Ampeg SVT-6 Pro Help Please

[cowpoke]

I am in the process of restoring an SVT-6 Pro that was blown. I am a collector of Ampeg gear and have brought back to life several different amps but this one is killing me. I am not an electronics guy, but am a biomed engineer and have a fair degree of technical skills.

I have replaced all of the mosfets, diodes, and resistors on both sides of the power section. I have also replaced all of the transisitors and tested all of the diodes and resistors on the main board. I also replaced IC2, but have not replaced IC1(TLO-74).

I am testing the amp outside of the case on a rubber covered piece of plywood with the preamp completely disconnected. This allows me to have better access to test points. I have only the transformer, switch and fuse attached. I dont know if this matters or not.

The main board powers up just fine, fans spin properly, tubes heat up, big resistors are hot etc. No puffs of smoke or sparks......

With only one set of mosfets in Channel 1 (+) I get -65V on the output. If I adjust AP1, the voltage on the output will increase from approx 30V to 70V DC. There is not increase in the voltage accross the .47 ohm source resistors. Actually there is no voltage at all across the source resistors.

With only Channel 2 (-) I get zero DC voltage on the output, but similarly I get zero voltage across the .47 ohm source resistors when adjusting AP2. There is no heat on any mosfets they do not get warm at all.

Any ideas would be greatly appreciated. You can see that some of the voltages seem a bit off, but nothing is really jumping out at me. I am concerned about IC1 (TLO-74) but I don't want to order one unless advised. I have spare parts for all other silicon parts and resistors.

Thanks a bunch.

[nickb]

On Ch1 what is the voltage on the collectors of Q3 & Q4 and also on pin 7 of the TL072 that feeds the two 220k resistors?

On Ch2 ,monitor the difference between the voltage between the collectors Q18 & Q19 as you vary the 10K bias pot. It will also help to know the voltages on B,C and E of Q22 with the set pot midway.

[MarkusBass]

You can see that some of the voltages seem a bit off, but nothing is really jumping out at me.

You're joking, right? You have -65V on the output and 17V on the input of the inverting opamp (IC1:A) and nothing is really jumping out at you?
The IC1 opamp consists of four opamps in one package and some of them are used for the fan but IC1:A is used as inverting opamp for Channel1(-). I think that the voltage you listed on the schematic looks like related to IC1:B, C, or D but not A.
So the measurements seem to be incorrect. Are you sure that you are doing the measurement on pin #2 of IC1?
If you really get -65V on the output of the power amp, you have still burnt transistors (Q3 and Q4 are suspected), or open tracks. When you find the problem (and fix it), I would measure voltages on R16 and R17.

Mark

[cowpoke]

You're joking, right? You have -65V on the output and 17V on the input of the inverting opamp (IC1:A) and nothing is really jumping out at you?
The IC1 opamp consists of four opamps in one package and some of them are used for the fan but IC1:A is used as inverting opamp for Channel1(-). I think that the voltage you listed on the schematic looks like related to IC1:B, C, or D but not A.
So the measurements seem to be incorrect. Are you sure that you are doing the measurement on pin #2 of IC1?
If you really get -65V on the output of the power amp, you have still burnt transistors (Q3 and Q4 are suspected), or open tracks. When you find the problem (and fix it), I would measure voltages on R16 and R17.

Mark

Mark,

Thanks a bunch. I will let you know what I find. Are you saying that IC1 is suspect ?

Mike

[MarkusBass]

Are you saying that IC1 is suspect ?

No, I'm saying that your measurements suggest that IC1:A is used for the fan but the schematic show that this is just voltage amplifier (inverter) for one of the power amps. So this problem has to be clarified. It is also possible that the schematic is incorrect. You have to verify whether the schematic matches the PC board, or not.
Also your measurements are not the one that are important here. Of course, it's nice to check that you have power supply but I think that you should focus on Q2, Q3, Q4 and R16 and R17 voltages. Have you checked D4-D7 Zener diodes?

Mark

[cowpoke]

Thanks Mark,

Is there a reason that I should not continue testing with only the (+ Half) main board, no preamp attached and the (- Half) removed completely?

Also should I test with only one pair of mosfets to prevent ruining parts?

I attached the full schematic. It is the correct one.

Thanks, Mike

[MarkusBass]

Is there a reason that I should not continue testing with only the (+ Half) main board, no preamp attached and the (- Half) removed completely?

I don't see such a reason. There is a note on the schematic that the power amps can be tested separately (with caution).

Also should I test with only one pair of mosfets to prevent ruining parts?

This should be done very carefully because when the current through this pair is to high, you may see some (magical) smoke.
General rule is that you can do a lot of tests but the assumption is that you understand the schematic of the amp. Otherwise, the result may be very poor.
I'm still waiting for the voltages I mentioned earlier (and clarification of the voltages on IC1:A). Without these voltages it will be very difficult to help you. Also, I hope that the MOSFETS are properly isolated from the heatsink.

Mark

[J M Fahey]

1) *for now* I am not concerned about IC1
2) this amp is basically a tube gain stage and a huge voltage follower which provides tons of current to speakers.
That two of them are bridged is just a detail, each half is stillan independent amp and can be troubleshooted on its own
3) we are clearly having big problems with the voltage follower/current booster pack, so let´s concentrate on it first.
4) thanks God it is not DC coupled to the tube section, so it can be analyzed and repaired on its own.
5) you have not supplied important data: we are worried about high DC voltage at the output of this voltage follower , you do not tell us what voltage is it following
Please answer NickB´s questions, asking exactly that

On Ch1 what is the voltage on the collectors of Q3 & Q4 and also on pin 7 of the TL072 that feeds the two 220k resistors?

Only then we can go on.

[cowpoke]

J M,

The collectors of Q3 and Q4 are reading -60V and -59V respectively. Pin 7 of T2 (TLO74) is 16.7V. This was tested with only one set of IRFP's.

[nickb]

J M,

The collectors of Q3 and Q4 are reading -60V and -59V respectively. Pin 7 of T2 (TLO74) is 16.7V. This was tested with only one set of IRFP's.

The TL072 pin 7 section is trying to correct the error but not succeeding. It looks like Q3 is dead although it could be associated components. Test or substitute it first, if that doesn't help measure the voltage on it's C,B &E.

[cowpoke]

ks for your help guys.

I replaced Q3. D2 was also bad.

I also replaced Q2, Q4, Q5 and Q6.

No I am getting zero DC volts accross J32 and J33. when I measure each of these to ground, I get -70V DC on each.

IC2 (TLO-72) pin 1 and Pin 7 are reading 16.6V.

CEB for Q3 is -70, 69, 98 V DC.

Q4 CEB is -75,-95,-95 VDC.

Q22 is -78, -78, -75 VDC.

I still get zero volts across the 0.47 ohm 5 watt resistors and therefore cannot adjust the bias.

Any ideas? I know I am missing something huge.

Thanks again,

Mike

[MarkusBass]

You are missing a lot of things.
First, you marked some components with red circle on the schematic but you haven't explained what exactly the red circle means. I thought that you marked components that were replaced initially. But now you are again replacing the same components. So, please start with an explanation what exactly the red circles mean.
Also, looking at the voltages on the schematic, it seems that D2 was OK. And now you say it was bad and you replaced it. How did you find out that it was bad?
You asked several times whether you can work with Half(+) power amp only. And now I see that you are working with both power amps. It's because you are measuring voltage on J33, which is output from Half(-) power amp. Looking at the voltages you have -70V on outputs of both power amps. But in the first post you said that you have 0V on the output of Half(-) power amp. Can this be somehow explained?
The voltages that you provided for Q3 and Q4 do not mean much because of the accuracy. It is very important whether between base and the emitter you have 0.6V, or exactly 0V. From your measurements it is hard to tell. Of course, the voltage -70V on collector of Q3 is incorrect but I'm not sure whether this is caused by failed (again) Q3, or Q4. Are you sure that you replaced Q3 and Q4 with correct type transistors?
The voltage on Q22 makes me wonder what exactly you are fixing. Q22 is in the other power amp. And I suggest that you fix both power amps separately - start with the Half(+) power amp.
In order to be able to see voltage drop on ballast resistors, you need to have 8-10V difference between collectors of Q3 and Q4. But, of course, the voltages there cannot be -70V as you have now. You have to fix the circuit consisting of Q3, Q4 and Q2. Have you checked the trimmer? Maybe start with measurements of voltages on Q2: base-emitter and base-collector. Also check what is the voltage drop on R10 (in order to see what current is flowing through Q2).

Mark

[nickb]

ks for your help guys.

I replaced Q3. D2 was also bad.

I also replaced Q2, Q4, Q5 and Q6.

No I am getting zero DC volts accross J32 and J33. when I measure each of these to ground, I get -70V DC on each.

IC2 (TLO-72) pin 1 and Pin 7 are reading 16.6V.

CEB for Q3 is -70, 69, 98 V DC.

Q4 CEB is -75,-95,-95 VDC.

Q22 is -78, -78, -75 VDC.

I still get zero volts across the 0.47 ohm 5 watt resistors and therefore cannot adjust the bias.

Any ideas? I know I am missing something huge.

Thanks again,

Mike

I certainly agree with Mark that at least one decimal point on all the readings would be helpful.

There is still something far wrong around Q3. The base at 98V is about right - I'd expect it to be 3.3V less than the nominal '100V' rail. The emitter is 69V but should be about 98.7V. This suggests that R14 is open.

[cowpoke]

Mark,

Circles in red please disregard.

On D2, I tested it and it was shorted. Zero both ways.

I finally noticed that Q17 MPS-A06 had a bad lug and that the relay was not working, thus CH1- was not working because the relay was not being activated. This is why I hooked up CH1- and tested. Now I get excatly the same voltage on both J33 and J32. Approximately -70.2V thus the voltage between is zero.

I will remeasure and reply with the more accurate readings. I just wanted to know if we were in the ballpark.

By the way, in my previous post I there is a typo. I meant to say CBE and not CEB.

Thanks again.

[MarkusBass]

Now I get exactly the same voltage on both J33 and J32. Approximately -70.2V thus the voltage between is zero.

On outputs of both power amps you should have 0V. -70V means that both power amps have failed and they have most probably the same problem. I still suggest fixing one power amp at a time.

Mark