Marshall avt 150. TDA 7293 issues

B12a3d · 19th May 2011, 11:57 PM

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

audiohead · 20th May 2011, 12:35 AM

Is your pre-amp working?
The LEDs on the front panel working?

Art

B12a3d · 20th May 2011, 01:58 AM

Yes the LEDs are all working, how do I check the pre amp?

amptech · 30th May 2011, 10:06 PM

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!

MC64 · 17th October 2012, 10:29 PM

Quote:

Originally Posted by B12a3d

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??

amptech · 18th October 2012, 05:59 PM

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.

MC64 · 18th October 2012, 08:34 PM

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

19th May 2011, 11:57 PM	#1
B12a3d diyAudio Member Join Date: May 2011	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

18th October 2012, 08:34 PM	#7
MC64 diyAudio Member Join Date: Oct 2012	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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20th May 2011, 12:35 AM	#2
audiohead diyAudio Member Join Date: Mar 2005 Location: Los Angeles, california	Is your pre-amp working? The LEDs on the front panel working? Art

20th May 2011, 01:58 AM	#3
B12a3d diyAudio Member Join Date: May 2011	Yes the LEDs are all working, how do I check the pre amp?

30th May 2011, 10:06 PM	#4
amptech diyAudio Member Join Date: Jan 2009 Location: Glasgow	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!

18th October 2012, 05:59 PM	#6
amptech diyAudio Member Join Date: Jan 2009 Location: Glasgow	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.