I did a more complete acetone cleaning and it looks like a complimentary pair of TO-92 drivers (?) on the damaged side of the power supply needs to be replaced, the cases are cracked near the emitters which are connected to the array of mosfet gates. They are MPSA56 and MPSA06.
The pnp device has a collector to ground resistor that is supposed to be 10 ohms I believe (based on comparison to the other side), but is reading erratically around 190 ohms, and the case has come off. In addition there is a 100 ohm resistor on the damaged side showing the black stripe.
I ordered all parts. Thank you for any comments.
The pnp device has a collector to ground resistor that is supposed to be 10 ohms I believe (based on comparison to the other side), but is reading erratically around 190 ohms, and the case has come off. In addition there is a 100 ohm resistor on the damaged side showing the black stripe.
I ordered all parts. Thank you for any comments.
The amp just passed basic sound, sounds correct, doesn't seem to be doing anything unusual. I only tested around a minute at high output.
The amp itself makes a high-pitched sound. It is not very loud, but is definitely there. I have not changed the rail/din caps yet, I wonder if this could be the issue. Any comments are appreciated, thank you.
The amp itself makes a high-pitched sound. It is not very loud, but is definitely there. I have not changed the rail/din caps yet, I wonder if this could be the issue. Any comments are appreciated, thank you.
I have replaced all liquid electrolytics and all parts mentioned above. It makes and high tone which can be heard in the amp and both channels.
With no signal drive, just power/remote connections the power supply makes a kind of whistling noise, and there is a high tone, like a hum, in both channels. It is much louder in the channel that corresponds to the side of the power supply I did not repair. Otherwise the amp seems to work normally.
The mosfets in the other side are marked the same as those I replaced, NDP 7060 date code 9924. They do not appear to be shorted, and I don't observe any obvious damage. Would it be typical for failure of the mosfet array on one side to cause failure on the other?
The mosfets in the other side are marked the same as those I replaced, NDP 7060 date code 9924. They do not appear to be shorted, and I don't observe any obvious damage. Would it be typical for failure of the mosfet array on one side to cause failure on the other?
In most amps, all of the FETs fail when any fail.
What is the DC voltage on the gate legs of the FETs on each side?
What is the DC voltage on the gate legs of the FETs on each side?
Thank you Perry.
I have it apart right now, to inspect more carefully for damage.
One of the FETs on the unrepaired side is burned slightly on its face right under the clamp, and there is related scoring on the clamp itself. I just couldn't see it until I removed the clamp. I took pictures.
I am going to replace all 9 on the other side now and test, since it seems likely it will help.
I have it apart right now, to inspect more carefully for damage.
One of the FETs on the unrepaired side is burned slightly on its face right under the clamp, and there is related scoring on the clamp itself. I just couldn't see it until I removed the clamp. I took pictures.
I am going to replace all 9 on the other side now and test, since it seems likely it will help.
The driver transistors and the 10 ohm resistors in series with the PNP driver transistor collectors commonly fail.
OK I replaced those and it is on now. There tone is still there, but not as bad. The amp itself isn't making the noise anymore. Maybe outputs on that side? I can measure if that helps.
There are 5 2N6491 and 5 2N6488 for outputs in the bad channel.
It looks like someone has repaired this channel before.
One of the 2N6491 does not have a matching date code. There are 3 different date codes among the 2N6488s. I don't know if the lack of matching date codes itself might explain the problem. Several resistors have also been replaced. Nothing looks burnt or measures short.
It looks like someone has repaired this channel before.
One of the 2N6491 does not have a matching date code. There are 3 different date codes among the 2N6488s. I don't know if the lack of matching date codes itself might explain the problem. Several resistors have also been replaced. Nothing looks burnt or measures short.
Replacing the other nine mosfets improved the problem. There is no longer noise coming from the amp itself, just a tone in both channels (heard at speakers) without any drive signal. Like an elevator bell that is constant. Gain makes no difference. One side (same as before) a little worse than other.
I went over my work to look for any bad soldering, etc. I replaced lots of parts, but everything looks OK.
Since the last change was in the power supply and things improved, seems likely there are still damaged parts in it. I don't know how to proceed. I replaced two small transistors on the worst side and a few resistors (mentioned above), but not on the other. The rectifiers seem OK, I don't know if they go bad this way sometimes. If I play the gain high, you almost can't hear it (which again makes me think it is just being put right on the outputs by the supply). Somebody mentioned the zener diodes on the mosfet gates being a problem sometimes. Thank you for any suggestions I will update.
I went over my work to look for any bad soldering, etc. I replaced lots of parts, but everything looks OK.
Since the last change was in the power supply and things improved, seems likely there are still damaged parts in it. I don't know how to proceed. I replaced two small transistors on the worst side and a few resistors (mentioned above), but not on the other. The rectifiers seem OK, I don't know if they go bad this way sometimes. If I play the gain high, you almost can't hear it (which again makes me think it is just being put right on the outputs by the supply). Somebody mentioned the zener diodes on the mosfet gates being a problem sometimes. Thank you for any suggestions I will update.
They are different. One is 1.88 V, the other is 2.14 V. That is measured wrt rca shield. The gate diodes seem OK.
Using the shield 'might' be OK but I'd suggest using the RCA shield as the ground for audio side readings and use the main ground for power supply readings.
What's the resistance between the RCA shields and the main ground?
Did you check the 10 ohm resistors and the driver transistors?
Do all of the gate to source voltages match on each bank of the PS FETs?
Pull that Zener for now.
What's the resistance between the RCA shields and the main ground?
Did you check the 10 ohm resistors and the driver transistors?
Do all of the gate to source voltages match on each bank of the PS FETs?
Pull that Zener for now.
I have a 2nd revision of this amp, and it is using 1N4742A in a similar location, although the circuit is slightly different.
OK. I'm get 1.97V, 2.17V taken that way now for the gate DC voltages.
1.4 Ohms
Yes. They all read fine. I replaced at least one of them that showed heat banding.
Yes. 1.97 VDC on all 9 on one side, 2.17 VDC on all 9 on the other.
Correction, I am not certain a checked all driver transistors. I replaced the MPS A56/A06 pair on the worst side, but not the other. I did order replacements for the other side, however. Are these what you mean?
There are two power transistors standing on end that I did not touch. All 18 NDP7060 have been replaced.
There are two power transistors standing on end that I did not touch. All 18 NDP7060 have been replaced.