Philco vtvom 7001 line caps? - Printable Version
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Philco vtvom 7001 line caps? - KCMike - 06-10-2018
While checking out the vtvom I found it had 62v ac. between the chassis and earth ground. Did not matter which way it was plugged in.
It has 2 -.25 line bypass caps from the primary of transformer to chassis.
Put an 1000 ohm resistor between the chassis and earth ground and measured across it and have 12 volts. So I cut the ground side of the .25 caps and jumpered in 2-.01 Y-type disc safety caps. It now reads 51v ac chassis to earth ground and.05v ac across the resistor.
Two questions I need help with.
Is the 51v with the new caps a phantom voltage?
Can I use the .01 safety caps to replace the .25 caps or should I order the larger value safety caps?
Any help greatly appreciated.
RE: Philco vtvom 7001 line caps? - Chas - 06-12-2018
Quote:Can I use the .01 safety caps to replace the .25 caps or should I order the larger value safety caps?
Yes, the .01mf is sufficient bypass without having an excessive AC line current pass-through. A higher value of .03mf could, with other equipment on the same GFI branch circuit, cause a nuisance tripping of the circuit breaker. The OEM value of .25mf is extreme by todays standards. Such a value even with NEW "Y" capacitors would give an uncomfortable tickle and under certain conditions a disturbing shock, if accidentally combined with another device in use the shock could be very hazardous. For the VTVOM it could make low level AC voltage measurements impossible.
I do not know the instrument being repaired, if it has a power transformer, the windings can be leaking to the core. That would be a problem, an open style transformer can be disassembled and the core more effectively insulated to reduce the shock. However the primary could be leaking to the secondary and often that is in direct reference to the chassis. If so, then the transformer would have to be replaced.
YMMV
Chas
RE: Philco vtvom 7001 line caps? - KCMike - 06-12-2018
Thanks Chas. Went ahead and put the .01 safety caps in it. The tester does have a transformer so I'll check it for possible primary to secondary leakage.