old candohm resistors probs - Printable Version

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old candohm resistors probs - Texasrocker - 10-19-2007

I just completed a full electronics restoration on a Canadian made Philco 38- C9. Its same American chassis "counterpart model" is Philco 38-9. The Canada model has the shadowmeter. I had to replace the old orig Philco Canada version 20-40 cycle AC trans with a good used American 60 cycle version which worked out perfectly as I had a very good spare. After bringing the radio up slowly on my variac, using an ampmeter inline to monitor current the radio came back to life and played perfectly for over an hour on my workbench. I conducted all voltage checks under load, and everything was perfect. I always include a 1-amp fuse on one side of the AC line to protect the AC trans. Luckily I did on this radio because when I did the quick "off-back on" surge test with the on-off switch, it took the 1 1/2 amp fuse out and I saw smoke coming from behind the old "candohm" rivited to chassis bias resistor where the HV centertap from the AC trans connects with neg of a filter cap. This is not my 1st rodeo with these vintage pieces of "tapped resistor " dramas at all. I have seen many of these old style resistors fail under-load many times. I will be installing a new section of 5-W wirewounds to replace the bad orig candohm to give this radio a new life again. I suggest to all techs to permanantly remove and replace the old candohm-style resistors in all your future restorations. Seems to me that was a cheap way for vintage radio mfgrs to "get by" only using these type cheap components to last the 90 day orig warranty period from the getgo back in the late 30s? Those pesky-rascals left in place for "originality" purposes if still working during restorations surely can fail, sooner than later, under load!! Carefully removing those orig candohm resistor-bank fastening rivets with a dremmel-tool and small rock bit leaves plenty of room to mount new terminal-strips to get a nice fit back in place with dependable new small type wirewounds. Even though the vintage Candohms resistors banks always read good "point-to-point" if the nichrome wire is still good, their main prob is a quick-short to chassis ground thru the old fish-paper direct to chassis ground because of orig bad-design in my opinion. Orig candohms designs suck and should be replaced for reliability in ALL vintage electronics restorations wherever you find them! Just my .02. Randal (TxRockr)

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