[Boatanchors] an OT Question

[John Schmitz]

Look for bad solder connections first. I did TV repair for many years and
the number one problem was bad solder connections specifically around the
heavier components on the circuit board. The flyback, other transformers,
large power resistors, connectors soldered to the board, etc. Second leading
problem was dried up electrolytics. Start your checks in the power supply,
most TV problems end up being in the area were power is handled.
Specifically the scan power supplies. These are voltages derived from the
flyback transformer. If you have a schematic you will see that there are
several supplies derived off the secondary of the flyback transformer. A
quick voltage check you may want to make first is if you have +150volts to
the Horizontal output transistor. DO NOT check it right at the Horizontal
Output collector with a multimeter. If your high voltage is up, and you may
not realize it, there will be thousand volt pulses on the collector which
will do your meter no good. I like to do it with a scope probe. Hold the
probe close to the flyback, If pulses are there, you will see them without
touching anything. If you see no pulses then the Horz ckt probably is not
running. Then move the scope probe to the collector of the Horz Output (only
if there are no pulses). If you have +150volt approx at the Horz Output then
troubleshoot the scan supply. If you do not have +150volts on the Horz
output then troubleshoot back to the AC line power supply.

One last work of caution. Many TV's long eliminated the power transformer
and rectify right off the AC power line (kinda like the old AC-DC sets). You
must use an isolation transformer on the TV before connecting test equipment
for checks for your protection. I have been zapped pretty bad due to this
before. Use an isolation transformer.

John


-----Original Message-----
From: boatanchors-bounces at mailman.qth.net
[mailto:boatanchors-bounces at mailman.qth.net]On Behalf Of Gerry Steffens
Sent: Sunday, March 29, 2009 9:05 PM
To: boatanchors at mailman.qth.net
Subject: [Boatanchors] an OT Question


This is only related to Boatanchors 'cause I use it in the shop where I work
on BAs.

I have an old, modern (silicon based) 12 inch TV/VCR.

It worked fine for many years.  Then one day it would not turn on either
with the remote or the power switch.  Checked it every couple days sometimes
unplugged for a few days.  No go.

Put it on the shelf for a while.  About a year later took it out to dispose
of it.  Couldn't resist trying it again.  It worked!  Put it in service in
the shop, again.  Worked for a couple of weeks.

It quit again.  Let it set for a couple of days unplugged - nothing.

Set for 3 or 4 more weeks.  It worked.  Turned it off last night and then
power strip turned off (effectively unplugged).  Today turned on strip.  No
go????

Any ideas what to look for?

Any caps in this type of circuit?  Intuitively the trouble is either a cap
or the solid state device.

I know, I should scrap the thing and just work on the Bas.

Gerry



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