Ampeg BA115 blows fuses

[clintstone024]

So my bass player buddy brought me over his ampeg that his kid blew up. His son's exact words were "it just stopped working" further investigatio revealed he had it completely dimed at the time it quit. I'm a tube amp guy but i figured I would give it a shot. I disconnected the outputs of the power transformer and tested the voltage and it seems good, unsoldered the rectifier and it tests fine. at this point I did some research and found out these amps are notorious for blowing the output MOSFET's. so I unsoldered them and tested, sure enough one was shorted internally, so I ordered some online (which was easier said than done as they are discontinued) installed the new ones and it still pops the fuse instantly. I did some more testing, pulled the driver transistors and they test ok, put them back in and tested some of the obvious trouble components like the big zeners and and other diodes that are commonly short cct issues, nothing. So I got mad and stuffed it away for a couple of months and gave it another go yesterday. put a new fuse in and it held for about 10 seconds this time before it popped, tried another and it popped instantly, I'm guessing this was just the filter caps taking current? anyways this one has me stumped, and if it wasn't for the fact this is my freinds amp I would probably toss it. any hints as where to go next.
here is the schematic https://ampeg.com/support/files/Sche...(103XXC_1).pdf

[The Dude]

Firstly, build or use a light bulb limiter so you stop blowing fuses and potentially doing more damage to the amp. Where did you get the replacement parts? If ebay or some other non-reputable seller, it's possible you got "fake" FET's. FWIW: Digikey has both in stock. Unhook the speaker while you troubleshoot the amp and do not hook it up until you verify there is no DC on the output and the amp is working. I'd first try getting it to stay on with the LBL and see if you have DC on the output. If FET or FET's were bad, l'd be looking at D6-9 and the driver transistors. Also check any resistors in the driver/output section. Some may have burned open when FET's shorted. There aren't many parts in that entire output/driver circuit. It wouldn't take long to just do a quick check of all of them.

[Enzo]

And when you installed new outputs, did we make sure the insulating wafer was in place?

[clintstone024]

Yes the wafers were installed, I will try it with the current limiter, and yes the speaker was disconnected while testing, I will check for dc on the output

[glebert]

Do you have the bias pot set all the way to cold to start? Should be all the way clockwise iirc.

[clintstone024]

ok, I had a chance to fire up the amp with my lbl and it stays on, bulb glowing bright obviously. I quickly checked voltage at both 270 ohm 5W resistors and I have +12.2 R68 volsts and -12.2 volts R66. downstream of R68 is +5.8V while R66 is only -1.9V, am I correct in thinking the problems probably lies in the negative side, I desoldered and checked Q1, Q3 and Q4 and they tested ok. I'm thinking I have to desolder one leg of D8 and D9 to test, correct?

[clintstone024]

So I desoldered one leg of d6 to do and they tested ok with my meter, I'm tempted to unsolder these fets and test them again.

[glebert]

So I desoldered one leg of d6 to do and they tested ok with my meter, I'm tempted to unsolder these fets and test them again.

When you had the FETs out did you test to see if the +/-40V rails were solid?

I recently did a BA115HP (basically same amp) and I wanted to make sure the voltages going to the FET gates were correct, but to get that to work without the output FETs you have to remove U3. I would probably do that again if I needed to so I could know that the biasing and driver circuitry was acting as expected.

[g1]

What the readings at those two 270R's indicate is more draw from the negative low voltage rail. That would be the +/-16V supplies. Check the negative side zener, D12.
What surprises me is there is so little voltage going IN to those resistors. The bulb I guess is lit very bright?

[clintstone024]

Unsoldered both fets again and sure enough Q5 was shorted out. I measured the resistance of the bias pot, 470 ohms. I adjusted it, at first I felt a faint snap on my little screwdriver as I initially turned it. So I exercised it back and forth a few times and now it feels good. I set it for 0 Ohms and installed new fets.
To my delight the current limiter glowed for a milliseconds and then went dim, I now had 32 volts on my rails. Removed the limiter and I now have an amp that is not blowing fuses and I have positive and negative 38 volts on my rails.
now the schematic has bias instructions to set the bias pot to achieve 5mV across R61, I did this and so far so good. Dare I hook it up and try it out?

[glebert]

Are you getting any DC at the speaker connection? If that is close to zero I would hook up the speaker while amp still on the light bulb limiter just in case.

You can also try the headphone out to see if you are getting clear sound.

[clintstone024]

I have 0.2vdc on the speaker connection, I hooked up the speaker and tested it, sound is clean but I have very low volume, as in 10 sounds like is actually about 1 or 2.

[g1]

How about through CD input, same low volume?

[Jon Snell]

How about through CD input, same low volume?

I already asked that question on the TDPRI post he enquired on earlier this evening. No reply yet though.

[clintstone024]

Just had a chance to try it with the cd input, definitely seems louder.