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Marshal JTM60's having trouble...

9/21/2004 12:33 PM
Marshal JTM60's having trouble...
I've got a couple of JTM60's (2x12, 3x10) that are having the same problem... After a few minutes of playing (the time varies), the volume on these amps drops to a very low level. I haven't had a chance to take a close look yet, but I know these are somewhat notorious for having problems and wondering if someone was familiar with this particular problem?  
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9/22/2004 2:18 AM
Mark Black

Have you tried running a patch cord from send to receive of the effects jacks (to bypass the internal switch contacts)? Even if that works it doesn't necessarily mean the jack is bad or dirty (see next).  
Most of my experience with JTM60's has involved a mass retouch of the lousy soldering, specifically at all jacks, pots, switches, tube sockets, and anything that is large (big caps, big resistors, diode bridge), as well as every multipin. Also don't forget the fuseholder and AC inlet jack.  
These are a pain to disassemble to where you can get to the solder side of all the boards but I think the results are worth it, and in fact view that retouch as similar to replacing spark plugs in an engine before attempting to tune it. At least you will know bad solders are not the problem and the amp will probably last longer before it needs service again.
9/22/2004 6:57 AM

Thanks Mark,  
As soon as I can them fully dissassembled, I will definitely retouch the solder joints. I generally do that on any circuit board manufactured amp. Hopefully, that is all it is. As I had mentioned earlier, these particular amps are also pretty notorious for power supply problems, and wondering if anyone has had these symptoms related to any P/S specific, or power section problems?  
9/22/2004 11:47 AM
Mark Black

Hi Chad -  
It strikes me that a JTM60 may have been my intro to shorted output tranny problems (see "Silver Jubilee" thread below). As I remember I could play the amp at low volumes but when cranked the arcing would start (drastically reducing the volume) and continue until powered off.  
I believe that may also have been the model where I encountered a lead for the bias supply winding pulled loose from the power tranny. If memory serves it was one of two small white wires exiting directly from the winding tape, as opposed to being attached to the terminal lugs, and was still embedded in the tape but no longer connected to the winding. That didn't completely kill the bias supply but knocked it down a lot, causing red-plate of the power tubes. Probably not the problem you have though...  
And of course there is the bridge rectifier (DB1, I think) which feeds the preamp tube heaters. There is almost always overheating of the solders and sometimes actual failure of the device. These could probably benefit from a larger device or a heatsink or both.  
Looking forward to hearing what you find!
9/22/2004 11:52 AM
Mark Black

By the way I think the output tranny arcing/shorting was audible directly from the tranny when I was using a dummy load.  
Good luck -  
9/22/2004 4:37 PM

Thanks again Mark,  
I will keep a closer look in those areas, if you come across anything else, please let me know.  
9/22/2004 9:29 PM
Mark Black

Coincidentally enough I just got a JTM60 in for repair today (in pristine physical condition for a change) with the complaint "Amp turns on, with no power to tubes".  
It will be interesting to refresh my memory on this amp while keeping your problem in mind.  
I'll let you know if I get a new idea...  

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