AR88 BFO problem.

[Steve_G4HTZ]

Originally Posted by theredhouseinn

There are 4 of those mica mold caps inside the bfo can, change them for the values marked on them as one of them is a different value to the cct diag. The bfo will not tune corectly if you use the cct values, took me a while to figure this out on my AR88 when I switched on.
The mains switch is well known for failing, so I fitted a mains relay to switch the tx on so that the mains switch only operated the relay coil.
I have the same problem with the bfo failing after about 30 minutes. The only part that I have not changed yet is the 6j5, but if I switch off and on again the bfo works. One day I will try another valve. The 6j5 tests perfectly on the valve tester.
John.

Hi John
After seeing your post I did the same …When the BFO stopped working after about 20mins …turned mains off and then back on again …BFO restored …in fact BFO hasn’t failed since switching it back on this morning ….maybe it will be the same tommorow? …definitely a strange one .

Thanks for all suggestions…sorry no scope here …haven’t had chance to open the 88 up yet to try and see what if anything has already been modified ….it does have the 2nd headphone socket , adjacent to the original one ….think that may have been done during a MOD overhaul.

[Jon_G4MDC]

I'm thinking what about the heater in the 6J5 or the heater wiring/pins in the base and socket...?
Goes open circuit when hot?

[vu2nan-nandu]

I second G4MDC Jon's suggestion.

Ages ago, I had a similar experience with an AA5 AC/DC radio having series-connected heaters. After a few minutes of operation it would gradually go dead with no glow from the tube heaters. But all the heaters would test okay for continuity after the tubes were cool to the touch.

The faulty tube was isolated after a hurried check was carried out, wearing oven mitts!

Nandu.