Ampeg SVT-350T

[Off-Beat]

hey all, first off as this is my first post I'd like to say great forum! Lots of helpful people here.

I have been learning the dark art of amp repairs over the past year so I am still new to a lot of it.
I have an svt 350t on the bench atm. The problem with it (aside from being dirty and beat to shit) is that there is no sound. At the beginning the voltages all seemed ok to me and I could trace the signal all the way to the output jack.
I found a rectifier diode that seemed to be open circuit so I replaced it like for like correct way around etc.. Since then the voltages seem to constantly rise up and down on both the preamp and output board... I have been working on a few amps in between so I haven't got back to it since. I'm guessing there is possibly a short somewhere or a power transformer issue?

I should also add that when probing around with the scope I accidentally touched the base of one of the output transistors and a nice big spark flew off it... It seems to still be ok but as I'm still new to this I'd like to know why that would have happened?

Any suggestions?

[The Dude]

Firstly, welcome to the place!

Some questions:
1) Did you replace the diode with an original part number?
2) Which diode did you replace and do you have a schematic?
3) Is them amp stable enough to run?
4) Is the amp outputting any DC voltage? If so, do not hook up a speaker until that is taken care of.
Thoughts:
1) It's highly unlikely you have a transformer problem.
2) If the amp powers on and there is no DC at the output, run a signal into the "amp in" jack in the rear and see if you get sound. Also run the "preamp out" jack to another amp and see if you get anything. This will help us narrow down preamp, power amp, or power supply problem. After we see the results of this test, it will be easier to point you in the right direction.

[Off-Beat]

This is the schematic I am using http://bmamps.com/Schematics/ampeg/A...Schematics.pdf
I replaced the diode before I had the schematic with the same part number. Not sure on the diagram which diode it is until I can have another look, it's one right next to the smoothing caps.
The schematic is for the 200T, all I could find but it seems to be near identical to the 350T.

When you say is the amp stable to run, what should I be looking for?
Thanks for the info il try that tomorrow.

[g1]

As far as your voltage readings going up and down, this often happens it a ground gets disconnected. If the unit is disassembled, it may have been relying on chassis connections to complete the ground paths.

[mozwell]

sch...

[Off-Beat]

Thanks for the help guys. g1 was right in regards to the voltage issue. There was a loose ground connection which I have now fixed. I ran the pre amp out to another amp, the signal was very weak and some nasty pops started to come through. I also ran a signal into the amp input at the back as Dude suggested and there was nothing at all.

Feeling a bit lost with it now but I don't want to give up!

Thanks mozwell

[52 Bill]

I also ran a signal into the amp input at the back as Dude suggested and there was nothing at all.

No sound at all or no noise/popping? Which jack did you use?

If the signal coming from the preamp is weak and distorted check the low voltage power supplies.

[Off-Beat]

Yeah nothing at all from the power amp input at the back. Il check the voltages on the pre amp tomorrow. Hope it's something simple!