[TechAssist] JVC TV model AV27CM3- top half of screen stretched with foldover [FIXED]
- From: Gary McCartney <number63@xxxxxxxxxxxxx>
- To: techassist@xxxxxxxxxxxxx
- Date: Wed, 09 Oct 2002 09:58:11 -0400
A strange chain of events lead to the fix on this set.
I had replaced D534 with a diode that supposedly crossed to ECG552, just
to test the unit.
Later on as I was measuring voltages in the vertical area, I heard a pop
and saw smoke. D534 and R534 were toast. I reinstalled the originals as
these parts were not the cause of the problem anyhow. Then set would not
start up, so I thought maybe the shorted diode may have taken out the
vertical chip with it, and this was not allowing the set to start. I
unsoldered the vert. IC and noticed that JVC sure didn't make very large
solder pads for the vert. IC pins. They had white paint around each pin
hole. I scraped off this white paint so that each pin had a decent sized
pad for the solder to hold to. I also soldered the whole vertical
section as someone had mentioned to me that there may be poor
connections which are un-noticable. Installed new vert IC and plugged
set in. Didn't start up. Noticed a 1.25A fuse near the power regulator
was open. Changed that and then set started up and vertical linearity
problem was solved.
I don't know exactly what solved it. Either it was my new LA7838 vert.
IC which I really doubt, or it was the poor excuse for solder pads to
the IC that JVC designed, or it was the solder connections in the
surrounding vertical circuitry.
The set has ran for about 10 hours and working fine. Thanks to all who
had suggestions or who provided the nice technical material to assist
me.
Gary McCartney
McCartney Electronics
7134 Fife Rd, RR 7
Guelph Ontario Canada N1H 6J4
Fax: (519)821-1530
email: number63 (at) inetsonic.com
My second posting:
I still haven't solved this set.
Measured B+ to vertical chip. I get 28.7V with 1.5 V p-p of ripple.
Replaced D534 rectifier and C534 filter cap but no change in ripple.
Also measured output of 12 V regulator I changed earlier. Measured
11.94V, with no noticable ripple. Scoped input pulse to vert. chip at
pin 2 and got nice clean square wave at 8V p-p and very high duty cycle,
maybe 95%. I don't have waveforms or schematic in print to verify my
measurements to be correct. Scoped vertical output waveform on pin 12
and get 55V p-p pulse with a blip in the slope which doesn't look
normal. I drew a picture of this and posted it for you to view:
http://www.number63.ca/waveform.jpg
Also replaced yoke. I found one in my stock which appears to be the
exact same yoke only out of a Philips set and the connectors were a bit
different. I switched the connectors from the JVC yoke and tried it. No
change in problem. If anyone is interested, here are the yoke numbers
(from the stickers on the yokes). Sorry I don't have record of the
Philips TV model.
Original JVC yoke: CE20240-AOA
Philips yoke: 362075-7
Changed a few more small caps and resistors in the vertical section.
Problem has not improved. Any help appreciated.
Gary McCartney
McCartney Electronics
7134 Fife Rd, RR 7
Guelph Ontario Canada N1H 6J4
Fax: (519)821-1530
email: number63 (at) inetsonic.com
My original message:
This is a 27" JVC tV model AV27CM3. No Sams listed but there is one for
model AV27CM4, # 3379.
The vertical linearity is off all the way from the bottom but once it
hits the center of the screen, it starts stretching so bad that there is
about 3" of foldover.
Working without a schematic. I have replaced:
IC421, LA7838
IC422, 12V regulator
ZD425, 75V 1W
C425
C426
C430
C421
C536
C530
I removed D421 and checked it but I don't know what it is. marking on it
is 3N. It looks like a glass 1W zener but symbol on board shows it as a
regular diode. Checks ok anyways.
Does anyone have a schematic of the vertical circuit plus hopefully
waveforms?
Thanks for any help.
--
Gary McCartney
McCartney Electronics
7134 Fife Rd, RR 7
Guelph Ontario Canada N1H 6J4
Fax: (519)821-1530
email: number63 (at) inetsonic.com
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