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.
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.
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.
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 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.
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?
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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.
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.
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.
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 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.
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?