Ampeg PF-500 Fault

[gerry113377]

Hello!
I have an AMPEG PF-500 bass amp. I am attaching the forum link with the schematics below. I have version J03. It recently developed a fault condition, where the fault light stays on all the time. I have a little electrical skill so I decided to take a look. I saw 2 bulging caps. The bad caps were C80 & C81. I decided to replace all the following caps. C77, C79, C80, C81, C6, C21, C8 &C29. I then found 2 shorted mosfets, Q10 & Q11 which I replaced. I checked and tested most of the other components on the board and didn't see any other obvious problems. I turned it on without a speaker and the fault light was gone and let it run for 20 minutes. I then plugged in a 6-string electric guitar to a 30w 8ohm guitar speaker, played for 15 minutes, no fault, everything worked fine. I could turn up the gain, volume, bass, mid-range, treble with no issues. Then I plugged in a 450w 8ohm AMPEG cabinet, with a bass guitar, struck my first note and it faulted immediately. I checked for a short in the cabinet and cable and it checked out at 6ohm, no problems. Checked mosfets Q10 & Q11 and both are shorted again. I removed both Q10 & Q11 and left them out, turned it on and the fault light is gone. I checked DC voltage across C141 and I have 157VDC.  So, now I am hoping someone can point me in the right direction on how and what to troubleshoot next. Thanks!

https://music-electronics-forum.com/forum/schematic-requests/30935-ampeg-pf-500

https://music-electronics-forum.com/filedata/fetch?id=848500&d=1517569621

[Jazz P Bass]

Usually the diodes & resistors for the mosfets get changed out as a matter of course.
(D39/40, R99/100)
Along with the gate driver ic (IRS20957S).
Unfortunately, that ic is out of production.
The only source is China.

[Loudthud]

Then I plugged in a 450w 8ohm AMPEG cabinet, with a bass guitar, struck my first note and it faulted immediately.

When you plug or unplug a speaker cable equipped with phone plugs on the speaker cabinet end, you are briefly shorting the output of the amplifier. If the amp was powered on, the brief short could have killed the amp.

[gerry113377]

They stock the gate driver ic at mouser. I will order the diodes and resisters along with the ic and let you know what happens after I change them, thanks. The speaker cable was never plugged in or pulled out with the amp turned on but thanks for that bit of knowledge.

[gerry113377]

I finally found the issue. Both sides of the board are different circuits, so when I soldered in the mosfets, the solder didn't make it to the otherside, leaving the top of the board with an open circuit. Live and learn. Thanks for your help!

[g1]

Both sides of the board are different circuits, so when I soldered in the mosfets, the solder didn't make it to the otherside

The 2 sides of double-sided boards are connected by 'feed throughs' or 'vias' which are like tiny pipes at the through holes where the component legs go.  They are not just connected by solder wicking through.
What often happens is the vias get pulled out along with the component legs when the parts are removed.  It can be very difficult not to tear them out unless you have a vacuum de-solder station or use something like 'chip-kwik'.  If you look close at the parts that were removed, you may find the vias still on the legs of the old parts.
Sometimes you have to repair the board by running tiny wires through the through holes and connecting to the traces on either side. 
Sometimes you can get lucky with the solder connecting to both sides, like you did.   

[Jazz P Bass]

Yeah, it is a bummer when the plated through hole comes out.
As G1 stated, running a small wire up through the hole is best for a good mechanical connection.

[Kaz Kylheku]

Some circuit boards are better at saying good bye to pads than others.

To minimize lifting, never pull on a part while the solder is cold. When removing a stubborn part, concentrate a small amount of force on one terminal and apply heat to it. It will move when the solder liquefies.

One reason that pads come off is that they get overheated. When we install a through hole part, we have to heat he pad and the terminal, for a good solder connection. When we are removing, we are destroying the connection. We can therefore concentrate the heat into the terminal. We don't have to heat the pad.

If you repeatedly mod the same part of the device, swapping a part many times, the repeated heat stress can unglue the pads. When you're installing, work quickly; don't overheat pads.

[gerry113377]

Great info for next time! I did have trouble removing the mosfets. In the process the "pad" became separated from the "through hole". Did I say that right? I never knew there was a through hole. All is well for now. Thanks!!!

[g1]

the "pad" became separated from the "through hole". Did I say that right?

'Through hole' in general refers to the older system with holes in the boards.  The components are called 'through hole' type, which means they have the type of leads that go through the holes in the board.
The modern technology is SMT or 'surface-mount' where there are no holes in the board and the component leads are flat.

With through-hole system, when the board is double-sided there are the little pipes connecting the traces at top and bottom sides of board.  They can be called 'plated through-holes', 'vias', or 'feed-throughs'.
When the board is single-sided, the pads don't connect through, they only connect to the component lead on the trace side.

[Tassieviking]

If I have to remove any components I usually cut all the legs off first and then I can remove them one by one.
Components are cheap and unless I have to remove an expensive part just for a test the legs get snipped.

Why do I always get a cold shiver when I hear or say snipped?
It must be a guy thing I guess.