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Old 17th Jan 2022, 9:56 pm   #1
Charles Wallis
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Join Date: Mar 2021
Location: Edinburgh, Scotland, UK.
Posts: 16
Default Ferguson 383A restoration and FM unit fix

I acquired this 65 year old 5 valve superhet (pic 1) over 30 years ago at a jumble sale and been using it on a regular basis ever since. I noticed that that reception on FM had become very weak only receiving the strongest channels with volume full up and an external aerial to help. Something was clearly wrong and I set about trying to make it good again. This was my first valve project though I did have some knowledge of them from my youth. All the posts I read pointed to V1, ECC85 as a potential cause of failure.

First up I desoldered and removed the chassis and built a wooden frame to support it upside down so I could easily work on the underside. I wired back in the AM ferrite rod, an external FM dipole aerial, a spare loudspeaker with a series lightbulb limiter for power, so it was fully functional on the bench (as was, pic 5).

Then with the Trader sheet I went through all the accessible components checking them where feasible with a multimeter. I checked the DC anode voltages and some were well below what they should be, though the metal rectifier was clearly past its prime, reducing the HT voltage. Many of the resistors were high. Of particular note were R2 (open circuit) and R4 which supply the anodes of V1 the VHF amplifier and mixer /oscillator. I thought that might explain the low anode voltage and poor reception.
So I set about replacing several resistors. The work was quite fiddly due to the congested wiring around the RF stages. I also replaced some small capacitors where access was tight, and all the electrolytics. So far it was no better.

Now the power supply. I removed the old metal bridge rectifier and mounted a tag board in its place on which I mounted a bridge of four 1N4007 silicon diodes and 3 new electrolytic smoothing caps to replace the old 3 in 1 can (which on testing was fine!). I put in a 330ohm 5w resistor to get the HT about the right voltage.
The FM was by now if anything, worse. The V1 anode voltages seemed to drop as low as 25v associated with crackling interference. The crucial clue was that these low voltages were at switch on before the valve would be drawing current, suggesting that the fault was not the valve but another component within the FM unit. There was nothing for it but to take it apart but I knew they were very fiddly to work on.

First I desoldered the 6 connections underneath and removed two retaining screws. Then I turned the chassis over removed the remaining 3 connections and the unit came free. I gently removed the metal shield to reveal the innards. It is a small metal frame with all the components tightly wired within. Clearly something was shorting the V1 anode to earth. See my other post about this. The suspects were C2 which looked fine, and as suggested by a member, C4 the trimmer cap (top left pic 2). This connects anode to earth so would be exposed to full HT at switch on. It looked OK to me but the only thing was to test the unit out of the shield so I could measure what was going on. I soldered it a all back again and switched on. Lo and behold there was a small fireworks display with arcing around the ceramic trimmer. I suspect the improved HT voltage and new resistors had actually made it worse.

Now how to fix it? My worry was that the standard triimer caps today are rated at 200v, which would be exceeded by the HT and it might all happen again. Higher voltage rated trimmers were not only expensive but the wrong shape to fit in the tiny space allowed. After some consultation on this forum I came up with a solution, which was to protect it with a larger 100pF capacitor in series and I was advised to put a 1 megohm resistor across the trimmer to shunt any DC developing across it. The trimmer would be to an RF earth, but without much DC across it.

The next challenge was how to wire in these components in the very cramped FM unit, with the trimmer under the adjustment hole in the shield. I cut a small bit of PCB stripboard and soldered on the 3 components, then fixed the PCB in using the bolt that had held the the ceramic trimmer with grommets under to insulate it (pic 3). I then wired it to the V1 anode and earth and slid it all back into the shield. A grommet in the adjustment hole was to stop the shaft of a screwdriver shorting the trimmer screw to earth as it would be held 'live' by the larger protecting capacitor.

I once again soldered the FM unit back in and secured it on the main chassis. After all this fiddly effort and false hopes would it work at all?
I held my breath as I powered it on and waited for it to warm up. Then it burst into life and for once the FM came in very loud and very clear! Success at last.
All that remained was to reattach the tuning chord that connected the FM variable inductor pulley sytem to the AM tuning and cursor. I put in a new three core mains cable and properly earthed the chassis, and added a fuse to the live line for safety (pic 4). The only remaining problem is the waveband switch that has a cog wheel connection to the actual knob. The cog was worn so would not turn when re intalled. The solution was to take the cog off and turn it round, which works for now, but I suspect is not a long term solution. I put the chassis back in the cabinet and gave it a spray of polish to buff it up.
I am very glad I did not buy an expensive and scarse ECC85 valve as it was clearly fine all along and would not have fixed the problem. And it looks and sounds great!
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Old 19th Jan 2022, 10:06 am   #2
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Location: Doncaster, South Yorkshire, UK.
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Default Re: Ferguson 383A restoration and FM unit fix

A great job, and an excellent and detailed write-up.

Well done for sticking at it and achieving a great result!

Chroma 04
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Old 19th Jan 2022, 8:00 pm   #3
Charles Wallis
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Location: Edinburgh, Scotland, UK.
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Default Re: Ferguson 383A restoration and FM unit fix

Thanks Adrian. I don't suppose there are many of these radios left now but I hope its of help if anyone else gets stuck on the FM unit. They are really fiddly to work on and the worst thing is you can't examine it without desoldering it and then you have to solder it back in to test it. There was very little online about how to fix them apart from warnings about being tricky.
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Old 19th Jan 2022, 10:03 pm   #4
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Location: Croydon, Surrey, UK.
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Default Re: Ferguson 383A restoration and FM unit fix

Originally Posted by Charles Wallis View Post
Thanks Adrian. I don't suppose there are many of these radios left now
There are quite a few of these about. There have been at least three on eBay recently (at the same time) and there may well be a couple on there now. I had two of being a donor set to replace a faulty IF transformer and speaker). I still have the donor chassis (most of it) and passed the good repaired one on.

We had one of these sets as the family radio back in the 70's. I fitted a socket on the back connected across the volume control so that I could record 'Pick Of The Pops' with Alan Freeman on Sunday afternoons....!
On Spike Milligan's headstone......I told you I was ill...
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