Reproduction vintage TRF radio repair. - UK Vintage Radio Repair and Restoration Discussion Forum

cinema1 · 30th Jun 2008, 4:24 pm

Hello,
could any one help with the follow,
I have built a two valve trf radio the valves they are EF36--6J5 i built it from practical wireless, It is a medium wave radio.
Radio lancashire is the dominant station i receive, I can just pick up five live in the back ground. (CAN NOT TUNE ANY OTHER STATIONS)
The rf signal at the anode of the EF36 is coupled back through a trimmer capacitor to to the grid circuit of the EF36.
When i ad just the trimmer with my hand the radio comes alive with stations
take my hand away and the stations become week.
does any body know why.
THANK YOU. ERIC.

Brian R Pateman · 30th Jun 2008, 4:59 pm

First of all, welcome to the forum!

Secondly I hope you don't mind but I've re-titled this to make the subject of the thread a little clearer.

Thirdly, and more to the point, what sort of aerial are you using? This would probably have been designed for use with the ubiquitous long wire so common until the ens of the 1960s.

Try a few feet of wire draped around the picture rail or similar. You are acting as an aerial capacitively coupled to the trimmer and that's why the performance increases when you touch it or bring your hand close to the front end.

Regards,

Variometer · 30th Jun 2008, 5:44 pm

Hi Eric,
Are you able to show us a circuit of the set? Normally, the feedback is via a small variable capacitor into a coil wound either next to, or over the grid coil. This is the reaction coil. If it is connected the wrong way round, you won't get any feedback and maybe your hand was supplying it instead. With two valves, the stations should be coming in at a very good volume!
Bob

cinema1 · 1st Jul 2008, 1:33 pm

HI, BRIAN--moderator.
BOB--VARIOMETER.
I will try the aerial wire suggested by brian.
I will reverse the reaction coil connections suggested by BOB.
kind regards, ERIC.

PJL · 1st Jul 2008, 2:43 pm

As well as the wise advice about aerials I would mention that you will never get the set to work properly with capacitive coupling from your hand to the reaction control. As soon as you move your hand away the reaction setting will be wrong.

The coupling could be via the knob or through the front panel. Try a big plastic knob, reverse the connections to the variable capacitor, or screen the inside of the front panel...Peter

30th Jun 2008, 4:24 pm	#1
cinema1 Tetrode Join Date: Jun 2008 Location: Preston, Lancashire, UK. Posts: 50	Reproduction vintage TRF radio repair. Hello, could any one help with the follow, I have built a two valve trf radio the valves they are EF36--6J5 i built it from practical wireless, It is a medium wave radio. Radio lancashire is the dominant station i receive, I can just pick up five live in the back ground. (CAN NOT TUNE ANY OTHER STATIONS) The rf signal at the anode of the EF36 is coupled back through a trimmer capacitor to to the grid circuit of the EF36. When i ad just the trimmer with my hand the radio comes alive with stations take my hand away and the stations become week. does any body know why. THANK YOU. ERIC.

30th Jun 2008, 4:59 pm	#2
Brian R Pateman Nonode Join Date: Jun 2005 Location: Western Lake District, Cumbria (CA20) - UK Posts: 2,136	Re: Reproduction vintage TRF radio repair. First of all, welcome to the forum! Secondly I hope you don't mind but I've re-titled this to make the subject of the thread a little clearer. Thirdly, and more to the point, what sort of aerial are you using? This would probably have been designed for use with the ubiquitous long wire so common until the ens of the 1960s. Try a few feet of wire draped around the picture rail or similar. You are acting as an aerial capacitively coupled to the trimmer and that's why the performance increases when you touch it or bring your hand close to the front end. Regards, __________________ Brian

30th Jun 2008, 5:44 pm	#3
Variometer Retired Dormant Member Join Date: May 2008 Location: Preston, Lancashire, UK. Posts: 428	Re: Reproduction vintage TRF radio repair. Hi Eric, Are you able to show us a circuit of the set? Normally, the feedback is via a small variable capacitor into a coil wound either next to, or over the grid coil. This is the reaction coil. If it is connected the wrong way round, you won't get any feedback and maybe your hand was supplying it instead. With two valves, the stations should be coming in at a very good volume! Bob

1st Jul 2008, 1:33 pm	#4
cinema1 Tetrode Join Date: Jun 2008 Location: Preston, Lancashire, UK. Posts: 50	Re: Reproduction vintage TRF radio repair. HI, BRIAN--moderator. BOB--VARIOMETER. I will try the aerial wire suggested by brian. I will reverse the reaction coil connections suggested by BOB. kind regards, ERIC.

1st Jul 2008, 2:43 pm	#5
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Reproduction vintage TRF radio repair. As well as the wise advice about aerials I would mention that you will never get the set to work properly with capacitive coupling from your hand to the reaction control. As soon as you move your hand away the reaction setting will be wrong. The coupling could be via the knob or through the front panel. Try a big plastic knob, reverse the connections to the variable capacitor, or screen the inside of the front panel...Peter