Faulty guitar amp

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Bizzie
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#1 Faulty guitar amp

Post by Bizzie »

I have on loan with the option to buy a 50w Marshall guitar amp that sounds great when it works but it sometimes takes a while to come on and does so with a loud crack.

I was warned about this problem by the owner a friend of mine who said that it didn't use to do this but he hasn't used it for ages and this is what happened when he tried it before lending it to me.

Although I don't want to mess with it too much yet but I have had a quick look inside just after using it and there are two electrolitic's 2200uf which I think are part tof the power supply that although they are not out of shape were quite warm almost hot to the touch.

I have always thought that capacitors shoudn't get hot so could they be on the way out possibly and causing the problem
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Nick
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#2

Post by Nick »

certainly seems likely, other thing to check is the rectifeirs feeding the caps, though if one of them had gone short, I would expect the mains transformer to complain.
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Andrew
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#3

Post by Andrew »

Yes, seems like they're not right and probably, for safety sake, should be replaced anyway.

-- Andrew
Bizzie
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#4

Post by Bizzie »

There seems to be 1 large bridge rectifier close to the tranformer, what would be the best way to check it out without removing from the pcb is it just a case of checking the output with it turned on as I don't want to get too invasive yet as it I haven't actually bought it yet but would like to know if it would be easily repairable if I do.
Andrew
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#5

Post by Andrew »

Could be hard to test diodes in circuit...first thing to check is have you got meter with a diode test feature?

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Bizzie
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#6

Post by Bizzie »

After thinking about this for a while I have come to the conclusion that the bridge is alright as the power led is coming on but there is no sound so the problem could be elswhere.

As almost all the pots were crackly when it worked I decided to give them all a clean with deoxit and give them a thorough workout.

Everything was left to dry out for an hour before switching on and low and behold its now working again but without the loud cracking sound when switched on.


It seems to be OK now so the problem seems to have been one or more of the pots so I'll give it some use over the next week or so before commiting to buy but if the problem re-appears then I may give it a miss as if the rectifier is faulty it could have damaged other components and some of the IC's have had there numbers removed.
Bizzie
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Posts: 500
Joined: Mon Dec 17, 2007 1:22 pm
Location: North Lincolnshire

#7

Post by Bizzie »

Well looks like it still has a problem as it wouldn't come on again last night, the power led lights up OK as does the the one for the overdrive channel.

I think that I will give this back to my mate unless he's willing to let it go dirt cheap so I can risk the loss if it can't be repaired.
Bizzie
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Location: North Lincolnshire

#8

Post by Bizzie »

After discussing the problem with the owner he gave me the go ahead to do some surgery on the amp.

I managed to find the schematic on the web and found that the op amps run on a +and -15v supply this was checked on one of the ic's and I had +15 and -0.68v this was traced back to the power supply and found to be the same after the two 15v zenners so they were the first suspects.

After unsoldering both zenners the seemed to check out ok using my Multi meter so they were put back in but swapped around so the -15v leg had the one that previously had the zenner from the +15v leg and there was no change still no -15v supply.

Then all the electolytic caps in the power supply were changed one at a time and it tuned out that one of the 2200uf caps mentioned at the start of the thread was the culprit and the amp now powers up and runs as it should.

Trouble is now I suspect that all the electrolylitic's are probably getting a bit tired and should be changed but I didn't have all the right values to do it while it was stripped down so I'm not sure if I can be bothered now its back together and running ok.
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