Ferguson 383A FM unit problem - UK Vintage Radio Repair and Restoration Discussion Forum

Charles Wallis · 31st Dec 2021, 9:35 pm

I am trying to restore this 1950's valve radio that I have had for years. The FM reception has always been poor and I need some advice please. I have taken out the chassis and gone through it with the 1299 Trader Service Sheet. Many of the resistors had gone high and I have replaced most of them. I have also replaced all the electrolytic capacitors and changed the metal rectifier to silicon diodes with a 330 ohm dropper for the HT.
On MW and LW it now works extremely well with good volume and clarity for AM. The problem seems to be with the FM unit. It either works very faintly or crackles with interference. I have been through the anode and screen voltages. Most seem OK as listed below, but measuring the ECC85 anodes from points 3 and 4 on the tuner unit connections gives some very odd results. V1a either runs at 150 v and then can drop to just 25v. R2 the 10K HT dropper seems to get a bit warm suggesting a leak to ground in the unit. Worried that this is a short risk I decided to remove the unit and inspect it. It was very fiddly but actually came out OK. The inside looks OK with no signs of overheating and the components look OK as far as I can tell, see pictures. Restistance to ground from point 3 is very high and C2 comes in at 1500pF as it should. My next step was to get a new ECC85 as I gather they are prone to failure but wanted to be sure it was not something else first as they are expensive. I would be grateful for any ideas.

Valve Anode Voltage (Data sheet) Measured /Screen
V1a ECC85 (135) 25-150?
V1b (148) 145
V2a ECH81 (95) 106 /88
V2b (230) 233
V3 EF89 (215) 233 /145
V4 EABC80 (80) 77
V5 EL84 (258) 262 /217

Cobaltblue · 31st Dec 2021, 9:52 pm

Hi Charles and welcome to the Forum.

I have a funny feeling that those trimmer capacitors can go leaky.

I have certainly come across leaking trimmers on other sets in the past worth checking

The ECC85's can fail due to cathode poisoning, decades sometimes without HT the Radio only being used on AM.

Cheers

Mike T

Sideband · 1st Jan 2022, 2:21 am

I think I have a scrap chassis for one of these. If I can find it, you are welcome to the whole chassis for spares or just the FM unit.

ECC85's usually go low gain rather than intermittent.

Charles Wallis · 1st Jan 2022, 9:18 pm

Thanks Mike, I will check those trimmers if I can.

telstar · 4th Jan 2022, 4:23 pm

Hi Charles.
Welcome to the forum.
I have two Ferguson valve radios, the 382U "Firefly", and the later 384U "Finesse" (with push buttons). These are poor on V.H.F, although someone informed me that the two 680 K ohms resistors in the tuning assembly go out of tolerance and can cause the V.H.F to fail. If you have checked the components in the V.H.F tuner then these resistors are probably not to blame.
They are certainly great looking radios (I have not seen your 383A). My grandmother had a 382U radio in 1961 when she lived in Suffolk. I saw it when I had a holiday with my grandparents.
Keep trying with some more ECC85 and see if it works the trick. Someone told me that using an ECC81 valve and modifying the heaters, this often cures the fault when you can not find the original valve. The problem with most things are people have "panic bought" components up and that is why valves like ECC85/UCC85 are so hard to find.
Good luck
Mike

Charles Wallis · 4th Jan 2022, 9:18 pm

Thanks to Mike T. Correct diagnosis! I rewired the FM unit without the metal shield it normally lives in and switched on so I could see what was going on. The voltage on point 3 which Valve 1a anode, was about 60 v but variable with reasonable reception but intermittent interference. On closer inspection I could see arcing on C4, the trimmer cap from anode to ground. That would totally explain the variable voltage, high current draw and (hopefully) poor performance. It would also explain why the voltage seemed low even on cold start before the valve began to draw current.
Having found the problem I need to find a solution. C4 is a 2-10 pF variable cap. The problem is that it has to withstand full HT voltage of about 300v on start-up dropping to 135v when the valve warms up. Most of the variable caps on sale today seem to be rated at 200v. I worry that if I put one of these in it too will arc over again. Higher voltage caps seem very expensive or will not fit in the metal shield.
I thought of a possible solution but would value advice. That is to put a much larger high voltage cap (eg 100pF) in series with a variable caps to protect it from HT. It would have little effect on overall capacitance (1/c=1/c +1/c etc). So the trimmer cap would be to RF ground but not DC ground. Would that work?

I have checked those 680K resistors and they were bang on tolerance, unlike most of the others in the set.

Mr 1936 · 5th Jan 2022, 8:48 am

Hi

I think your idea of a series DC blocker will work if it is a low inductance ceramic type with short leads.

It then all comes down to a DC potential divider formed by the stray leakage resistances of the blocker cap and the shunt resistance of the trimmer cap circuit, which will both (hopefully) be very high. Maybe you could shift things even more in your favour by deliberately shunting the trimmer circuit with a 1 megohm miniature resistor, being mindful that this may add perhaps 0.5 pF in shunt.

Or you could try just using a 200V trimmer on the basis that the voltage rating is conservative !

ms660 · 5th Jan 2022, 11:13 am

I'm sure I made a reference to the faulty trimmer in an earlier post.

But the post seems to have been removed ??

Lawrence.

Charles Wallis · 15th Jan 2022, 9:33 pm

Success!
I have replaced the faulty trimmer cap with a new one rated at 200v with a megohm resistor in parallel, to 100 pF cap in series to ground as above. It was quite tricky due to lack of space. The result was spectacular. FM is loud and clear like never before. All voltages near where they should be. Project complete. I will write it up seperately giving more details.
Thanks to those who advised me.

31st Dec 2021, 9:35 pm	#1
Charles Wallis Triode Join Date: Mar 2021 Location: Edinburgh, Scotland, UK. Posts: 45	Ferguson 383A FM unit problem I am trying to restore this 1950's valve radio that I have had for years. The FM reception has always been poor and I need some advice please. I have taken out the chassis and gone through it with the 1299 Trader Service Sheet. Many of the resistors had gone high and I have replaced most of them. I have also replaced all the electrolytic capacitors and changed the metal rectifier to silicon diodes with a 330 ohm dropper for the HT. On MW and LW it now works extremely well with good volume and clarity for AM. The problem seems to be with the FM unit. It either works very faintly or crackles with interference. I have been through the anode and screen voltages. Most seem OK as listed below, but measuring the ECC85 anodes from points 3 and 4 on the tuner unit connections gives some very odd results. V1a either runs at 150 v and then can drop to just 25v. R2 the 10K HT dropper seems to get a bit warm suggesting a leak to ground in the unit. Worried that this is a short risk I decided to remove the unit and inspect it. It was very fiddly but actually came out OK. The inside looks OK with no signs of overheating and the components look OK as far as I can tell, see pictures. Restistance to ground from point 3 is very high and C2 comes in at 1500pF as it should. My next step was to get a new ECC85 as I gather they are prone to failure but wanted to be sure it was not something else first as they are expensive. I would be grateful for any ideas. Valve Anode Voltage (Data sheet) Measured /Screen V1a ECC85 (135) 25-150? V1b (148) 145 V2a ECH81 (95) 106 /88 V2b (230) 233 V3 EF89 (215) 233 /145 V4 EABC80 (80) 77 V5 EL84 (258) 262 /217 Attached Thumbnails Last edited by Cobaltblue; 31st Dec 2021 at 9:46 pm. Reason: Rule B8

31st Dec 2021, 9:52 pm	#2
Cobaltblue Moderator Join Date: Dec 2010 Location: Exeter, Devon and Poole, Dorset UK. Posts: 6,878	Re: Ferguson 383A FM unit problem Hi Charles and welcome to the Forum. I have a funny feeling that those trimmer capacitors can go leaky. I have certainly come across leaking trimmers on other sets in the past worth checking The ECC85's can fail due to cathode poisoning, decades sometimes without HT the Radio only being used on AM. Cheers Mike T __________________ Invisible airwaves crackle with life or at least they used to Mike T BVWS member. www.cossor.co.uk

1st Jan 2022, 2:21 am	#3
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 7,580	Re: Ferguson 383A FM unit problem I think I have a scrap chassis for one of these. If I can find it, you are welcome to the whole chassis for spares or just the FM unit. ECC85's usually go low gain rather than intermittent. __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman.....

1st Jan 2022, 9:18 pm	#4
Charles Wallis Triode Join Date: Mar 2021 Location: Edinburgh, Scotland, UK. Posts: 45	Re: Ferguson 383A FM unit problem Thanks Mike, I will check those trimmers if I can.

4th Jan 2022, 4:23 pm	#5
telstar Octode Join Date: Dec 2003 Location: Leeds, West Yorkshire, UK. Posts: 1,593	Re: Ferguson 383A FM unit problem Hi Charles. Welcome to the forum. I have two Ferguson valve radios, the 382U "Firefly", and the later 384U "Finesse" (with push buttons). These are poor on V.H.F, although someone informed me that the two 680 K ohms resistors in the tuning assembly go out of tolerance and can cause the V.H.F to fail. If you have checked the components in the V.H.F tuner then these resistors are probably not to blame. They are certainly great looking radios (I have not seen your 383A). My grandmother had a 382U radio in 1961 when she lived in Suffolk. I saw it when I had a holiday with my grandparents. Keep trying with some more ECC85 and see if it works the trick. Someone told me that using an ECC81 valve and modifying the heaters, this often cures the fault when you can not find the original valve. The problem with most things are people have "panic bought" components up and that is why valves like ECC85/UCC85 are so hard to find. Good luck Mike

4th Jan 2022, 9:18 pm	#6
Charles Wallis Triode Join Date: Mar 2021 Location: Edinburgh, Scotland, UK. Posts: 45	Re: Ferguson 383A FM unit problem. Thanks to Mike T. Correct diagnosis! I rewired the FM unit without the metal shield it normally lives in and switched on so I could see what was going on. The voltage on point 3 which Valve 1a anode, was about 60 v but variable with reasonable reception but intermittent interference. On closer inspection I could see arcing on C4, the trimmer cap from anode to ground. That would totally explain the variable voltage, high current draw and (hopefully) poor performance. It would also explain why the voltage seemed low even on cold start before the valve began to draw current. Having found the problem I need to find a solution. C4 is a 2-10 pF variable cap. The problem is that it has to withstand full HT voltage of about 300v on start-up dropping to 135v when the valve warms up. Most of the variable caps on sale today seem to be rated at 200v. I worry that if I put one of these in it too will arc over again. Higher voltage caps seem very expensive or will not fit in the metal shield. I thought of a possible solution but would value advice. That is to put a much larger high voltage cap (eg 100pF) in series with a variable caps to protect it from HT. It would have little effect on overall capacitance (1/c=1/c +1/c etc). So the trimmer cap would be to RF ground but not DC ground. Would that work? I have checked those 680K resistors and they were bang on tolerance, unlike most of the others in the set.

5th Jan 2022, 8:48 am	#7
Mr 1936 Heptode Join Date: Mar 2017 Location: Romsey, Hampshire, UK. Posts: 524	Re: Ferguson 383A FM unit problem Hi I think your idea of a series DC blocker will work if it is a low inductance ceramic type with short leads. It then all comes down to a DC potential divider formed by the stray leakage resistances of the blocker cap and the shunt resistance of the trimmer cap circuit, which will both (hopefully) be very high. Maybe you could shift things even more in your favour by deliberately shunting the trimmer circuit with a 1 megohm miniature resistor, being mindful that this may add perhaps 0.5 pF in shunt. Or you could try just using a 200V trimmer on the basis that the voltage rating is conservative !

5th Jan 2022, 11:13 am	#8
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Ferguson 383A FM unit problem I'm sure I made a reference to the faulty trimmer in an earlier post. But the post seems to have been removed ?? Lawrence.

15th Jan 2022, 9:33 pm	#9
Charles Wallis Triode Join Date: Mar 2021 Location: Edinburgh, Scotland, UK. Posts: 45	Re: Ferguson 383A FM unit problem Success! I have replaced the faulty trimmer cap with a new one rated at 200v with a megohm resistor in parallel, to 100 pF cap in series to ground as above. It was quite tricky due to lack of space. The result was spectacular. FM is loud and clear like never before. All voltages near where they should be. Project complete. I will write it up seperately giving more details. Thanks to those who advised me.