Peavey 400 LTD Help

[Pryde]

Hello everyone,
I would appreciate any guidance on troubleshooting my amp problem. I have built and modded some tube amps but never really dealt with solid state amps such as this one:

I have a old 1976 Peavey LTD 400 amp that was all original and was putting out very low, distorted signal. I just changed every electrolytic cap in the circuit, including the main filter caps as I knew this should be done no matter what.

Plugged it in after installing the new caps and it immediately blew the fuse? Hmm? I suspected I could have created a short somewhere but I went through all my solder joints and checked continuity and all checks out there?

First I need to locate some more fuses (125v 8a), but what should I start looking for/investigating with this issue? Thanks for the help!

[Enzo]

This is exactly why I always recommend FIX and amp before updating a lot of parts.

I can almost guarantee you installed a cap backwards. Did you wire all the negative ends to ground? The power amp runs on +50v and -50v. That negative supply needs its cap grounded at the positive and and the negative end is at -40v. COuld that be it? Backwards caps blow fuses.

What main caps did you use? The schematic says 6000uf at 45v. That is not a good spec for a 50v supply. I'd recommend at least a 63v cap. 6000uf is a non-standard value today. 5600uf or 6800uf would be the closer standards today.

My schematic say 6A for the main fuse. Is the fuse value printed on the rear panel of the amp? In any case 6A or 8A, just go to the hardware or someplace. You won;t find 125v fuses anywhere, I never see them, but 250v ones are everywhere.

But instead of throwing boxes of fuses at it, look up "light bulb limiter." Make one, it is simple, and use it.

[Pryde]

Thank you Enzo.

I will again re-check all cap orientation but I have already done this x6. I was meticulous and swapped one cap at a time. The only deviation was I replaced all the 2uf electros with 2.2uf poly film.

The original filter caps that I pulled where 5000uf 55v (big silver cans). I replaced them with 6800uf 63v.

I do have a light bulb limiter as well but in my haste I did not use it to power up (stupid I know)

[Jazz P Bass]

If you blew an 8 amp fuse you have a gross short.
The amplifier, working properly, should draw no more than 1/2 amp at idle!

[Pryde]

OK here is what I got:
Replaced fuse and powered up the amp with my light bulb limiter using a 75w buld WITHOUT the speaker plugged in and the bulb was dim and stable.

I then powered up again with the speaker plugged in and the 75w bulb light up and stayed very bright. So I am shorting somehow with the speaker plugged in only?

Thanks for the help

[Jazz P Bass]

Good old Peavey.
The schematic is part of the instruction manual.
Link: http://www.peavey.com/media/pdf/manuals/80344000.pdf
Seeing that you went from 'distorted output' to 'blowing fuses' I think, as Enzo pointed out, that you have a cap in backwards or there is a solder bridge somewhere.

[Pryde]

Thank you JP Bass.

That is the manual/schematic I have as well.
I have went through 6+ times and verified all caps are oriented correctly, continuity check for solder bridges on every part installed reveals no issues either that I can find.

Obviously there is a shorting issue but I can't seem to locate it on any of the replaced caps. The only "difference" I can see is the original filter caps had a plastic outer coating protecting them from the metal clamps they sit in. The new ones just have the normal aluminum shell. Also, I had to solder small ring terminals to the filter caps as they had small plug-in leads on them. I hooked all wiring up as per original using photos and a drawn diagram.

As far as all the small electrolytics, they are soldered in correct.

NOTE: Also I am getting -45v DC at the speaker terminal.

Any other paths to try?

THank you,

[Jazz P Bass]

Do not attach a speaker or a load until you suss out what is wrong.

[52 Bill]

Test the output and driver transistors for shorts. You did say that you have both + and - 50 volt supplies working, right?

[Pryde]

Test the output and driver transistors for shorts. You did say that you have both + and - 50 volt supplies working, right?

Not sure where to test/verify both the - and + 50v supplies?

*EDIT: I am getting +45vDC off of the first filter cap and -45vDC of the second one

[nashvillebill]

Man, I sure wish those early Peavey schematics had labels on the components...

Did you pull and then re-insert the output transistors as part of your recap? Could be one of them is shorting as you reinserted them. Try pulling them, them retesting with the DBT--and a dummy load, not your speaker!

[Pryde]

WHAT AN IDIOT (me)

I simply had a positive lead plugged into a wrong male connector on the power amp board. Jeez I always overlook the simple things and assume the worst...

BUT: Now the amp powers up the way it should (no blown fuses) but I am back to square one with low, distorted output. Should I start a new thread with this issue or keep this one going?

sorry to waste everyone's time with my stupid mistake.