Problem with AMPEG ba115hpt

[Pentodo]

Hi, long time reading you and finally my first post comes in!

Something strange happens to this AMPEG ba115hpt, you can find its schematic here: AMPEG BA115HPT Service Manual free download, schematics, eeprom, repair info for electronics

I connect a CD to the CD INPUT and a little crack sound like some component breaking and now you can hardly hear the sound. I am getting signal in the MOS gates but the output is nearly quiet with the level at max. I checked bias and supply voltages and they all are ok.

I already change the NE5532 (U3) with no improvements. I wonder how nearly 2V sinusoidal is reaching MOS gates and getting no amplification. Any suggestion are appreciated.

[DrGonz78]

Do you have the same symptoms now when you plug into the main inputs at the preamp of the amp too? Did you try looking at that headphone jack? The cliff jacks sometimes just need a good cleaning, but not sure what made that crackle sound as it went faint. I would definitely verify whether the preamp signal is having the same problem too.

[Pentodo]

Preamp is fine but not its signal is not amplified since the power amp is not working correctly. I guess that the noise I heard pluggin the CD was one of the semiconductors failing, may be the input signal was to strong or that comp was already damaged.

I will start replacing all of them but for the power MOS, those are the only parts that I do not have on the table.

[soundguruman]

I suspect the opto-isolator has crapped out.

[Pentodo]

But the opto just acts as a compressor of the signal doesn't it? its photoresistance is around 50 ohms when the amp is off, how coult it uneable the power amp from correctly working? I will extract it and test resistance change by feeding the LED wires and find out and see if there is any change in power amp performance.

Sorry people, just realized that I was confussing gate with drain.....that is what happens when you mess around with electronics too late in the night. So, I am not sure if the MOS are getting signal at the gates, I will chek it later and hope that only the drivers BJTs are bad. Hold your horses, I will update the real situation of the amp ASAP.

[soundguruman]

But the opto just acts as a compressor of the signal doesn't it? its photoresistance is around 50 ohms when the amp is off, how coult it uneable the power amp from correctly working? I will extract it and test resistance change by feeding the LED wires and find out and see if there is any change in power amp performance.

Sorry people, just realized that I was confussing gate with drain.....that is what happens when you mess around with electronics too late in the night. So, I am not sure if the MOS are getting signal at the gates, I will chek it later and hope that only the drivers BJTs are bad. Hold your horses, I will update the real situation of the amp ASAP.

just for fun, I would take out the photoresistor side of the optocoupler

[Jazz P Bass]

If you follow the supplied test points & you have a 250mv's at TP5 (pg 3) & that signal comes out of U3B pin 7 (TP 10/ Pg 1) at 2 .0 Vac, then everything there is fine.

I would test the output signal from the mosfet's into a dummy load.

With a 2.0 Vac signal at the gates, you should see a 26.3 Vac signal on the load.
Assuming that you have verified the + &- 50 volt rails as good & clean.

[Pentodo]

Yes Jazz, finally got aceptable results in comparison to the test points, it seems some bad contact in the speaker leads. Still wondering what the popping noise was. I am getting a nice output again and here comes the second problem, incredible static noise when I plug a bass or a guitar.

This static noise changes (reduces) when If I turn down the vol pot from the instrument, with a micro there is not so much noise. I muted the preamp and test the power amp and is noiseless unless everything is turn to max. I replace U1 hopping for some kind of grounding problem in that op-amp but nothing chaged.

I noticed something which I don't know might mean something. If I check bias without load the difference between positive and negative rails MOS source resistors is around 1mV, if I checked with load is over 10mV. Is this normal? Could be causing this static noise? Actually I guess it has something to do with grounding problem but still uneable to figure it out.

[Jazz P Bass]

If the noise can be reduced by the volume control, then the problem lies before the volume control, not in the power amp.

[Pentodo]

The noise is reduced by the instrument volume, by the amp volume and can even filter-out with the tone stack. So I was supecting the problem was among the tone stack and the input jacks (it only appears with an instrument plugged) until I touched the CD input signal-pin and the noise was produced even whithout an instrument plugged. The CD connection is after the preamp, I guessing this amp has a major grounding problem.

[DrGonz78]

I hate to be a pest but did you try cleaning out the headphones jack just to rule it out?

[Pentodo]

DrGonz78 be all the pest you need, I really appreciate all your comments and Jazz P Bass. Yes I cleaned pots and jacks. I can reduce the static noise by a cap placed in parallel to C3 but of course it also affects the signal from the bass guitar. It seems like the cable even being shielded is getting lots of noise, too sensitive to magnetic fields around, even bypassing the tube stage I keep getting all that static noise.

[Jazz P Bass]

The noise is reduced by the instrument volume, by the amp volume and can even filter-out with the tone stack. So I was supecting the problem was among the tone stack and the input jacks (it only appears with an instrument plugged) until I touched the CD input signal-pin and the noise was produced even whithout an instrument plugged. The CD connection is after the preamp, I guessing this amp has a major grounding problem.

'Touching ' an input jack tip Will produce noise.
That is not a good test.

I would start at the input jacks.
Resolder the PCB pads & verify that the grounds are intact.

Have you tried either/both inputs?

J1 is a bit demanding as it requires that both tip & ring be grounded for J2 to function properly.

[Pentodo]

You are right Jazz P Bass, touching is not very good and safe test, actually it was by accident, and definetly it did help to much. I have done some fourier and this guy is acting like an antenna getting all kinds of noise. The problem is reproduced in both inputs. I will follow Jazz P Bass suggestion and review the PCB, resoldering and checking out all comps. The noise is being catched in the input, so the problem has to be in that first stage. I will let you know the results.