Cossor 524 low sensitivity on FM - UK Vintage Radio Repair and Restoration Discussion Forum

Malcolm G6ANZ · 15th Apr 2021, 3:52 pm

I am revisiting this set. I recapped it a few years ago and replaced a couple of low emission valves and it has worked OK. It now has poor sensitivity on FM. During investigation I found that by pulling on the tuning cord going to the slug in L3 the sensitivity dramatically improved. This coil is tuned with C4 (4pF) its the small grey one behind the black wire. So do these caps fail? or should I be looking at something else?

Malcolm

Mr 1936 · 15th Apr 2021, 6:07 pm

Hi Malcolm

Assuming that stations still appear at their correct places on the dial, it would seem that it's the RF stage tuning that is incorrect. The inductor is fairly bombproof (assuming the slug on a string is intact), so more likely to be the parallel capacitor C4. This type of low value fixed capacitor is normally pretty reliable, but it's not impossible that it has drifted or even gone O/C. Do you need more or less core into the former to improve sensitivity ? You could try temporarilty removing it and replacing with a new low tempco ceramic or even a miniature trimmer.

If the stations have "moved" on the dial then it's the LO that's wrong and you are simply retuning the RF stage to compensate for the error.

Malcolm G6ANZ · 15th Apr 2021, 6:26 pm

The string is intact and unadjusted. I have to pull the tuning slug right out so that it is right out of the coil, with it as far as it can the sensitivity still doesn't peak. The stations appear to be at the right point on the scale.
Am I right in thinking that even though its in the anode circuit the voltage across it is low since it is not referenced to chassis so a 50v one would be OK?
Malcolm

Mr 1936 · 15th Apr 2021, 8:58 pm

Hi

If you need to pull the slug out to reduce inductance, it implies that the capacitance is higher than it should be.

DC wise, there is no voltage across this capacitor (it's shorted out by the inductor) so no worries about voltage rating.

Cobaltblue · 15th Apr 2021, 9:07 pm

I have a couple of these and never had a problem with the FM alignment.

That said have you followed the Mechanical and RF alignment in the Cossor service sheet?

There are adjustments to the cords and two trimmers

Cheers

Mike T

Malcolm G6ANZ · 15th Apr 2021, 9:22 pm

As far as I know the mechanical set up has not been touched, I did an RF alignment when I first did the restoration and it all came up ok, but even then it was insensitive. I didn't touch the FM front end mechanics at that time as I was still learning and didn't want to disturb anything that would be difficult to put back.
So the mechanical setup hasn't changed, the RF tuning slug needs to be fully out to get a decent sensitivity, the tracking of the LO is OK so I'm tempted to change the 4pF capacitor.
What can possibly go wrong

15th Apr 2021, 3:52 pm	#1
Malcolm G6ANZ Octode Join Date: Feb 2006 Location: Bristol, UK. Posts: 1,042	Cossor 524 low sensitivity on FM I am revisiting this set. I recapped it a few years ago and replaced a couple of low emission valves and it has worked OK. It now has poor sensitivity on FM. During investigation I found that by pulling on the tuning cord going to the slug in L3 the sensitivity dramatically improved. This coil is tuned with C4 (4pF) its the small grey one behind the black wire. So do these caps fail? or should I be looking at something else? Malcolm Attached Thumbnails

15th Apr 2021, 6:07 pm	#2
Mr 1936 Heptode Join Date: Mar 2017 Location: Romsey, Hampshire, UK. Posts: 524	Re: Cossor 524 low sensitivity on FM Hi Malcolm Assuming that stations still appear at their correct places on the dial, it would seem that it's the RF stage tuning that is incorrect. The inductor is fairly bombproof (assuming the slug on a string is intact), so more likely to be the parallel capacitor C4. This type of low value fixed capacitor is normally pretty reliable, but it's not impossible that it has drifted or even gone O/C. Do you need more or less core into the former to improve sensitivity ? You could try temporarilty removing it and replacing with a new low tempco ceramic or even a miniature trimmer. If the stations have "moved" on the dial then it's the LO that's wrong and you are simply retuning the RF stage to compensate for the error.

15th Apr 2021, 6:26 pm	#3
Malcolm G6ANZ Octode Join Date: Feb 2006 Location: Bristol, UK. Posts: 1,042	Re: Cossor 524 low sensitivity on FM The string is intact and unadjusted. I have to pull the tuning slug right out so that it is right out of the coil, with it as far as it can the sensitivity still doesn't peak. The stations appear to be at the right point on the scale. Am I right in thinking that even though its in the anode circuit the voltage across it is low since it is not referenced to chassis so a 50v one would be OK? Malcolm

15th Apr 2021, 8:58 pm	#4
Mr 1936 Heptode Join Date: Mar 2017 Location: Romsey, Hampshire, UK. Posts: 524	Re: Cossor 524 low sensitivity on FM Hi If you need to pull the slug out to reduce inductance, it implies that the capacitance is higher than it should be. DC wise, there is no voltage across this capacitor (it's shorted out by the inductor) so no worries about voltage rating.

15th Apr 2021, 9:07 pm	#5
Cobaltblue Moderator Join Date: Dec 2010 Location: Exeter, Devon and Poole, Dorset UK. Posts: 6,875	Re: Cossor 524 low sensitivity on FM I have a couple of these and never had a problem with the FM alignment. That said have you followed the Mechanical and RF alignment in the Cossor service sheet? There are adjustments to the cords and two trimmers Cheers Mike T __________________ Invisible airwaves crackle with life or at least they used to Mike T BVWS member. www.cossor.co.uk

15th Apr 2021, 9:22 pm	#6
Malcolm G6ANZ Octode Join Date: Feb 2006 Location: Bristol, UK. Posts: 1,042	Re: Cossor 524 low sensitivity on FM As far as I know the mechanical set up has not been touched, I did an RF alignment when I first did the restoration and it all came up ok, but even then it was insensitive. I didn't touch the FM front end mechanics at that time as I was still learning and didn't want to disturb anything that would be difficult to put back. So the mechanical setup hasn't changed, the RF tuning slug needs to be fully out to get a decent sensitivity, the tracking of the LO is OK so I'm tempted to change the 4pF capacitor. What can possibly go wrong