JVC HR A630 VCR chicken or the egg ?

[N Cook]

Dead power supply. I've the schematic for the JVC HR J725 of which the ps
seems much the same. Secondary side of the switch mode transformer fed
forwards through an opto isolator to the HV switching power FET.
No oscillation ,all circuit protectors OK, removing the main driver FET it
tests ok. How to locate such a situation where there is no obvious
oscillator/controller IC.

 

[N Cook]

Many JVC's need a 1 uF at 250 volts (or so) in the primary, and a 120 uF at
6.3 volts cap on the secondary. Obviously one might just replace all suspect
electrolytics, though...

Mark Z.

I'd removed both the switching power FET and the biasing driver 2SC3616
because there was a 220K resistor between G and S which was not on the
schematic of another JVC I was looking at and confusing the cold checking.
I removed the 2uF,50V to the G as well, nothing seemed to be wrong with it
but was intending to replace with 2x 1uF,63V in parallel but accidently put
them in series so only 0.5uF but it worked all the same. I'm not convinced
it was a problem with that electrolytic but its the only thing I actually
changed.