[TechAssist] Magnasonic TV (Emerson??) model GCT5536, dark picture
- From: Gary McCartney <number63@xxxxxxxxxxxxx>
- To: techassist@xxxxxxxxxxxxx
- Date: Thu, 27 Jun 2002 01:23:21 -0400
This is a Magnasonic TV (Canadian no-name brand) model GCT5536. No FCC#,
made in 1992. It is a 19" set. I believe this chassis could possibly be
Emerson but not sure about that. It uses TA8680BN as the jungle chip and
a no-name Chinese picture tube (chungaua or similar name).
I actually have two identical sets, one works fine. The other one has
dark picture (video can be seen but very dark). The problem is in the
front end video circuitry as opposed to the flyback screen adjustment.
I have been comparing voltages and waveforms with the set that is
running fine. No schematic but I do have a pinout sheet for a TA8680N
IC.
The problem appears to be the video waveform on the TA8680BN, IC301 pin
38 being too low. THe video signal going through the chip is at the same
level as the good set until the signal comes out the chip at pin 38. On
the good set, this waveform is 0.5V p-p with NTSC test pattern on
screen. The bad set shows approx. 0.20V p-p at pin 38. Good set measures
3.4V dc at same pin and bad set measures 2.5V dc. If I take a low value
resistor and jump from pin 43 (where video enters chip) to pin 38, the
picture becomes somewhat brighter. Have checked for something loading
down pin 38 but no luck. This makes it appear as problem is the TA8680
chip, however I have replaced it with a used chip, then again with a
brand new chip. The measurements are the same with the new chip, however
the picture is slightly brighter.
Why am I losing amplitude of the video signal inside the jungle chip? B+
to chip is bang-on 9.0V. Changing chip doesn't help and can't find
anything externally loading it down. I need to get the 0.20 V p-p signal
back up to 0.5V p-p as is the case with the good running set and then I
am confident that the screen will have plenty of brightness.
Does anyone have a suggestion as to a model cross to Emerson or some
other brand or even have a schematic for this DOG?
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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