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Re: Marshal JTM60's having trouble...


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9/28/2004 12:42 PM
scott
Re: Marshal JTM60's having trouble...
well... i took the amp apart and started probing, looking, sniffing. turned it backwards and forwards, upside down. decided to plug it in and meter some values at certain points so i could compare it with values at the same points after the volume drops out. i started playing while waiting for the problem to happen ( generally < 5 min. ), and you can guess what happened. the amp played fine for over an hour, no problems whatsoever. so my twisting and turning could have connected/unconnected some bad solder/loose wire/faulty connection/short ? i'm afraid to put it back together because i know as soon as i do it will fail again. so i'll keep playing, see if it fails and retouch the solders ( as everyone here has said is really helpful for these marshalls ). i hate intermittance.
 
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9/29/2004 11:40 AM
Mark Black

Sorry to hear that. Welcome to "Technician Hell".  
 
Intermittents really are the worst, not the least because they degrade the trust level (however tenuous) between a customer and a tech. If the amp goes back working and immediately does the same thing for the customer at a gig it's just all bad...  
 
If it's your own amp it can just make you crazy.  
 
So let's see what might have changed:  
 
Indeed things have been moved around and may have wiggled something back into better contact (or out of contact).  
 
Screws are removed which may have been contacting something they shouldn't have.  
 
The chassis is open to the air and better able to get rid of heat.  
 
Things that were grounded together and/or shielded by each other are not.  
 
Just some observations to help with the idea flow as you look at the amp. Sometimes it helps to put the job away for a while and come back with a fresh perspective.  
 
Here's a great article about a nasty intermittent problem (not related to yours I'm sure but interesting anyway): http://everything2.com/index.pl?node_id=1071844  
 
Good luck once more!  
 
-MB
 
9/29/2004 5:55 PM
Enzo

Intermittants are the hardest thing to find.  
 
Put it back together and see if it screws up that way. For the reasons suggested above, it may well be screws hitting something or whatever. When things are bolted up there may be a bit of flex put on the boards.  
 
With the chassis out heat cannot build up as well. Fold up a towel a couple layers thick and lay in on the circuit board to help it heat up. DOn't leave it unattended - fire hazzard.  
 
Push on the board itself to see if flexing it makes a difference.  
 
I have a magnifying glass next to me always. Inspect the solder on everything in sight looking for cracks.
 
10/4/2004 9:25 AM
Chad

Hello gents,  
 
I'm having the same intermittent problem on both I'm working on. Unfortunately, doing my best to recreate the problem, ie. containing the heat, pushing on the board, etc., and just not having much luck. Touched up suspect joints, but not really convinced they were the problem. Just about ready to "shotgun" it, and for the cost of the parts and time involved, just replace those common problem parts just over the power tubes. Not there yet, but getting close.  
 
Frustrated,  
Chad.
 
10/7/2004 8:34 AM
Dave Rich
My JTM 60 had some problems. Bad speaker jack. It was made of tin foil and plastic. I also kept blowing the heater fuse. The heater voltage on mine was high, well over 7 volts IIRC. I ended up putting a small resistor in series with the heaters. BTW, the heater voltage to the power tubes is AC. It is then rectified to DC for the preamp tubes. The geniuses at Marshall mounted the first two filter caps directly above the EL 34's which were cooking them. I replaced these.  
 
So check out your filter caps and your heater voltage. also try installing a cooling fan. It's a great sounding amp but I hate not being able to rely on it.
 
10/8/2004 12:28 PM
Richie

Its been a while since i worked on a JTM 60,and the JTM30.. Bad OTs..the heater circuit problems..and lots of loose solder joints. Might check those small computer grade caps,they tend to come loose.  
These amps have flimsy chassis,and can be twisted very easy,so mounting and unmounting them might make a difference,or change the problem..and all those jumper leads.. More like a Peavey with a Marshall name on it..  
 
Richie
 

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