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ST-70 Restore... And What Next?

adnick said:
Interested in what you’ve discovered, any resolution to the right channel issue?

Was the rectifier ok?


Good luck,
Andy
Click to expand...
I had to order fuses, since I had no slo-blo's on hand. I ordered another rectifier as well... two, actually, just in case. The fuses arrived last night, but I haven't wanted to try the same rectifier, since I've read that a bad one can cause additional damage. I'm not sure if this is correct or not, but spending $17 to NOT find out is OK with me!
As to the silent right channel... I tested a lot of stuff on it after that incident. I measured resistance from RCA input to many points in both channels, and results were very close. I tested tube socket pins to where they should go, based on schematic, and all was good. I checked for stray wires making contact where they shouldn't, and didn't see any. I touched up a couple potentially questionable solder joints just to make sure. I was about to do some testing on the output transformers, but I ended up being tired at that point. I put a meter to the speaker output terminals, when there was no audio from the right channel, and there was voltage present. I'll test that again with a test tone and compare to the left channel when I get the new 5AR4.
The only things that're different now from when I tested it pre-recapping are
  • replaced selenium rectifier with diode/resistor
  • replaced resistors underneath
  • replaced all caps; polarity is correct
  • replaced can cap, wiring is correct
  • speakers were connected to 4 ohm outputs vs 8 ohm like previously (different test speakers now)
I'll report back when I can test further.
 
When I had my st70 I loved it! But sold it before a move across country. Having 2 young kids was also a reason as well. So back to solid state just for ease of use.
Anyways, if I recall, there was a day when I had an issue with no sound output, one or both channels (I can't remember which). I ended up checking all the resistors and I think I had one that had failed on the driver board. Switched it out and I was back in business.

If you haven't figured it out yet. Check the values on all resistors and see if you have anything that seems suspiy.
 
jcm2012 said:
When I had my st70 I loved it! But sold it before a move across country. Having 2 young kids was also a reason as well. So back to solid state just for ease of use.
Anyways, if I recall, there was a day when I had an issue with no sound output, one or both channels (I can't remember which). I ended up checking all the resistors and I think I had one that had failed on the driver board. Switched it out and I was back in business.

If you haven't figured it out yet. Check the values on all resistors and see if you have anything that seems suspiy.
Click to expand...
I did a lot of testing and troubleshooting from schematics... followed signal paths on both channels to make sure nothing was misconnected (none), tested point to point on both channels (no difference), made sure solder connections were good (they appeared to be). Because I felt it was something simple, and because I need to do something, anything, I retouched all the solder connections on the right channel.
When my new 5AR4's arrived, I tested it again, and all was well. Perhaps it was a bad solder joint after all. I can't see how a rectifier tube could cause no output on one channel, but that and the solder joints are all I did. At any rate, it sounds really nice - nice and quiet with no pops or anything, and the sound quality is good. I'm happy with how it eventually turned out.
 
That’s fantastic that it works, but understand you wanting a direct reason for the previous issues...

Solder joints have caused me problems in the past, and human induced problems are always harder to find.


Best of luck,
Andy
 
If the problem comes back, in addition to solder joints, check the wiring between the input jacks and the input grids of the output tubes. The wire Dynaco supplied with these amps was solid core, which can break inside the insulation if it's flexed too much, and time likely hasn't improved matters.
-Adam
 
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