Dynaco St70 Static

Have you done anything to make sure the static is actually originating in the amp? If not, WITH AMP OFF, remove both interconnects and power it up again. If the static's still there, it's the amp; otherwise, look elsewhere.

If the amp is at fault, I'd suggest that you check the resistances of the power traces on the PC-3 board AFTER YOU UNPLUG THE AMP AND MAKE SURE THE HIGH VOLTAGE IS FULLY DISCHARGED. They should read close to the residual resistance of the meter leads, but I've seen some read about 60 ohms and guess what! They made crackling noises. If this is your problem, try reflowing solder over the traces AFTER YOU UNPLUG THE AMP AND MAKE SURE THE HIGH VOLTAGE IS FULLY DISCHARGED. Check for solder bridges before powering it back up.

Good luck.

--Al

Jayboard,

It would be a heck of a coincidence for both bias pots to go flaky at the same time. I don't believe in that kind of coincidence in electronics (which does not make it impossible, just improbable). It is far more likely to be something common to both channels, such as the power supply or the bias supply.
BTW, I've worked as an electronic technician for over 30 years. I should be getting good at it any day now :-)

Bill,

If you haven't done this already, try replacing the selenium rectifier (the one with "fins" in the middle of the underside of the chassis) with a silicon diode (Good advice even if the SR is working OK. Should cost you under 2 bucks (including a terminal strip to mount it on) if you can install it yourself; if not, if you live in or near New Jersey, I can install it for you. Be absolutely sure to observe correct polarity--the banded end should be attached to the transformer wire. This is a negative power supply). Selenium rectifiers are notorious for going bad (and if it opens totally, you'll lose bias and have fried output tubes for lunch. Y'all want crispy resistors with that?).

--Al