should I get a matched pair, or will just replacing the offending transistor be sufficient?

Quote from: js1970Should I get a matched pair, or will just replacing the offending transistor be sufficient?

"Matched pair"?  Man that's as flash as a rat with a gold tooth.  No, a simple replacement should do just fine.

I hope you are quite certain about the health of the other transistors, in particular the associated driver transistor.

   I'm fairly certain the other transistors are fine. I tested them out of circuit using the diode mode on my DMM. The respective output transistors were consistent with each other and the drivers checked out too. The funny thing is, the driver transistor I shorted across (Q310) is opposite the output transistor I found faulty (Q313). At least, I thought it odd. Is this cause for concern?
   

Well that is a little odd, but it's hard to say, even knowing exactly what got shorted to what.

What we can say is that like any blow-up repair it's important to "hasten slowly", use your limiting lamp, carefully check supply and half rail voltages, be suspicious, don't assume, when in doubt stop and have a good think.  Be alert to the possibility of another fault still lurking undetected.

 I've got the new output transistor in, and everything powered up. When all said and done, all voltages are back to where they were previously. Except the halfrail voltage. That went up to 69.4mV.
  Satisfied, I checked the voltage across collector/emitter to find the same 1.8V. There was no change in value when adjusting the trimmer. While I had some downtime, I discovered a diode I believe faulty. It is the D306 diode off of the scheme,and it had a drop of .091V one way and .109V another. Pretty sure that's a no good. All other similar diodes had a drop of .581V one direction only. These were D303,304 and 305. It doesn't seem as though this would effect my cold bias problem does it? Seems more in line with my little blow out earlier.
 

Quote from: js1970D306 diode off of the scheme,and it had a drop of .091V one way and .109V another. Pretty sure that's a no good.

I'm pretty sure you are right, that ain't no diode no more.

It's one of the gating diodes into the protection circuit, so I would replace it and check the other gating diode D305, and protection transistors Q312 and Q311.

Yes this may have been because of your slip, still, fingers crossed eh?

Once SVR302 starts to vary the voltage between the bases of Q309 and Q310 you are really getting somewhere. (but keep that limiting lamp in circuit until you get the bias under control - don't want to go blowing any more transistors).

  Hey everyone.I finally got my hands on the transistors I needed. After I found I had a faulty diode(D306), I checked transistors Q311,312 as Roly had suggested. I  found Q312 to be bad.
  I swapped the offending components and did some metering. Here's what I found
     TP1 to g~ 16.1 mV
     TP2 to g~  -1.5mV
     TP1 to TP2~  19.2 mV
     Halfrail ~ 9.8 mV
   V btw Q309 & 310 ~ 2.2 V   this varies with trimmer. This voltage trended down to     2.15V after a bit. That is to say, with thermistor 302 btw my fingers, and feeling the transistors for warmth. All ran well with Q314 & 319 getting warmest, almost hot.
   This was  all while plugged into my limiter lamp.
   From what was discussed in previous posts, the 2.2 V btw the bases of Q309 & 310 has me feeling optimistic. The other measurements, however, seem a little inconsistent with each other to my untrained eye. What do you think?
   

Quote from: js1970TP1 to TP2~  19.2 mV
     Halfrail ~ 9.8 mV
   V btw Q309 & 310 ~ 2.2 V   this varies with trimmer. This voltage trended down to     2.15V after a bit. That is to say, with thermistor 302 btw my fingers, and feeling the transistors for warmth. All ran well with Q314 & 319 getting warmest, almost hot.

I think that is looking a lot more healthy.

Idle Current:

19.2mV across 2 * 0.47 ohms

I = E/R

0.0192/(2*0.47) = 0.0204 = 20mA

That looks very reasonable to me.

9.8mV half rail offset is good.

Response to trimmer and finger heat is good.

Uneven heating is not so good.  You should check that you have about 10mV across each of the 0.47 ohm emitter resistors (to confirm that they are all intact, none open circuit).

If all seems to be well electrically it could be due to these two devices having worked a bit loose/not being firmly clamped down, or that the heatsink compound under them has dried out and become ineffective.

On a low level test (with limiting lamp) this should now sound a whole lot cleaner than before because the output stage is now correctly biased.


You may now care to move up in steps with your limiting lamp, confirming all is still well (which I expect it will be) then to full mains, and finally a high power soak test into your dummy load.  You will need to keep a close eye on it to make sure there is no inclination to go into thermal runaway.

   I checked the emitter resistors. This is what I found in mV:
     R325~ 14.4        R324~12.8
     R327~6.2           R326~8.6
     R329~ 5.3          R328~2.4
     R331~ 5.1          R330~10.2
   These were taken at idle,no load,no signal.
 
   In a previous post, I mentioned that a couple output transistors seemed warmer than others, almost hot. After some research, I found I should have fastened everything back to the heatsink. I have since done this.
   All is running well with bias voltage starting at 2.388V and after an hour at idle I'm at 2.350,with all hardware at even temperatures.
  Curious, I plugged the bass in and started thumping away. That is, signal in,no load and still watching my bias. With the volume up I noticed the bias Voltage jumping erraticly as I hit thee strings, sometimes going as low as .8V. Is that normal? Im hoping I stumbled across a normal audio phenomenon. What do you think?
   Still curious, I plugged into load. Sure enough, when the bais went below 2V or so, it resulted in crossover distortion. When I stopped playing the bias went back to a comfortable 2.343V. It went as far as opening the relay at a certain point. Where to look next?

Quote from: js1970After some research, I found I should have fastened everything back to the heatsink.

Arrrggghh...   :(

Quote from: js1970With the volume up I noticed the bias Voltage jumping erraticly as I hit thee strings, sometimes going as low as .8V. Is that normal? Im hoping I stumbled across a normal audio phenomenon. What do you think?

The bias is a static setting, so when you superimpose tens of volts of signal you could get anything at all.

Quote from: js1970Still curious, I plugged into load. Sure enough, when the bias went below 2V or so, it resulted in crossover distortion. When I stopped playing the bias went back to a comfortable 2.343V. It went as far as opening the relay at a certain point. Where to look next?

Yes that still doesn't sound 100% healthy, however it may be an artifact of trying to drive it too hard via your limiting lamp and the supplies are sagging.  The relay is detecting DC offset on the output half rail. (so at least we now know that is working)

Since it now seems to be idle biased and maintaining the half rail I'd be inclined to try a larger wattage limiting lamp and see if your symptoms get better.

Another reason I suggest using a sig gen or MP3 player as a test signal source is that it is generally fairly compressed and a lot less peaky that whacking guitar strings.

    I ran though it again to ,hopefully, get a different result. I ran the hartke through a 100W floodlight bulb, and into the 4x10 cab. For a signal, I have a Roland micro cube that has a tone gen. function. I plugged the signal into both the active and passive inputs on the head, both having the same result.
    Power on, fired up nice, circuit protection relay clicked closed.
    Again, bias started at 2.440V measured btw driver bases.
    At very low volume, situation normal; even signal, minor fluctuations of the bias voltage, limiter bulb holding steady.
    As volume increases; bias voltage drops sharply while my limiter bulb sympathetically, if not proportionately, gains briteness. As the tone decayed so did the bulbs britness to its steady low volume illumination.
    Needless to say, I'm reluctant to go full mains.
    Why does the bias drop like that with increased signal?
    You said the bias is a static setting, which i take to mean it is set  for the power amp to run most efficiently to amplify a signal, but with the signal effectively stealing voltage from this setting, how can the power amp be working sufficiently, let alone efficiently?
    Perhaps my apprehension is due to my inexperience, and if it is I do apologize.
Just trying to make sense of it all

Quote from: js1970I'm reluctant to go full mains.
    Why does the bias drop like that with increased signal?
    You said the bias is a static setting, which i take to mean it is set  for the power amp to run most efficiently to amplify a signal, but with the signal effectively stealing voltage from this setting, how can the power amp be working sufficiently, let alone efficiently?

Uh huh, this is about what I'd expect.

Limiting lamp, increased signal, increased current, supply voltage droops, bias circuit responds.

Quote from: js1970I ran the hartke through a 100W floodlight bulb

In my workshop if that didn't produce smoke I'd call it "fixed".   :dbtu:

---

To recap; the bias is required because the transistors in the top and bottom of the "totem poll";

(http://www.ozvalveamps.org/repairs/solidstateamprepair/outputcctshp.jpg)

... don't start to turn on at zero volts but about 0.6 volts, making a deadband of around 1.2 volts while the signal crosses zero.

(http://www.ozvalveamps.org/repairs/solidstateamprepair/directcoupling.jpg)

This is extreme Class-B and produces horrible crossover distortion.

(http://forum.allaboutcircuits.com/attachments/dd-png.73373/)

To eliminate this the output bases are driven with an inbuilt bias between them to bring both to the edge of conduction while idle, and to ensure that there is a smooth change over of conduction between them at signal zero crossings.

(http://www.ozvalveamps.org/repairs/solidstateamprepair/bias.jpg)

Away from zero the bias has little effect since one or other of the output transistors will be in major conduction and the other off, even 'tho "biassed".


{if you really want to get into the nitty-gritty you need to look inside the devices, at their models. 

(http://www.maximintegrated.com/en/images/appnotes/697/E37Fig2A.gif)

Particularly with power devices, you don't just have an "ideal" transistor.  In this case you need to consider that each electrode has its own parasitic resistance in series, so that when the base current goes up the actual pin voltage will be somewhat higher than the theoretical B-E voltage of the ideal (internal) device.  But you can't actually get to that, you have to deal with the real-world at the leads, and that includes electrode bulk resistance that gets significant under some conditions.

Similarly a power FET may look like an ideal switch under DC conditions, but the gate has a huge parasitic capacitance, so if you want to run one in Class-D at 100kHz clock rate you have one hell of a problem driving that gate capacitance fast enough so it doesn't get horribly inefficient and lossy.

Water is wet, rocks are hard, and devices have real-world limitations you just have to work around.}

HTH

  Thanks Roly,for the info. Your patience hasn't gone unnoticed or unappreciated.
  With that being said....
  I ran the amp at full mains ,only to have it overheat. After about 15 min, the protection relay clicked open, and the heatsink was hot to touch.
  I let it sit awhile and checked everything through the limiter.
   TP1 to G ~ 8.6 mV
   TP2 to G ~ 6.2 mV
   TP1 to TP2 ~ 2.6 mV
   Halfrail 6.6 mV
  I think these are good, yes?
  Then I disconnected the limiter and ran full, btw TPs it started at roughly 63mV and was rapidly rising. I shut er down without the other supporting measurements.
  So basically, the amp tests good under limited power, and drives hard on full.
As I'm writing this, l can't help but wonder if maybe R321 or R317 has gone bad. My thinking being, all values are good with the limiter providing the resistance needed. Take away the limiter....danger Will Robinson.
   

 

Quote from: js1970l can't help but wonder if maybe R321 or R317 has gone bad

Well don't just stand there muttering "short circuit" lad - lengthen it!  Out with your multimeter and prove the point to your satisfaction one way or the other.

Quote from: js1970I ran the amp at full mains ,only to have it overheat. After about 15 min, the protection relay clicked open, and the heatsink was hot to touch.

I assume this was at or near idle, not fully driven.  If 15 mins of full drive opens the thermal cutout then I wouldn't be impressed, but I also wouldn't be too worried (and I'd fit a fan).

But the thermal tripping after 15 mins only on idle is a real worry.

It could be that the idle bias is set too high, but it certainly seems to be that the thermal compensation, thermistor TH302, which is supposed to reduce the bias as the rig gets hot, isn't doing it job.

This thermal compensation is more normally a couple of forward-biased diodes.  As they get hot the idea is that their forward voltage drop falls in step with the B-E junctions of the drivers and OP transistors, and ideally the bias current remains rock steady from freezing to roasting.  Ideally.

In more professional rigs you see these compensation components actually mounted on the heatsink, which makes very good sense since that is what they are supposed to be responding to/measuring.

In consumer stereos they are almost always fitted on the PCB nearby on the assumption that, given the amplifier duty, they will be about the same temperature.  In high power rigs this is not a safe assumption, and simply having it out of its case can effectively decouple the sensor from the thing it is supposed to be sensing.

Now you previously observed that heating the thermistor with your fingers caused the bias current to drop, so that suggests that the circuit itself is working properly, just that it isn't having enough effect to hold the bias under control.


However;

Quote from: js1970the protection relay clicked

All this does is look for standing DC on the output, it isn't the thermal cutout.

So I'm wondering; you previously set the bias current, but wasn't that with a Limiting Lamp in series?

Perhaps you should back the bias off, connect the amp direct to mains, then carefully re-set the bias to the correct value on full mains voltage, and closely observe it as the rig warms up.

Let's see where that leaves us.

   Heh heh, yes I hear ya. I metered all of the 2W resistors to find good values. R317 ,however, was super hot and was found with 83V and 43V on the legs relative to ground. This was at idle under full power and bias set.
   I reset the bias like you said. At idle btw the bases of Q309/310 I have 2.540V and trending down as it warmed up. Also, btw the TPs, I started at 24mV which dropped steadily to settle about 16 mV +/- 1mV.
   These measurements were taken over 30 min at idle. The components were slightly warm, except for R317 which was very hot.   
   Should I be concerned that this resistor got hot,or that the others did not?
   Does it seem like a lot of voltage on it?

Quote from: js1970I reset the bias like you said. At idle btw the bases of Q309/310 I have 2.540V and trending down as it warmed up. Also, btw the TPs, I started at 24mV which dropped steadily to settle about 16 mV +/- 1mV.
   These measurements were taken over 30 min at idle.

This all looks quite good indeed.  The bias may be a bit over-compensated for temperature, but that's not particularly abnormal and far better that than going into thermal runaway.


R317
What we've got here is a differential or Long Tailed Pair, Q304 and Q305 (common tail above, loads below).  The Q305 side is the drive for the power amp, the R317 side its differential partner.  Now what they have done is run these two LTP loads into a current mirror, Q306 and Q307, and if you suspect that something is amiss I would have these transistors out for checking (however it is not uncommon for power resistors to run too hot to touch.)

{Note that the preceding stage is also a LTP but the other way up, and a current mirror has been used to set the tail current here.}

To be specific: on the circuit I'm looking at <Ha3500.pdf> R317 is a 15k/2W in the collector of Q304.

83v and 43V on 15k

I = E/R
(83-43)/15 = 2.66666667mA

P = E * I

(83-43) * 2.66666667 = 106.66666680mW

This seems very reasonable, a tenth of a watt ain't much, so something doesn't fit here, your observation doesn't make sense with my circuit.

   I did some research and looked into the Long Tailed Pair circuit. I'm trying to get the gist with regards to what you said. I'm not sure what doesn't fit. Is it that I'm getting 40V difference at R317 ,or that if 2.67 mA are flowing through it,it should not be so hot.
 

The Long Tailed Pair, a.k.a. a differential amplifier or emitter/cathode-coupled amp.

(https://www.st-andrews.ac.uk/~www_pa/Scots_Guide/audio/part1/fig7.gif)
(the interlocking circles at I_E is a constant current sink, but often just a resistor)

The "long tail" comes from the shared emitter (cathode) resistor.  This is normally a high value connected to a fairly high negative voltage, and therefore approximates an infinite resistance connected to an infinite negative voltage, the result of which is (almost) constant current.

A refinement is to make this common tail an active current sink (source) so that within the limits of available voltage it really does act like and infinite resistance connected to an infinite negative voltage.

(http://www.learnabout-electronics.org/Amplifiers/images/constant-current-source.gif)


The "differential" comes from the fact that it amplifies the difference between the pair Bases (grids) and also has a differential output available at the collectors (anodes).  As such it is a very sensitive amplifier, 'tho easily driven into overload.  Ideally the diff. pair are quite deaf to any signals in common mode to both inputs.

(http://www.eie.polyu.edu.hk/~ensurya/lect_notes/commun_cir/Ch2/Fig2-29.jpg)
Because the total tail current is constant, so therefore must the sum of the Collector currents be.  As one collector current rises from zero so must the opposite Collector current fall by exactly the same amount, so the Collector currents (and voltages) mirror each other.


The "Emitter/cathode coupling" comes from the signal passing from one device to the other via their shared Emitter/cathode circuit.

The LTP structure forms the basis for many op-amps (look at op-amp datasheet internal circuits (http://www.digchip.com/datasheets/parts/datasheet/321/LM741.php)).


(http://www.sentex.ca/~mec1995/tutorial/xtor/xtor4/4xtor16.gif)

In audio power amps the signal input is generally the non-inverting (+) input while the other is the inverting (-) input and is connected to the main output to provide overall Negative FeedBack.

Because of the high gain the LTP is sensitive to any difference between the two active devices, in particular the thermal change in the Base-Emitter voltages.  For this reason they are sometimes matched pairs, and you encounter twin transistors with six legs so they are tightly thermally coupled (apart from being inherently matched twins on a common chip).

{on the other hand Elektor made use of this "shortcoming" by using two fixed-bias LTP's at right angles  as a no-moving-parts wind speed and direction sensor, the wind cooling the upwind device more and producing a voltage proportional to the vector of the wind.}

Quote from: js1970I'm trying to get the gist with regards to what you said. I'm not sure what doesn't fit. Is it that I'm getting 40V difference at R317 ,or that if 2.67 mA are flowing through it,it should not be so hot.

Simply that your two observations are contradictory.

Your voltage measurements suggest that the resistor is dissipating one-tenth of a watt (and these numbers look pretty sensible), which is almost nothing and you should be able to hold the resistor between your fingers and feel a bit of warmth.

Yet the resistor is getting very hot.

These can't both be right.

I'm inclined to place more trust in a burnt finger than your measurements or my calculations, but there is an anomaly here, and "following the bit that doesn't fit" is always profitable when faultfinding.

I can't guess what the answer will be, but these two observation are uncomfortable bedfellows - there is something that needs to be resolved here.

   After reading your last post, I diode tested the transistors and they seemed up to snuff. So I cleaned off the cooling fan, fastened everything down tight, and ran it. I had to keep the amp at sensible volumes, but nevertheless, I ran it for about an hour. I must say that thing is a beast. Punchy, defined, with gobs of power. At times, it seemed almost too punchy, too powerful ( if indeed, there is such a thing).
   Satisfied, I took the lid off and checked the resistor with the back of my finger. Barely warm! Maybe my original observation was a bit askew, or maybe the thermal management of the fan and heatsink made it a non issue. I'm going to take it  to a jam and see how she does.
    Roly, I'd like to thank you and all from the forum again for your help with this fault finding mission. I like to think I learned a little bit, and fear I have set on a new hobby to which my lovely ,understanding ,bride will be thrilled.

As long as somebody is receptive I'm happy to go on 'splainin' 'til the cow come home.

Quote from: js1970After reading your last post, I diode tested the transistors and they seemed up to snuff. So I cleaned off the cooling fan, fastened everything down tight, and ran it. I had to keep the amp at sensible volumes, but nevertheless, I ran it for about an hour. I must say that thing is a beast. Punchy, defined, with gobs of power. At times, it seemed almost too punchy, too powerful ( if indeed, there is such a thing).

That's "fixed", yup.   :dbtu:

Quote from: js1970Satisfied, I took the lid off and checked the resistor with the back of my finger. Barely warm! Maybe my original observation was a bit askew, or maybe the thermal management of the fan and heatsink made it a non issue. I'm going to take it  to a jam and see how she does.

A bit of (forced) air movement over a component can make a very large difference to its temperature, but it's this final good result that is the key point.

Quote from: js1970fear I have set on a new hobby to which my lovely ,understanding ,bride will be thrilled.

Ahhhh.  Now, a couple of tips.

The first is that electronics, building or faultfinding, is actually quite addictive.  My wife (my third :o) is a puzzle buff, crosswords, Suduku's, and the like.  I get a similar charge from solving faults on the bench, and this "charge", endorphins or whatever, is quite addictive.


Secondly, electronics generally isn't horribly messy like working on car engines or building surfboards, but it can be a bit ... untidy.

{My wife's ex worked on big engines and had one in bits in every room of the house except the bedroom, where he had been clearly warned, so when she came home and found another one pulled down in the bedroom and grease on the bedsheets that was it.  Finish.}

For a time we lived in a small flat and there was nowhere that could be set aside for working on electronics projects, so I had a large cardboard box I could put on the dining table and work on stuff without any danger of damaging the surface or leaving solder splashes everywhere, and which could be packed up and hidden away to restore what girls like to think of as domestic normality.

Some flat dwellers have even built specific boxes that open into a mini-workshop or Ham station where it can all be hidden away.

So my hints are;
1. look after her in bed.
2. promptly fix everything that isn't working properly, blinds, heaters, door catches, etc, and she'll be a lot more tolerant of your electronic man-interests.
3. Chocolate.  It's like beer to us or catnip to cats, but only in small quantities of the highest quality you can find.  Here we have "Tim-Tams" which are not particularly up-market, but the dark chocolate ones are very effective at calming upset partners;

(http://www.chocablog.com/wp-content/uploads/2009/04/timtam-sweet-surrender-2.jpg?0247c8)

And if she ever starts complaining about "all these wires everywhere" gently remind her that you don't work on cars (and make certain that nothing electronic ever turns up in your bed - they really don't like that).

It helps a lot if she's into something creative herself such as music, photography, painting, dressmaking, pottery, writing that killer novel, radiation polymer chemistry, &c&c.

4. don't take my advice on relationships.

ATB to you both.

https://www.youtube.com/watch?v=m2ou-WIxfLY (https://www.youtube.com/watch?v=m2ou-WIxfLY)