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Old 27th Sep 2018, 12:22 pm   #1
vic0239
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Default Ferguson 621U - help needed

Not posted in this part of the forum previously, but would appreciate some advice from the experts.

Whether wisely or not I purchased a Ferguson model 621 U (schedule D) mainly because I liked the look of it and it was described as "working well". On arrival it was dead and I found that the UY85 had not survived the journey, it was out of the socket and its heater o/c. While waiting for a replacement I removed the chassis from the case and set about cleaning many years of muck from everything I could and gave the components a cursory examination. Nothing looks untoward to my untrained eye and when the new valve arrived the set sprang to life and I can receive stations on FM and faintly on MW.

However, my issue is that I noticed that the wire coil L8 is broken and I was wondering what the best repair option would be. Could I carefully solder the two ends together or would that damage the ferrite rod?

There aren't many previous threads on this set, so I'm unsure what else I might need to change. I've checked the mains cable is wired correctly and will probably remove the external loudspeaker connection. Should I change C54 as well?

Sorry for the basic questions, but this is my first restoration!

Thanks ... Andy.
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Old 27th Sep 2018, 1:26 pm   #2
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Default Re: Ferguson 621U - help needed

Soldering temperatures won't affect the ferrite rod. Even if its properties change when hot, they'll recover when it cools down.

Something must have bashed it quite hard to do that!

Take care not to create a shorted turn in the process.

If C54 is the output grid coupler then yes, unless it's ceramic or a Mullard mustard or other plastic film type.
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Old 27th Sep 2018, 3:50 pm   #3
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Default Re: Ferguson 621U - help needed

Thanks Chris.

I will attempt to mask the windings of the coil.

Not sure about the type of capacitor, the value according to the service data is 3900pF. Can you tell from the attached if it is ceramic?

Andy.
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Old 27th Sep 2018, 3:54 pm   #4
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Default Re: Ferguson 621U - help needed

Yes.
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Old 27th Sep 2018, 3:58 pm   #5
vic0239
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Default Re: Ferguson 621U - help needed

Is that yes it is ceramic or just confirmation of your capability in identifying the component?
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Old 27th Sep 2018, 4:05 pm   #6
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Default Re: Ferguson 621U - help needed

The reddish coloured one is a ceramic type, the other two? look like ceramic types as well.

Lawrence.
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Old 27th Sep 2018, 4:48 pm   #7
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Default Re: Ferguson 621U - help needed

Should be fine, then.

Is this a set with valveholders straight into Paxolin type PCB? If so you may get nasty problems with conductive scorched areas round the output valve causing bias upset. "Floating" of the connections to the control grid may be needed.
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Old 27th Sep 2018, 4:51 pm   #8
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Default Re: Ferguson 621U - help needed

Quote:
Originally Posted by ms660 View Post
The reddish coloured one is a ceramic type, the other two? look like ceramic types as well.
Many thanks.
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Old 27th Sep 2018, 4:54 pm   #9
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Default Re: Ferguson 621U - help needed

Quote:
Originally Posted by Herald1360 View Post
Is this a set with valveholders straight into Paxolin type PCB? If so you may get nasty problems with conductive scorched areas round the output valve causing bias upset. "Floating" of the connections to the control grid may be needed.
It is. I'll check it for that. Looking at my photos the board could do with a bit more cleaning anyway! As to "Floating" of the connections to the control grid", that has floated way above my head.
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Old 28th Sep 2018, 12:25 am   #10
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Default Re: Ferguson 621U - help needed

One way to do it is to drill a clearance hole through the grid pin, coupling capacitor and grid leak resistor grid ends solder pads then use some single core insulated wire to remake the connections "in the air". The other ends of the components will hold them in place well enough.

Check for any positive grid volts coming from elsewhere than the coupling cap or the grid itself before jumping in, though.
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Old 28th Sep 2018, 10:52 am   #11
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Default Re: Ferguson 621U - help needed

One thing you need to be aware of, although only pin 2 is assigned to the control grid in the UL84, pin 1 is connected to pin 2 inside the valve. This means you will need to include pin 2 in the work that Chris mentioned above.
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Old 28th Sep 2018, 4:15 pm   #12
vic0239
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Default Re: Ferguson 621U - help needed

Many thanks to all who have replied with your very useful advice.

I have successfully repaired the coil and stations on MW are now very much better and I can't see any signs of scorching around the UL84, so will leave this for now. Just a little tear in the speaker to repair and a blast of switch cleaner and hopefully I will be able put it all back together - job done.

Thanks ... Andy.
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Old 4th Oct 2018, 3:25 pm   #13
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Default Re: Ferguson 621U - help needed

Need a bit more help with this set please.

Radio was fine on all bands for about 1 hour when it started to crackle and pop. Coming and going at first, but then quite loud irrespective of volume control setting with stations still audible. Then AM stations became inaudible followed by the FM stations, the crackles persisted. When subsequently switched on from cold the crackle is still there, but stations can again be heard. I switched off at this point. Tried a spare UL84 with same result. I'm assuming that the problem is in the output stage, but am unsure what to check/replace next. Any assistance welcome.

Thanks .. Andy.
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Old 4th Oct 2018, 9:02 pm   #14
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Default Re: Ferguson 621U - help needed

I would suggest a few voltage measurements around the output valve, particularly the grid, cathode and anode. PCB radio's are a PITA as the output valve and rectifier heat causes the PCB to be conductive or can cause dry joints. This is not helped by the set designers pushing the output valve to it's limits.

When I have restored them, I have made design changes to reduce the anode current (increased the cathode resistor and the HT dropper resistor).
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Old 5th Oct 2018, 1:09 am   #15
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Default Re: Ferguson 621U - help needed

If the crackling is unaffected by the volume control, check the UABC80. With luck, there will be a dry joint on one of its pins or a component connected thereto. I say "with luck" because there is a nastier plausible failure mode which I'm sure you agree with me in hoping it isn't .....

The wavechange switch in an AM/FM set switches the HT to the anodes of the UCC85 and the triode of the UCH81, and it is possible for HT to track where it is not wanted. This can cause all manner of strange behaviour, and the fault probably will sound different on FM and AM.

If you can get the room really, really dark (you will also have to fit a temporary resistor in place of the set's scale lamps), you might be able to see a faint glow when the set crackles.
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Old 6th Oct 2018, 5:59 pm   #16
vic0239
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Default Re: Ferguson 621U - help needed

Thanks for your replies. Had some time to look at this today and think I have made progress.

I had a good look round the printed circuit joints and did some measurements on the valve holders - all looked ok in terms of dry joints. I did find what looked like an old repair to the circuit board track near the UY85 which didn't measure too well. I remade this, but no change in the crackling. However, my lights-out checks did reveal some sparking at the top connector to the output transformer. These are push fit and look quite blackened and tightening the sleeve did not give a secure connection. So I have temporarily soldered it to the transformer tag and the crackles have practically gone. I'm hoping when I do the same to the others the crackles will be gone - fingers crossed!
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