• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

AMC CVT 3030 one more try - or dumpster

Some years ago, when trying to set bias and center voltage, i accidently had a small spark between the screwdriver and somewhere at the pcb. Shut the amp down. One fuse blown- replaced it- powered it up. The amp sounded distorted and weak. Checked bias again - one tube on each channel was responding low on bias-set - less than 3volts. The other tubes responded to setting - 6v.
Never went further with center voltage adj. because i noticed the pot´s were worn and broke due to previus owner(-s) attemts.
Stoved the amp away, instead i got my hands on a old Dynaco st70. The Dynaco was no problem to fix and tune. There is a lot written on the web about tuneups for an old st70. Step by step instructions and pictures.

Last winter i decided to fix the AMC again. New caps, resistors etc. Moved the bias and center-volt pots + meterpoints to the upper side of the pcb. Installed ceramic tubesockets and new el34´s. Ewerything was perfect i thought. Fired up the amp, adjusted bias -all good 6volts on all four. Moved to the centervolt adj. Should read + - 5 volt, but read 200 volts (+-5 v) instead!!
Yes, i have double-tripple-qadro-checked every single thing i i did for tuneup. The mosfet´s on the poweramp module has been checked "onboard" and read ok. Any ideas anyone?
 
Why i bother to fix it; simpy because it´s a very good amp. I dont know if it´s because that it´s a "pure class a" but it outperforms my Dynaco st-70. The AMC has (had) a very detailed sound. Exact and soft at the same time. When running warm it went kind of 3-dimensional, it was like having the artist and instruments in the room. You didnt hear "the sound of" a guitar, you heard the guitar.
The starting guitar riff in Roy Orbison´s "The only one" in right channel was stunning. I´we never heard that on a another amp. My St70 don´t come close to that punch. Another quality was how the AMC handled voices. Was wery surprised to hear that it was Sting singing in Dire straits "Money for nothing". Never even thougt about that before. Another tune i´we listned to hundreds of times is Pink Floyd "Time". First time i heard it in the AMC-amp, i almost fell out of the chair whem the bells start to ring.
 
I wouldn't dumpster it, someone could always use it for parts or tinkering. I had a few of those amps and found some strange things in their design. I check the site you posted for the schematic but did not see it....so maybe you can post it. One thing to eliminate right off the bat is if the tubes were damaged. Next all the semi conductors.....check all the diodes for reverse leaks and I seem to recall diodes in the power tube circuitry as well soldered right to the octal sockets.
 
Thank you all for the respons. Yes, the AMC is a bit strange in internal design. Thats why i spent so much time to re-design it.
And again, yes i have checked (+rechecked x5) every single diode, cap, resistor, board etc and it´s all good.
The reading of 200 volts in center volt adj. is on both power amp modules!
The only thing i havent done (yet) is to reinstall all new transistors on each board.A good friend of mine ,a skillfull tv-repair-man, made a check on all these transistors and said they was all good. This was done on the pcb. I don´t know how this circuit works, but is it possible that the mosfets on board is balancing the whole circuit? And- it might be a possibility that it´s easy to miss a bad transistor when checking it on the pcb?
 
I wouldn't dumpster it, someone could always use it for parts or tinkering. I had a few of those amps and found some strange things in their design. I check the site you posted for the schematic but did not see it....so maybe you can post it. One thing to eliminate right off the bat is if the tubes were damaged. Next all the semi conductors.....check all the diodes for reverse leaks and I seem to recall diodes in the power tube circuitry as well soldered right to the octal sockets.

Yes, i will go for your advise. I am making a list for every transistor/semiconductor on the power amp pcb for making a order. But i am having problem to find two mosfet on my suppliers list. They are Q508+Q509 marked bss92ph3, and Q501+Q502 marked bss101 on the schematic. Probably out of production. Any ideas for a good replacement?
Thanks