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Marantz PM66KI amp - resistor keeps dying!
Marantz PM66KI amp - resistor keeps dying!
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Old 11th February 2019, 12:47 AM   #1
dfear is offline dfear
diyAudio Member
Join Date: Feb 2016
Default Marantz PM66KI amp - resistor keeps dying!

Recently a customer of mine brought a Marantz PM66 KI amp in for repair. I was told that one of the channels wasn't working. I was told that the left channel wasn't working and that the volume knob was also loose. The customer also wants to upgrade the two huge caps to Mundorf AGs, but that's not relevant to the current problems I am having.

I tightened the nut on the volume knob, which seemed to cure most of the problems with the amp.

as per:
Marantz PM66KI amp.

I changed Q764 Q762 Q752 Q754 Q758 Q756 Q760 QN02 QN03 anyway.

Also I did:

"once rebuilt set volume to minimum , balance to center position , set the bias VR to center position (R756 or R755).

Set your digital (not analogue) multimeter to mV and connect to the two outer test points of the output resistor (R767 or R768). "

For the first minute or so every thing was OK, then the problems started. First R760 went up in smoke, then R764 started glowing red hot in the center of the resistor. I switched off the amp ASAP.

I tested R760, which was dead and R764 which started at around 200 ohms then quickly rose to 100 ohms plus and then the meter showed O.L.

I replaced R764 with a 330 ohm 5% 2W resistor (PR02000203300JA100 | Vishay PR02 Series Axial Metal Film Fixed Resistor 330Ω +-5% 2W +-250ppm/degC | RS Components) and R760 with a 100 ohm 5% 1/4W resistor (FRN25J100R | TE Connectivity FRN25 Series Axial Thin Film Fixed Resistor 100Ω +-5% 0.25W +-350ppm/degC | RS Components) as I couldn't find a 1/6W 100 ohm one.

Also I removed the new transistors Q756 and Q760 and replaced them with the original ones I removed from the amp. I tested both with my LCR tester. The new 2SA970 (which is Q756) was shown as a 2 diodes, where as the original one shows up as PNP transistor.

I turned on the amp and R764 glowed, then went up in smoke; straight away. R760 *didn't* fail and hasn't failed yet.

I replaced R764 for the 2nd time and removed the new 2SD3033A (which is Q758) and replaced it with the original one.

I turned on the amp and R764 glowed, then went up in smoke; straight away.
I have checked Q760 and Q756, which seem overly hot.

What should i do next?
I plan to replace the new 2SD3033 (Q758) with the original one and replace the burnt out R764. I this the correct next step? or should I be checking and replacing other components?

Thanks for any help anyone can give.

The service manual: http://testing.dfear.co.uk/pm66ki.pdf
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Old 11th February 2019, 08:37 AM   #2
mbz is offline mbz  Australia
diyAudio Member
Join Date: Jul 2013
First up, build a dim bulb tester, a current limiting device. All this smoke is likely to
damage/stress other components. Will try and look at schematic.
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Old 11th February 2019, 08:48 AM   #3
mbz is offline mbz  Australia
diyAudio Member
Join Date: Jul 2013
Check Q704, possible c-e short?
Build the DBT so you can power up without cooking things. Then need to take some voltage
measurements. If you are lucky Q704...
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Old 11th February 2019, 08:53 AM   #4
Mooly is offline Mooly  United Kingdom
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Join Date: Sep 2007
Marantz PM66KI amp - resistor keeps dying!
I'm afraid that replacing a whole bunch of semiconductors wasn't a great move. You should have proceeded to do normal checks and tests on the amplifier in its original faulty state rather than replace parts in hope.

At this stage, using the DBT that mbz advices above is the first thing to do.
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Old 12th February 2019, 01:49 AM   #5
stocktrader200 is offline stocktrader200  Canada
diyAudio Member
Join Date: Jan 2008
Location: Calgary
if those resistors get hot, the bias is way off and the final output pair are weak or dead.
there should only be 1.2v across the 330 ohm. the 100 should see even less.
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