Great job. I am currently doing the same radio. Mine had similar problems , failed output transistors , a cracked pcb , and mine had a trace that had got real hot in the past , and had been repaired ,, which I had to refix more better 🙂 I didn't want to 'fuss around' and fix the PA so I went with a IC amplifier module instead , ditching almost everything after the detector. This radio came to me in too poor a shape to be a worthwhile restoration , so mods were not going to reduce it's value. My transistors exhibited very odd test results likes your did. At times I thought that I was experiencing some sort of madness., because it didn't make sense. I always suspect Germaniums anyway , so I wasn't surprised. I have also replaced all resistors , and all the caps besides the ceramic and polystyrene caps - which thanks to your video I will now pay attention to. I tested mine on a similar tester to yours and they all looked good. This is still a work in progress , but I expect it to be successful.
Thanks for your very interesting comments, good luck with the one you are fixing! Polystyrene caps are certainly on my radar now, from the feedback I have got so far looks like the ESR goes up in them. It is hard to measure ESR on small value caps though. I have tried a simple method of comparing the phase shift of a capacitor under test with that of a known one using the CRO, so a video on that may be worthwhile. Germanium output transistors in car radios came in during the late 1950s when the rest of the radio was still valves - amazing! Yes they are hard to test! Cheers, Mark.
I've found the same issue with an Astor car radio, with a faulty polystyrene cap in the oscillator. Replacing it fixed it. I think they develop high ESR, which is not picked up by most capacitance meters. I've noticed that the null on a bridge is much less with some capacitors. I've found many mica caps to be so inflicted. I recently did an STC 680 of 1935 and every mica cap in it (outside the I.F. cans) measured OK with a multimeter, but had a poor or no null on a bridge. The set worked great only after every one of them had been replaced.
Thanks for your interesting comments, especially regarding ESR. Since reading them I got my ESR meter out which reads to 99 ohms and tested what the smallest value capacitor I could measure with it was. That was 96 ohms for 0.012uF. How are you measuring ESR for snaller values? Since making the video I verified both faulty caps have > 100 meg ohms insulation resistance. Also looking at the phase shift at 100kHz with a 10k ohm series resistor on the CRO in XY mode, I found the faulty caps again behaved as if they had values < 10pf. This subject may qualify for a video on its own! Mark.
An ESR type capacitor checker will show if a cap has developed a significant series resistance. A cap that has a high equivalent series resistance may actually test bigger in capacitance than it actually is on some DVM capacitance checkers because the ESR affects the charge/discharge timing. ESR meters are essential to testing electrolytics. They are also great for measuring very low value emitter resistors.
Thanks for your comment re ESR. I certainly have found my ESR meter very useful for testing electrolytics. However the max capacitance I can measure with it is 0.012uF (which has an ESR of 96 ohms) as the meter has a max reading of 99 ohms. How do you measure the ESR of smaller capacitors?
