|
Title: Strange problem with 75A-4 Post by: W1RC on March 16, 2014, 05:13:22 PM The A-4 developed a strange problem on 20m. It was dead on 20. We removed the bottom panel and the band came to life. however when we touched the crystal cluster it went dead again. This also happened when my hand was close to the crystals but not touching them. Radio works fine on 20 with no bottom cover. We are still trying to figure it out. Anyone ever experience the same problem?
73, Mchael, W1RC Title: Re: Strange problem with 75A-4 Post by: W7NGA on March 16, 2014, 05:34:43 PM Just tweak the oscillator coil a tad to keep it oscillating under load.
It's not critical but you do need to keep it running and not obsess on dial accuracy. dan W7NGA Title: Re: Strange problem with 75A-4 Post by: Todd, KA1KAQ on March 16, 2014, 06:14:21 PM Needs new micas. Mine did this until I moved the coils quite a bit. Eventually it will catch up, because I didn't address the root cause, only bypassed it temporarily.
Title: Re: Strange problem with 75A-4 Post by: kb9r on March 16, 2014, 08:03:05 PM Michael,
First note where L-14 is set for the 20 meter xtal. Set your selectivity to the widest position. Then with the calibrator on turn L-14 one way or the other a 1/4 turn or so to restore operation. If that doesn't work go plus or minus 1/2 turn, etc. Understand that you will be changing frequency while doing this so the dial may have to be moved accordingly. If that fails to restore operation, then you may have to replace the 20 meter xtal (16.5 mc) or the mica cap associated with it. (C-42 I think) (Don't forget that V4 can sometimes cause strange oscillator problems) Let us know what happens, Joe - KB9R Title: Re: Strange problem with 75A-4 Post by: N8ETQ on March 16, 2014, 08:39:03 PM incorrect long screws driving into the wiring could do it. /Dan Title: Re: Strange problem with 75A-4 Post by: W2VW on March 16, 2014, 08:58:59 PM I know a guy with a completely original 75A-4. No deadly caps or parts changing ::) AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands
Once in a while it will not oscillate on the 20 meter het osc. A slight tweek of the slug and advice to change bands until it works have carried it a few years. It may need to be repaired correctly someday. |