BA-115HPT

[nanodocl]

Hello Everyone,

I am having a problem fixing a BA-115HPT.
The amp works but it makes a horrible noise. I think is because 5VDC is coming out to the speaker. I tried to fix this adjusting the dc offset trimmer but all mosfets died in less than a second. I move back the trimmer to previous position and replace the mosfets but the amplifier still noisy. I did check the power supply and weird thing is that the rails are not compensated. Positive rail is +50VDC and negative is -40VDC and the 330R resistor that is located on the negative rail, gets very hot. I had check if the main transformer was uncompensated AC voltages, but its perfectly matched. I had checked all transistors, diodes and capacitors.
Any tips or information will be appreciated.

Thanks in advanced
Nano

[Enzo]

Do not connect a speaker or load until you are sure the amp is not making DC on the output. The 330 ohm resistor gets hot probably because the zener diode it serves has shorted. Check D12. And that would drag down the -50. Plus it means the -16 is missing.

If -16 is missing, that means the op amps, particularly the one in the power amp, are running on +16 and zero rather than +15 and -15. It will try to work, but it will center on +8v rather than zero volts. And that will throw the whole amp that far positive.

Unfortunately you tried to adjust it out. There is only one trim pot on this board as far as I know, and it is NOT a DC offset. If you have a later revision that includes on, so be it, but that is un-crate like. That control is the bias control, and you may well have baked the MOSFETs by turning the bias too far.


So check those things and tell me if I am onto something of if I have missed the boat.


And look up "light bulb limiter" and make one and use it.

[nanodocl]

Thanks Enzo, you are right about the bias pot, it is not a DC offset. I did tried to adjust to 15-20mv on r72-r75 .33 ohm resistors but didn't get equal voltages. I probably get DC voltage at output because rails still decompensated.
Zener D12 its ok. The rails voltage for opamps are ok. I did check Capacitance and ESR on every capacitor of the board, they are all ok.
Diodes and transistors seems to be ok when I measured the with --i>|-- multimeter function, I do not know what to do anymore.

Thanks for helping
Nano

[Enzo]

In that case I would look at the main rail that measures low. How much ripple is on that supply?

[nanodocl]

Enzo, you were right, 16v zener diode was defective. For some reason the forward diode measure with the multimeter was correct, but all went back to normal when I replace it.
Many thanks for the tip, the amplifier is running perfectly.