Peavey Musician 400 MK III, Thermal shuntdown & OT

[Feufeu]

Hello at all, sorry for my bad English, i'm French .

We have a Peavey Musician Bass MK III with stange problem.

Amp Working, but after several minutes headsink become very hot and amp stop to work in thermal shuntdown.

After check we have found 2 transistors with realy bad values on right bank. we have remplaced the 4 transistors in bank.
Before turn on amp, have check mesure between E,C,B's transistors and ground, check in not shorted connexions
But have arround 1 ohm between Q8,9,10,11's Collector and ground , strange ... !

Plug amp with a light bulb limiter wired in series, but ligthing immediately ... I suspected this ...


This amp have output transfomer, I disconnect this, annnnnnnnnd ... amp working ...
I measure the windings, have 0.44 & 0.88 ohms between ground plug on schematic and 2 other wirrings, I find it very weak !!!?

OT seems shorted
how did the amp work before ?!?

Does amp can working without this OT ?
Or maybe other problems ?

Thanks in advance for your help.

Regards.

[Enzo]

The transformer is a 4 ohms to 8 ohms transformer, so it will have a very low resistance.

[Jazz P Bass]

"But have around 1 ohm between Q8,9,10,11's Collector and ground , strange "

This statement does not make any sense.

The Collector of Q8-Q11 is tied to B+ (+52Vdc).

[Feufeu]

Thanks for your reply

The transformer is a 4 ohms to 8 ohms transformer, so it will have a very low resistance.

I do not find these values, have around 1 ohm between Black & grey or Black & white ... i think is shorted, if i plug this, bulbe limiter light immediately.

If i understand , with transformer adapt impédance, for example , if i plug 4 ohms speakers in 8/4 ohms output , power amp see 2 ohms for maximumpower ?

Also; amplifier can working without this transformer ? in Musicizn MK IV, it's same power output stage without this OT.

This amp can working without this OT ?

Thanks for you help.

"But have around 1 ohm between Q8,9,10,11's Collector and ground , strange "

This statement does not make any sense.

The Collector of Q8-Q11 is tied to B+ (+52Vdc).

Sorry for the mistake, I meant Emitter

[Enzo]

Yes, the amp operates without this transformer. It will SEEM like a short if there is any DC on the amp output.


The transformer will not measure 4 or 8 ohms, because your meter does not measure impedance. It is very unlikely to be shorted.

[Dave H]

...the amp operates without this transformer. It will SEEM like a short if there is any DC on the amp output.

I'm guessing it has a small DC offset.

Feufeu, measure the offset voltage (DC) on the output. If there's any DC on the output a high (DC) current will flow through the low resistance of the OT turning on the bulb. Disconnect the OT and the current is limited by the resistance of the speaker so the bulb doesn't light and it seems like it's working OK.

[Feufeu]

Yes, the amp operates without this transformer. It will SEEM like a short if there is any DC on the amp output.


The transformer will not measure 4 or 8 ohms, because your meter does not measure impedance. It is very unlikely to be shorted.

Arfff, Have no impedance meter, just good multimeter... that's why values ...

I'm guessing it has a small DC offset.
..
Feufeu, measure the offset voltage (DC) on the output. If there's any DC on the output a high (DC) current will flow through the low resistance of the OT turning on the bulb. Disconnect the OT and the current is limited by the resistance of the speaker so the bulb doesn't light and it seems like it's working OK.

Hello Dave,
have around 7mVDC on speaker output, seems good value

One question, What creates DC offset and when offset is not good how can adjust DC offset, if is possible on this amp ?
Thanks really for your reply

EDIT:
Have plug OT for test, and amp working, Enzo have reason, OT working,
I think I was scared when I saw the lamp lit and the low measure of the OT

I Bench amp on 4R dummy load and scope, look if amp have always thermal problem after transistors remplacement ...

Amp cut after 15 min , thermal shuntdown .. initial problem always present ...

As you have an idea who creates this problem ? BIAS to hot ?