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Old 23rd Nov 2015, 11:46 pm   #1
ronbryan
Octode
 
Join Date: Jan 2003
Location: Guildford, Surrey, UK.
Posts: 1,535
Default Ferguson 401RG with AGC problem.

The Ferguson 401RG is a 1955 era AM/FM radiogram featuring an unusual push-pull EL84 output stage and Long, Medium and Short wave bands plus VHF and Gram selected by a multi-wafer rotary switch on the receiver. The amplifier and receiver are mounted in separate chassis and the on/off, volume and tone controls are fitted to a small sub-panel. All of the sub-assemblies are connected via cables with plugs at one end. Howard produced this one for me to repair.

When trying to find out why there was no reception on either AM or FM, I noticed that there were a few volts of positive bias on the control grid of the ECH81 mixer. A new ECH81 made no difference. There was also a large black 30k Welwyn pan-climatic resistor in parallel with the AM AGC decoupler fitted between the bottom end of the aerial coils and chassis that had clearly been added during a previous repair. Removing this resistor caused an even higher positive voltage on the mixer grid so it was likely to have been fitted as a 'work-around'.

The section of the wave-change switch that switched the aerial coils (the wafer nearest to the indexing mechanism) also had contacts that switched off the 330V HT to the radio and magic eye, when Gram was selected. It seemed that the paxolin rotor on that wafer was leaking HT on to the high impedance mixer grid-leak circuit and that the 30k resistor had been added to pot down the leakage and as it happened, most of the AGC intended for the mixer.

I tried cleaning the switch with Servisol and IPA to remove any surface contamination and waiting days for it to dry but it had no lasting effect. I then removed the HT wiring from the switch and re-connected it to the various loads. This cured the problem, but of course the radio and magic eye were still running when Gram was selected.

Various solutions have been proposed on the Forum for this sort of switch leakage problem on Bush VHF sets, including a reed switch powered from a low voltage supply derived from the heater AC. I did consider this, but all of my reed relays were rated at 250V max and were pcb mounted, so I tried to think of alternatives that would not be too conspicuous. A cam operated micro-switch was also considered but making the cam, providing fine adjustment for the switching point and installing it somewhere on the switch shaft seemed all to difficult at the time.

I finally decided to add another wafer to the wave-change switch, to perform just the simple HT switching function, if I could get the right type. I had some luck by locating on eBay some unused 1 pole 12 way wafers which had the same fixing centres. When they arrived, I noticed that they were of an improved design with nylon rotors, but the main thing was that they fitted the switch actuator and could be orientated and wired to do the switching function I wanted.

Fitting the extra wafer next to the aerial switching wafer was not easy, because the two rear wafers had to be removed first. The long shaft going through the double front panel prevented the switch from being tilted and the various chassis-mounted trimmers and tag-strips prevented the wafers being slid off. There were also loads of wires and components to be carefully disconnected as you would expect. When the new wafer was added, I was going to cut the existing brass spacers to suit, but managed to find four accurately made 4BA paxolin spacers to fit either side of the new wafer, which saved a bit of time as I do not have a lathe.

Anyway, I managed to complete the job, including wiring up the new wafer as the HT switch and re-connecting the wiring in two or three days, so I was relieved to hear the set burst into life when I powered it up.

Ron
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