Post by 6A8G. on Feb 27, 2022 12:33:00 GMT 12
My latest restore is a Neeco five valve superhet. I’ll try to write this up as a separate article, but I thought I’d share an interesting problem…
I brought the meter into play & started measuring. The HT resistor was warm but not in distress, but when I measured the volts I found NEGATIVE volts??? Then a very embarrassing light-bulb turned on… this circuit has the field coil in the B- part of the circuit. Sure enough the HT centre tap was connected to the speaker field, the other side going through two resistors, one being the unstressed carbon one then through what I can best describe as a tiny toaster element, connected to the chassis.
OK I then un-soldered all the wires from the 5Y3 cathode & measured each one. The one with the short was connected to the X61M, EF39, EBC33 plates & screens. (Not the original valves.)
A check on the plate of the X61M revealed no volts at all… how come I could still hear very quiet stations? I disconnected IFT1 & sure enough there was a short from the B+ terminal to ground.
After un-soldering & bending components this way & that I was able to remove it. When I managed to extricate it from the can I saw the problem. Now I’ve read about this sort of thing in “From the Serviceman who tells” but he was dealing with primary to secondary shorts – he found this out the hard way when he got a belt from the grid cap of the IF amplifier.
Anyway, I was able to carefully un-solder the wire from the coil, slip a bit of heat-shrink over it the re-install. After more jockeying I was able to re-solder the beastie into the circuit & switched on.
It sounded fine.
BTW if anyone come across one of these IFT's it looks like this...
Happy rebuilding all...
John:)
