Hi folks;

Wrestling with this Mits, intermittent shutdown. I got the print for the VS5001
which seems to conform. I'm at home now so when I give the "Q" numbers I can't
be 100% sure, but you'll get it. They way I see it, Q4A1 is the Vsweep loss
detector and Q504 is the overvoltage detector.

This set will shutdown after about 10 minutes, but about ½ the time it shuts
down immediately. I did fix the SMPS and the voltages are rock solid after the
usual caps and PCB work. I found there was no Vsweep at that point and threw in
the LA7838 and of course the 100uf. Seems to work for about ten minutes. After
that I changed the 3300s, they actually were decent ESR but they were leaking
(fluid not electrons). The foil wasn't black on the output coupler, but it was
on the main Vcc filter. Runs 10 minutes again.

Like I said about ½ the time it shuts dowm immediately and now I have Vsweep,
so the closest place to look is Q504 collector. Not there. Then after
subsequent tries it is sometimes there, but I deem that a supply startup
anomoly, usually C of Q504 stays low. So I go to Q4A1, yes it is tirggered
whenever it shuts down even though there is Vsweep.

When it shuts down immediately, the B of Q4A1 reads a sawtooth, but when it
runs the top is clipped about 20% of the time. This amounts to the base voltage
when Q4A1 conducts.

The plot thickens. (I know yall hate me when I say that) This thing is also
blowing vertical ICs (LA7838), but not immediately, nor are they running hot. I
may have answered part of my question, and I did put the keyboard down to think
for a few minutes, OK, there is a pincushion transformer off the yoke return,
if the connection between that and the flyback breaks, well you have
inductance, reduced feedback and subsequent overdriving of the V output stage,
with inductance so you get a voltage spike. Now when it blows the IC, the main
VCC to the chip discharges normally, but the "boosted" supply to the output
stage is not loaded, in fact it stays charged up for quite a while. Now if a
voltage spike is wasting the thing, it should be shorted. Right ?

At this point I'm not totally in the dark, but would appreciate comments and
suggestions on if you think I'm going in the right direction. If you know
something about the LA7838 I don't know I'd love to hear it. Even if it has an
internal fuse, there is some DC on the output pin. Shouldn't the bottom output
transistor be shorted ? It's the same on two blown ICs.

For now I think I'll chalk this up to a wierd thing about the LA7838 and check
that NS PIN ckt very carefully. Maybe I should disconnect the transformer from
the flyback and short that winding and see if it will run all day ? This might
help get the job out, but it doesn't explain why at least the output pin of the
IC(s) isn't shorted to ground after it fails.

Yes I have checked the "across the yoke" components, but since it's
intermittent, who knows for sure ? A case history would come in real handy
about now.

Thanx in advance and thanx for reading, especially after the "plot thickens"
part.

Try the following:

1. SHUTDOWN CAN BE CAUSED BY NO VERTICAL DEFLECTION. SINCE THE VERTICAL
CIRCUIT IS POWERED BY THE SWITCHED POWER SUPPLY THE UNIT MUST BE TURNED ON
TO TROUBLESHOOT THE CIRCUIT. THIS CAN BE ACCOMPLISHED BY TEMPORARILY
DISCONNECTING THE VERTICAL INPUT TO THE X-PROTECT CIRCUIT. ** TO PREVENT
DAMAGE TO THE CRT'S YOU MUST UNPLUG THE CRT PCB'S. ** TO DEFEAT THE
X-PROTECT CIRCUIT, UNSOLDER D4A1 LOCATED OFF THE COLLECTOR OF Q4A1. IF THE
UNIT POWERS ON AT THIS POINT, YOU HAVE CONFIRMED A VERTICAL FAILURE AS THE
CAUSE FOR SHUTDOWN. ** D4A1 MUST BE REINSTALLED IN CIRCUIT TO PREVENT CRT
DAMAGE. **
2. CHECK PIN 1 OF IC401 FOR 10.7VDC. IF LOW OR MISSING CHECK FOR SHORTED
D4000 (PART #264P487030) LOCATED OFF PIN 1. (PCB-MAIN) 3. CHECK FOR
DEFECTIVE IC401. IC401 PART 272P239040, OR INSTALL PT# IC-401, THIS KIT
CONTAINS CAPACITOR C920, D4000 AND IC401. BE SURE TO MOUNT C920 ON FOIL SIDE
OF PCB.
I hope this helps you.
R.

"JURB6006" bravely wrote to "All" (10 Sep 03 01:41:16)
--- on the heady topic of "Mit. VS5010 intermittent shutdown."

JU From: (JURB6006)
JU For now I think I'll chalk this up to a wierd thing about the LA7838
JU and check that NS PIN ckt very carefully. Maybe I should disconnect the
JU transformer from the flyback and short that winding and see if it will
JU run all day ? This might help get the job out, but it doesn't explain
JU why at least the output pin of the IC(s) isn't shorted to ground after
JU it fails.
JU Yes I have checked the "across the yoke" components, but since it's
JU intermittent, who knows for sure ? A case history would come in real
JU handy about now.

JU Thanx in advance and thanx for reading, especially after the "plot
JU thickens" part.

Have you changed the diode across the pins of the LA7838?

Asimov
******

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