Discussion in 'Amateur Radio Amplifiers' started by KM5L, Jan 26, 2021.
Im starting to see bandswitches on ebay for the 922. Are they any good?
The new ones made by Switchcraft I thing it is sells for $180, a bit pricey but it is a superior switch. BUT, I’ve seen one of them burned up too. So it’s NOT a substitute for stupidity.
what usually kills those bandswitches?
9 times out of 10 it’s operator error. All the ones I’ve seen were due to that.
I added fuses to BOTH primaries of the filament transformer to hopefully protect in the event of a G-F short. Why both? Because I determined that a fair amount of current can be sourced with only one of them connected. I figured this out when I was testing adding some series R to the primary in order to drop the filament V a bit (mine was at 5.32VDC). I have not monitored the temp of the transformers yet but I did monitor the socket bases/underneath because I heard people saying there was inadequate cooling down there. Even on AM I didn't see temps above 120°F. Maybe I'll check the big iron next.
I've attached a redline schematic of the mods I've done to my 922...feedback welcome!
The simple bias mod is even more effective and should be done anyway even if you put in fuses.
Lou, I did not realize a diode is needed on the grid meter. What diode number do u recommend?
The TL922 needs meter protection. A 1n5408 from B- to ground, and back to back diodes across the plate meter which can easily be accessed at the meter board pins up top.
Can you describe or provide a drawing for the "simple bias mod"? I see it mentioned all around but have yet to find a schematic or any details. Also, when you say "back to back" diodes, you mean in parallel facing opposite directions, right?
In parallel in opposite direction yes. The bias mod is simple. It’s outlined at W8JI webpage in his discussion on the subject and I believe he has a photograph.