10-28-2011, 07:17 PM
morzh Wrote:Arran
Thanks,
yes those are carbon resistors, and I ordered everything from Mouser yesterday. All 5 of them.
Plus, what bugs me (I am sure it does not matter) - the mica cap bypassing the detector's (24) anode, which is supposed to be 250pF, when disconnected and measured, was 400pF.
I thought mica caps were long-term stable. I am sure this does not matter much in this case, but ordered another mica cap for the 250pF just in case.
I want to have all resistors right, so to compare the DC voltage values with the chart first, otherwise it becomes a guessing game. My non-tube troubleshooting experience tells me "get all values right, then troubleshoot". Usually makes all that much easier.
The last thing I will check is those potted caps and the two caps in metal boxes (so far I measured two of the potted caps I could easily unsolder, and their values were right where they are supposed to be). Hopefully the rest is like this also.
Those potted caps would be the first thing I would change, even if they test good now it's like playing a game of Russian Roulette, I would say that at least 75% of the problems in old radios are caused by leaky, shorted, and open, paper caps, the black Bakelite block condensers especially. The ones in the steel can sometimes work and sometimes they don't, some old timers will tell you that they don't need replacing and will last forever but these caps are 80 years old now and are not to be trusted. As for the micas those don't increase in value, sometimes they leak, sometimes they go open or short, if there is a 400 mmf mica in there and it looks factory it could be a factory revision, there are early and late model 20s so they may have decided to increses the value from a 250 to a 400 or reduce the value from a 400 to a 250 for some reason.
Regards
Arran