Unknown Home Built TRF radio - Page 3 - UK Vintage Radio Repair and Restoration Discussion Forum

davidgem1406 · 3rd Nov 2019, 2:31 pm

Hello See_Mos,
Thanks for your input.
Yours is very similar with slight changes to the chassis, whilst the cabinet is identical.
It looks to be using the same coil set as well.

The capacitor and transformer that you mention. Is that the one on the top side of the chassis? If so that looks to be the transformer for the valve heaters, that would make the capacitor to be across the AC supply.
The smaller one underneath looks to be the output but I can't see a capacitor across that.
Although, in both cases, it is difficult to follow the wiring through.

I will try a capacitor across the output primary, also reduce the value of the pot.
Hopefully that will cure one problem, and help the other.

Thanks,
Regards Dave.

See_Mos · 4th Nov 2019, 11:56 am

Yes, it should be across the mains so on the transformer on the top as can be seen from the photos and it is C10 on the diagram just above the transformer.

I fired mine up and I am surprised by how sensitive it is.

davidgem1406 · 7th Nov 2019, 5:30 pm

Hi See_Mos,

I added the capacitor, but it had no effect at all. Maybe because the set I have is using a valve rectifier so the cap is only doing AC transient suppression, it will have to be replaced with an X2 type anyway.

Yes the sensitivity on mine is good, it picks up all the stations available here.

I have noticed that the aerial and oscillator coils are much the same. Mine however, in each case, has a single turn of solid wire between the top 2 windings. I assume that has been added during the build and just increases the coupling between the 2 windings.

As for, what I call, the growling problem I have not found any reason for it so far. But it is definitely caused by the incoming signal. It also varies with the content of that signal.

I now wonder if those extra turns of wire could be responsible, or something in the layout. Although that is much the same as the original except for the rectifier where I have added a 91R resistor to each anode. Also, I added extra smoothing 10uf to the HT supply to V1 & V2

Dave.

davidgem1406 · 21st Nov 2019, 5:24 pm

Well, I have now tried a multiple of things without any success in clearing the distortion problem.

What I did find was that V2 (6SJ7) was micro phonic, I tapped it a couple of times, there was a sudden loud 50 Hz buzz. I switched off, tested the valve and it had a H/C short that was intermittent. The valve was replaced a few days later as I did not have one to hand.
On switch on nothing happened, the H.T. had vanished, the A.C. series resistors got hot.
Only one thing could do that, the rectifier valve was a dead short Heater to Cathode.
Fortunately I had one so normal service was resumed. I then checked the other 2 valves by replacement. So all 4 valves were replaced and still the distortion continued.

Today I decided to check each stage for audio distortion using an external audio input using the radio from a Sony Walkman. I started at the output stage (V3) then followed by the preceding stage (V2) and finally the first stage V1. None of the stages produced distortion. So audio wise all things are good.

I have tried bridging some existing capacitors, and adding some in one or two places. Moving some wiring about.
I also removed the single turn of wire that had been added to each of the coil assemblies. This made no difference whatsoever to anything. Why they were there I know not.

Nothing I have done has changed the distortion problem in any way.
It is there on all stations, and both wavebands. Is far more prevalent on music than speech.
It has to be RF coming through on the audio.

I am out of ideas, so any suggestions and help would be very much appreciated.

Regards to all,
Dave.

3rd Nov 2019, 2:31 pm	#41
davidgem1406 Heptode Join Date: Feb 2011 Location: Sheerness, Kent, UK. Posts: 936	Re: Unknown Home Built TRF radio Hello See_Mos, Thanks for your input. Yours is very similar with slight changes to the chassis, whilst the cabinet is identical. It looks to be using the same coil set as well. The capacitor and transformer that you mention. Is that the one on the top side of the chassis? If so that looks to be the transformer for the valve heaters, that would make the capacitor to be across the AC supply. The smaller one underneath looks to be the output but I can't see a capacitor across that. Although, in both cases, it is difficult to follow the wiring through. I will try a capacitor across the output primary, also reduce the value of the pot. Hopefully that will cure one problem, and help the other. Thanks, Regards Dave.

4th Nov 2019, 11:56 am	#42
See_Mos Hexode Join Date: Nov 2003 Location: Ossett, West Yorkshire, UK. Posts: 483	Re: Unknown Home Built TRF radio Yes, it should be across the mains so on the transformer on the top as can be seen from the photos and it is C10 on the diagram just above the transformer. I fired mine up and I am surprised by how sensitive it is.

7th Nov 2019, 5:30 pm	#43
davidgem1406 Heptode Join Date: Feb 2011 Location: Sheerness, Kent, UK. Posts: 936	Re: Unknown Home Built TRF radio Hi See_Mos, I added the capacitor, but it had no effect at all. Maybe because the set I have is using a valve rectifier so the cap is only doing AC transient suppression, it will have to be replaced with an X2 type anyway. Yes the sensitivity on mine is good, it picks up all the stations available here. I have noticed that the aerial and oscillator coils are much the same. Mine however, in each case, has a single turn of solid wire between the top 2 windings. I assume that has been added during the build and just increases the coupling between the 2 windings. As for, what I call, the growling problem I have not found any reason for it so far. But it is definitely caused by the incoming signal. It also varies with the content of that signal. I now wonder if those extra turns of wire could be responsible, or something in the layout. Although that is much the same as the original except for the rectifier where I have added a 91R resistor to each anode. Also, I added extra smoothing 10uf to the HT supply to V1 & V2 Dave.

21st Nov 2019, 5:24 pm	#44
davidgem1406 Heptode Join Date: Feb 2011 Location: Sheerness, Kent, UK. Posts: 936	Re: Unknown Home Built TRF radio Well, I have now tried a multiple of things without any success in clearing the distortion problem. What I did find was that V2 (6SJ7) was micro phonic, I tapped it a couple of times, there was a sudden loud 50 Hz buzz. I switched off, tested the valve and it had a H/C short that was intermittent. The valve was replaced a few days later as I did not have one to hand. On switch on nothing happened, the H.T. had vanished, the A.C. series resistors got hot. Only one thing could do that, the rectifier valve was a dead short Heater to Cathode. Fortunately I had one so normal service was resumed. I then checked the other 2 valves by replacement. So all 4 valves were replaced and still the distortion continued. Today I decided to check each stage for audio distortion using an external audio input using the radio from a Sony Walkman. I started at the output stage (V3) then followed by the preceding stage (V2) and finally the first stage V1. None of the stages produced distortion. So audio wise all things are good. I have tried bridging some existing capacitors, and adding some in one or two places. Moving some wiring about. I also removed the single turn of wire that had been added to each of the coil assemblies. This made no difference whatsoever to anything. Why they were there I know not. Nothing I have done has changed the distortion problem in any way. It is there on all stations, and both wavebands. Is far more prevalent on music than speech. It has to be RF coming through on the audio. I am out of ideas, so any suggestions and help would be very much appreciated. Regards to all, Dave.