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Messages - phatt

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Preamps and Effects / Re: LND150 Jose style preamp
« on: July 19, 2019, 09:03:12 PM »
There is something wrong with the footprint of the electrolytic caps,, the square pad is usually the positive terminal?? :o

Oh BTW,,, I hope you intend to Bread board this long before you etch a PCB... Cause there is a whole lot more to building such a design than just cramming everything onto a blank with no understanding of all the pitfalls of Hi Z circuits. Layout is key to low noise/hum.

I fixed a Fender Pro Junior some years back as it had a gawd damn awful hum. :grr
Tracked it down to the input of the second stage track running right beside the B+ track. Which goes to prove even the biggest names on the planet who you would think should know by now that it would cause a major hum issue?  but obviously NO :duh :duh :duh :duh xP xP xP :loco :loco :loco
Nothing wrong with PCB's but once etched and populated it's a nightmare to fix any major design flaws. :grr

BBoard test it first will tell you how to do layout the PCB tracks. (Hint) 8)
As the old saying goes "More haste = less speed" :lmao:

If you have full rail voltage at +C0 (the output) then it's a fair bet at least one power transistor is dead.
The problem with these circuits is that they are DC coupled and if one Transistor blows it usually takes out (or severely stresses) the other components.

In this case *DC coupled* stands for Direct Coupling of active devices.
There are no interstage capacitors to block the DC voltages so if a resistor or transistor goes out of spec then high currents flow and burn up transistors.
As these Amps are getting old then it's common for things go wrong.

As you can't take voltage readings with the board in place then it might be easier to replace TR20 to TR23 and build yourself a light bulb limiter Shown here;

Then if things are still not right you won't blow up brand new components when testing.

Depending on circuit design and what has actually blown, some Power amp circuits will still pass signal even with blown output Transistors.
In this case maybe Tr20 &Tr21 are still working giving the low output volume you noted.

You won't be able to balance the output (with VR2) at C0 to half voltage until the offending devices have been replaced.

And same goes for VR1 which adjusts the idle current for the power transistors.

Better minds here might have easier/better ways to fix this but I'd just replace the right hand side TR20 to TR23 and fire up with the limiter and see if you can centre the C0 point and set idle current.
Then run a signal through it.

Don't worry about VR3 until you get the DC working,,VR3 is just for balancing the AC signal entering the amp.


Amplifier Discussion / Re: Peavey Session 400 1976 troubleshooting
« on: July 12, 2019, 08:29:02 AM »
Thanks Phil - I appreciate the explanation very much. I need/want to comprehend the theory of operation of everything I work on. I have much to learn.
You are welcome. :tu:
This guy has a ton of well written pages and I highly recommend it;

Main page;

Click on Articles,, Projects,, and Site Map to find a ton of in depth info :dbtu: :dbtu: :dbtu:
I've found it easier to use the site map to find stuff of interest.

The Reverb circuit (preamp) schematic is posted earlier in this thread.
Durh :-[ silly me yep I should pay more attention,,  :crazy2:

I can't see the driver plug clearly but the return plug has 3 wires.
This makes me think there might be a ground missing for the drive.
Darn hard to explain this with words but always remember you have to check the ground path of circuits as well as the hot wires.
If driver output is grounded via the return plug it may not be connecting back through the tank. look at the schematic might help.

Sounds like you are mixing up test points;
CO is the Poweramp output which sits at Half the supply voltage.
The highest voltage in the amp is at +B (next to the 3 Amp fuse,, which supply the collectors of TR20 & TR22,, as well as TR24 & TR19.
Or pin 6 on the board.

PIN5 should read half voltage which is the *CO* voltage,
So if you have 100VDC on +B supply then *CO* will read 50VDC.
IF point *CO* also reads 100VDC then likely the power transistors are dead.

The fact that you have altered that trim pot means the amp is now overheating and pulling a LOT MORE Current dragging the supply lower likely to kill even more parts.

As I said before check those voltages which are all over the schematic,, the ones in little boxes,, they are the DC voltages you should be seeing if they are way off it gives a clue as to what might be stuffed.

If the Amp is working close to spec then you should see some where close to 137VDC at the Collectors of TR24,19,20,22. with all controls at zero.
if you only get 90~100 then something is likely blown and dragging the supply down.

Amplifier Discussion / Re: Peavey Session 400 1976 troubleshooting
« on: July 11, 2019, 08:34:46 AM »
I may not be reading it right but the best I can make out is that it all seems ok.

Easiest way to grasp this is maybe I try and explain the basic concept.
It's a split supply and the output sits at Zero VDC.
It's never perfect so always a little DC offset. So 160mV of DC on the output (Pos or Neg) is normal, as long as it's close to Zero.

So the Emitters of the top 3 Transistors are going to be very close to Zero VDC.(which is what i think I'm reading) While the Emitters of the bottom 3 Transistors are going to be within a bees dick of the Neg rail hence you noted -52Volts on those resistors.

The basic plan of these circuits is to keep the base's of the two input transistors which form a LTP (long tail pair) as well as the output as close to Zero as possible, Feedback (that 100k) helps to keep this in check.

The diode string (2 in that dotted box and the one below) bias the output Tr's. The 2 Tr's beside the bias are just for safety,, they limit the output current if the output is shorted out.

The Tr down the bottom left is the voltage gain and then sent to the output TR's which turn that into current to drive the low Z speaker.
Hope that helps.

without a full schematic No way to tell what might be wrong with Reverb.

Divide and Conquer.
To test the power amp separately from the preamp just lift that orange wire (comes off TR10 to point 0 on pcb)
That is the preamp out/power amp input. So rig up a temporary input socket, make sure it's got a ground connection to amp chassis and plug your guitar into that,,,
you should get a clean undistorted signal. it won't be super loud but if it can hardly be heard then the power amp has a problem.

Or better still plug in your mp3 player or other audio player with line out/headphone out  and it should blow you head off at full volume on your player.
If only weak signal is present then you know it's a power amp problem.

If Power amp test is good then you know the preamp is at fault.

Does the distortion work?

OK, maybe check those voltages shown on schematic,, see if any are way off.
TR5 looks like it's switching the Dist on/off ,,maybe it's not working right.
Others here will know more,, Phil.

Amplifier Discussion / Re: Ampeg SVT450
« on: June 28, 2019, 09:51:41 AM »
D4 might be shorted,,Maybe check the other Diode points marked *P*,, D5 & D6 and see if they have similar readings.
Those *P* points go to R40 Which looks like the peak detector circuit.
Maybe Q3 might be stuffed.

Amplifier Discussion / MaxTurbo mods
« on: June 19, 2019, 09:24:04 PM »
I picked this up cheap and took the chance it might be an easy fix but proved to be quite a task.
It worked but had very low output for the claimed 40~50Watts. more like 10Watt :duh
Obviously a no name import job but as it was a small 10Inch speaker, just the size I was looking for as a small grab and go amp.

It says *RSGA5010R* on the front and a small badge on the grill that says *Max Turbo*
I only found one small reference to it on the net giving the specs but nothing more.
Power chip is TDA7295 and runs on +/-40volt rails.
I doubt there would be many of these around but if you have one it's likely got the same design problems so if you need all the mods I did to get it working I'll up load the schematic of all the mods.
meantime I've traced the main preamp circuit and added the main mod that fixed the notch filter issue.

It took a while to work out the circuit because double sided PCB's are a royal PITA to back track.  :grr
I'm sure I will have missed something but these schematics will be fairly close. I've tried to label the part numbers as found on pcb.
I have not drawn up the power section as it seems like it's taken from the data sheets of TDA7295.

After a while I found some of the missing power was due to a bent pin of an IC that was never soldered. Wooden chop sticks are a great fault finding tool when you
probe around. ;)

Improved power but winding the treble up past 3/4 invoked some really harsh hi freq distortion more like a crackle which was evident from the start but now even moreso.

Along with the bass treble there is a notch filter knob but it did not seem to work in any meaningful way.
So I finished drawing out the schematic (after many attempts) and simulated the circuit.
And sure enough just as my ears had noted, that Notch filter network seemed to have little effect on the tone shaping.

Up shot,, U5 (which is what the notch control pot connects to) seems to make no sense.
Be aware I'm still not sure how this circuit is *supposed to work* so I'm guessing a lot here. :-[ :-\
After many sims I took a gamble and connected the *top of VR5b* to the *input of U7* via a 10k resistor and Bingo the notch sweeped and worked as I would expect and
the bonus was that horrible treble issue was gone.  :) :)

The cab is cheap compressed cardboard crap so keep it away from water and it should be ok.
This thing had 10 miles toooo much bass so I replaced most of interstage 100nF caps with 10nF which made the amp usable at full volume.
I plugged the ports on the front which also helped the sonic result.

As I had a short Reverb tank laying around doing nothing so added a Reverb circuit between preout and power in,, a nice addition. 8)
The send return on the front is useless as it only works on the clean selection so when OD is engaged the FX loop is bypassed. WTF???
I'm stuffed as to why they made it so, maybe another design stuff up??

Amplifier Discussion / Re: Fender Acoustasonic Jr. Squeal
« on: June 15, 2019, 09:07:04 AM »
OK Obviously a Electrical problem and not acoustic FBack.

Ok the reverb was faulty and you fixed it but if you are not familiar with reverb circuits there are a few thing that can catch you out.
1/ check the tank leads for correct in/out. The owner may have tried to fix it and messed up the plug connections. So don't assume it was correct when you got it.
(some rev circuit will squeal if plugs are reversed)
2/ check your work and make sure you did not inadvertently ground the wiring on the drive end of the tank.

Note that the driver circuit (U9A) output is not grounded,, while (U9B) the Pickup circuit IS Grounded.  If the leads have been reversed it will likely squeal like a pig.

The Ground lifted output of U9A has to be reflected in the tank connections as well.
If you follow the ground connections in the tank you will (should) find that the tank Case is only grounded on the Pickup end.

If unsure how to establish correct in/out then rule of thumb is the drive transducer will be the lower resistance. Ball park readings would be ~30-Ohms for drive Tx and ~200-Ohms for PU Tx.
Only the Pu Tx is grounded to reverb tank if both Tx's are grounded to case then you may have  wired it incorrectly when you fixed it.

Amplifier Discussion / Re: Fender Acoustasonic Jr. Squeal
« on: June 14, 2019, 03:17:40 AM »
Welcome D16,, :tu:
I assume you have an electric Acoustic plugged in when this problem happens and you are likely close to the amplifier?

If so then the *Phase switch is working as it should*,, that is why it's there to help stop Feedback. When using E/Acoustic guitar in close proximity to a speaker Feedback is always bound to happen. At higher volumes the phase switch may not be enough and it will squeal like a pig no matter what.  :'(

If it squeals with no guitar plugged in then Yes the Amp may have a problem, if not then the issue is likely the guitar not the amp.
All E/Acoustics exhibit this curse because they have a hollow cavity which will oscillate at it's resonant frequency. Larger bowels are more likely to squeal than smaller ones.
It may not be obvious to the casual observer but Ovation guitars have far less Feedback problems due to the rounded bowel.
The classic wood guitars may well give a better tones but with 2 opposing parallel sides the chances of feedback are very high once you amplify it.

I personally use a bowel back guitar for these reasons. As I'm mostly playing through amps Sure the unplugged sound is not as good as other guitars but once I'm plugged it's a big sound. For me THAT is when I want it to sound Great.

In My Observation if you want to play Amped Acoustics then Test for Fback problems before you buy.

Tubes and Hybrids / Re: ENGL Fireball E625 Head white noise issue
« on: May 31, 2019, 04:30:57 AM »
If you are not used to really hi gain amps then it might seem like there is something wrong.
Generally a lot of hi gain amp circuits make a fair bit of noise and in a small room it can be quite disconcerting.
But these things are for LIVE gigs and that noise would be drowned out by the high sound levels of the band.
As long as everything works and no other odd quirks then likely the amp works fine.
On the other hand if you can't play the guitar louder than the background noise then you may need to look deeper.

I once tweaked a Laney all valve amp that had insane noise on the hi gain setting and it was factory stock design.
A couple of simple tweaks reduced the noise to a usable level. :tu:

A mate years back purchased his first ever valve amp,, he rang me and asked about the noise problem,, my answer was ,,Welcome to the world of valve amps,, they do tend to hum and buzz a lot. ;)

Amplifier Discussion / Re: Transistors for Unicord Stage 65b Amp
« on: May 26, 2019, 06:08:52 AM »
Try searching this;

2sC693    F7F
2sA640     E76
2sC1175    E7F
2sC536     G7C
2sA659     E6G

google search with "2sc693" brings up this;
It's common ,,they don't print the whole part on the device.
Leave off the the end 3,, they will turn up in a search.

Be aware that a lot of other Equivalent transistors will likely work.
The main thing is to find a device with similar specs.
Main things to note are;
Maximum Collector-Base Voltage |Vcb|: 40 V
Maximum Collector Current |Ic max|: 0.05 A
Maximum Collector Power Dissipation (Pc): 0.1 W
Alltransistor has a Cross Reference Search.  :tu:
HTH,, phil.


The Newcomer's Forum / Re: Heathkit TA16
« on: May 11, 2019, 09:48:32 AM »
Nope ya goin' the wrong way, could lead to more mistakes xP
 Establish those voltages FIRST. 8|
The pic quality is good but we need the whole board
Some good clear pics of both sides of the board will help a lot ;)

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