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Marantz PM66KI amp - resistor keeps dying!
Marantz PM66KI amp - resistor keeps dying!
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Old 10th July 2019, 07:28 PM   #121
dfear is offline dfear
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Quote:
Originally Posted by Mooly View Post
Thanks for the kind words... its really appreciated and great to hear that your amp is all up and running now.

I'm not sure what to think of that modification list, you would need to look very very carefully at what is involved. Black Gate caps have been out of production for well over a decade... and there is a lot of scope for things to go wrong when undertaking something like that.
As per my customers' request I have replaced C801 and C802 with:
https://www.audiohobby.eu/en/mundorf...125c-2pin.html

I have reset the balance to centre and the volume to minimum. Also I have adjusted R764 to the lowest bias.

I have powered on the amp via the series lamp limiter and the buib is very bright and flickering. Also the power led on the front of the amp is NOT on.

Do I need to snip R754 again and adjust the value again?

Last edited by dfear; 10th July 2019 at 07:28 PM. Reason: spelling mistake
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Old 10th July 2019, 07:29 PM   #122
dfear is offline dfear
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Originally Posted by dfear View Post
As per my customers' request I have replaced C801 and C802 with:
https://www.audiohobby.eu/en/mundorf...125c-2pin.html

I have reset the balance to centre and the volume to minimum. Also I have adjusted R764 to the lowest bias.

I have powered on the amp via the series lamp limiter and the buib is very bright and flickering. Also the power led on the front of the amp is NOT on.

Do I need to snip R754 again and adjust the value again?
Also I have disconnected the speakers and the AUX inputs.
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Old 11th July 2019, 06:59 AM   #123
Mooly is offline Mooly  United Kingdom
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Marantz PM66KI amp - resistor keeps dying!
Replacing the main reservoir caps will not alter any of the DC conditions.

There are really only a few possibilities.

1/ You have fitted the caps incorrectly (polarity).

2/ Some solder has dropped onto the board somewhere causing a short.

3/ There is some physical problem with all the previous work such as a poor joint or crack in the print and the act of working on the board to replace the caps has shown up such a problem.

Recheck all your work very carefully before doing anything else.

The bulb flashing brightly and no LED means there is a near short on the power supply.

If you can see significant voltage across the 0.1 ohm resistors in either channels output stage then that shows where the current is going. If there is no voltage across those resistors then the short is elsewhere.
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Old 12th July 2019, 06:47 PM   #124
dfear is offline dfear
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Quote:
Originally Posted by Mooly View Post
Replacing the main reservoir caps will not alter any of the DC conditions.

There are really only a few possibilities.

1/ You have fitted the caps incorrectly (polarity).
Yes the C802 cap was fitted in reverse polarity.

Quote:
Originally Posted by Mooly View Post
2/ Some solder has dropped onto the board somewhere causing a short.

3/ There is some physical problem with all the previous work such as a poor joint or crack in the print and the act of working on the board to replace the caps has shown up such a problem.

Recheck all your work very carefully before doing anything else.

The bulb flashing brightly and no LED means there is a near short on the power supply.

If you can see significant voltage across the 0.1 ohm resistors in either channels output stage then that shows where the current is going. If there is no voltage across those resistors then the short is elsewhere.
Yesterday (11/7/19) I made an attempt to to sort out the problems. As I said above C802 was fitted wrong. in regard to C802, i matched the neg side of the cap with the pin marked ground on the underside of the PCB and NOT with the side marked with the dot on the upper side of the pcb.

Didn't make any difference; the amp still makes the series lamp limiter bulb v. bright and flickering. Also the power led is off.

I think I have managed to find where the short is: I am not sure if the short is just on C802 or is on C802 (Neg/-19V leg) and collector pins of Q764, Q760, Q756 and Q763.

I cleaned around the area of C802 and C801 caps with IPA and I also brushed the PCB several times (to remove stray solder).

After many rounds of de-soldering and re-soldering C802 (with short persisting), ic. cleaning pads and legs of caps with IPA and braid; then swapping the cap in C802 for the one in C801 - the short has gone!

I powered on the the amp. The power led is off. The bulb is v. bright and flickering.

shorts to ground:
C802 (Neg/-19V leg)
Q764 Collector
Q760 Collector
Q756 Collector
Q763 Collector

At 20.53 above shorts have disappeared.

20.55 Power on the the amp. No power led. The bulb is v. bright and flickering.

POWER OFF

20:57 - shorts to ground have reappeared.



12/7/19

Shorts to ground are still there.

at some point (i think whilst holding the positive probe on the NGE/-18V pin of C802), the short disappears.

19.00 Power on the amp. No power led. The bulb is v. bright and flickering.

POWER OFF

19.03 Retest C802, short has reappeared.


NOTES:

I am a bit flummoxed, sometimes the short is there and sometimes it isn't.

From memory, I don't think there is an voltage across R768. I haven't tested the 0.1x2 resistor on the other channel yet.

Even swapping the cap from C801 to C802 hasn't made the short go way permanently. It worked at first, then the short came back.

WHAT DO I do???????

I'll double check for voltage across both 0.1x2 resistors and report back.
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Old 12th July 2019, 07:15 PM   #125
Mooly is offline Mooly  United Kingdom
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Marantz PM66KI amp - resistor keeps dying!
A problem with the mounting of Q763 or Q764 could give a short symptom on the negative rail.

As I mentioned yesterday, you need to check ALL your previous work very carefully.

Make sure the transistors are insulated correctly from the heatsink.
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