Marshall JCM 900 making a crackling noise

[goldtop5]

Hey Guys!

I've got this Marshall JCM900 amp head (model 4500). That has a crackling noise when you strum a chord. It only happens on channel A. Channel B works fine.

I've checked for opened resistors, shorted caps and diodes. I've swapped both preamp and power tubes, changed out a broken preamp tube socket, and resoldered all connections. Still the same problem. By looking at the schematic, I'm starting to suspect a bad ic. Could this be possible? Any hints and ideas would be great, because the owner of this amp (the customer) won't leave me alone. He calls constantly.

Thanks!

[Alex R]

Customers are strange. Some get the idea you'll fix the amp really quick if they call you all the time, and will put it in the back yard and forget it if you don't. Others don't seem to want them back at all. Pressures in their lives get transferred to the repairman - but actually that shouldn't be so.

Why I'm writing is not to moan about customers but to say that I had a Laney with this problem and it turned out to be a coupling cap that looked as if it was soldered in but actually the wire hadn't gone through the board. I see you've resoldered everything though.

Input jack? Dodgy pot?

If you can feed a big signal in and make it crackle you could probably see it on a scope and trace it down to its origin - or use an audio probe and listen for it if you can bear the buzz. Audio probes are good, I have a nice old Emrad ss jazz guitar amp in at the moment that hisses too much; hard to see hiss on the scope but the audio probe picked it up and I traced it to its three different sources (a dropping resistor, a zener and a transistor) easily by listening for drops/lifts in hiss across components etc.

Generally opamps that go wrong either hiss/crackle all the time, or just die.

[goldtop5]

Hello, and thanks for responding! Yes, I have a bunch of customers who think that this business is a drive-thru service. They can't accept the fact that not everything is fixable right off the bat. There are some people who are in a hurry, because they have a gig. So when you fix the amp, call and tell them it's ready, they disappear for 3 months! This job is commissioned based only, so that really set me back financially. I was forced to work a second job on account of that. My boss lost so much money in this business that he's closing in 3 weeks. I will be on my own and will have to open up my own repair shop, but I will not deal with customers directly. I will only deal with music stores' repairs. This will eliminate traffic coming in and will ensure I get paid for the work done. It will also prevent eating up repair costs. I only have 2 years experience so I'm not going to be able to fix everything. I have to limit myself on what I can and cannot do. Lots of new gear these days are crap and all you do is spin your wheels and get nowhere.

I checked all the ic's, and sure enough, none of them are bad. It kinda sounds like a signal path that's partially broken down somewhere. I've been using my test guitar as a signal injector to probe through the circuit. Not a very strong signal, but it gave me a few hints. I don't trust myself enough to use a tone generator or a cd player being that I blew our last ones out. There are a few transistors and j-fets I haven't checked yet. The diodes seem to check fine while in circuit. The resistors seem to ohm out ok. But could there be some resistors drifting out of tolerance during power up?

[Mars Amp Repair]

hello,
the JCM 900 incorporates about 100 different amplifiers...do you know which jcm 900 is is?

Also, after stumming a cord, does banging the top of the amp cause any change in the sound? I have to think you've done this.

Have you tried injecting a signal at the return jack of the effects loop? (if it has one). This could help you eliminate or implicate the driver & power stages. Amps can be funny animals & just because it doesn't do it in channel 2, doesn't always mean you can eliminate the driver/power stages. The signal switching paths can be weird.

Hooking up a generator or CD player to the unit should not cause it to blow as long as you keep the output of the devices you're injecting at a low level. The guitar input jack is a very high gain point. Using an alternative signal source will free up your hands to investigate the circuit & help you be less likely to slip with your probe & short things out.
If you have a scope or audio signal tracer, you can trace the signal from stage to stage & possibly determine where the distortion is being generated. g

[goldtop5]

hello,
the JCM 900 incorporates about 100 different amplifiers...do you know which jcm 900 is is?

Also, after stumming a cord, does banging the top of the amp cause any change in the sound? I have to think you've done this.

Have you tried injecting a signal at the return jack of the effects loop? (if it has one). This could help you eliminate or implicate the driver & power stages. Amps can be funny animals & just because it doesn't do it in channel 2, doesn't always mean you can eliminate the driver/power stages. The signal switching paths can be weird.

Hooking up a generator or CD player to the unit should not cause it to blow as long as you keep the output of the devices you're injecting at a low level. The guitar input jack is a very high gain point. Using an alternative signal source will free up your hands to investigate the circuit & help you be less likely to slip with your probe & short things out.
If you have a scope or audio signal tracer, you can trace the signal from stage to stage & possibly determine where the distortion is being generated. g

It's the Model 4500 head.

[Mars Amp Repair]

wow,
that is one very weird circuit there in that pre-amp. I think audio signal tracing would be in order for a problem this weird. You should check the + & - inputs of the ic's with a voltmeter. It looks like in this amp they all should be near 0V as well as the outputs should be 0V too. If there is any large variation, you probably have a bad IC.

Also you might be experiencing parasitic oscillations...you'd need to hook a scope to the speaker outputs to view that, tho.

Other than that, can you beat on the thing & get the noises to change? glen

[goldtop5]

wow,
that is one very weird circuit there in that pre-amp. I think audio signal tracing would be in order for a problem this weird. You should check the + & - inputs of the ic's with a voltmeter. It looks like in this amp they all should be near 0V as well as the outputs should be 0V too. If there is any large variation, you probably have a bad IC.

Also you might be experiencing parasitic oscillations...you'd need to hook a scope to the speaker outputs to view that, tho.

Other than that, can you beat on the thing & get the noises to change? glen

I used an IC checker. I replaced a TLO72 chip with a JRC4558. I've checked my low +,- voltages on every IC, and the registered +,-15V.

[Amp Kat]

Did you do the test that Mars suggested to you running a signal into the return and see if it still crackles ? Your best off if you can isolate the problem to preamp or power amp. I'd also put that TLO-72 back in instead of the 4558. You can also take the send out to the front of another amp or it's return and test the preamp to see if it crackles. Not trying to beat a dead horse here but all of the crackle problems I've seen are due to tube sockets, bad tubes on both the 12AX7's or the power tubes especially Sovtek WXT's, bad connections and or leaky coupling caps but rarely the case on the caps. Try leaving the power tubes in and pull the preamp tubes out.