Peavey 400B BASS Mark III series Amp problem

[max3228]

Hello All,
I live in the UK [so its 240v 50Hz A/C mains voltage] and I have an old Peavey 400B/G Mk. III Series Amp. that belongs to my son-in-law [who plays keyboard] and he desperately hopes that I can get it working again.

I also have a Circuit Schematic [us Brits call em Circuit Diagrams] for this Amp. due to some kind person posting it in Maintenance,Troubleshooting & Repair on this good forum. A big thank you for that .
I come from a UK TV/CTV repair background from 1964-2000, then I was recruited to UK MoD, Hereford for 12 years and retired from there in 2012.

At first the amp. was just blowing 4Amp mains input fuses at switch on so I isolated the Output Modules by unplugging both their power supply molex plugs. This stopped the 4Amp mains input fuse blowing so I then Fluke DMM checked all the supply rails for good measure. All seemed well with the +52v,-52v,+15v and -15v although I haven't had chance to scope them for any sign of ripple yet.
So I then removed all the Power Output Transistors firstly from the left module and found that Q4,Q5,Q6 had gone short along with O/C emitter 10W W/W resistors R29,R30,R31. I checked Q13,Q14,Q15,Q16 with my DMM and they all seem OK.
So here are my Questions if you would be so kind:

1. Can I safely run this Amp. without any load / Loudspeaker?
2. Can this be a Driver or pre-driver issue leading to this high current draw or is it simply failure of O/P Transistors?
3. Is this likely to be down to the specially selected PV diode array [CR14] 13886 failing or perhaps something else?

I am familiar with putting a 60W light bulb in series with the mains input to limit current flow.
Sorry for this long post and all the questions but I am hoping someone can provide help and guidance !!

Regards,
Mike
Peavey Mark III Bass + PA Circuit Schematic.pdf

[The Dude]

1) Yes, you can safely run the amp without a load. In fact, you should until you can power it up on the limiter without issue. Also, before you hook up a load/speaker, verify that there is no DC voltage on the speaker output.

2) My experience is that, much of the time the problem is shorted outputs only. I would check drivers to be safe and associated resistors. If you have it on limiter on initial power up, the bulb will tell you if there are more problems. Replace any known shorted/defective parts and try it on the limiter.

3) This failure can be caused by a multitude of things, so I hazard to guess. Probably the most common causes are a short presented at the speaker output or amp is driven past clip for extended periods of time. But again, it could be lots of things including no reason at all except a part just gave up.

[glebert]

1. Can I safely run this Amp. without any load / Loudspeaker?
2. Can this be a Driver or pre-driver issue leading to this high current draw or is it simply failure of O/P Transistors?
3. Is this likely to be down to the specially selected PV diode array [CR14] 13886 failing or perhaps something else?

Have you replaced the shorted transistors? I think in general the smart thing is to replace transistors at least one stage upstream from the ones that failed, so if the O/P transistors fail replace the drivers, if the drivers failed too replace the pre-driver, etc.

[Enzo]

I have no reason to suspect the special diode unless something specifically points to it as the failure.

I agree often shorted outputs is as far as it goes... but not always.

Shorted output transistors often take out the emitter power resistors.

you not check Q3? That is the same type as the outputs but is really the driver. If the three outputs were shorted, I would replace Q3 for sure, regardless how it tests.

Please check Q2,12, and resistors R26,44. Also L1 may be replaced with a resistor, if so, it is not a mistake. Also R28.

[max3228]

Have you replaced the shorted transistors? I think in general the smart thing is to replace transistors at least one stage upstream from the ones that failed, so if the O/P transistors fail replace the drivers, if the drivers failed too replace the pre-driver, etc.

Firstly glebert, I want to thank you very much for your reply.

No I have not ordered the shorted transistors yet, and yes I agree with you I would always replace the driver [which DMM meters OK] and any "upstream" from the failed ones.
My son-in-law lives 95miles away [I know round the block for USA types] and I have not yet received the go ahead to order any parts yet.

[max3228]

I have no reason to suspect the special diode unless something specifically points to it as the failure.

I agree often shorted outputs is as far as it goes... but not always.

Shorted output transistors often take out the emitter power resistors.

you not check Q3? That is the same type as the outputs but is really the driver. If the three outputs were shorted, I would replace Q3 for sure, regardless how it tests.

Please check Q2,12, and resistors R26,44. Also L1 may be replaced with a resistor, if so, it is not a mistake. Also R28.

Enzo many thanks for your prompt reply,

Yes I have DMM checked Q3 [Driver] and it seems to be OK but If I get the go ahead to repair this PV Amp I would order all four MJ-15003 along with all four 10W power resistors.