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Old 19th May 2011, 11:57 PM   #1
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Default Marshall avt 150. TDA 7293 issues

I found this in an old thread but not specific to my problem so I'm posting this again. I replaced the TDA chips twice now. my hypothesis for the first replacements blowing was that I didn't have them attached to the fan housing when I turned it on and they overheated. So I've installed the second set and everything turns on and no blows yet. But I have no sound. Any ideas or help would be awesome I have a show tomorrow and need the amp
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Old 20th May 2011, 12:35 AM   #2
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Is your pre-amp working?
The LEDs on the front panel working?


Art
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Old 20th May 2011, 01:58 AM   #3
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Yes the LEDs are all working, how do I check the pre amp?
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Old 30th May 2011, 10:06 PM   #4
amptech is offline amptech  Scotland
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how to check the preamp....plug a signal in such as a microphone..guitar or cd player and slowly turn it up the pre gain pot on the clean channel then the master level and connect a speaker to the amp, now wait for the sound!
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Old 17th October 2012, 10:29 PM   #5
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Default avt 150

Quote:
Originally Posted by B12a3d View Post
I found this in an old thread but not specific to my problem so I'm posting this again. I replaced the TDA chips twice now. my hypothesis for the first replacements blowing was that I didn't have them attached to the fan housing when I turned it on and they overheated. So I've installed the second set and everything turns on and no blows yet. But I have no sound. Any ideas or help would be awesome I have a show tomorrow and need the amp
Hi I have a valvestate 2000 avi 150; Initially it was dead but I found two burned out 2.2 ohm resisters which I replaced. Everything now seems to work except that the volume is quite low. The fans dont work terribly well but the chips dont show any sign of damage, and they certainly dont heat up very much at the moment. If they were damaged I would have expected no volume at all or hum, etc. So any ideas please as to what might be the problem? The sound is clear and crisp just no real power??
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Old 18th October 2012, 05:59 PM   #6
amptech is offline amptech  Scotland
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These amps have many faults such as damaged o/p ic's to faulty preamp stage.. The preamp stage stage is fed via two .22 ohm resistors at the low voltage ac supply feeding the bridge diode unit.. Then there's the ht booster supply for the valve. Now the supply voltage is 47dc to the o/p ic these chips fail most of the time due to over heat etc.
Note that the psi caps are 22000@ 63v crap replace with 47000uf much better. Circuits diagrams can be downloaded from Marshall and schematic info. To check preamp just hook a music signal and use the send out to a spare amp..note damage pots also cut the signal.
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Old 18th October 2012, 08:34 PM   #7
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Default re VS 200 AVT150

Greetings,

Many thanks for your reply, and the many faults are noted; and that in part is the problem for the less experianced. Therefore I was wondering if you could attempt to be a little more specific, an educated guess perhaps? As I say the 2.2 ohm resisters were burned out so were replaced. The sound output shows no sign of distortion its just that if I feed a 1v audio AC from a signal generator over a band of frequency ranges the result is the same, low volume. Crisp ,clear, but low! Where would be a good place to start looking for this kind of fault do you think? The board shows now sign of visible problems, the ouptut chips show no sign of being over heated. The only notable problem is the fans rattle, and take a short time to get up to speed.

Many thanks

MC
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