21 Feb
2003
21 Feb
'03
5:40 a.m.
All,
yep, it's me again. The 4000 VLC started and ran perfectly. I left
it on for about 8 hours as a burn-in test. When I came back, it was
(apparently) still running - but the VT-320 it was talking through had
died. Arrgggh. I've done a little investigating, so hopefully I won't need
quite so many exchanges to get this one going.
The external symptoms of the VT320 were: no display; when power
switch is on, the LK201 lights flash about once a second. No other sign of
life. I turned it off, unplugged it, and opened it up. The power supply
board at the side has an output indicator LED which also flashes about once
a second.
I turned it off, unplugged the power supply board from the
mainboard, and turned it on again. In this condition (which I'll call
"unloaded"), the output indicator LED comes on and stays on. In addition,
the pins on the output connector go to more or less their correct voltage,
as determined by labels on the mainboard at the other end of that connector
harness:
Labelled Ground 5V 12V 18V
Actual Ground 6.5V 12.5V 22V
The power supply has a big (10-pin) transformer inside a cage (with
some other components) on it. The transformer seems to divide the circuitry
in two, inasmuch as only the "Ground" traces seem to go to both sides. On
the input (upstream, plugs into wall) side, there is a network connected to
a UC3842N IC. Google led me to a spec sheet for that, which calls it a
"Current-mode PWM Controller". The spec sheet also has a "typical
application" schematic for an "Off-line Flyback Regulator" which looks to
be at least somewhat similar to the upstream side of the power supply,
though I have not traced the latter out well enough to be sure yet.
There are also several large electrolytic capacitors on the input
side of the power supply.
Possible signs of trouble are there, in the form of scorch marks on
the PWB around two power resistors (? They have what looks like a powder
blue ceramic exterior, are marked 120 Ohms +/- 5% and ?? (maybe 47 Ohms),
and are much bigger than most of the resistors on the board).
In the "loaded" condition, the resistor with the worst scorch marks
around it (R523, as marked on the circuit board) has 0V across it, with a
small spike upward (as shown by the needle jerking on my analog VOM) each
time the output LED flashes. In the "unloaded" condition, my VOM shows 15V
across that resistor, and it gets pretty warm pretty fast.
Questions for the group:
1) Have I got good evidence that the fault this time is actually on the
power supply board, and not a pull-down on the mainboard, as it was on the
VAX 4000 VLC?
2) Is there a good way (without use of an ESR meter, which I haven't got)
to isolate which component is bad, or should I just replace the capacitors
on suspicion?
The power supply, at least, looks very easy to work on. Nice big
components, circuit board is only 2 layers (front and back) and not very
many traces on the component side, and the traces are exposed and easy to
get to in its operating configuration.
I'm being careful around the 120VAC (rectified to 160 DC in the
unloaded condition).
Thanks in advance for any help you can give me!
- Mark
21 Feb
21 Feb
7:23 a.m.
Mark Tapley wrote:
All,
yep, it's me again. The 4000 VLC started and ran perfectly. I left
it on for about 8 hours as a burn-in test. When I came back, it was
(apparently) still running - but the VT-320 it was talking through had
died. Arrgggh. I've done a little investigating, so hopefully I won't need
quite so many exchanges to get this one going.
The external symptoms of the VT320 were: no display; when power
switch is on, the LK201 lights flash about once a second. No other sign of
life. I turned it off, unplugged it, and opened it up. The power supply
board at the side has an output indicator LED which also flashes about once
a second.
I turned it off, unplugged the power supply board from the
mainboard, and turned it on again. In this condition (which I'll call
"unloaded"), the output indicator LED comes on and stays on. In addition,
the pins on the output connector go to more or less their correct voltage,
as determined by labels on the mainboard at the other end of that connector
harness:
Labelled Ground 5V 12V 18V
Actual Ground 6.5V 12.5V 22V
The power supply has a big (10-pin) transformer inside a cage (with
some other components) on it. The transformer seems to divide the circuitry
in two, inasmuch as only the "Ground" traces seem to go to both sides. On
the input (upstream, plugs into wall) side, there is a network connected to
a UC3842N IC. Google led me to a spec sheet for that, which calls it a
"Current-mode PWM Controller". The spec sheet also has a "typical
application" schematic for an "Off-line Flyback Regulator" which looks to
be at least somewhat similar to the upstream side of the power supply,
though I have not traced the latter out well enough to be sure yet.
There are also several large electrolytic capacitors on the input
side of the power supply.
Possible signs of trouble are there, in the form of scorch marks on
the PWB around two power resistors (? They have what looks like a powder
blue ceramic exterior, are marked 120 Ohms +/- 5% and ?? (maybe 47 Ohms),
and are much bigger than most of the resistors on the board).
In the "loaded" condition, the resistor with the worst scorch marks
around it (R523, as marked on the circuit board) has 0V across it, with a
small spike upward (as shown by the needle jerking on my analog VOM) each
time the output LED flashes. In the "unloaded" condition, my VOM shows 15V
across that resistor, and it gets pretty warm pretty fast.
Questions for the group:
1) Have I got good evidence that the fault this time is actually on the
power supply board, and not a pull-down on the mainboard, as it was on the
VAX 4000 VLC?
2) Is there a good way (without use of an ESR meter, which I haven't got)
to isolate which component is bad, or should I just replace the capacitors
on suspicion?
The power supply, at least, looks very easy to work on. Nice big
components, circuit board is only 2 layers (front and back) and not very
many traces on the component side, and the traces are exposed and easy to
get to in its operating configuration.
I'm being careful around the 120VAC (rectified to 160 DC in the
unloaded condition).
Thanks in advance for any help you can give me!
- Mark
Look at the flyback transformer. That is the ceramic thing with the thick
wire attaching to the CRT tube. Does it have a crack around it with
hardened "ooze" coming out of it?
90% of the time the vt320's fail, it is this transformer shorting out, which
takes the horozontal transistor(?) with it. It heats up, cracking open, and
spewing out it's magic ooze. If this is the case, you can replace both of
these parts, and it will work again. DEC used some really crappy flyback
transformers in these things.
However, it would probably be cheaper just to replace the whole terminal
in that case. The transformer will probably cost at least $30.00, and the
transistor around $5.00. Then comes the fun part of trying to extract
and replace them without destroying the circuit board.
7:44 a.m.
Look at the flyback transformer. That is the ceramic
thing with the thick
wire attaching to the CRT tube. Does it have a crack around it with
hardened "ooze" coming out of it?
Will look when I get home. I don't remember there being a wire that went
from the PS board to the tube, other than ground straps
from the cage
assembly. There is a ceramic transformer-looking thing and the wires
on it
are rusty, which is one of my second-order questions (how to get rid of the
rust and arrest the corrosion process).
90% of the time the vt320's fail, it is this
transformer shorting out, which
takes the horozontal transistor(?) with it. It heats up, cracking open, and
spewing out it's magic ooze. If this is the case, you can replace both of
these parts, and it will work again. DEC used some really crappy flyback
transformers in these things.
Is that transistor on the PS board as well? If so, why does the PS board
appear to work "unloaded"?
If that's the fault, are the scorch marks under the blue resistors normally
there, or unrelated?
However, it would probably be cheaper just to replace
the whole terminal
in that case. The transformer will probably cost at least $30.00, and the
transistor around $5.00. Then comes the fun part of trying to extract
and replace them without destroying the circuit board.
Sigh. I may look on it as a learning experience, if I can find a
replacement transformer that fits.
- Mark
8:12 a.m.
Mark Tapley wrote:
It isn't on the power supply board, it is on the main logic board,
located under the display. Looking at the screen, it will be on the
right, about halfway back. Probably the biggest single component
on that board. A big ceramic blob, with the thick wire coming
out of the top.
The transformer is a big white thing with a high voltage cable
plugged into the side of the video tube (with a rubber gasket
to protect you from the high voltage). Be careful, because it charges
the tube to over 10,000 volts, and can give you a very nasty jolt.
You're PS is probably be Ok if it is the flyback that is shot.
I believe it is a switching power supply, and they give weird
readings when unloaded.
It is also on the main logic board. It is mounted near the flyback,
attached to a metal heatsink. It has a "plastic" washer to insulate it
You have to use the correct model. I used to be able to special order
them from a local electronics shop, but haven't bothered in recent years.
Look at the
flyback transformer. That is the ceramic thing with the thick
wire attaching to the CRT tube. Does it have a crack around it with
hardened "ooze" coming out of it?
Will look when I get home. I don't remember there being a wire that went
from the PS board to the tube, other than ground straps from the cage
assembly. There is a ceramic transformer-looking thing and the wires on it
are rusty, which is one of my second-order questions (how to get rid of the
rust and arrest the corrosion process).
90% of the time
the vt320's fail, it is this transformer shorting out, which
takes the horozontal transistor(?) with it. It heats up, cracking open, and
spewing out it's magic ooze. If this is the case, you can replace both of
these parts, and it will work again. DEC used some really crappy flyback
transformers in these things.
Is that transistor on the PS board as well? If so, why does the PS board
appear to work "unloaded"?
If that's the fault, are the scorch marks under the blue resistors normally
there, or unrelated?
from the heatsink, but that is often melted too.
However, it
would probably be cheaper just to replace the whole terminal
in that case. The transformer will probably cost at least $30.00, and the
transistor around $5.00. Then comes the fun part of trying to extract
and replace them without destroying the circuit board.
Sigh. I may look on it as a learning experience, if I can find a
replacement transformer that fits.
- Mark
8:56 a.m.
Look at the
flyback transformer. That is the ceramic thing with the thick
wire attaching to the CRT tube. Does it have a crack around it with
hardened "ooze" coming out of it?
Will look when I get home. I don't remember there being a wire that went
from the PS board to the tube, other than ground straps from the cage >90% of the time the vt320's fail, it is this
transformer shorting out, which
I would put it at closer to 98% :-) Those transformers are really unreliable.
>takes the horozontal transistor(?) with it. It
heats up, cracking open, and
Not always. I've had VT3xx's where the transistor was not defective (but
the flyback certainly was!)
spewing out
it's magic ooze. If this is the case, you can replace both of
these parts, and it will work again. DEC used some really crappy flyback
transformers in these things.
Is that transistor on the PS board as well? If so, why does the PS board
appear to work "unloaded"? If that's the fault, are the scorch marks under
the blue resistors normally
there, or unrelated?
>However, it would probably be cheaper just to replace the whole terminal
>in that case. The transformer will probably cost at least $30.00, and the
>transistor around $5.00. Then comes the fun part of trying to extract
>and replace them without destroying the circuit board.
Oh come on. Those parts are not difficult to desolder without damage. Or
at least I've never had any problems doing so.
The transistor is a standard part (I forget just what it is, but a good
TV spares place would have one). The transformer is non-trivial to find,
AFAIK DEC never supplied them as a spare part. I believe there are 3rd
party replacements for it, but I can't remember where to get them.
-tony
9:11 a.m.
Will check the flyback xformer, and rip it off the board (OK, I'll desolder
it...) if it looks guilty. Probably will take the HOT, if I can identify it
unambiguously, as well. Then it's back to the parts-shopping game. Sigh.
> If that's the fault, are the scorch marks
under the blue resistors normally
> there, or unrelated?
Still curious about this. The scorch marks make me nervous - can or should
I try to heat-sink at least the resistor with the bad scorches around it?
Also, what if anything should I do about the rusting smaller transformer on
the PS board?
- Mark
9:32 a.m.
Will check the flyback xformer, and rip it off the
board (OK, I'll desolder
it...) if it looks guilty. Probably will take the HOT, if I can identify it
unambiguously, as well. Then it's back to the parts-shopping game. Sigh.
The HOT is the power transistor next to the flyback. IIRC it's in a TO220
package.
> If that's the fault, are the scorch marks
under the blue resistors normally
> there, or unrelated?
Still curious about this. The scorch marks make me nervous - can or should
I try to heat-sink at least the resistor with the bad scorches around it?
22 Feb
22 Feb
7:34 a.m.
Mark Tapley wrote:
Will check the flyback xformer, and rip it off the
board (OK, I'll desolder
it...) if it looks guilty. Probably will take the HOT, if I can identify it
unambiguously, as well. Then it's back to the parts-shopping game. Sigh.
It's probably normal. If you want intresting burn marks, look at the
LA210, which would discolor a large area of the case above the power
supply. Customers always asked me if that was normal. I think a lot
of the DEC terminal/printer stuff left scortch marks somewhere.
>If that's the fault, are the scorch marks
under the blue resistors normally
>there, or unrelated?
>
>
Still curious about this. The scorch marks make me nervous - can or should
I try to heat-sink at least the resistor with the bad scorches around it?
Also, what if anything should I do about the rusting
smaller transformer on
the PS board?
Sounds like someone spilled something liquid into the terminal at some
point. Heat + liquid + wire = Instant Rust.
If you just want to use the terminal, I wouldn't bother. More likely to
break something than any cleanup would fix. If you want something
for people to look at, then you might be concerned. I'd only be bothered
about it if it was too excessive, then I'd try scraping off the excess
with a knife, or probably just swap in another power supply from a
"dead" one (which I usually had a couple of).
After the flyback, I had problems with badly scratched picture tubes,
and dead tubes. I still don't know how they could scratch them up so
badly.
If it were a vt420, after the flyback (probably from the same compny)
I'd look at one of the cables going from the CRT to the main board.
The contacts would heat up, and either burn the connector out, or weld it
to the motherboard.
8:06 a.m.
After the flyback, I had problems with badly scratched
picture tubes,
and dead tubes. I still don't know how they could scratch them up so
badly.
Easy. Pinheads would put any CRTs face down on any dirty floor or
pavement and grind it around. Just a slight move same thing. Even
two CRTs facing each other and touching while jousling either one
have same effect.
Cheers,
Wizard
8:22 a.m.
jpero(a)sympatico.ca wrote:
Some of them were scratched about halfway through the glass,
in a single long streak. If it was always one customer doing it, I
could assume a royal idiot, but these came from multiple businesses.
I don't know how they gouged it so deeply, without breaking
the tube entirely. They just wanted them replaced because the
scratch made it hard to read in that area. It's not something I'd
like to have sitting a few feet from by face anyway.
After the
flyback, I had problems with badly scratched picture tubes,
and dead tubes. I still don't know how they could scratch them up so
badly.
Easy. Pinheads would put any CRTs face down on any dirty floor or
pavement and grind it around. Just a slight move same thing. Even
two CRTs facing each other and touching while jousling either one
have same effect.
7:16 a.m.
Tony Duell wrote:
The flyback is on the main logic board in these terminals. The PSU is
just the 'low voltage' supplies.
No, all the deflection circuit parts are on the main logic board.
Many of the ones I had to repair had been left on for a long time in
that condition (...mumble...customers...), and had often burnt the
curcuit board, enough to cause the copper to come loose from the
fiberglass backing (delaminate?). Removing the components on
these, without losing a lot of the copper was a major pain.
Sometimes, though, I was lucky and it wasn't too badly fried, and
wasn't too hard to fix (just needed a good, hot iron for the flyback), but
usually they waited a couple of days before complaining that their
"computer" wasn't working, and it had a nasty smell, and smoke was
coming out of it (and should they turn it off?).
Look at the flyback transformer. That is the ceramic
thing with the thick
wire attaching to the CRT tube. Does it have a crack around it with
hardened "ooze" coming out of it?
Will look when I get home. I don't remember there being a wire that went
from the PS board to the tube, other than ground straps from the cage
>90% of the time the vt320's fail, it is
this transformer shorting out, which
>
>
I would put it at closer to 98% :-) Those transformers are really unreliable.
>takes the horozontal transistor(?) with it. It
heats up, cracking open, and
>
>
Not always. I've had VT3xx's where the transistor was not defective (but
the flyback certainly was!)
spewing
out it's magic ooze. If this is the case, you can replace both of
these parts, and it will work again. DEC used some really crappy flyback
transformers in these things.
Is that transistor on the PS board as well? If so, why does the PS board
appear to work "unloaded"?
If that's the fault, are the scorch marks
under the blue resistors normally
there, or unrelated?
>However, it would probably be cheaper just to replace the whole terminal
>in that case. The transformer will probably cost at least $30.00, and the
>transistor around $5.00. Then comes the fun part of trying to extract
>and replace them without destroying the circuit board.
>
>
Oh come on. Those parts are not difficult to desolder without damage. Or
at least I've never had any problems doing so.
The transistor is a standard part (I forget just what
it is, but a good
TV spares place would have one). The transformer is non-trivial to find,
AFAIK DEC never supplied them as a spare part. I believe there are 3rd
party replacements for it, but I can't remember where to get them.
I usually replaced it anyway, since I didn't really trust it after what
it had
just gone through, and I had the thing disassembled anyway. I got the
transistors from Radio Shack (special order), but the transformer cost
me $35 to $40 from the one place I could find them.
At the time, it was cheaper than a new VT320, but I don't think that is true
any more, especially if you have to add labor to the cost.
26 Feb
26 Feb
1:57 p.m.
Kevin:
90% of the time the vt320's fail, it is this
transformer shorting out, which
takes the horozontal transistor(?) with it. It heats up, cracking open, and
spewing out it's magic ooze.
and Tony:
Alas not. Go back to the main board of the VT320 and
look at the flyback
transformer (the one with the thick red EHT cable to the side of the
CRT). Most likely it will be physcially cracked.
pinpointed the problem. My fever went down enough today (bad flu) to take
the thing apart farther. Just as predicted, the transformer in question has
an ugly-looking crack near the EHT cable, with Magic Spooge leaking out and
solidifying. Alas.
Eldest daughter and I took turns with the soldering iron and solder-sucker,
and got the busted transformer off. (The little spring-operated solder
suckers are actually pretty effective.) It says:
88 26
SAMPO
FDAO17
16-27975-01
HI POTTED
Googling for a source for a replacement transformer is next on the task
list. Will also order a replacement for the transistor at the same time, in
case the transformer alone doesn't fix the problem.
My thanks and compliments to Kevin and Tony - I have no idea how you (and
the list collectively) maintain such an incredible knowledge base.
- Mark
21 Feb
21 Feb
8:47 a.m.
All,
yep, it's me again. The 4000 VLC started and ran perfectly. I left
it on for about 8 hours as a burn-in test. When I came back, it was
(apparently) still running - but the VT-320 it was talking through had
died. Arrgggh. I've done a little investigating, so hopefully I won't need
quite so many exchanges to get this one going.
The external symptoms of the VT320 were: no display; when power
switch is on, the LK201 lights flash about once a second. No other sign of
A very common problem, and it's not the PSU in the VT320. Even though, as
you've correctly noticed, the PSU is 'tripping'.
life. I turned it off, unplugged it, and opened it up.
The power supply
board at the side has an output indicator LED which also flashes about once
a second.
I turned it off, unplugged the power supply board from the
mainboard, and turned it on again. In this condition (which I'll call
"unloaded"), the output indicator LED comes on and stays on. In addition,
the pins on the output connector go to more or less their correct voltage,
as determined by labels on the mainboard at the other end of that connector
harness:
Labelled Ground 5V 12V 18V
Actual Ground 6.5V 12.5V 22V
OK, so the SMPSU is doing something. Try it with a dummy load -- a
suitable light bulb -- between each of the output rails and ground (or at
least the 5V rail and the 12V rail). My guess is that the PSU voltages
will become correct.
1) Have I got good evidence that the fault this time
is actually on the
power supply board, and not a pull-down on the mainboard, as it was on the
VAX 4000 VLC?
Alas not. Go back to the main board of the VT320 and look at the flyback
transdormer (the one with the thick red EHT cable to the side of the
CRT). Most likely it will be physcially cracked. Shorted turns in this
transformer is a very common problem in the VT3xx series. Sometimes it
takes the HOT (horizontal output transistor) out with it. But the PSU
starts tripping owing to excessive load.
-tony
7970
days inactive
7975
days old
12 comments
4 participants
participants (4)
-
ard＠p850ug1.demon.co.uk -
jpero＠sympatico.ca -
kth＠srv.net -
mtapley＠swri.edu