sound city 50R problem

[M Fowler]

You guys are making me miss living in California

[selloutrr]

Did you look at the heater supply and grounds?

That's next and last thing....unless I can get selloutrr over here to show me god

I'd be over now if I wasn't in Oklahoma City on business. I should be back in CA by mid May. I could use a good beer OK has some F'd up liquir control.
Ill contact you and follow up when I get back if its still giving you grief and not used for batting practice I can come to you or you can come to me and put it on the bench and figure out where the grimlen is hiding.

[selloutrr]

If we have gone down the logical possibilities lets get creative.
Working with what we know is good.
Tubes, PT voltages, rectifier voltage, bias section, OT? I know you said something about MM. But have you tested it personally? Is the circuit clean? Could power be jumping from somewhere... Shortest path to ground. Are all bolts and screws tight? Sockets, pots are clean? Resistors are in spec?... Turn a cap of compressed air upside down and spray ice on the resistors in question after the circuit is hot. Is the standby switch arching? You chop stocked with no change so it's probably not wire dressing besides '70's fenders were a hot mess and silent. Caps have all been tested... I agree 6v is not much however after being amplified through the circuit I'm sure it's a bitch of signal to noise ratio. Tubes are not microphonic.., all chassis grounds and board stand offs are making contact. Meter all grounds. The trick now is to isolate what works and what is left until its down to just one part. If you have a bench power supply you could rule out the PT entirely. Swap an OT and know the iron is good. Is it possible you missed or bridged or reversed a component when the new caps went in?
Take a night off drink a beer or more and try it on a fresh hangover
Don't get frustrated we will figure it out.

[chief mushroom cloud]

thanks selloutrr
I'm down sick w/ a bad cold now....but will put it back on the bench later this week. The ONLY thing I haven't done now, is swap/temp in a known working PT.
I have the power amp and power supply circuit totally isolated from everything else. I'm going to look at the heaters next.

[selloutrr]

Time away and fresh eyes often fixes everything.
Double check everything mentioned as though you were servicing the amp for the first time. Then futz with the PT... If nothing else it helps isolate and reinforce your looking in the right place and when you find it it won't be multiple issues causing one problem.

Oh geez... Speaking of multiple issues those are the worst. You run circles going mad that nothing fixes the problem only to find every little bit does help but not alone. Nothing is really wrong but a sum of the wrong makes a noticeable wrong. If the PT isn't the issues we will address this and nip its gremlin ass! Either way you will end up with a studio silent amp.

Go drink a bottle of green NyQuil and chase the lepricon

[chief mushroom cloud]

Time away and fresh eyes often fixes everything.

wise advice.
it's fixed.
I tore down the entire power amp, and rewired it...new twisted heater wiring, soaked and scrubbed the old McMurdo sockets, all new ground wiring. Whatever was causing the problem...it's gone now. B+ on plates is dead flat, hum barely audible.
Thanks all for suggestions and help...hope this can be a help to anyone else that encounters a ghost problem in a repair.

A CLEAN AMP IS A HAPPY AMP

[martin manning]

Good news! Those things that don't appear on the schematic are always the hardest to find! The only sad thing is that the root cause is still unknown. What's your best guess?

[jelle]

My guess would be a bad ground.

[chief mushroom cloud]

Good news! Those things that don't appear on the schematic are always the hardest to find! The only sad thing is that the root cause is still unknown. What's your best guess?

As Jelle suggested, but it's idling at less current now (about 80mA)...so I'm guessing leftover arc from a previous catastrophic failure (it's still on the chassis, but I got it off the sockets). But also possible bad repair work after that failure. or a combination of both. I scrubbed and re-tinned the sockets, and twisted the heater wiring.
one thing I've noticed about these old Octal sockets, is that the plating doesn't solder very well, unless you burn them a lot of flux and pre-tin the terminals. I've been doing that w/ NOS McMurdos I get from UK.

[selloutrr]

Congratulations!