asdf53 wrote on 2026-02-27, 09:07:

Couldn't you measure the resistance across C203 with a multimeter to determine leakage? If you put the red multimeter lead on the negative side of the capacitor, the transistor is reverse-biased, so the only current path is directly through the capacitor. Depending on what else is upstream, a low reading could result in a false positive, but if it's high, the capacitor is likely good.

The idea is quite good, but not ideal. If you reverse bias an electrolytic cap, that may cause leakage that isn't present while the capacitor is forward biased (in capacitor sense). An electrolytic cap is chemically similar to an electrolytic rectifier operated in reverse bias (in rectifier sense). So just applying the reverse bias may increase leakage of the capacitor, causing false positive results (capacitor seems broken although it isn't). On the other hand, the leakage of that capacitor in the PCjr monitor seems marginal, that it, it can be re-formed by operating the monitor for some time, making the low-intensity colors suddenly visible. To get a clear "leaky" result, you would need to make sure to test the capacitor while it has "maximum leakage", not while it is partially reformed. Measuring the capacitor after the low-intensity colors were visible may result in a false negative result.

Probably, the idea is still good enough. Typically, these capacitors might be re-formed for a short time, but the dielectric barrier will break down after some time without applying voltage, so performing the test after the monitor was off for a day will likely not present a false negative result. Also, the (up to) 2.5V probe voltage is likely insufficient to cause significant leakage through a 10V cap if that cap isn't marginal anyway.

Thanks for the suggestion, asdf53, and for the explanation, mkarcher!

Unfortunately, C203 is located in a pretty crowded place in the neck board. When I was able to navigate two probes in there, the red one to its negative, and the black one to its positive lead, the readings on the multimeter just kept jumping around -- from more than 500 to 0 -- never settling on a number. Although I was fairly confident they were touching the appropriate leads, because of the amount of capacitors and other elements in the way, I couldn't be sure they weren't touching anything else. I didn't see them touch anything else, but, like I said, there was a lot of stuff in the way.

If the unstable measurement (jumping between 500 ohms and nearly 0 ohms) was taken correctly, it can be the result of the capacitor being unstable (0 Ohm = a lot of leakage current, 500 Ohms = probably kind of OK), but I wouldn't take the results under these conditions too seriously. As you need to further disassemble the monitor anyway to be able to get a good reading, you could also try to desolder the capacitor or snip a leg, which will give a definitive result whether this capacitor is the cause of your issue.