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Old 7th July 2012, 05:20 PM   #1
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Default Loud amplifier hum

Had a problem with blown fuses in my marshall avt50 head. I replaced the tda7293 chip, and now I can turn it on without the fuse blowing, but there is a very loud hum when I plug in a speaker. I can't find any components that look questionable, so does anyone have some suggestions on what I should test first?
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Old 7th July 2012, 05:22 PM   #2
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If it is humming with no inputs attached then I would look at the PSU.

Check capacitors in PSU and the rectifiers.
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Old 7th July 2012, 05:25 PM   #3
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Also depends on what you call "LOUD".

An often overlooked diagnostic tool is the smaller cap. Not many guys carry large stocks of huge audio grade caps because they are too expensive. Most amps will operate reasonably well with relatively cheap caps. NOT AT ALL like we want the final product to sound, but replacing a faulty 22000uF cap with a cheap 1000uF cap will help point you down the road as to where the fault might be.
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Old 7th July 2012, 05:35 PM   #4
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By loud, I mean that the hum is louder than I would normally play, and loud enough that I doubt I could hear my guitar if it were plugged in. I don't mean background noise.
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Old 7th July 2012, 05:42 PM   #5
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As the rectifiers are pennies I would change the rectifiers and the PSU caps for starters.

Working from the PSU end can only help matters.

If you try working from the ouput end you could end up destroying new components that are replacing old ones.

If you start blowing fuses then there could be output issues.

Last edited by KatieandDad; 7th July 2012 at 05:44 PM.
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Old 7th July 2012, 08:07 PM   #6
Enzo is offline Enzo  United States
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Does your speaker cone move one direction and stay there when you power on? That means ther is DC on the output. That will damage your speaker. Disconnect your speaker and measure for DC voltage between the speaker wires. Is there?

DC on the output here means the TDA7293 is blown, or one of the main power supplies is missing.

Doesn;t matter if the parts LOOK goode, they have to BE good. Bad parts usuall;y look just like good parts.
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Old 7th July 2012, 10:06 PM   #7
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I wonder if I might have an input grounded somewhere. Any tips on finding that?
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Old 12th December 2012, 10:36 PM   #8
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Hi, I have an AVT50 with fuse and tda issue...after changed R58, TDA7293 two times and fuse 6 times, I found something!! On LT+ I have 32VDC( it is right) but also 67V AC .. Is it wrong??? Also changed BR1, ZD5, ZD6 ( old components are OK after test, but I changed this because other post)

If this value is not OK...Which component needs to be changed to avoid that failure??? On LT- I have 32VDC with 0v AC...

Currently, I changed BR1, ZD5 and ZD6, main fuse ( 6 times ), TDA7293 ( two times) and R58...Please help me!!
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Old 13th December 2012, 03:44 PM   #9
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Quote:
Originally Posted by moonmark View Post
Hi, I have an AVT50 with fuse and tda issue...after changed R58, TDA7293 two times and fuse 6 times, I found something!! On LT+ I have 32VDC( it is right) but also 67V AC .. Is it wrong??? Also changed BR1, ZD5, ZD6 ( old components are OK after test, but I changed this because other post)

If this value is not OK...Which component needs to be changed to avoid that failure??? On LT- I have 32VDC with 0v AC...

Currently, I changed BR1, ZD5 and ZD6, main fuse ( 6 times ), TDA7293 ( two times) and R58...Please help me!!
You can't measure AC on a DC rail using a multimeter (or at last almost all of them), you will get totally incorrect readings, and probably nothing is the polarity is wrong.

There's no reason EVER to set your meter to AC when measuring DC supply rails, and it's not something you should be doing. Presumably you're not an electronics engineer?, or you wouldn't (or at least shouldn't) be attempting it.

+/-32V sounds about right for the supply rails.

If you mention what the actual fault is, I'll try and offer advice.
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Old 13th December 2012, 06:39 PM   #10
Simon B is offline Simon B  United Kingdom
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Quote:
Originally Posted by Nigel Goodwin View Post
.......If you mention what the actual fault is, I'll try and offer advice.


Of course I'm sure I'm missing something here, but if he knew what the "actual fault" was, apart from the already mentioned very loud hum, then.........
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