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hartke3500 blown transistor

Started by js1970, October 12, 2014, 08:00:21 PM

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js1970

   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.

Enzo

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.

js1970

   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.

DrGonz78

#3
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
"A person who never made a mistake never tried anything new." -Albert Einstein

js1970

#4
   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.

Enzo

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.

js1970

   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.

js1970

    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?

Roly

+/-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.
If you say theory and practice don't agree you haven't applied enough theory.

js1970

   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.

Roly

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

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?

If you say theory and practice don't agree you haven't applied enough theory.

js1970

   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.

Roly

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:
If you say theory and practice don't agree you haven't applied enough theory.

js1970

   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.







Roly

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.
If you say theory and practice don't agree you haven't applied enough theory.