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
Mark
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
Mark