M
[email protected]
- Jan 1, 1970
- 0
Hello all!
I was inside a ViewSonic A70 17" VGA monitor (2000 vintage) this evening,
and noticed a couple of odd things. The first was a wire that didn't go
anywhere. It was plugged (not soldered) into the main circuit board and
the connector was labeled "PUS" on the circuit board. It was generally
in the area of the base drive for the HOT. It was maybe an 18 or 20 gauge
insulated white wire. It came up from the circuit board, was taped to the
plastic housing on top of the yoke, ran to the CRT glass and ran through a
couple of wire holders that also held a coil about 6" diameter around the
CRT neck, then ran up to the top of the CRT and along the degauss coil
from right to left, doubled back and ran from left to right, and stopped
with its end insulated by a piece of heat-shrink. That's it... only
connected at one end. Any idea what this wire is for?
Also, the flyback had a wire looped through its core. It was a brown
wire, about 18 gauge. Both ends of this wire ran back to the main circuit
board, near the SMPS, and what I think was the SMPS controller chip. The
wire ends were labeled "FB+" and "FB-". My guess on this is that it picks
up the frequency of the flyback and feeds it to the SMPS so the power
supply can adjust its frequency to match the scan frequency, to minimize
any interference with the picture. Or is there a better explanation?
The reason I was in it was that a cat pissed in it while it was turned on,
resulting in a totally dead set. A quick inspection revealed a smoking
crater in the circuit board, about 1/4" (6 mm) diameter, between the C and
E legs of the HOT. I pulled the HOT and it tested OK. I cleaned out as
much of the burnt circuit board as I could with a knife, and cleaned up
the carbon and remaining cat piss with cottons swabs and alcohol. I fired
it up without the HOT, intending to measure the B+, and got periodic arcs
between the C and E traces, which is more than I was getting before. I
quickly shut it down and reinstalled the HOT, using a short jumper wire on
the collector as it was just hanging in space. I fired it up again, and
hey, it works! It will be reinstalled in a room that is free of cats.
Thanks for any ideas on the mysteries I found!
Matt Roberds
I was inside a ViewSonic A70 17" VGA monitor (2000 vintage) this evening,
and noticed a couple of odd things. The first was a wire that didn't go
anywhere. It was plugged (not soldered) into the main circuit board and
the connector was labeled "PUS" on the circuit board. It was generally
in the area of the base drive for the HOT. It was maybe an 18 or 20 gauge
insulated white wire. It came up from the circuit board, was taped to the
plastic housing on top of the yoke, ran to the CRT glass and ran through a
couple of wire holders that also held a coil about 6" diameter around the
CRT neck, then ran up to the top of the CRT and along the degauss coil
from right to left, doubled back and ran from left to right, and stopped
with its end insulated by a piece of heat-shrink. That's it... only
connected at one end. Any idea what this wire is for?
Also, the flyback had a wire looped through its core. It was a brown
wire, about 18 gauge. Both ends of this wire ran back to the main circuit
board, near the SMPS, and what I think was the SMPS controller chip. The
wire ends were labeled "FB+" and "FB-". My guess on this is that it picks
up the frequency of the flyback and feeds it to the SMPS so the power
supply can adjust its frequency to match the scan frequency, to minimize
any interference with the picture. Or is there a better explanation?
The reason I was in it was that a cat pissed in it while it was turned on,
resulting in a totally dead set. A quick inspection revealed a smoking
crater in the circuit board, about 1/4" (6 mm) diameter, between the C and
E legs of the HOT. I pulled the HOT and it tested OK. I cleaned out as
much of the burnt circuit board as I could with a knife, and cleaned up
the carbon and remaining cat piss with cottons swabs and alcohol. I fired
it up without the HOT, intending to measure the B+, and got periodic arcs
between the C and E traces, which is more than I was getting before. I
quickly shut it down and reinstalled the HOT, using a short jumper wire on
the collector as it was just hanging in space. I fired it up again, and
hey, it works! It will be reinstalled in a room that is free of cats.
Thanks for any ideas on the mysteries I found!
Matt Roberds
