Our 20 year old (1984) JC Penney microwave, model 5919-00-40, ceased to
work last night. It acted like the fuse had blown, which it had, but
that didn't cure the problem. Further investigation revaled that the
problem lies in the control pad, not the actual part that does the
cooking and can kill you. I took out the circuit board, which contains
the display, keypad, etc. and controls the microwave, and looked it
over to see if there were any obvious problems, such as another blown
fuse, which there weren't. I saw some little strips of wire on the
board in various places marked 9V, 12V, etc. and when I connected a
meter between them and the ground wire I got no reading. I verified
that the transformer was providing electricity to the board, and the
output frpm the connectors matched the values printed on the circuit
board.

By accident I shorted out part of the transformer for the board, the
10v section, and to my surprise the display lit up and the buzzer
beeped, like it would after a power failure. Then it would sort of
slowly fade out. I didn't repeat this too many times because I didn't
want to ruin the power transformer or anything.

From this, can anyone figure out what might've happened? I really don't
want to scrap this as it is so simple to repair, as far as the layout
goes.


Regards,

Tim

writes:

Our 20 year old (1984) JC Penney microwave, model 5919-00-40, ceased to
work last night. It acted like the fuse had blown, which it had, but
that didn't cure the problem. Further investigation revaled that the
problem lies in the control pad, not the actual part that does the
cooking and can kill you. I took out the circuit board, which contains
the display, keypad, etc. and controls the microwave, and looked it
over to see if there were any obvious problems, such as another blown
fuse, which there weren't. I saw some little strips of wire on the
board in various places marked 9V, 12V, etc. and when I connected a
meter between them and the ground wire I got no reading. I verified
that the transformer was providing electricity to the board, and the
output frpm the connectors matched the values printed on the circuit
board.

By accident I shorted out part of the transformer for the board, the
10v section, and to my surprise the display lit up and the buzzer
beeped, like it would after a power failure. Then it would sort of
slowly fade out. I didn't repeat this too many times because I didn't
want to ruin the power transformer or anything.

From this, can anyone figure out what might've happened? I really don't
want to scrap this as it is so simple to repair, as far as the layout
goes.

You say you measured the outputs of the transformer? So, the 10 V output
measured 10 VAC

Also, you said a fuse actually did blow?

The circuitry between the transformer and the DC outputs should be quite
simple. If you aren't getting any DC voltages at, something is likely
open - a bad solder joint or fusable resistor.

The effect you describe of shorting the 10 VAC output from the transformer
sounds strange. Possibly you were pushing on something in the vicinity
and the act of shorting having an effect was just a coincidence.

I could speculate but better to just trace the circuit for one of the DC
outputs and check each component and the wiring. It shouldn't be too hard
to find the problem now that you know the rest of the circuit probably works.

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You say you measured the outputs of the transformer? So, the 10 V output
measured 10 VAC


Correct.

Also, you said a fuse actually did blow?


The main one did.

The circuitry between the transformer and the DC outputs should be quite
simple. If you aren't getting any DC voltages at, something is likely
open - a bad solder joint or fusable resistor.


That's probably it. I'll spend an afternoon working on it.

The effect you describe of shorting the 10 VAC output from the transformer
sounds strange. Possibly you were pushing on something in the vicinity
and the act of shorting having an effect was just a coincidence.


I shorted the ground of the circuit board and a lead from the transformer
that was directly connected to a fuse of the circuit board, not the main pwr
supply fuse, and that produced the effect described. I thought the same as
you - that it might be a nearby component - but repeated shorting confirmed
that indeed one part of the transformer was being shorted.

I could speculate but better to just trace the circuit for one of the DC
outputs and check each component and the wiring. It shouldn't be too hard
to find the problem now that you know the rest of the circuit probably
works.


Yes. Shouldn't be too difficult