Dynaco St70 Static

My great sounding Dynaco St70 started making a static sound through both left and right sides. I powered down and put in a spare set of tubes. Powered up slow with my variac until static started again. Its sounds like a needle at the end of a record sound - like a scratching sound. Where should I start looking for problems? This unit is stock. Thanlk you.
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.

I fixed a scratchy, crackling sound coming through one channel by cleaning the output tube pin sockets and the biasing pot contacts with a contact cleaner. I did this all at once, but my guess is that it was the potentiometer that was the culprit. That was months ago, and the problem has not reappeared. It's certainly possible this channel-specific problem is showing up in both channels of your Dyna -- worth a look.

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 :-)


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?).

I have restored over 50 st-70's and common problems are if its a kit form unit cold solder connections and poor quality output sockets per EL-34's can cause static noise,
you have had some very good advice on replacing selenium rectifier, I have also replace the two filter caps on the bias circuit.
When replacing the rectifier to standard diode the voltage drop may be much less so your output tubes may not bias up to 100 ma per pair, 1.56v across cathode resistor, you can change the 10k 1watt resistor connected between bias pots and ground to 8.2k 1 watt this will give you a wider range in the bias circuit should tubes not bias up to spec's
I would also check all connections between 7199 board and power capacitor, be sure to tighten ground connection where every ground is connected to chasses that is a problem point of the ST-70's since its a mechanical connection.
Be sure all tubes fit snug in sockets, you may want to clean pin connections of the 4-EL-34's, 5AR4 and 2-7199's with contact cleaner.
Best Regards