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Valvestate 8100

Started by Vitrolin, October 27, 2013, 11:18:41 AM

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Vitrolin

a friend gave me a valvestate vs8100 it did not work, i found out that the -15 volts were not there, i changede C61, C62 &  R101 they were looking suspicious. i now had +8 -6 volts or so. i replaced Zener diodes no change, then i removed TR8-9-10. and then +/-15 volts were on.
then i replaced them and it seemd maintain voltages.
i applyed signal and its output was not pretty. i then saw that problem was IC1. i took awhile since i started applying signal at input of IC8B and halfling my way back through the circuit.
but replacing IC1, it was shortet between + & - pins, it then was up and running.
so i dragged it to a fiends studio to see what i could do and try it with different speakers. it worked for a while, and i played around a lot with settings and then i wanted to try it with another cab, nothing but silence... voltmeter told me no +/-15V.
i tried some of the same procedures from last time but no luck this time and its pissing me off.

i hope someone have had a similar experience or maybe a usefull input.
i attached a schematic

Roly

The most obvious reason for IC1 to fail would be excessive input voltage, which it has no protection against; but really you have little option but to treat this as a whole new fault (checking IC1 first of course).

You may be able to locate the problem (most likely one of the op-amps if both supplys are shorted) with the finger test; touch the top of each IC and see if one seems hotter than the others.

HTH

If you say theory and practice don't agree you haven't applied enough theory.

Vitrolin

hi there
ive been a bit busy but this weekend i took some time look at the amp
there vas noting to find with the "finger test" so i started to remove IC one by one, after a few removals with no luck i decided to check if the power amp it self was working so i liftet the pins of R83&78 aplyed +- 15volt from the bench supply and a sine signal at IC8 pin7, hooray a nice amplified sine wave output.
back to removing after having removed 6 IC i startet to think why isnothing getting warm
...well 40V (unreguletet) across 270R thats about 150mA, not strange that nothing is warming up. so i connectet my bench supply with no current limit. and yes i got warm...hot it drew 2,5A and IC was smoking.
so problem located. IC10 was of course the last IC to be removed becuase of how they are lined up.
I will also replace IC9

Roly

Quote from: Vitrolinit drew 2,5A and IC was smoking.

Well, that's one way to find a faulty IC.   ;)
If you say theory and practice don't agree you haven't applied enough theory.