Civilian Wartime Radio - Further Notes

[Sparks]

Thanks - The AVC capacitor is brand new, so no reason why it shouldn't be working!
Bob

[Sparks]

I have just removed the chassis and checked the AVC wiring again, and it is all OK.
Bob

[pwdrive]

Could it be the volume control?

Lawrence.

[boater sam]

Easy to check, monitor the AVC line with a DMM whilst tuning a strong signal. It could be that the frequency changer is a bit down on emision.
Does the signal at the grid of the output valve have the same distortion?

[Sparks]

The original volume control (1M) was faulty. The only one I had with a switch was 500K. After I got the set working, I took the 500K out and replaced iot with a 1M (Without switch) just to see if it made any difference. It didn't, so I put the 500K back.
The ECH35 may well be low on emmission, but I have tried several (all used) but with no change. As I said before, whatever I build always has distortion on the local station that is only about 5 miles away at the most. I have come to the conclusion that it is just overloading it.
It really is a very powerful set, although I would not have thought it with only three valves.
I have only tried two output valves (6V6) with no change, but have just found a new-looking one that I will try.
Bob

[Ed_Dinning]

Hi Bob, you could try an L C rejecter circuit set to your local station's frequency in the aerial circuit.

Ed

[Sparks]

Hi Ed,
Thanks - I never thought of that!         It really is a powerful station, and even with a three-foot wire aerial, it is blasting in!    
Bob

[Murphyv310]

Have a look at the RF Envelope of the offending station on the scope and see if it is over modulating, the fact it's distorted on other sets makes me think this is a problem not with the radios.

[Sparks]

I have a GW Oscilloscope GOS-310 but I seldom use it these days, and have no idea how to look at the RF envelope of the station!
When I did use oscilloscopes, it was mainly for repair of shipboard radar installations where I was just looking at various waveforms generated within the equipment. I have never even tried to use it on valve radio receivers!

[Murphyv310]

Hi.
You would need to use a x10 probe and look at the waveform on the IF amp grid. Adjust the scope so you can see the audio clearly in the carrier. If you see bits missing in the middle of the carrier it is overmodulation most likely from the transmitter. Compare it with a nice clear station and you should see the difference.