Kevin I remember I had put another 1000microfarad in parallel, I believe, with C48 and that solved my hum. Another time I got some strange audio, there were some bad solder joints at the output headphone jack. My advice would be to check thoroughly all the joints at the audio section.
Consider that if you use low impedance headphones you will probably hear anyway some hum (placing a 600ohm resistor in series would greatly reduce it) Maybe it might be the 50Khz oscillator (bfo) the became offset for some reason or the 405/505Khz oscillator. In this case, I believe that you would hear "hum" just in USB/LSB mode, but not in AM mode. One method I use to check which stage might have some hum problem is to see on a computer the spectral analysis. I would check which audio frequency you actually hear that is annoying. Then I would check it at the IF (point 4 of T8). At point 4 of T8 the signal is at 50khz, I convert it (trhough a mixer and a signal generator) to between 0 and 20Khz and I see the output on the screen. If there is still hum I check previous stages at 455Khz output and so on. 73 Frank IZ2OOS 2010/3/11 Kevin LaHaie <[email protected]>: > 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 > > > _______________________________________________ > Drakelist mailing list > [email protected] > http://mailman.zerobeat.net/mailman/listinfo/drakelist > _______________________________________________ Drakelist mailing list [email protected] http://mailman.zerobeat.net/mailman/listinfo/drakelist
