Ampeg SVT-6 Pro Help Please

[nickb]

Mark,

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

Thanks again.

OK, so Q13 CBE is. -70, 69, 98

The base should be 3.3V less than the "100v" rail. I'll hazzard a guess that it actually 98V since that is what the emitter of Q3 is at. So the base should be at 98-3.3 = 94.7V and the emitter should be about 95.4V. It would appear that Q3 is still bad and the zener D2 is not doing it's job.

Remove Q3 and D2. We need to figure out why they have gone bad again. Check that R22 really is 4.7K and there are no shorts on that PCB trace. If you are 100% certain that all is good, go a put a new zener in. Power up and check for 3.3V across it.

Recheck CBE of Q4 with decimal point included we are looking for the base to be 0.7V more than the emitter.

[cowpoke]

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

Since they both have the same problem, could it be something coming from the preamp like the "Tube Gain"? Just trying to think outside the box.....

[cowpoke]

OK, new development on the power supply. C16 blew when I powered up today.

After further inspection and removing a bunch of silicon glue, C31 also looks bad.

I will be ordering replacements and also checking the associated diodes.

What do you think caused this?

[MarkusBass]

Since they both have the same problem, could it be something coming from the preamp like the "Tube Gain"? Just trying to think outside the box.....

Thinking outside the box is usually a bad idea. The tube stage is isolated from the power amp with C6 and C7 high voltage capacitors. I would think of something more obvious like missing supply voltage, or a shorted MOSFET, or not working DC-servo circuit IC2 (and Q3 and Q4 transistors).

Mark

[cowpoke]

Do I need to wait to get the replacement C16 to test Q3, Q4? Is there a better replacement for these caps?

While I am ordering another set of parts, is there anything else I should get? I plan on getting a few more Q3, Q4's. I have a bunch of spare diodes, and a new set of matched mosfets,......maybe another IC2 (TLO-72)?

Parts are cheap here. Shipping is the most expensive part.

All resistors checked out to be correct.

The PCB is a mess. The previous owner did a hack job trying to repair before I bought it. I thought 50 bucks for this was a good deal, now I am really wondering if this thing will ever make an ampeg sound.

I have compared every trace to the schematics and had to jump a few damaged lines. I have verified time and time again.

You guys have been very helpful. Hopefully with my new parts, I can give you better measurements and solve this problem. This has been the most difficult Ampeg that I have ever worked on.

[MarkusBass]

Do I need to wait to get the replacement C16 to test Q3, Q4?

You are joking again, right? C16 forms a power supply for Q4. How would you like to test Q4 without power supply? Unless you desloder the transistor and test t it outside the PC board.

Is there a better replacement for these caps?

This is a very bad idea. You need to bring the amp to the state as it was when it left factory. Do you think that Ampeg has put a bad quality cap into the amp? The cap blew because most probably the voltage applied to it was to high. Check whether the transformer is connected correctly o the board.

The PCB is a mess. The previous owner did a hack job trying to repair before I bought it.

I think that this is the most important message about the amp and you should provide it in the first post. I've seen many Ampegs like this repaired incorrectly. Are you sure that the previous owner repaired it correctly? I doubt it since he was not able to fix it completely.
Do you have any burned tracks on the PC board, or carbonized PC board? Can you restart the thread and provide some photos of the PC board (the bottom side if there are any burnt tracks)?

Mark

[clintronics]

I am repairing one of these today. One observation is R28,R29,R30,R31,R136 that all burned up are all 1/4W resistors, but the latest version of the schematic has upgraded them to 1/2W now. There are 20 of these that should be upgraded.

[nevetslab]

I am repairing one of these today. One observation is R28,R29,R30,R31,R136 that all burned up are all 1/4W resistors, but the latest version of the schematic has upgraded them to 1/2W now. There are 20 of these that should be upgraded.

I was servicing one of these last December, and posted the following, and how you have to power it up outside of the chassis to get at the lower power amp stage for setting bias on the MosFET's.

https://music-electronics-forum.com/...ad.php?t=47862

[clintronics]

Thanks for the warning on mixing up the trim pots. I was planning on setting the lower bias with the upper completely removed. I would measure the voltage using two wires I solder to the bottom and then placing the PCB back into the case for the power up.