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Title: HRO-60 AC Hum Post by: KW4H on July 06, 2021, 06:09:47 PM With the recap finished, defective resistors replaced, and wiring checked and double-checked, the HRO-60 on the bench is back in its case for now and powers up (passes the smoke test). Voltages check out against the chart. However -- the audio output is one giant 60 Hz hum. I could use some help. Here's what I've been able to determine so far:
1. The level of the hum fully varies with the volume control. 2. B+ is clean. 3. The hum is only audible when the control switch is in CW or AM. However...see (4) below... 4. Using a scope at the Select-O-Ject socket, pin 5, I can see the hum not only in CW or AM mode, but also when the control switch is in NFM (I don't have the NFM module). 5. Pin 5 of the Select-O-Ject socket is jumpered to pin 1, which goes down to the noise limiter (V-12). The hum exists all over V-12 -- plates and cathodes -- regardless of mode. 6. I've backed up one step further to V-9 (2nd Detector). The hum exists there, but appears to stop at T-8. C-57 tests good and I replaced C-54 for good measure. AVC on/off switch has no effect. Would appreciate any and all advice. Title: Re: HRO-60 AC Hum Post by: W1NB on July 07, 2021, 01:08:30 PM The first thing that comes to mind, if it is indeed 60 Hz hum, and not 120 Hz, is to look for a tube with a heater/cathode short. AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands
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