Ampeg SVT III Pro DC leaking to output problem

[Domsob1974]

Hi,

first of all, I am not a tech, but I hired one to work on my SVT III pro. When I bought it (used). It would sound distorted sometimes unless I (as weird as it sounds) hit my bass strings hard and then it would "snap out of it". After awhile my speaker in the cab died. I replaced it and the other one died pretty quickly. So, I took it to my tech and he is having a bear of a time figuring this out. Here is the info I can give you guys and I am going to forward your replies back to him.

"I have a problem with the negative transistors not turning on. The DC on the output is probably low current, I am testing with no load. It is almost like Q4 is not turning on."

"I have had that thing on the workbench four times and can't figure what I am missing. it wants to out 60 volts dc on the speaker output and I am pretty sure I have replaced every output, driver pre-driver, bias and over-current transistor in that circuit."

Anyone have any ideas? We are all out of ideas. It just keeps smoking my speakers and this guy is GOOD and knows his stuff but this unit makes him want to jump off a bridge.

Thanks in Advance,

Dom

P.S. If I didn't include enough information let me know and I will get any info from him and post it here (and I will also inform via this thread the results of the steps that you guys advise to take.

Yes, we have a schematic and an oscilloscope (He has been doing this for years).

[DrGonz78]

I was going to say post the schematic but I am pretty sure this is the correct one. Is it correct?

[Domsob1974]

I was going to say post the schematic but I am pretty sure this is the correct one. Is it correct?

Here it is

[The Dude]

I'm a little confused by the initial post. You say, "it wants to out 60 volts dc on the speaker output". If this were the case, you would definitely hear it and the amp would not be operational. Yet, you infer that your were using it. Is this a new symptom? Is the 60VDC constant, or does it just happen occasionally? Making inferences from your description, it sounds like an intermittent to me. I would pull the output/PS board and check it over for cracked/cold solder joints. Either that, or I am not correctly understanding your description of the symptoms.

[Domsob1974]

When I got the amplifier it was fully operational. I ended up using it for a few months, but sometimes it would sound as if it was it seemed as if the only thing that I could do to rectify it was to smack on the bass strings really hard and it would snap it out of It. then, one day at practice, I noticed a burning smell. It ended up torching one of my woofers. So, I thought that maybe I just had it turned up too high so I went ahead and installed another woofer and it did the same thing. I took it to the tech and the text said that it was leaking DC voltage in to the speaker outputs and that's why it keeps on blowing the speakers. The quotes that I put in my original post are correspondence from him as to what he has done so far to try to chase this Gremlin out. Does that make sense?

[The Dude]

I would start by actually measuring the output for DC (with no speaker attached, of course). Let us know if it is constant or intermittent. While you are monitoring the DC, give the amp a couple of whacks with your fist and see if the DC on the output changes or not. I still suspect an intermittent connection.

[Domsob1974]

I have not hooked any load to it. When turned on, it slowly rises to about +60V on the output rail. It is almost like the negative bank is not turning on. It has to be a driver, but I swear they are all new. As are all the outputs. To avoid any over current situation, I have a 150 watt light in series with the line as a current limiter, it barely glows and voltage rails are close to normal."

[The Dude]

If it rises slowly, it might be heat related. Aside from tapping around to look for intermittent problems, I might also try some freeze spray while monitoring the output. Some part might be problematic when it warms up.

[Enzo]

Has to be a driver?

If you had a bad transistor I'd expect it to snap right up to the 60v rather than drift.

Your 60v rails are OK? How are the two 100v rails the drivers rely on? Do they remain up as it drifts?

Did you check the resistors associated with the transistors?

Q2 is the bias transistor and AP1 is the trimmer for it. What DC voltage appears on them? ANy leg, doesn't matter. If that also goes to 60v, then everything to the right of it is likely working, as the job of the outputs is to follow that. Look at the gate voltages on the outputs, are they telling the MOSFETs to turn on to 60v?

It would be exceedingly rare for C6,7 to fail, but just in case, pull tube V2 and see if the DC goes away.

[nosaj]

I have not hooked any load to it. When turned on, it slowly rises to about +60V on the output rail. It is almost like the negative bank is not turning on. It has to be a driver, but I swear they are all new. As are all the outputs. To avoid any over current situation, I have a 150 watt light in series with the line as a current limiter, it barely glows and voltage rails are close to normal."

New does not always mean good. It happens.
nosaj

[Domsob1974]

"I am tempted to lift C6 & 7. the DC servo can't pull hard enough to center the amp. Maybe a leaky cap there could be the hex. I am weary and it is time for me to leave my shop. I will try these things first thing tomorrow afternoon (and report back findings). I don't want to pull the tube, the heater circuit is tied to +16."

[Enzo]

So much easier to just pull the tube. Seems unlikely BOTh caps would be real leaky, and the tube is the only source of voltage through those caps.

Don't assume all problems are bad transistors, open resistors can have the same effect. So can broken or cracked copper traces and bad solder joints. ANy interboard ribbons are suspect, or the solder to the male headers.

Divide and conquer, are all the gates tracking this rise in output? is the bias voltag - the space in voltage between the opposing gates - staying constant, even as the voltage rises? And the voltage at the bias transistor, what is it doing? Please don;t start lifting caps just to see if they are maybe it maybe not. That is guessing. FIND the problem, then it will be clear WHAT the problem is.

[Domsob1974]

D6 & D7 need replacing, keeping the 9240's from turning on. One of those zeners measures 192 ohms, both directions. Those diodes are nestled way down between the heat sink and main caps, hard to see. Maybe next time I should hi-lite each part on the schematic as I test them.