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Old 24th Jan 2020, 12:04 pm   #13
cathoderay57
Nonode
 
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,358
Default Re: Eddystone 730/4 BFO not working

Hi Steve, as I read it the Eddystone manual indicates 90v (J-) anode and 20v (D-) grid 2 of the BFO valve V12. These are for the higher impedance voltmeter so more likely to mirror what you will measure with a DVM. The EMER circuit also shows 90v anode but there is an amendment that changes the grid 2 resistor from 68k to 330k and a commensurate drop of the grid 2 potential from 20v to 7v. Can you post what you have actually measured please? If the valve has tolerable HT voltage then the listening test on an adjacent receiver is a good idea in theory. However, The BFO is intended to run at a frequency either side of the receiver's IF which is 450kHz. The snag is you are unlikely to have a receiver that will receive signals at 450kHz (=666 metres). However, you might be able to pick up the second harmonic at 900kHz. The whistle should come and go and change note as you adjust the BFO control. If it does then it is likely that the BFO has drifted and you should be able to tune it back by very carefully adjusting the core in the BFO coil (T7). If it is an iron dust core be careful not to split it with a screwdriver. A plastic toothpick or knitting needle filed with a screwdriver flat is recommended. If the dust core is too tight to move, then put a small drop of white spirit on the top and leave to soak. Don't use methylated spirit as it can dissolve the outer plastic coil former. If the BFO isn't oscillating, and you have good HT then there are some paper capacitors nearby that might be leaky and need replacing such as C123 and C116 (0.01uF / 10nF). Also worth checking C114 and C115 (0.5uF / 500nF) paper. Good luck, Jerry
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