Marshall AVT50 Guitar amp repair

[davidkien68]

HI,

I have a Marshall AVT50 which has stopped working. The speaker has been blown but there are also other problems. I can get output from it by plugging a CD player into the CD in jack and unsing headphones in the headphone socket. Both of the clean and drive channel LED's light up but I only get a slight hum from the drive channel through the headphones. The main fuse is OK.

Clearly I need a new speaker, but there is obviously another issue. The guys at Marshall that I've spoken to (parts dept) suggested that the power stage of the amp could be blown. Does anyone have exoprience of this amp/problem?

Richard.

 

[KrisBlueNZ]

Hello Richard (or is it David?)

I have no experience of that amp or that problem, but I do have the schematics. You can get them at http://elektrotanya.com/marshall_avt150_avt275.pdf/download.html. And they show that the headphone socket is driven separately from the main amplifier. So that explanation could be right.

The power amps are TDA7293s (it looks like there are two of them) and they are available in two versions from Digi-Key:
http://www.digikey.com/product-detail/en/TDA7293HS/497-8865-5-ND/1665069 ("HS" package with 90° leads for mounting flat against the board)
http://www.digikey.com/product-detail/en/TDA7293V/497-2170-5-ND/599502 ("V" package with straight leads for mounting vertically)

The circuit doesn't have any speaker protection, so it's quite possible that when the original TDA7293(s) failed, the speaker(s) were damaged by high-voltage, high-current DC. If you replace the TDA7293(s), make sure that the outputs are sitting at 0V before you connect any speakers to them!