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Hello everyone,
I'm turning to you with a problem that I'm unfortunately at a loss for answers or have run out of ideas.
Some time ago I received a defective Peavey Musician Series 400 with the statement "power transformer broken".
After a brief examination, it turned out that the mains transformer was working perfectly (even the fuse was still OK), but one of the end transistors was missing and the remaining three 60142 or 2N3772 in this branch had been replaced by 2N3055.
A 1µF film capacitor had also been added directly to one of the transistors in parallel with the 0.05µF capacitor in the V+ branch. But more on that later.
I started the repair by measuring the remaining four 2N3772s, identifying another faulty one (the driver).
I also measured the high-load ceramic resistors, and the one of the missing transistor had a very high resistance.
I then ordered five 2N3772s and fitted them, replacing all the ceramic resistors with new parts. The small electrolytic capacitors also had to be replaced as they were already bulging.
During the first test with a cheap high-power car speaker, which I use as a standard test speaker due to its low price (gift) and high power handling, the amp worked wonderfully at first and I was delighted.
At a certain volume, however, the amp started to hum loudly and the input signal was no longer amplified.
To date, this has been the case both with the internal preamp and when feeding a signal directly into the power amplifier. The error is also 100% reproducible.
In the humming state, the humming signal can also be measured directly at the input transistors with the oscilloscope at the same level as at the loudspeaker output.
If I remove the added 1µF capacitor as a test, the amp turns immediately unstable.
I cleaned and measured the contacts to the housings of the 2N3772, they have 0 Ohm according to the multimeter.
The measured voltages are shown on the scan. The areas marked in green are piece for piece measured parts of the circuit or completely correct voltages.
As I have mainly worked on tube amps up to now, I feel a bit overwhelmed and would be pleased if I could get some suggestions as to which direction I can look in.
Many thanks in advance,
Gabriel
P.S.: Apologies for the maybe sometimes strange phrases, but I used DeepL to translate this text.
peavey_400_module_b_series_sch.pdf
I'm turning to you with a problem that I'm unfortunately at a loss for answers or have run out of ideas.
Some time ago I received a defective Peavey Musician Series 400 with the statement "power transformer broken".
After a brief examination, it turned out that the mains transformer was working perfectly (even the fuse was still OK), but one of the end transistors was missing and the remaining three 60142 or 2N3772 in this branch had been replaced by 2N3055.
A 1µF film capacitor had also been added directly to one of the transistors in parallel with the 0.05µF capacitor in the V+ branch. But more on that later.
I started the repair by measuring the remaining four 2N3772s, identifying another faulty one (the driver).
I also measured the high-load ceramic resistors, and the one of the missing transistor had a very high resistance.
I then ordered five 2N3772s and fitted them, replacing all the ceramic resistors with new parts. The small electrolytic capacitors also had to be replaced as they were already bulging.
During the first test with a cheap high-power car speaker, which I use as a standard test speaker due to its low price (gift) and high power handling, the amp worked wonderfully at first and I was delighted.
At a certain volume, however, the amp started to hum loudly and the input signal was no longer amplified.
To date, this has been the case both with the internal preamp and when feeding a signal directly into the power amplifier. The error is also 100% reproducible.
In the humming state, the humming signal can also be measured directly at the input transistors with the oscilloscope at the same level as at the loudspeaker output.
If I remove the added 1µF capacitor as a test, the amp turns immediately unstable.
I cleaned and measured the contacts to the housings of the 2N3772, they have 0 Ohm according to the multimeter.
The measured voltages are shown on the scan. The areas marked in green are piece for piece measured parts of the circuit or completely correct voltages.
As I have mainly worked on tube amps up to now, I feel a bit overwhelmed and would be pleased if I could get some suggestions as to which direction I can look in.
Many thanks in advance,
Gabriel
P.S.: Apologies for the maybe sometimes strange phrases, but I used DeepL to translate this text.
Attached Files