RF alignment question. - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

PJL · 27th Sep 2016, 10:09 am

Typical MW range is 550-200Metres or 0,54 to 1.5MHz
Add .465 for LO and we have 1.0 to 2.0MHz
The actual here is 1.5MHz to 2.21MHz so quite a bit out (the equivalent of 150pF).

According to the trader these are iron core so check the core has not fallen out or moved. There is a possibility the problem is with deterioration of the core material.

If that round (ceramic plate?) capacitor in the right of your pic is C9 (MW tracking) then I would replace it.

PJL · 27th Sep 2016, 10:46 am

There should be a mark on the dial indicating the alignment position for fully open/closed. What is the range of the dial? My favourite is the inductor and you should be able to confirm this by twiddling the MW trimmer when the capacitor is fully open which should still have a significant impact on the frequency.

Voxophone · 27th Sep 2016, 4:51 pm

I have good news to report!

I removed the MW tracker capacitor pictured previously from the set and having measured it with my 'scope I'd place the valve at roughly between 130 and 160pF. It's therefore way off from its 430 pF design value.

I made a temporary replacement using two 220 pF ceramics in parallel. Installing this improved the MW alignment considerably. I am now able to pick up Radio 4 just 22 kHz or so out of position. The error increases up to 150 kHz at the highest frequency stations but this may reduce if the r.f. is realigned.

The set is now very usable on all bands. The MW volume is extremely high - presumably because the aerial and LO circuits are now on speaking terms. Several stations can be heard at decent listening volume with no external aerial and the volume control at minimum. This set does have a 12" flying lead for the aerial connection however which may be all it needs.

It's a pity the loudspeaker is only 4"!

ms660 · 27th Sep 2016, 6:16 pm

Well, there yer go, life ain't so bad after all, you did the right thing first off, by checking the frequency range of the oscillator with your 'scope.

Lawrence.

kalee20 · 27th Sep 2016, 8:41 pm

I've generally found that the oscillator gets less lively as frequency is increased, not decreased. I have a Cossor 501 where the thing used to stop altogether on SW, for the last third of the dial. A new frequency changer valve plus other components - including rewinding the SW oscillator coil - cured it, but it's still weak at the HF end.

If the aerial tuned circuit is not in tune with everything else, signals can still get through rather well. Tuning follows the oscillator, all the RF tuning has to do is be selective enough to stop the image frequency (and other spurious frequencies) getting through. I bet some manufacturers deliberately made it rather low-Q, so they didn't have to get the tracking particularly accurate...

Station X · 28th Sep 2016, 8:12 am

Two posts moved to a new thread here:-

https://www.vintage-radio.net/forum/...d.php?t=130019

PJL · 28th Sep 2016, 1:05 pm

So much for the iron core theory - pleased to see it was a much easier fix.

27th Sep 2016, 10:09 am	#21
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: RF alignment question. Typical MW range is 550-200Metres or 0,54 to 1.5MHz Add .465 for LO and we have 1.0 to 2.0MHz The actual here is 1.5MHz to 2.21MHz so quite a bit out (the equivalent of 150pF). According to the trader these are iron core so check the core has not fallen out or moved. There is a possibility the problem is with deterioration of the core material. If that round (ceramic plate?) capacitor in the right of your pic is C9 (MW tracking) then I would replace it. Last edited by PJL; 27th Sep 2016 at 10:21 am.

27th Sep 2016, 10:46 am	#22
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: RF alignment question. There should be a mark on the dial indicating the alignment position for fully open/closed. What is the range of the dial? My favourite is the inductor and you should be able to confirm this by twiddling the MW trimmer when the capacitor is fully open which should still have a significant impact on the frequency.

27th Sep 2016, 4:51 pm	#23
Voxophone Hexode Join Date: Oct 2015 Location: Sunderland, Tyne and Wear, UK. Posts: 340	Re: RF alignment question. I have good news to report! I removed the MW tracker capacitor pictured previously from the set and having measured it with my 'scope I'd place the valve at roughly between 130 and 160pF. It's therefore way off from its 430 pF design value. I made a temporary replacement using two 220 pF ceramics in parallel. Installing this improved the MW alignment considerably. I am now able to pick up Radio 4 just 22 kHz or so out of position. The error increases up to 150 kHz at the highest frequency stations but this may reduce if the r.f. is realigned. The set is now very usable on all bands. The MW volume is extremely high - presumably because the aerial and LO circuits are now on speaking terms. Several stations can be heard at decent listening volume with no external aerial and the volume control at minimum. This set does have a 12" flying lead for the aerial connection however which may be all it needs. It's a pity the loudspeaker is only 4"!

27th Sep 2016, 6:16 pm	#24
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: RF alignment question. Well, there yer go, life ain't so bad after all, you did the right thing first off, by checking the frequency range of the oscillator with your 'scope. Lawrence.

27th Sep 2016, 8:41 pm	#25
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,081	Re: RF alignment question. I've generally found that the oscillator gets less lively as frequency is increased, not decreased. I have a Cossor 501 where the thing used to stop altogether on SW, for the last third of the dial. A new frequency changer valve plus other components - including rewinding the SW oscillator coil - cured it, but it's still weak at the HF end. If the aerial tuned circuit is not in tune with everything else, signals can still get through rather well. Tuning follows the oscillator, all the RF tuning has to do is be selective enough to stop the image frequency (and other spurious frequencies) getting through. I bet some manufacturers deliberately made it rather low-Q, so they didn't have to get the tracking particularly accurate...

28th Sep 2016, 8:12 am	#26
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,288	Re: RF alignment question. Two posts moved to a new thread here:- https://www.vintage-radio.net/forum/...d.php?t=130019 __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

28th Sep 2016, 1:05 pm	#27
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: RF alignment question. So much for the iron core theory - pleased to see it was a much easier fix.