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j4976 7th July 2012 05:20 PM

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?

KatieandDad 7th July 2012 05:22 PM

If it is humming with no inputs attached then I would look at the PSU.

Check capacitors in PSU and the rectifiers.

KatieandDad 7th July 2012 05:25 PM

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.

j4976 7th July 2012 05:35 PM

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.

KatieandDad 7th July 2012 05:42 PM

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.

Enzo 7th July 2012 08:07 PM

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.

j4976 7th July 2012 10:06 PM

I wonder if I might have an input grounded somewhere. Any tips on finding that?

moonmark 12th December 2012 10:36 PM

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!!

Nigel Goodwin 13th December 2012 03:44 PM

Quote:

Originally Posted by moonmark (Post 3282366)
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.

Simon B 13th December 2012 06:39 PM

Quote:

Originally Posted by Nigel Goodwin (Post 3283230)
.......If you mention what the actual fault is, I'll try and offer advice.

:rolleyes:

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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