"T o d d P a t t i s t" wrote in message
...
I'm at the end of a long repair of this Mitsubishi CS-3535R.
I've replaced the switching regulator and related caps to
get the primary side working, the 30 volt regulator
transistor and related zener with bias resistors that burned
up, a burned zener and bad cap near the "black level
corrector IC," a couple of fuses and finally the vertical
output IC - a Sanyo LA-7838 and all the caps in the vertical
section.

The initial failure involved smoke and the repair was a
pain, as I smoked several components on the main board and
saw smoke once in the vicinity of the CRT neck. The last
bit of repair was the replacement of the LA-7838, which I
replaced with an NTE sub.

The picture currently has a slightly brighter horizontal bar
about 20% down from the top. The scan lines are slightly
closer together there, and I suspect that it is the closer
scan lines that make it appear brighter there, but it may
actually be brighter. The picture is slightly compressed at
the line, but looks OK above and below. The caps were all
replaced in the vertical section hoping to get rid of the
vertical problem (which was not there prior to the power
supply problems.)

I've gotten advice from some here whose advice I really
respect, and the consensus is that it's probably not the
vertical IC, despite the fact that I used a sub. However,
having replaced all the electrolytic caps in the vertical
section, I'm not sure where to go now.

I'm going to put a scope on the output, and I expect I'll
see a kink in the ramp at the 20% point. I thought I'd
scope the power to the vertical IC too, just to see if it's
fluctuating, but it goes through a pair of zeners and a
large cap, and the voltages look OK on the DMM.

Anyone want to give some more advice?
Thanks.

The natural function of the wing is to soar upwards and carry that which
is heavy up to the place where dwells the race of gods. More than any
other thing that pertains to the body it partakes of the nature of the
divine.

Plato, 'Phaedrus.'

Sounds like you either missed the vertical boost cap, usually about 100uF at
35V, near the vertical output IC, or else a B+ filter near the flyback,
something like 4.7uF at 160V or 10uF at 160V, etc. If it's the boost cap,
the vertical output IC will be running hot.

Mark Z.

On Wed, 12 Oct 2005 09:38:54 -0400, T o d d P a t t i s t
put finger to keyboard and composed:

"Mark D. Zacharias" wrote:

Sounds like you either missed the vertical boost cap, usually about 100uF at
35V, near the vertical output IC,

There were two of these - both replaced. One on pin 8
(second B+) and one on pin 9 (Pump Up Output).

What about the corresponding diode? Could it be leaky?

-- Franc Zabkar

Please remove one 'i' from my address when replying by email.

On Wed, 12 Oct 2005 09:38:54 -0400, T o d d P a t t i s t
put finger to keyboard and composed:

"Mark D. Zacharias" wrote:

Sounds like you either missed the vertical boost cap, usually about 100uF at
35V, near the vertical output IC,

There were two of these - both replaced. One on pin 8
(second B+) and one on pin 9 (Pump Up Output).

or else a B+ filter near the flyback,
something like 4.7uF at 160V or 10uF at 160V, etc. If it's the boost cap,
the vertical output IC will be running hot.

If you suspect this, where would you put the scope to verify
(as long as I've got it out and hooked up?)

Here's a photo of the current LA7838 output.

http://delpet.com/images/LA7838Output.jpg

What voltage are you measuring on pin 13? Is it significantly higher
than pin 8? If not, then you will be running out of headroom. I'd
replace D4E2 just in case. A 1N4007 should be OK. I'm also curious as
to where the LH trace at pin 9 goes.

-- Franc Zabkar

Please remove one 'i' from my address when replying by email.

On Fri, 14 Oct 2005 09:40:32 -0400, T o d d P a t t i s t
put finger to keyboard and composed:

Franc Zabkar wrote:

What voltage are you measuring on pin 13? Is it significantly higher
than pin 8? If not, then you will be running out of headroom. I'd
replace D4E2 just in case. A 1N4007 should be OK. I'm also curious as
to where the LH trace at pin 9 goes.

I had no chance to work on this yesterday, but in the 5
minutes this morning before heading out, I quickly put the
scope on pin 13 (vertical drive) and on the 30V supply which
connects directly to pin 8. The 30V supply/pin 8 looks
reasonable - a bit of ripple, but no more than 5%, perhaps
less.

Then I looked at pin 13. It looks like a 5% on duty cycle
30V signal - 95% of the time off! That would mean I'd be
looking at a 30V drop on forward biased D4E2. Whatcha think
- time to replace it? :-) Seems reasonable to think it blew
when the IC blew, and the vertical drive is somehow coming
via pin 8 instead of pin 13.

My memory is hazy, but I do know that I once had an obscure problem
that was cured by replacing this diode in another set. I'm sorry but I
can't recall if I observed the waveform on a scope. I'm still puzzled
by the trace going to the left. It doesn't appear in the application
diagram in the datasheet.

-- Franc Zabkar

Please remove one 'i' from my address when replying by email.