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Author Topic: Valvestate 8100  (Read 2832 times)

Vitrolin

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Valvestate 8100
« on: October 27, 2013, 11:18:41 AM »
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

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Re: Valvestate 8100
« Reply #1 on: October 27, 2013, 01:34:43 PM »
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

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Re: Valvestate 8100
« Reply #2 on: December 15, 2013, 03:09:02 PM »
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

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Re: Valvestate 8100
« Reply #3 on: December 16, 2013, 07:12:24 AM »
Quote from: Vitrolin
it 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.