john@frog.UUCP (John Woods) (09/22/89)
In article <1929@cbnewsl.ATT.COM>, fjs@floyd.ATT.COM (Fred Shubert ,2A-154,3094,ATTBL) writes: > 2) My nuker has one large diode/resistor assembly (packaged-sealed) > that is connected between gnd and the .8uF bypass cap (LARGE!!). > Its resistance is approx. 15M ohms. Does this sound right?? Could be. Diode/resistor assemblies generally have a rather high turn-on voltage (like .6v per diode), so the obvious way of measuring the resistance (a typical VOM/VTVM/DVM/whathaveyou) is nearly useless. A better way is to take a 12V battery and a current meter (and a limiting resistor of, say, 100 ohms) and determine the current through the diode. If it is high one way and negligible the other way, the diode string is probably good. Probably. If it is high both ways, it is obviously bad. If it is negligible both ways, try 24 volts, and if it is still low both ways, it is likely open. (It seems unlikely that they would need to string together more than 20 diodes, but if they found 50PIV diodes cheap enough, who knows...? :-) Note that one or two diodes might be shorted even if it looks good, which will eventually result in total failure when stressed by the actual HV of operation (the other diodes are stressed just a little harder than the design called for, so one more fails, etc...) You might be able to check that if you can find a friend with the same oven who would be willing to let you check the voltage drop across his/her diode string; that should be the same (roughly) as from yours. (Don't check it in active operation, of course :-). -- John Woods, Charles River Data Systems, Framingham MA 508-626-1101 ...!decvax!frog!john, john@frog.UUCP, ...!mit-eddie!jfw, jfw@eddie.mit.edu