J
JURB6006
Guest
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 ˝ 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 ˝ 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.
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 ˝ 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 ˝ 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.