[ARC5] Progress on R-25/ARC-5 receiver

[Kenneth G. Gordon]

Well, I have gotten the subject receiver working, and it works quite well. 

Calibration, after touching-up, is excellent, sensitivity seems to be very good, 
although not as good as my TS-940, stability is excellent, staying locked on 
2.5 Mhz WWV with the BFO on (to detect any drift) for hours. Drift, when it 
occurs at all, is only a few cycles at most.

The power supply I am using is a nice little unit that was built at least 40 
years ago on a small chassis with mini-switches for Fil. and B+, and 
incorporating a 24VAC transformer for filaments. B+ is about 210 VDC under 
load, and the power transformers stay quite cool.

Power is fed via some longish wires to an old dynamotor base. The builder 
even mounted a bakelite safety shield over the dynamotor base where the 
leads are connected. It is also fused in both the primary and the HV 
secondary, and has a front-panel mounted neon pilot light.  The job is very 
well done, unlike so many others I have seen.

There is some residual hum, especially at low volume, as I think the ancient 
can-type FP electrolytic filter cap in the power supply is weak, but I'll fix that 
in due course.

I have used the receiver to monitor some SSB and CW communications on 
160 meters, and it stays in tune for hours at a time.

However, I have to keep the RF gain low to prevent distortion, as is usually 
the case with too-little BFO injection, a simple diode detector, and no audio 
gain control.

Audio quality on AM is adequate, and I have listened to the many AM BC 
stations that have taken up residence on the high end of the BC band, 
despite the fact that there really isn't much there worth hearing.

Of the original caps in the receiver, all but two were bad, either leaky or 
open, and I replaced them all with 0.05 MFD 1KV disk ceramics, and one 
eletrolytic in the cathode of the 12A6. To me, the disks are a bit too large, 
and look somewhat out-of-place, but I couldn't easily find 500 VDC jobs, so 
used what I had.

Surprisingly, the triple 0.22 mfd can was OK.

Thanks to Keith Harrington, I have received a replacement original audio 
transformer which I will install in the near future.

Thanks a heap, Keith. :-) 

In the meantime, I found an old Radio Shack 10 watt 70 volt line transformer 
with 4, 8, and 16 ohm outputs. The 0.68 watt tap on the primary calculates 
out to 8K ohms impedance, and I temporarily hooked that up. Although it 
works OK, and the 12A6 doesn't get too hot, I think the audio passband 
needs to be much narrower as I am hearing a lot of what I consider to be 
unnecessary highs in the audio.

To sum up, I continue to be very impressed with the design and 
implementation of the ARC-5 receivers. I stand by my original statement that 
they are possibly the finest single-band receivers ever built, or at least of 
those I have ever used.

I would like to incorporate this receiver into a semi-portable 160 meter station 
using one of the horribly hacked ARC-5 transmitters I have here, rebuilt for 
160.

I will keep the list informed of progress.

Ken W7EKB