21-08-2019, 05:07 PM
The quiescent current is supposed to be 10mA. It started at 8.5mA and very gradually rose to about 13mA.
So I examined the caps.
2 x 50uF were only rated 6V. Leaking about 2mA and almost no capacitance. Replaced with 47uF
I forget what the 8uF were. Not good. Replaced with 10uF that measured 10.1uF. The original were +80 / -20 % tolerance.
There were 4x 0.1uF paper caps. Even at only 8V the leakage varied intermittently from 20uA to 50uA approx. Replaced with ceramic.
There were 2x 0.01uF (10nF) black crumbling Hunts. One is emitter resistor RF decoupling on the mixer/osc VT1. The other is IF decoupling junction of D1 and R12 volume. I probably could have left them, but I replaced them with small ceramic discs.
Oddly the PCB didn't seem designed for the fitted paper caps, the ceramic ones fitted perfectly. Those paper caps were obsolete rubbish a few years earlier than the radio production, but UK makers continued to use the obsolete UK paper caps. Partly because ceramic and polyester were more expensive and also tariffs on imported components. Even Eddystones of then and later used a lot of paper dielectric caps though later ones had a higher quality encapsulation than the crumbly black & brown Hunts and the card + wax cases.
The quiescent current is now 10mA and the set comes on instantly. At least unlike a valve set (battery or mains) the rubbish caps weren't going to do harm. Performance & volume is slightly better. I made a note of the connections of volume control/power and the two sets of transformer wires and disconnected them all from PCB for ease of work after the first two caps.
Photos soon.
So I examined the caps.
2 x 50uF were only rated 6V. Leaking about 2mA and almost no capacitance. Replaced with 47uF
I forget what the 8uF were. Not good. Replaced with 10uF that measured 10.1uF. The original were +80 / -20 % tolerance.
There were 4x 0.1uF paper caps. Even at only 8V the leakage varied intermittently from 20uA to 50uA approx. Replaced with ceramic.
There were 2x 0.01uF (10nF) black crumbling Hunts. One is emitter resistor RF decoupling on the mixer/osc VT1. The other is IF decoupling junction of D1 and R12 volume. I probably could have left them, but I replaced them with small ceramic discs.
Oddly the PCB didn't seem designed for the fitted paper caps, the ceramic ones fitted perfectly. Those paper caps were obsolete rubbish a few years earlier than the radio production, but UK makers continued to use the obsolete UK paper caps. Partly because ceramic and polyester were more expensive and also tariffs on imported components. Even Eddystones of then and later used a lot of paper dielectric caps though later ones had a higher quality encapsulation than the crumbly black & brown Hunts and the card + wax cases.
The quiescent current is now 10mA and the set comes on instantly. At least unlike a valve set (battery or mains) the rubbish caps weren't going to do harm. Performance & volume is slightly better. I made a note of the connections of volume control/power and the two sets of transformer wires and disconnected them all from PCB for ease of work after the first two caps.
Photos soon.
