Just picked up a 2NT/2C station that had sat in a (heated and dry) attic
for 20 years. Very nice condition, 100pct original, a little chassis
corrosion but a very nice set. As always, I did a visual inspection
first, and then took all of the normal precautions when first applying
A/C to it.
On a variac, I brought up voltage slowly and by the time only 25vac was
reached, a very loud hum appeared on the speaker and headphones. OK,
electrolytics (typical)
I replaced the 3 higher voltage electrolytics in the can with discreet
parts under the chassis, found one of the low voltage filter caps to be
split so replaced that one (C48 I believe). All voltages appear good on
the scope with normal ripple visible only with the most sensitive
setting on the scope (5mv)
Nothing reduces the hum, which is very loud. No controls affect the
output in any way, I removed all tubes, still the hum.
Doesn't appear on either of the 2 driver transistors, or the base /
emitter of the final output transistor, only on the collector and of
course the headphone and speaker terminals.
The scope shows a malformed AC waveform as the hum, so it's obviously
being amplified since the audio stage of this receiver is all solid state.
So, output transformer? Audio PA transistor?
Hoping for some insight where to go from here!
Thanks
Kevin K7ZS
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