troubleshooting Fender 3016 amp

[mikebarg]

I'm working on a Peavey 3016 PA/amp. I've been searching for a schematic , but haven't found one yet. I emailed Fender last week but haven't gotten a repley yet. When I got it, it would blow fuses when turned on. I found a bad diode in the PS and replaced it. It now powers on but... The limit led comes on and stays on, I hear the speaker thump a little and see it deflect ( and stay partially deflected)when I turn the amp on. But, I get no audio out to the speaker. I tried the channel 1 Hi Z input and I tried the power amp input but nothing. I have included the amp block diagram to give a general idea of the configuration. any Ideas where to start. I'm kinda flying blind without a schematic.

[Alex R]

You will have DC on the output, that's what's moving the speaker. The first thing I would look for is shorted output devices.

[mikebarg]

the schematic

I followed up with a contact at Fender and received this schematic.

[Jazz P Bass]

Fender 3016

The speaker moving is not good!
You have a Vdc on the speaker out.
That means a failed output section.
Please disconnect the speaker when testing the amp.
You have to find what is passing Vdc to the output.
A working amp will show anywhere from 0mv to maybe 100mv.
Test the output with a voltage meter. Betcha you will measure a lot more than that.
As mentioned above, your output transistors are a good place to start looking for failed parts.
If they are indeed shorted, also check (replace) the drivers before them.
Depending how hard the section failed, you may find the bias transistor shorted.
All the resistors associated with any failed component should also be checked.

[mikebarg]

Check output transistors

I pulled the 3 mj15022 and 3 15023 transistors and checked them. they all checked fine. I put eveything back and turned the amp on (with no speaker attached)to check some voltages and now it's blowing the 2 fuses on the board.

[Jazz P Bass]

Fender 3016

Looks to me like there are (4) each of the output transistors.
Remove all of them.
Verify your power rails. + - 70Vdc. +- 15Vdc.
Diode check the driver transistors Q14, 15, 16 & 17.
Check all the transistors in the output section.

[Enzo]

Power off. Check for continuity from the cases of each of those MJ power transistors to chassis. If you find continuity, they are shorted to ground. Possible missing insulating washer.

The two fuses look to be guarding the low voltage supplies, so check for shorted rectifiers there as well as shorts across the rails.

[mikebarg]

checks

I removed and check all 8 power transistors. All checked good. I put them all back. I checked continuity from the power transistors cases to ground on the board. They all read either 11.4K or 12.4K.

[mikebarg]

dang! smoke!

Well I managed to smoke a few components on the board. I had all of the power transistors pulled and plugged in the unit to check the voltages and withing a second or two , two transistors and surrounding components smoked. BTW, I don't think the schematic exactly matches my unit. The Component numbering doesn't seem to match. Notice, I burned R38. Which on the schematic is a 5W resististor connected to Q21. See the pic of the board.

[Jazz P Bass]

Fender

You have a severe short to ground somewhere.
Are you testing without a load (speaker)?
Drain the power rails. + & -. Use a 1K 2W resistor to ground.
Remove the driver transistors. Q15 & Q17. Check for shorts.
Remove the clamping diodes. CR25 & CR21. Check them on diode check.
(you do have a DVM with diode check, right?)
Check all the transistors in the output section.
Measure the resistance from the power rails to ground.
Should read high K ohms.
Build a lamp limiter before you power it up again.

[Enzo]

Not generally a good idea to run it woth the outputs removed. Then the drivers have nothing to share current with them.

[mikebarg]

correct schematic

I finally got the correct schematic from Fender. It was under a Sunn 6350 amp. The parts that I smoked were q5 -MPS W92, q14 -MPS W42 , R13 - I think it's 150 ohms, and r38 - again I think 150 ohms.