06-05-2012, 07:43 PM
I bought on of these dinky little TRF radios at the Golborne swapmeet, and I've finally got round to having a closer look at it. I noticed that the mains switch on the pot (which as per the circuit is in the neutral line) had been by-passed by the 'phantom fiddler' through whose hands the set had passed. I suspected that the switch was therefore open circuit, as it proved to be. No problems I thought - I can easily fit a new 47k switched log pot. Then I noticed a little faded label on the pot stating 'anti-log'. Maybe if it were just a straighforward volume control, a normal log pot would have sufficed, but it doubles as the reaction control, so probably wouldn't have worked correctly. I did actually plot the resistance curve by adding a temporary pointer and paper dial to the shaft of the pot, noting the resistance at 20 degree intervals, plotting the resuts on a graph to satisfy myself that it was indeed an anti-log law pot, as it was.
Anti-log pots aren't easy to come by, especially switched ones, but I managed to track down a new un-switched one on e-Bay, so I've ordered one, thinking that I might be able to swap the track with a new switched pot of the same diameter. However, nothing ventured nothing gained, so I carefully bent back the tabs that hold the pot together, flooded the switch section with Servisol, and operated the switch several times in the hope that it would clean the tarnished contacts, as indeed it did. I also cleaned the pot track, then reassembled it and it all works fine. I've now wired the mains lead correctly via the switch.
When the anti-log pot arrives, I'll still see if I can transfer the track to a switched pot of the same diameter. There are other tricks that can be used to create an antilog pot using resistor networks, but I'd rather keep it as simple and original as possible. I've attached a pic which shows the various types of pot tracks, and a photo of the pot on my Princess, where it can be seen that the switch had been by-passed. I was lucky enough to find on eBay two of the three knobs that I need to replace the missing knobs, and will have a good furtle around at the NVCF to see if I can find another. (No such luck at Cowbit).
A few waxies to replace, and burnt out dropper resistors, then I'll see if it comes to life!
Anti-log pots aren't easy to come by, especially switched ones, but I managed to track down a new un-switched one on e-Bay, so I've ordered one, thinking that I might be able to swap the track with a new switched pot of the same diameter. However, nothing ventured nothing gained, so I carefully bent back the tabs that hold the pot together, flooded the switch section with Servisol, and operated the switch several times in the hope that it would clean the tarnished contacts, as indeed it did. I also cleaned the pot track, then reassembled it and it all works fine. I've now wired the mains lead correctly via the switch.
When the anti-log pot arrives, I'll still see if I can transfer the track to a switched pot of the same diameter. There are other tricks that can be used to create an antilog pot using resistor networks, but I'd rather keep it as simple and original as possible. I've attached a pic which shows the various types of pot tracks, and a photo of the pot on my Princess, where it can be seen that the switch had been by-passed. I was lucky enough to find on eBay two of the three knobs that I need to replace the missing knobs, and will have a good furtle around at the NVCF to see if I can find another. (No such luck at Cowbit).
A few waxies to replace, and burnt out dropper resistors, then I'll see if it comes to life!
Regards, David.
BVWS Member.
G-QRP Club Member 1339.
'I'm in my own little world, but I'm happy, and they know me here'
BVWS Member.
G-QRP Club Member 1339.
'I'm in my own little world, but I'm happy, and they know me here'
