[ARC5] Interesting occurrence...

[Kenneth G. Gordon]

Well, that R-25/ARC-5 I got working the other day has recently exhibited an 
interesting (to me) phenomenon:

First, an explanation: this unit had been rather badly hacked (among other 
things, extra holes in the case) and all the remaining "flower-pots" were bad. 
Some were missing.

Overall, it didn't look all that bad, so I replaced the bad caps with 0.05 mfd 
1KV disk ceramics, cleaned everything cleanable, and plugged tubes into it.

The power supply that I am using was built back in the 1970s by someone 
who did a pretty fair job. As it came, it output about 230 VDC and 24 VAC, 
and had separate switch to turn the filaments and the B+ on and off. 

However, the only DC filter was a triple 40 MFD at 450 VDC upright can cap 
with all its sections in parallel. No choke or RC pi-filter.

When connected to the receiver via an adapter made from a dynamotor 
base, the receiver exhibited quite a lot of hum, especially at low audio levels.

So, I added a 3K ohm 5 watt resistor between the input filter (2 sections of 
the can-cap in parallel) and the output cap. This brought the voltage down to 
about 145 VDC, so I moved the screen tap to the high end of the screen-
dropping resistors.

However, the hum was gone. I suppose I should reduce the value of the filter 
resistor by 1/2 and restore the screen connection to the center-tap point.

I used the receiver for a couple of weeks, and was quite pleased with its 
stability, excellent calibration, and sensitivity. I then replaced the 12SK7s in 
the RF and 1st IF with 12SG7s. This dropped the noise down by about 1/2 
and increased the sensitivity noticeably (unmeasured).

When I would go to bed, I would turn off the B+ but leave the filaments on.

I started to notice that the overall audio level was slowly dropping, and last 
night, it was almost down to zero. 

In fooling with it, I found that when the tuning reached about 2.7 Mhz, the HF 
oscillator quit completely.

So, I replaced the original 12K8 with one I pulled from another receiver. 
Although this revived the receiver, it couldn't be accurately calibrated any 
longer, the low-end calibration cap being maxed out before reaching 
accurate calibration.

Also, the antenna peaking cap was now way over against the left stop for the 
peak over most of the tuning range.

I am now wondering if leaving the filaments on, but no B+, for extended 
periods has a deliterious effect on the tubes, or if some other component is 
failing?

Anyone have an idea?

Eventually, I want to use this receiver to monitor 160 meters and WWV.

Ken W7EKB