Earthing Problem???

[MicksTape Productions]

hi i have just acquired an early 90's model 8100 Marshall Valvestate Amplifier.

It's a solid state, 100 watt (4 ohm out) guitar amp. Made in the UK.


I plugged it to test it, and it produces a ridiculously low volume re-production of whatever signal is being fed to it, guitar, keyboard, but it's almost inaudible over the high frequency squelch it also emits.

If you are, or know an engineer/tech, it's most likely an easy enough problem to fix, as these things are as straight forward to work on as a lego castle. Apparently....

any help would be apreciated as would a referral to a good trustworthy tech in my local area (Central Coast NSW Australia) in the case that i cant fix/meddle with it myself.

I have a basic knowledge of circuitry but am unsure of certain variables regarding voltage i/o and the like and therefore would prefer to pay someone if it is a problem that i in my limited techness can handle.

Thanks for your time.

[Enzo]

Get something insulated, like a wooden chopstick, and while the amp is playing something so you can hear the sound, press down on each contact in the external speaker jack, and on the headphones jack. if any of those restore the sound, replace the jack. It might be possible to save the jack by reforming (fancy word for bending) the contact.

Also, push against all the rectangular cement power resistors to see if any have broken free of thier solder.

[MicksTape Productions]

cheers mate

thanks for the post Enzo, i will endeavour to try and use your tips in a safe manner. i hope its something simple.

thanks again

[mykey]

hi i have just acquired an early 90's model 8100 Marshall Valvestate Amplifier.

It's a solid state, 100 watt (4 ohm out) guitar amp. Made in the UK.


I plugged it to test it, and it produces a ridiculously low volume re-production of whatever signal is being fed to it, guitar, keyboard, but it's almost inaudible over the high frequency squelch it also emits.

If you are, or know an engineer/tech, it's most likely an easy enough problem to fix, as these things are as straight forward to work on as a lego castle. Apparently....

any help would be apreciated as would a referral to a good trustworthy tech in my local area (Central Coast NSW Australia) in the case that i cant fix/meddle with it myself.

I have a basic knowledge of circuitry but am unsure of certain variables regarding voltage i/o and the like and therefore would prefer to pay someone if it is a problem that i in my limited techness can handle.

Thanks for your time.

check the 15 volt supplies first, you should see + and - 15 volts but one will be off.
what usually goes wrong is the mosfet power transistors or the low voltage supply drops out because of a short. ZD1, ZD2 the zeener diodes overheats and short out because one of the op amps in the preamp is shorted. check the temperature of all the op amps one at a time until you find the hot one. That's the shorted op amp. after replacing the bad op amp, replace the zeeners + and- 15 volts should come back up and the amp will work again usually. OK sometimes it's just a bad zeener. well it's been a while since i had to fix one of those.

[Enzo]

How does a short across the supply stress the zener? A short bypasses current around a zener.