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Success Stories If you have successfully repaired or restored a piece of equipment, why not write up what you did and post details here. Particularly if it was interesting, unusual or challenging. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 26th Dec 2020, 5:53 pm   #1
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 1,296
Default Eddystone 750 BFO Trouble

My Eddystone 750, that had been an excellent performer on SSB, providing that I backed off almost completely the RF and IF gain controls (on account of the deliberately low level of BFO injection), started giving BFO trouble. Resolving SSB became nigh on impossible unless I off-tuned a lot more than normal, using the main tuning dial, in which case the audio level dropped to almost inaudible. There was also a high pitched hiss which varied in pitch as I adjusted the BFO tuning knob. Sometimes it seemed OK but as soon as I touched the BFO knob the problem kicked off. Reception was fine on AM. I suspected BFO instability so I checked the anode supply voltage to the 6BA6 (V9) which was about 148v. I could not measure test point Y, the actual anode voltage, which is only accessible from within the BFO coil can and should be 75v following R44, a 47k anode load resistor, according to Eddystone service data. Maybe a bit high then - but at least I had got anode volts. Out came the BFO coil can and I opened it up. The anode and grid 1 resistors (both 47k) checked out OK. I decided the silvered mica caps were almost certainly innocent so all that was left was 3 x 10nF decouplers, one on grid 2 and one each between the heater connections and chassis. See attached. The heater decouplers had turned into low value resistors and the grid 2 decoupler seemed to have lost capacity. I decided not to muck about testing the caps any further and just replaced all 3 with new. Reassembly was straightforward and normal service resumed. Jerry
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Old 26th Dec 2020, 6:00 pm   #2
G6Tanuki's Avatar
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 9,815
Default Re: Eddystone 750 BFO Trouble

I had 'overload issues' with the AGC on "Steady Eddy", my 840A bedside radio - which after much annoyance turned out to be one of those 'rat-turd' capacitors buried deep within the coil-box as part of the RF amp control-grid feed, which had decided to forget about doing the capacitance-thing and had re-identified as a 70KOhm resistor.

See them, replace them. They're not to be trusted.
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Old 26th Dec 2020, 6:05 pm   #3
Join Date: Jan 2003
Location: Guildford, Surrey, UK.
Posts: 1,755
Default Re: Eddystone 750 BFO Trouble


When you need to measure a valve electrode voltage, or inject a signal when the valve base is inacessible, but the valve is exposed, such as in a FM tuner for example, I have used a short piece of 1/0.6mm silver-plated ptfe wire stripped at the end and looped tightly around the valve pin to make a connection.

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Old 26th Dec 2020, 9:35 pm   #4
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 1,296
Default Re: Eddystone 750 BFO Trouble

Thanks for the replies - useful tips. Jerry
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Old 7th Jan 2021, 12:12 pm   #5
John Caswell
Join Date: Jul 2009
Location: Wokingham, Berkshire, UK.
Posts: 368
Default Re: Eddystone 750 BFO Trouble

Hi all,
I think these components are Hunts capacitors.
If so, I have had horrendous problems with them in Quad QC22 preamplifiers in the tone control circuits, so much so, that I now replace them on spec. There is nothing consistent about them as to the "fault" high, low, open, short, I have had them all.
If there are any others in this Eddystone 750 I would most certainly contemplate replacing them.

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