amp crapped out/help/ where to start

turtle · Post by **turtle** » Sat May 10, 2008 6:35 pm

Twas playing the other day and the amp just ceased to play.
don't see a fuse , suppose there must be one.
obviously I checked chords and what not.

So I don't even know where to begin.

Talk me through Mike?
anyone?

it's a Marshall JTM 60

James · Post by **James** » Sat May 10, 2008 6:40 pm

Sloan · Post by **Sloan** » Sat May 10, 2008 6:47 pm

Sometimes there are fuses inside the amp chasis. Not sure about the JTM 60 though, it's worth checking out.

Sloan · Post by **Sloan** » Sat May 10, 2008 6:49 pm

Just found this info:

The most common faults on these (which could cause this problem) are: Output Transformer (including loose push-connects); speaker wiring (again including loose push-connects); speaker jack; the large resistors on the center board near the PT end.

It's worth replacing the incredibly poor quality speaker hookup cable and soldering all the speaker connections, whether or not you think it's the cause here; leave a little extra length and the zip-tie the cable to the underside of the tube cover to prevent it getting snagged and stressing the jack.

They also have a few other common faults which won't be the cause of this problem (because they either cause total failure or different symptoms). One of them, which you might as well fix when you're in there, is to change the filament fuse on the center board from T6.3A to T10A - Marshall simply got the value wrong and it occasionally blows for no reason other than the startup current going up to (or even above) 10A for about a second, so 6.3A really isn't enough safety margin to guarantee it won't blow. 10A is more than enough protection against a real filament short, and this is one of the very few cases where increasing a fuse value is the right thing to do.

Sloan · Post by **Sloan** » Sat May 10, 2008 6:51 pm

Here is a good thread:
http://electrical.com/phpBB2/viewtopic. ... a78d1d0cb1

turtle · Post by **turtle** » Sat May 10, 2008 6:54 pm

Thanks Sloan............I have to go through the motions and actully move the amp from the corner it's buried and bring it out into the open so I can check.

Post by **Mike** » Sat May 10, 2008 7:11 pm

Simple things to try:

1. Are all the tubes lighting up (take any metal cans off the preamp tubes)? If not suspect bad preamp/power amp tubes and fuses.

2. If they're lighting check your speaker cable is good with a DMM. Then to elimate the preamp inject signal into the return of the FX loop from a pedal or pream out of another amp.

turtle · Post by **turtle** » Mon May 12, 2008 3:20 pm

Mike wrote:Simple things to try:

1. Are all the tubes lighting up (take any metal cans off the preamp tubes)? If not suspect bad preamp/power amp tubes and fuses.

2. If they're lighting check your speaker cable is good with a DMM. Then to elimate the preamp inject signal into the return of the FX loop from a pedal or pream out of another amp.

So the fuse is fine and the tubes glow red. Might also mention I was playing it when it stopped.
I'm not sure I understand the last suggestion ... injecting signal

thanks too sloan for the link

Post by **Doog** » Mon May 12, 2008 3:34 pm

Mike's basically saying to plug your guitar into the return of the FX loop to see if the preamp is the culprit by bypassing it.

IMPORTANT!
Turn your guitar's volume all the way down first- by bypassing the preamp, you're also gonna be bypass the master volume, so essentially would be playing as loud as the amp gets!

Post by **Mike** » Mon May 12, 2008 3:37 pm

I would have thought most Master Volumes would be post FX Loop and either Pre or Post Phase Inverter.

Also check continuity on your speaker and guitar leads, try the speaker with a different amp etc.

Post by **Doog** » Mon May 12, 2008 3:40 pm

I was kinda wondering too, but the few times I've tried this, the master volume musta before, cause it was looooooooooooud.

Although I know jackshit about amp wiring, I can say it was loud.

Post by **Mike** » Mon May 12, 2008 3:50 pm

I'm thinking that was a solid-state amp probably?

Anyway Lou, report back after these experiments and I'll have more for you. It's worth dropping the chassis to see if anything looks obviously bad, check the rectifier (black solid lump) and any traces/components for any burn/scorch marks.

euan · Post by **euan** » Mon May 12, 2008 3:52 pm

You have a JTM60 buried in a corner? Aren't the known for overheating?

turtle · Post by **turtle** » Mon May 12, 2008 4:27 pm

euan wrote:You have a JTM60 buried in a corner? Aren't the known for overheating?

Not that kinda buried!

Actually the only over heating was from my smoking leads.........................(wakes up from dream)

checking now Michael and Andrew

turtle · Post by **turtle** » Mon May 12, 2008 4:29 pm

Doog wrote:Mike's basically saying to plug your guitar into the return of the FX loop to see if the preamp is the culprit by bypassing it.

IMPORTANT!
Turn your guitar's volume all the way down first- by bypassing the preamp, you're also gonna be bypass the master volume, so essentially would be playing as loud as the amp gets!

highlighted................. = Parallel or series?
maybe you guys can explain the difference to me one day

turtle · Post by **turtle** » Mon May 12, 2008 4:35 pm

so..................no go through loop..........parallel or series

I have no other amps here to check anything ie speaker.......one is at band practice site...other/others sold

Post by **Mike** » Mon May 12, 2008 4:36 pm

Hook it up to some stereo equipment.

turtle · Post by **turtle** » Mon May 12, 2008 5:24 pm

speakers work

at least there is one advantage to being old.............stuff laying around....I found a Laney Theatre 150T X5 PA head in the attic!

turtle · Post by **turtle** » Wed May 14, 2008 11:20 am

any more suggestions?

Post by **Mike** » Wed May 14, 2008 11:20 am

If you can see nothing obviously bad on the PCB it needs to go into the shop I'm afraid.