marantz PM 94 - keeping it cool

[Jazzmuso]

I read short write up found at http://audiokarma.org/forums/index.php?threads/pm-94-or-1300dc.319785/#post-3978224

Just checking if there are any further ideas on this topic. We (Cape Town Hi fi Club) have a restored and functional PM 94 amplifier and all fear that it runs a bit hot. We love the sound and build quality but fear heat damage will lead to malfunction. Any ideas for cooling and keeping it cool will be useful and welcome.

Thanks

 

[Jazzmuso]

Just got some input that the cover of the amplifier is a poor design - the slits in the cover are too narrow for proper heat dissipation and sometimes directs heat back onto the heat sinks. I am surprised that there has been no response from any members here. No one using this amplifier, a classic Marantz here on the Marantz sub-forum on AK? Or does nobody here have heat problems with their PM 94?

 

[restorer-john]

The PM94 is rare, hence not many people own one. That said, I have a PM94 and a PM95. The issues on the PM94 is not the heat per-se, but the way the PCBs are plugged together on each amplifier module. The expansion and contraction of the multipin connectors on the stacked twin PCBs on the heatsink, plus the relatively average soldering, all result in major and spectacular failures in that unit.

The units run hot- they are intended to, and the heatsinks and ventilation is otherwise adequate (if you don't stack stuff on the amp).

You should also consider, the unit runs twin rails, uses a commutating supply and the switching circuitry for the Quarter A is on a separate PCB, mounted vertically and it also can be the cause of problems. The PM95 solved all the problems in design of the PM94, and is IMO a much better amplifier (it uses the same MOSFET output stage but has a switchable Class A driven by optocoupled switching and lower voltage secondary taps) That said, the PM95 gets a TDA1541S1 DAC (essentially the same circuitry as the DA12LE) whereas the PM94 has a superb phono stage with twin LCOFC MC step up transformers that the PM95 lacks.

I have a pair of PM94 channel modules here if you want me to point out the problem areas in pics.

 

[Jazzmuso]

Thanks for your response John, makes a lot of sense. The pics would be really useful to pass onto our main tech in the club.

 

[airtime]

The issues on the PM94 is not the heat per-se, but the way the PCBs are plugged together on each amplifier module. The expansion and contraction of the multipin connectors on the stacked twin PCBs on the heatsink, plus the relatively average soldering, all result in major and spectacular failures in that unit..

That is the same problem the SC-9/3650 preamps had, The pin board contact solder pads heat up. Eventually they loosen or lift up then need reflowing or repairs. In the preamp it only causes an annoying intermittent channel. But I'm sure in an amp it could cause bigger problems.

 

[restorer-john]

OK, here's some of the PM94 amp board pics and also a comparison with the PM95 amp module.

Below is a PM94 board with the mosfets and rail switcher Trs pulled off to repair and clean. Notice the 4 multipin connectors. These are the cause of the PM94 self destructing. The matching sockets on the driver board that is stacked on top need to be carefully cleaned, resoldered and spring re-tensioned.



Here you can see the boards stacked and the pins sitting in their connectors. The PCB also needs to be cleaned of lots of brown glue and/or corrosion from said brown glue.



Close up of the other pins, seated. The rail switching transistors also need to have their soldered legs checked as they are under tension and sometimes the pcb trace can lift- note, it is a double sided trace board.




This is the PM94 and the PM95 boards side by side. Note the single PCB on the PM95, the Mosfets are soldered to twin-width tinned copper pads on the upper side of the PCB and once the legs are formed and soldered carefully, there is no tension on the legs or the PCB. That said, the bias transistor IS under tension, and resulted in a cracked joint and a blown set of Mosfets in this case.


Another comparision of single channel modules, the PM94 has a thicker finned heatsink, whereas the PM95 has a taller heatsink- surface area and dissipation would be similar and both are more than adequate for Class A running.



(PM95) Can you see the problem? Pretty disgusting on a AU$5999 TOTL amplifier. It was like that, from the day it was made- both channels had similar central leg poor placement. The bias/vbe and drivers were soldered and then bent over at the factory, as they mounted the PCB to the heatsink, most leads bent over in a nice curve, but the centre lead didn't- it pushed the thin trace off the PCB and hung there for 20 years without anyone ever knowing. Cost a full set of ultra rare Toshiba Mosfets...




PM95 New Mosfets about to be soldered- see the way they improved the mounting? There is the ability for the legs to expand and contract with a step-bend in them and the tinned plate on the PCB means no more dry joints on the output devices either. Optocoupled Class A switching of bias.


Look at all that yummy copper plate- it's everywhere, the whole amp is covered in it, both the PM94 and the PM95.



Some new Mosfets, waiting to be installed after matching...



Hope that helps.

John

 

[Ds2000]

Wow informative.

 

[Jazzmuso]

Thanks very much - I am sure our technical guys will make much more sense of it than I can. Appreciated

 

[October]

here we go, resurrecting an old thread. i have a question regarding the cooling of a PM-94 amp. can anyone describe - and show with pics - how their auxiliary cooling system works? i have access to a largefan that i can use to cool my amp. i'm just wondering what others are doing.

 

[Oerets]

here we go, resurrecting an old thread. i have a question regarding the cooling of a PM-94 amp. can anyone describe - and show with pics - how their auxiliary cooling system works? i have access to a largefan that i can use to cool my amp. i'm just wondering what others are doing.

A simple solution I've used before on there equipment in want of better cooling. Using the fan out of a laptop computer cooler. The type used under the laptop and plugged into the usb. Just find one at a thrift and bust open use a 5V wall wort for power. Then plug in the switched outlet on the unit needing the air. I put feet on the fan and have the air sucking out not blowing in air to unit.

 

[krautmaster]

I use a pair of cooling fans on top of every amp & receiver in the house to pull the hot air out of the case. I have four set up at all times, and just rotate them in-out of service every three or four months. I have: Onkyo M508 (220 watt monster), Onkyo A-10, Onkyo A-7, Marantz 1152DC, Marantz 2252B, Marantz 2275 (my personal favorite) and an Onkyo TX-4500MK II.

https://www.ebay.com/itm/Receiver-A...eed-control-/360321397254?hash=item53e4d43206

 

[October]

sorry for the delay in replying. but, thank you for the info. i have a 10" computer fan but unsure how to hook it up. i'd like to be able to plug it in to the switched outlet at the back of the receiver so it will come on when the receiver is turned on.
thank you for the ebay link. i might do a search on there for something useable.

 

[Oerets]

You may get a pop or static when using the receivers switched outlet. The little laptop coolers fans with a wall wort don't seem to cause this when in use.

 

[October]

You may get a pop or static when using the receivers switched outlet. The little laptop coolers fans with a wall wort don't seem to cause this when in use.

oh, man.....good to know. thank you for that.