How do I identify some of those old black Philips jellybean caps from I guess the 1950s? It can be rather difficult tracing them down with the schematics. Maybe someone knows an easier way. Then, what to replace them with. I was thinking modern poly caps. They seem to be the best all around.
I don’t know if you can identify them straight from the schematic, but when you look under the chassis, you will easily recognize them. They usually have slightly tapered ends, and imprints in small yellow characters. They look like they have been covered with a thick layer of tar, and are often very dirty and must definitely be replaced. I replace them with poly caps indeed. I can tell you that all the replaced black capacitors I measured with a Heathkit IT-11 Capacitor Checker show terrible electrical leakages, starting at voltages well below the rated voltage. You will also notice that the tar-like outer layer is easily melted, and can sometimes even be missing a fraction due to the presence of some hot component nearby. On the Philips radio forums they are often called ‘tar tubers’.
Note however that the mustard-colored capacitors also found in old Philips gear, are usually very good, close to specs, and hardly need being replaced ! Some people even salvage these mustard-colored capacitors from old instruments and receivers.