Pic for Discussion entitled, "St-70, Left Channel, 60 Hz Hum"
DynacoGuy- Posts : 16
Join date : 2020-10-06
DynacoGuy- Posts : 16
Join date : 2020-10-06
- Post n°2
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Last edited by DynacoGuy on Fri Nov 27, 2020 6:50 pm; edited 2 times in total
DynacoGuy- Posts : 16
Join date : 2020-10-06
I put callouts on the pic.
A couple of things I could adjust,
but not sure it would make a difference.
The center taps, power xfmr and
the Heat CT GNDs could be
Removed form the ground buss,
tied together and I'd add a wire to
Locate them on Xfmr & 3rd prong GND lug.
In another thread somewhere, can't recall,
There should be continuity only on pin 2 to pin 2
and only on pin 7 to pin 7 on each pair of power tubes.
Is that correct?
In this amp pin 2 and pin 7 have continuity throughout
the amp on either side and in board.
Any ideas?
DynacoGuy- Posts : 16
Join date : 2020-10-06
peterh wrote:What a nightmare !
A few observations : Shorting inputs should never increase any hum. This seems to indicate a
groundloop somewhere
Removing the 7199 tubes : if hum remains will localize the area to the EL34 / bias / B+
( unbalanced powertubes might cause hum )
How/where is the SDS board connected to ground ? That could be your source, and a source that
will disappear if you rebuild with a cancap ( restoring the original config)
As for diagnosing, a scope should be able to find and measure the hum, just turn sensitivity up.
Having a value of the hum will make it much easier then to resort to listen with the amp
moved from the workbench.
I followed the Van Alstine protocol,
Open inputs.
left right swap:
power tubes, hum stayed in left channel.
7199 swap, hum stayed in left channel.
Full tube complement with shorting plugs installed:
Increased hum and Hum is now in both channels.
Pulled 7199s, no hum.
I have not added any additional caps other
than the 8 in the power supply.
The original multisection was disconnected and
attached loosely with one glob of solder on a twistlock.
When I tried to find the hum with the scope I couldn't find it.
Unless I had brain fart and didn't recognize it.
I couldn't hear it on my bench speakers...nothing.
I went back and placed it into the system, then
with extension chord plugged it into two different
home power nodes...no difference. Hum remains.
DynacoGuy- Posts : 16
Join date : 2020-10-06
Look at the common output!
There was no reference to ground on either,
so I ran a ground wire from each common output
to the ground buss in the amps center; the same
location used by Dynaco.
DG
There was no reference to ground on either,
so I ran a ground wire from each common output
to the ground buss in the amps center; the same
location used by Dynaco.
DG
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