Marantz PM-94

[StuartMoore]

I have been using a Marantz PM-94 Amp (partnered with a CD-94II) for about 14 years. The PM-94 has failed a couple of times and the problem both times was the main power connector on the power amp boards. Because of the high temperatures, the solder joints get thermally fatigued and eventually end up dry.

The same fault was starting to occur again and I decided to buy some high melting point solder and do both sides 'once and for all'. I did the repair, installed the amp back into the system, switched on and all worked as expected - for about two minutes, then 'click' the speaker protection relays dropped out. I have not been able to get the relays to energise since and also the front panel lights are off.

On the face of it, you would say that there is a fairly simple power supply problem like a dead rectifier or similar. However, there does not seem to be a simple PSU for that part of the circuit (I have been unable to find a service manual or schematic). The mains input relay does operate and the class A and class AB supplies are working and the amps are getting hot indicating that they are powered and biased (although one side gets slightly hotter than the other, indicating a slight bias imbalance).

I would really appreciate some help with this since it is probably a simple repair and I really like the amp. I would have to pay a lot of money to replace it with something as good.

Thanks

Stuart Moore

[Tube_Dude]

Hi Stuart

Maybe a solder bridge that short when the amp gets hot ...(dilatation)

[djmiddelkoop]

Hi Stuart,

at least I have the schematics, but they are on a large paper format. So posting is not possible.
If you give me your email address, I can try to send you scanned parts of it.

At least you will have something to start with.

Dick.

[StuartMoore]

Hi Dick,

I would be really grateful for some schematics. Anything you can give me will be much appreciated. My email is: smoore@ttm.uk.com and just about any size file is ok.

Thanks in advance

Stuart Moore

[destroyer X]

I will be happy to see this schematic, i am a colector and a constructor and tester.

My system can hold easy 10M files, no problem, please...send it to me.

thanks in advance.

regards,

Carlos

[Quattor]

I'd love to have that schematic as well. If you could be kind enough to send it to me, please. Thank you very much!

[djmiddelkoop]

Ok, guys, it's to large to post it here, so mail your email address
to:

fam.middelkoop@quicknet.nl


and i'll be happy to send it over, but be patient, I'm a very busy guy.

Dick.

[djmiddelkoop]

Stuart,

I just spoke to an onwer of a PM94 that had the same problem when he put together his amp after a small fix.

His problem came from the contacts between the output drivers board and the rest of the poweramp.
The heat had worn the contacts, after he had soldered them everything was ok again.

Dick.

[CederPM94]

Hi Stuart

Do yourself the favor of resoldering all the connections between the driver board and the output board. Make sure that the kobber leads on the board are not broken around the connectors. I had one broken kobber lead on mine.

If you want to know how to adjust it let me know. It's not that easy to figure out from the schematics.

Best regards,

Carsten

[StuartMoore]

Hi All,

Thanks for all your help. I only just found this group while trawling the web for PM-94 info. I am overwhelmed by the response to my thread.

Dick, thanks a million for the schematics. I have found a dead rectifier - DU85 (DN85 on the board) which is OC on the AC side. I guess its time was up and it was sheer coincidence that it blew just after a repair. Knowing where to prod was a great help. Hopefully this is the only fault and all will be well when I have replaced the rectifier (no doubt I will be back here if it doesn't).

I have inspected all the contacts between the boards and they all look good. As I said in my original post, the real problem has been the main power connector on the driver board. The power connections between the boards will be less stressed (I think) because they only carry power to the Class B side which I doubt get used much.

Carsten, I would like to know how to set the Class A bias on this amp. I would like to make sure they are balanced and maybe I could reduce the bias slightly to reduce heat dissipation (and therefore perhaps improve reliability). If I do get it working again I will probably fit some extractor fans on the top cover to encourage more air movement.

Thanks again

Stuart Moore