28 Apr
2017
28 Apr
'17
2:32 a.m.
Hi all,
A colleague and I are trying to get a VT220 working again as it recently
died on us. We are hoping to set up a few items for the mid-80s
(including this terminal) to show the graduands what it would have been
like if they were doing their CompSci degree 30 years ago.
It looks to me like the flyback is dead. There is a lot of soot and
there looks like there is some damage to the top of the transformer,
better seen in the second image.
http://www.cs.nott.ac.uk/~psxasj/sparse/flyback1.jpg
http://www.cs.nott.ac.uk/~psxasj/sparse/flyback2.jpg
The terminal powers on and does the usual beeping but nothing is
displayed on the screen. Does anyone have any advice about what to do
here? Are there any sources of compatible flyback transformers?
We have a second VT220 which exhibits the same behaviour, hopefully for
a different reason so we can try and cobble two into one.
Any thoughts / advice would be greatly appreciated.
Thanks,
Aaron.
--
Aaron Jackson
PhD Student, Computer Vision Laboratory
http://cs.nott.ac.uk/~psxasj
28 Apr
28 Apr
5:27 a.m.
Hi all,
A colleague and I are trying to get a VT220 working again as it recently
died on us. We are hoping to set up a few items for the mid-80s
(including this terminal) to show the graduands what it would have been
like if they were doing their CompSci degree 30 years ago.
It looks to me like the flyback is dead. There is a lot of soot and
there looks like there is some damage to the top of the transformer,
better seen in the second image.
http://www.cs.nott.ac.uk/~psxasj/sparse/flyback1.jpg
http://www.cs.nott.ac.uk/~psxasj/sparse/flyback2.jpg
I think the soot is fairly typical of what accumulates on high voltage
components in a city environment. I'm not sure this indicates any damage to
it but it may well be faulty without showing any signs of damage.
The terminal powers on and does the usual beeping but nothing is
displayed on the screen. Does anyone have any advice about what to do
here? Are there any sources of compatible flyback transformers?
I have a VT220 which also appears to have a faulty flyback. This results in
it drawing too much current from the power supply. I am not certain the
flyback is faulty but I have eliminated most of the other components which
are likely to be responsible.
I have looked for replacement flybacks or equivelants but I have never found
any. Someone mentioned that they might have VT220 parts available on the list
some time ago but I didn't get anywhere in following that up.
We have a second VT220 which exhibits the same behaviour, hopefully for
a different reason so we can try and cobble two into one.
I have a second VT220 which works and I have used it to compare readings
with my faulty one. I also tried swapping some of the suspect components
but this failed to take suspicion off the flyback. I am reluctant to try
swapping the flybacks over in case I cause damage to the working VT220.
While the two flybacks appear likely to be electrically similar, I am not
certain of this and they are physically different - one is PCB mounted
and the other is chassis mounted with flying leads to the PCB as far as
I recall. They have different part numbers, 16-21181-01 and 16-26299-01.
Any thoughts / advice would be greatly appreciated.
Sorry, I haven't been able to provide much help or hope.
Regards,
Peter Coghlan
Thanks,
Aaron.
--
Aaron Jackson
PhD Student, Computer Vision Laboratory
http://cs.nott.ac.uk/~psxasj
6:07 a.m.
Thanks for the information, Peter. It makes me want to try and test a
few of the components around the flyback.
Thanks again,
Aaron.
Peter Coghlan via cctalk writes:
Hi all,
A colleague and I are trying to get a VT220 working again as it recently
died on us. We are hoping to set up a few items for the mid-80s
(including this terminal) to show the graduands what it would have been
like if they were doing their CompSci degree 30 years ago.
It looks to me like the flyback is dead. There is a lot of soot and
there looks like there is some damage to the top of the transformer,
better seen in the second image.
http://www.cs.nott.ac.uk/~psxasj/sparse/flyback1.jpg
http://www.cs.nott.ac.uk/~psxasj/sparse/flyback2.jpg
I think the soot is fairly typical of what accumulates on high voltage
components in a city environment. I'm not sure this indicates any damage to
it but it may well be faulty without showing any signs of damage.
The terminal powers on and does the usual beeping but nothing is
displayed on the screen. Does anyone have any advice about what to do
here? Are there any sources of compatible flyback transformers?
I have a VT220 which also appears to have a faulty flyback. This results in
it drawing too much current from the power supply. I am not certain the
flyback is faulty but I have eliminated most of the other components which
are likely to be responsible.
I have looked for replacement flybacks or equivelants but I have never found
any. Someone mentioned that they might have VT220 parts available on the list
some time ago but I didn't get anywhere in following that up.
We have a second VT220 which exhibits the same behaviour, hopefully for
a different reason so we can try and cobble two into one.
I have a second VT220 which works and I have used it to compare readings
with my faulty one. I also tried swapping some of the suspect components
but this failed to take suspicion off the flyback. I am reluctant to try
swapping the flybacks over in case I cause damage to the working VT220.
While the two flybacks appear likely to be electrically similar, I am not
certain of this and they are physically different - one is PCB mounted
and the other is chassis mounted with flying leads to the PCB as far as
I recall. They have different part numbers, 16-21181-01 and 16-26299-01.
Any thoughts / advice would be greatly appreciated.
Sorry, I haven't been able to provide much help or hope.
Regards,
Peter Coghlan
>
> Thanks,
>
> Aaron.
>
>
>
> --
> Aaron Jackson
> PhD Student, Computer Vision Laboratory
> http://cs.nott.ac.uk/~psxasj
8:08 a.m.
Thanks for the information, Peter. It makes me want to try and test a
few of the components around the flyback.
It would also be good to check the stuff like voltages on the base of the CRT
and that the CRT heater is glowing. You could have a problem that is nothing
to do with the flyback. (I'm assuming the brightness control is turned up).
Thanks again,
You're welcome.
Regards,
Peter Coghlan.
Aaron.
8:38 a.m.
Yes we checked for the glow. It's very dim but it is there. I will bring
one home this weekend and check some of the surrounding
components. Hopefully it is something very simple to replace. I'll have
a probe.
Thanks!
Aaron.
Peter Coghlan via cctalk writes:
Thanks for the information, Peter. It makes me want to try and test a
few of the components around the flyback.
It would also be good to check the stuff like voltages on the base of the CRT
and that the CRT heater is glowing. You could have a problem that is nothing
to do with the flyback. (I'm assuming the brightness control is turned up).
Thanks again,
You're welcome.
Regards,
Peter Coghlan.
>
> Aaron.
> 
9:01 a.m.
On Fri, Apr 28, 2017 at 4:38 PM, Aaron Jackson via cctalk
<cctalk at classiccmp.org> wrote:
Yes we checked for the glow. It's very dim but it
is there. I will bring
What do you mean by 'very dim'? It should be a distinct orange glow,
best seen looking
up the base of the CRT.
I normally start by checking the CRT pin voltages. There's nothing on
the pins of
a monochrome CRT that can't be measured with a normal multimeter. The sort of
voltages I would expect are :
pin 2 -- cathode -- 70V-100V
pin 1 or pin 5 -- control grid (the 2 pins are linked inside the CRT) -- perhaps
10V-20V less than the cathode
pin3,pin4 -- heater. One will be grounded (0V). The other will be around 11V-12V
pin6, pin7 -- anodes. I forget which is the first anode and which is the second
(focus) anode, but expect a few hundred V -- say 400V-800V on each of them.
The higher voltages (cathode, anodes) will come from the flyback transformer
in the horizontal output stage, so if they're all missing, suspect problems
there. If the heater voltage is low, suspect PSU problems.
I assume you have the printset (I think I got it from bitsavers). The monitor
section circuit looks very standard to me.
-tony
one home this weekend and check some of the
surrounding
components. Hopefully it is something very simple to replace. I'll have
a probe.
Thanks!
Aaron.
Peter Coghlan via cctalk writes:
>>
>> Thanks for the information, Peter. It makes me want to try and test a
>> few of the components around the flyback.
>>
>
> It would also be good to check the stuff like voltages on the base of the CRT
> and that the CRT heater is glowing. You could have a problem that is nothing
> to do with the flyback. (I'm assuming the brightness control is turned up).
>
>>
>> Thanks again,
>>
>
> You're welcome.
>
> Regards,
> Peter Coghlan.
>
>>
>> Aaron.
>>
9:41 a.m.
Hi Tony,
Thanks for the information, that is all very useful! I think I will open
up my VT420 which is working nicely to compare the glow in the base
followed by checking the voltages, as you describe.
Thanks again!
Aaron.
Tony Duell writes:
On Fri, Apr 28, 2017 at 4:38 PM, Aaron Jackson via
cctalk
<cctalk at classiccmp.org> wrote:
Yes we checked for the glow. It's very dim
but it is there. I will bring
What do you mean by 'very dim'? It should be a distinct orange glow,
best seen looking
up the base of the CRT.
I normally start by checking the CRT pin voltages. There's nothing on
the pins of
a monochrome CRT that can't be measured with a normal multimeter. The sort of
voltages I would expect are :
pin 2 -- cathode -- 70V-100V
pin 1 or pin 5 -- control grid (the 2 pins are linked inside the CRT) -- perhaps
10V-20V less than the cathode
pin3,pin4 -- heater. One will be grounded (0V). The other will be around 11V-12V
pin6, pin7 -- anodes. I forget which is the first anode and which is the second
(focus) anode, but expect a few hundred V -- say 400V-800V on each of them.
The higher voltages (cathode, anodes) will come from the flyback transformer
in the horizontal output stage, so if they're all missing, suspect problems
there. If the heater voltage is low, suspect PSU problems.
I assume you have the printset (I think I got it from bitsavers). The monitor
section circuit looks very standard to me.
-tony
29 Apr
29 Apr
6:34 a.m.
The heater and control pins appear to be giving sensible voltages. The
logic of the board is fine, I can type on the terminals keyboard and I
get the correct characters on the other end of the serial cable. The PSU
is putting out 31v which seems fine?
I'm used to discharging the tube before fixing things inside CRTs
(usually I only attempt to fix simple things like a broken toggle
switch), but I have not managed to get a spark off this monitor. The
tube doesn't seem to get charged up at all. I've measured the
capacitance of nearly all caps and they seem fine, diodes seem to be
working fine. Does this mean it is most likely the flyback transformer?
Are there any other bits I should be wary of and test properly?
Thanks again for your help. Your voltage listing and advice in general
has been very useful.
Aaron.
Tony Duell writes:
On Fri, Apr 28, 2017 at 4:38 PM, Aaron Jackson via
cctalk
<cctalk at classiccmp.org> wrote:
Yes we checked for the glow. It's very dim
but it is there. I will bring
What do you mean by 'very dim'? It should be a distinct orange glow,
best seen looking
up the base of the CRT.
I normally start by checking the CRT pin voltages. There's nothing on
the pins of
a monochrome CRT that can't be measured with a normal multimeter. The sort of
voltages I would expect are :
pin 2 -- cathode -- 70V-100V
pin 1 or pin 5 -- control grid (the 2 pins are linked inside the CRT) -- perhaps
10V-20V less than the cathode
pin3,pin4 -- heater. One will be grounded (0V). The other will be around 11V-12V
pin6, pin7 -- anodes. I forget which is the first anode and which is the second
(focus) anode, but expect a few hundred V -- say 400V-800V on each of them.
The higher voltages (cathode, anodes) will come from the flyback transformer
in the horizontal output stage, so if they're all missing, suspect problems
there. If the heater voltage is low, suspect PSU problems.
I assume you have the printset (I think I got it from bitsavers). The monitor
section circuit looks very standard to me.
-tony
> one home this weekend and check some of the surrounding
> components. Hopefully it is something very simple to replace. I'll have
> a probe.
>
> Thanks!
>
> Aaron.
>
>
> Peter Coghlan via cctalk writes:
>
>>>
>>> Thanks for the information, Peter. It makes me want to try and test a
>>> few of the components around the flyback.
>>>
>>
>> It would also be good to check the stuff like voltages on the base of the CRT
>> and that the CRT heater is glowing. You could have a problem that is nothing
>> to do with the flyback. (I'm assuming the brightness control is turned up).
>>
>>>
>>> Thanks again,
>>>
>>
>> You're welcome.
>>
>> Regards,
>> Peter Coghlan.
>>
>>>
>>> Aaron.
>>>
7:42 a.m.
On Sat, Apr 29, 2017 at 2:34 PM, Aaron Jackson <aaron at aaronsplace.co.uk> wrote:
The heater and control pins appear to be giving
sensible voltages. The
logic of the board is fine, I can type on the terminals keyboard and I
get the correct characters on the other end of the serial cable. The PSU
is putting out 31v which seems fine?
Err, the PSU has 3 voltage outputs, +5V, +12V, -12V. I am not sure where you
are measuring 31V between, but that doesn't sound 'fine' to me.
On the other hand, if the +12V rail was 31V the CRT heater would be burnt
out. If the +5V was 31V then the logic ICs would be totally fried so it wouldn't
respond to the keyboard.
So I suspect a measurement error....
I'm used to discharging the tube before fixing things inside CRTs
(usually I only attempt to fix simple things like a broken toggle
switch), but I have not managed to get a spark off this monitor. The
tube doesn't seem to get charged up at all. I've measured the
capacitance of nearly all caps and they seem fine, diodes seem to be
working fine. Does this mean it is most likely the flyback transformer?
According to the schematic of the monitor section (p16 of the 17 page
.pdf file I have) there is a bleeder (discharging) resistor between the EHT
output and ground inside the flyback. This would discharge the CRT in
a few seconds I think. So you probably wouldn't be able to get a spark.
I assume you don't have an EHT meter.
Are there any other bits I should be wary of and test
properly?
Thanks again for your help. Your voltage listing and advice in general
has been very useful.
Do you have a TV-rate video monitor with a composite video input? If
so, connect it to the BNC socket on the logic board. That's a video
output. If you get no video there, then you need to troubleshoot the
video logic first.
-tony
8:19 a.m.
Seems to be working fine as a terminal hooked up to the TV which is good
news. I am sure you are correct about me having made a measurement error
there...
I'll take another look at the schematics.
Thanks again,
Aaron.
On Sat, Apr 29, 2017 at 2:34 PM, Aaron Jackson
<aaron at aaronsplace.co.uk> wrote:
The heater and control pins appear to be giving
sensible voltages. The
logic of the board is fine, I can type on the terminals keyboard and I
get the correct characters on the other end of the serial cable. The PSU
is putting out 31v which seems fine?
Err, the PSU has 3 voltage outputs, +5V, +12V, -12V. I am not sure where you
are measuring 31V between, but that doesn't sound 'fine' to me.
On the other hand, if the +12V rail was 31V the CRT heater would be burnt
out. If the +5V was 31V then the logic ICs would be totally fried so it wouldn't
respond to the keyboard.
So I suspect a measurement error....
I'm used to discharging the tube before fixing things inside CRTs
(usually I only attempt to fix simple things like a broken toggle
switch), but I have not managed to get a spark off this monitor. The
tube doesn't seem to get charged up at all. I've measured the
capacitance of nearly all caps and they seem fine, diodes seem to be
working fine. Does this mean it is most likely the flyback transformer?
According to the schematic of the monitor section (p16 of the 17 page
.pdf file I have) there is a bleeder (discharging) resistor between the EHT
output and ground inside the flyback. This would discharge the CRT in
a few seconds I think. So you probably wouldn't be able to get a spark.
I assume you don't have an EHT meter.
Are there any other bits I should be wary of and
test properly?
Thanks again for your help. Your voltage listing and advice in general
has been very useful.
Do you have a TV-rate video monitor with a composite video input? If
so, connect it to the BNC socket on the logic board. That's a video
output. If you get no video there, then you need to troubleshoot the
video logic first.
-tony
8:22 a.m.
On Sat, Apr 29, 2017 at 4:19 PM, Aaron Jackson <aaron at aaronsplace.co.uk> wrote:
Seems to be working fine as a terminal hooked up to
the TV which is good
news. I am sure you are correct about me having made a measurement error
there...
So the logic side is working. The problem is very likely to be in the
monitor circuit,
page 16 of the schematics.
What test gear do you have? Do you have a 'scope, for example?
Can you get anying on the screen if you adjust the brightness preset?
-tony
8:44 a.m.
Unfortunately I do not have an oscilloscope anymore. I've been trying to
find a cheap one on eBay for a few months but the prices seem very high
these days. I might be able to borrow one. For now I just have a true
RMS multimeter.
I've tried both extremes of the contrast and brightness but the screen
is completely off. This is the amber version of the 220 if that makes
any difference. I can't hear any high pitched whining from the flyback,
which I can hear from my 420. All the large caps seem to be giving a
very close reading considering I haven't taken them off the board.
This terminal was working fine until about two months ago. It was in a
air conditioned server room and probably hadn't been powered off for 5
years or so until we had a long power cut, after which it didn't
work. Does this provide any insight?
Thanks,
Aaron.
Tony Duell writes:
On Sat, Apr 29, 2017 at 4:19 PM, Aaron Jackson
<aaron at aaronsplace.co.uk> wrote:
Seems to be working fine as a terminal hooked up
to the TV which is good
news. I am sure you are correct about me having made a measurement error
there...
So the logic side is working. The problem is very likely to be in the
monitor circuit,
page 16 of the schematics.
What test gear do you have? Do you have a 'scope, for example?
Can you get anying on the screen if you adjust the brightness preset?
-tony
8:50 a.m.
On Sat, Apr 29, 2017 at 4:44 PM, Aaron Jackson <aaron at aaronsplace.co.uk> wrote:
Unfortunately I do not have an oscilloscope anymore.
I've been trying to
find a cheap one on eBay for a few months but the prices seem very high
these days. I might be able to borrow one. For now I just have a true
RMS multimeter.
I've tried both extremes of the contrast and brightness but the screen
is completely off. This is the amber version of the 220 if that makes
any difference. I can't hear any high pitched whining from the flyback,
which I can hear from my 420. All the large caps seem to be giving a
very close reading considering I haven't taken them off the board.
If you have a few hundred volts on pins 6 and 7 of the CRT then the
horizontal output stage must be working. I believe you've checked
this. I assume you used the terminal's ground (e.g. the outside of that
BNC connector) as the reference for all voltage measurements.
The colour of the screen phosphor should make no difference at all.
AFAIK the only difference between the various colours is the CRT.
What voltages do you measure on the 'ends' of the brightness
preset? With respect to terminal ground (as above) of course?
This terminal was working fine until about two months ago. It was in a
air conditioned server room and probably hadn't been powered off for 5
years or so until we had a long power cut, after which it didn't
work. Does this provide any insight?
Normally the power supply fails to start when that happens, but we
know the power supply is fine since the logic side is working.
-tony
9:45 a.m.
I have measured these again because I thought they seemed a bit
strange. I must have probed the wrong pad on the circuit board earlier
(my probes don't fit in the CRT header).
Here are all the voltages and they don't seem too promising:
pin 1, green, 4.3v, should be 50v to 80v
pin 2, yellow, 6.6v should be 70v to 100v
pin 3, brown, 12v - fine
pin 4, black, 0v - fine
pin 5, white, 0v, fine, goes to case ground
pin 6, red, 0v, should be 400v to 800v
pin 7, blue, 0v, should be 400v to 800v
The voltage from the brightness control ranges between 2.4v and 5.5v,
and matches pin 1 above.
Tony Duell writes:
On Sat, Apr 29, 2017 at 4:44 PM, Aaron Jackson
<aaron at aaronsplace.co.uk> wrote:
Unfortunately I do not have an oscilloscope
anymore. I've been trying to
find a cheap one on eBay for a few months but the prices seem very high
these days. I might be able to borrow one. For now I just have a true
RMS multimeter.
I've tried both extremes of the contrast and brightness but the screen
is completely off. This is the amber version of the 220 if that makes
any difference. I can't hear any high pitched whining from the flyback,
which I can hear from my 420. All the large caps seem to be giving a
very close reading considering I haven't taken them off the board.
If you have a few hundred volts on pins 6 and 7 of the CRT then the
horizontal output stage must be working. I believe you've checked
this. I assume you used the terminal's ground (e.g. the outside of that
BNC connector) as the reference for all voltage measurements.
The colour of the screen phosphor should make no difference at all.
AFAIK the only difference between the various colours is the CRT.
What voltages do you measure on the 'ends' of the brightness
preset? With respect to terminal ground (as above) of course?
This terminal was working fine until about two months ago. It was in a
air conditioned server room and probably hadn't been powered off for 5
years or so until we had a long power cut, after which it didn't
work. Does this provide any insight?
Normally the power supply fails to start when that happens, but we
know the power supply is fine since the logic side is working.
-tony
10:29 a.m.
On Sat, Apr 29, 2017 at 5:45 PM, Aaron Jackson <aaron at aaronsplace.co.uk> wrote:
I have measured these again because I thought they
seemed a bit
strange. I must have probed the wrong pad on the circuit board earlier
(my probes don't fit in the CRT header).
Here are all the voltages and they don't seem too promising:
pin 1, green, 4.3v, should be 50v to 80v
pin 2, yellow, 6.6v should be 70v to 100v
pin 3, brown, 12v - fine
pin 4, black, 0v - fine
pin 5, white, 0v, fine, goes to case ground
pin 6, red, 0v, should be 400v to 800v
pin 7, blue, 0v, should be 400v to 800v
The voltage from the brightness control ranges between 2.4v and 5.5v,
and matches pin 1 above.
This looks very much like a problem with the horizontal deflection
circuit. Maybe the flyback transformer, maybe not.
Without a 'scope, troubleshooting is going to be difficult. But
start by testing the horizontal output transistor (Q202 on
the printset). For a start, just check it for shorts and opens. But
if it has failed, don't just replace it, as a problem with the flyback
transformer can destroy the new transistor.
-tony
8:50 a.m.
On 04/29/2017 08:34 AM, Aaron Jackson via cctalk wrote:
The heater and control pins appear to be giving
sensible voltages. The
logic of the board is fine, I can type on the terminals keyboard and I
get the correct characters on the other end of the serial cable. The PSU
is putting out 31v which seems fine?
I'm used to discharging the tube before fixing things inside CRTs
(usually I only attempt to fix simple things like a broken toggle
switch), but I have not managed to get a spark off this monitor. The
tube doesn't seem to get charged up at all.
If everything is working, even a
small monochrome monitor
should give a bit of a crackle (might have to listen closely
in a quiet place) and you should be able to feel the
electrostatic field on the hairs on the back of your hand
for a few seconds after powering on.
I've measured the
capacitance of nearly all caps and they seem fine, diodes seem to be
working fine. Does this mean it is most likely the flyback transformer?
Since it doesn't smoke, I'm thinking the horizontal sweep
transistor may have gone out. It may have shorted, which
then may have burned up the primary winding in the flyback
(if left on long enough). I'd check that transistor for a
short, and then you can trace a few traces to identify the
primary terminals on the flyback and see if they are an open
circuit.
Jon
9:48 a.m.
Hi Jon,
I can hear a light crackle but not a whine.
Are you referring to Q202? The centre pin is 11v, the other two are at
0v.
Thanks,
Aaron
Jon Elson writes:
On 04/29/2017 08:34 AM, Aaron Jackson via cctalk
wrote:
The heater and control pins appear to be giving
sensible voltages. The
logic of the board is fine, I can type on the terminals keyboard and I
get the correct characters on the other end of the serial cable. The PSU
is putting out 31v which seems fine?
I'm used to discharging the tube before fixing things inside CRTs
(usually I only attempt to fix simple things like a broken toggle
switch), but I have not managed to get a spark off this monitor. The
tube doesn't seem to get charged up at all.
If everything is working, even a
small monochrome monitor
should give a bit of a crackle (might have to listen closely
in a quiet place) and you should be able to feel the
electrostatic field on the hairs on the back of your hand
for a few seconds after powering on.
I've measured the
capacitance of nearly all caps and they seem fine, diodes seem to be
working fine. Does this mean it is most likely the flyback transformer?
Since it doesn't smoke, I'm thinking the horizontal sweep
transistor may have gone out. It may have shorted, which
then may have burned up the primary winding in the flyback
(if left on long enough). I'd check that transistor for a
short, and then you can trace a few traces to identify the
primary terminals on the flyback and see if they are an open
circuit.
Jon
11:36 a.m.
Since it doesn't smoke, I'm thinking the
horizontal sweep
transistor may have gone out. It may have shorted, which
then may have burned up the primary winding in the flyback
(if left on long enough). I'd check that transistor for a
short, and then you can trace a few traces to identify the
primary terminals on the flyback and see if they are an open
circuit.
Hmm, not good news. zero ohm between pin one and three of the transistor
you mentioned. To be fair the board around it looks a little brown, but
I assumed this was just because it would have got hot. Could replacing
this cause damage to the flyback? I seem to have a low resistance on
what I think is the primary coil.
I am wondering if there was also a power surge when we had the power
failure that killed the terminal. If so, perhaps this is the only reason
why the transistor may have died.
Thanks,
Aaron.
10:59 p.m.
On Sat, Apr 29, 2017 at 7:36 PM, Aaron Jackson via cctalk
<cctalk at classiccmp.org> wrote:
Is this with the transistor soldered to the board or removed? If the latter then
the transistor is shorted base-emitter. If the former then you are measuring
the resistance of L200 and the secondary of T201 (horizontal drive transformer)
in seroes which will be essentially zero.
Since it
doesn't smoke, I'm thinking the horizontal sweep
transistor may have gone out. It may have shorted, which
then may have burned up the primary winding in the flyback
(if left on long enough). I'd check that transistor for a
short, and then you can trace a few traces to identify the
primary terminals on the flyback and see if they are an open
circuit.
Hmm, not good news. zero ohm between pin one and three of the transistor
you mentioned. To be fair the board around it looks a little brown, but I assumed this was just because it would have got hot.
Could replacing
this cause damage to the flyback? I seem to have a low resistance on
what I think is the primary coil.
Most flyback windings have a very low DC resistance. In any case the most
common failure is a short between a couple of turns of such a winding which
will ruin the AC performance of the transformer but have almost no effect
on the resistance. That's why you need to test such a transformer with a
ringing tester, not an ohmmeter.
-tony
28 Apr
28 Apr
2:42 p.m.
I have most VT100 family, some VT220 and VT330 parts, including all 3
colors of new VT330 CRTs.
Please contact me off list if interested.
On Fri, Apr 28, 2017 at 7:27 AM, Peter Coghlan via cctalk <
cctalk at classiccmp.org> wrote:
Hi all,
A colleague and I are trying to get a VT220 working again as it recently
died on us. We are hoping to set up a few items for the mid-80s
(including this terminal) to show the graduands what it would have been
like if they were doing their CompSci degree 30 years ago.
It looks to me like the flyback is dead. There is a lot of soot and
there looks like there is some damage to the top of the transformer,
better seen in the second image.
http://www.cs.nott.ac.uk/~psxasj/sparse/flyback1.jpg
http://www.cs.nott.ac.uk/~psxasj/sparse/flyback2.jpg
I think the soot is fairly typical of what accumulates on high voltage
components in a city environment. I'm not sure this indicates any damage
to
it but it may well be faulty without showing any signs of damage.
The terminal powers on and does the usual beeping but nothing is
displayed on the screen. Does anyone have any advice about what to do
here? Are there any sources of compatible flyback transformers?
I have a VT220 which also appears to have a faulty flyback. This results
in
it drawing too much current from the power supply. I am not certain the
flyback is faulty but I have eliminated most of the other components which
are likely to be responsible.
I have looked for replacement flybacks or equivelants but I have never
found
any. Someone mentioned that they might have VT220 parts available on the
list
some time ago but I didn't get anywhere in following that up.
We have a second VT220 which exhibits the same behaviour, hopefully for
a different reason so we can try and cobble two into one.
I have a second VT220 which works and I have used it to compare readings
with my faulty one. I also tried swapping some of the suspect components
but this failed to take suspicion off the flyback. I am reluctant to try
swapping the flybacks over in case I cause damage to the working VT220.
While the two flybacks appear likely to be electrically similar, I am not
certain of this and they are physically different - one is PCB mounted
and the other is chassis mounted with flying leads to the PCB as far as
I recall. They have different part numbers, 16-21181-01 and 16-26299-01.
Any thoughts / advice would be greatly appreciated.
Sorry, I haven't been able to provide much help or hope.
Regards,
Peter Coghlan
Thanks,
Aaron.
--
Aaron Jackson
PhD Student, Computer Vision Laboratory
http://cs.nott.ac.uk/~psxasj
4 May
4 May
9:30 a.m.
I am taking another look at this.
In this VT220 there is a "Billion" transformer attached to the side
which gives AC to the board via a two pin connector. My multimeter
(which is supposedly true RMS) tells me that there is 246v going into
the transformer (fine, I'm in the UK) and 40v coming out. Is this the
right voltage that the board should be getting? Maybe it is supposed to
be 110v. Would setting the switch on the back to 110v bypass this
allowing me to try it from a 110v transformer?
The logic side of the terminal is fine. I can hook it up to a TV and use
it, but there is no high voltage to power the tube.
The layout of the board in this terminal is slightly different to what
is shown in the block diagram in the schematics, but of the circuit
diagram appears to match for the most part.
Getting a bit confused here.
Thanks,
Aaron.
Aaron Jackson via cctalk writes:
Hi all,
A colleague and I are trying to get a VT220 working again as it recently
died on us. We are hoping to set up a few items for the mid-80s
(including this terminal) to show the graduands what it would have been
like if they were doing their CompSci degree 30 years ago.
It looks to me like the flyback is dead. There is a lot of soot and
there looks like there is some damage to the top of the transformer,
better seen in the second image.
http://www.cs.nott.ac.uk/~psxasj/sparse/flyback1.jpg
http://www.cs.nott.ac.uk/~psxasj/sparse/flyback2.jpg
The terminal powers on and does the usual beeping but nothing is
displayed on the screen. Does anyone have any advice about what to do
here? Are there any sources of compatible flyback transformers?
We have a second VT220 which exhibits the same behaviour, hopefully for
a different reason so we can try and cobble two into one.
Any thoughts / advice would be greatly appreciated.
Thanks,
Aaron.
2557
days inactive
2563
days old
20 comments
5 participants
participants (5)
-
aaron＠aaronsplace.co.uk -
ard.p850ug1＠gmail.com -
cctalk＠beyondthepale.ie -
elson＠pico-systems.com -
useddec＠gmail.com