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Mono composite screen issues
- Thread starter tezza
- Start date
tezza
Veteran Member
Hmm...the power supply part certainly looks similar. I only have two transistors in the general PSU area, but there may be a third one associated with power functions connected near by that I didn't consider part of the circuit.
Thanks Chuck, I'll print it out and take it home tonight. Assuming it is very similar, any particular thing I should be looking for regarding the fault?
Tez
Thanks Chuck, I'll print it out and take it home tonight. Assuming it is very similar, any particular thing I should be looking for regarding the fault?
Tez
Chuck(G)
25k Member
I haven't looked at the schematic carefully. Let us know if it's similar and I'll look through my pile of schematics for more examples.
tezza
Veteran Member
Hi Chuck,
I looked at the schematic. The PSU part looks quite similar but not quite the same. There are only two transistors instead of three for example. Also, KA2915. Is that an IC? There are no ICs in this monitor. It's vintage is about 1981.
I've now replaced all 20 caps. No difference. I think it's time I called it a day. It could be any one the many diodes, transistors or resistors on the board. In time, the problem might be found but I've just got too many other projects I want to get onto, rather than grope in the dark with this one. I'll put it in the parts bin. It can stand there mockingly with the Mac SE, which also defeated me.
The other projects involve resurrecting an IBM XT (no sign of life), a beautiful Osborne 1 someone gave me (no sign of life from the disk drives) and clean up three donated PS/2 Model 30s (and sell two). I also want to check out a donated ICL "Computerphone" (based on the Sinclair QL), clean up and sell off about 8 other spare computers from the 1980s (to make room for this lot), stock up my TRS-80 Model IV with software, write more repair blogs, update my System 80 site with details of the NZ networking modification (and dump the ROMS) and de-yellow a whole lot of System 80 gear.
So you can see, I have enough to be going on with. :D
Even though we couldn't crack this one, as always, the help was much appreciated guys.
Tez
I looked at the schematic. The PSU part looks quite similar but not quite the same. There are only two transistors instead of three for example. Also, KA2915. Is that an IC? There are no ICs in this monitor. It's vintage is about 1981.
I've now replaced all 20 caps. No difference. I think it's time I called it a day. It could be any one the many diodes, transistors or resistors on the board. In time, the problem might be found but I've just got too many other projects I want to get onto, rather than grope in the dark with this one. I'll put it in the parts bin. It can stand there mockingly with the Mac SE, which also defeated me.
The other projects involve resurrecting an IBM XT (no sign of life), a beautiful Osborne 1 someone gave me (no sign of life from the disk drives) and clean up three donated PS/2 Model 30s (and sell two). I also want to check out a donated ICL "Computerphone" (based on the Sinclair QL), clean up and sell off about 8 other spare computers from the 1980s (to make room for this lot), stock up my TRS-80 Model IV with software, write more repair blogs, update my System 80 site with details of the NZ networking modification (and dump the ROMS) and de-yellow a whole lot of System 80 gear.
So you can see, I have enough to be going on with. :D
Even though we couldn't crack this one, as always, the help was much appreciated guys.
Tez
pavery
Experienced Member
Back on the job
Back on the job
Hi All. I'm picking up where Tezza left off with this monitor. Firstly I should say my 'monitor theory' is little better than his, but my electronics understanding maybe more advanced.
To recap symptoms: please revisit Pg 1 of this thread. Please note I have an identical, (but working) monitor here for comparative measurements, but we don't have a schematic for these things.
I have since verified power supply operation with a scope to the good unit - it measures the same, ie good.
Ok, looking at the Video amp: Tracing back from the CRT neck there is one wire labelled "K" on the PCB. I assume this is the Cathode going to Video amp. The others from the neck go to the HOT. However I have noticed G2 (as labelled on PCB) is 250VAC on bad monitor, but 306VAC on good. Is that a concern or perhaps a side effect of possible Video amp trouble?
2nd observation is "K" wire at PCB measures 31VAC verses 206VAC on good monitor. These are measured with no signal input & raster just showing. Before I start the Herculean task (for me anyway) of sketching up the video amp circuit, I just want to make sure I'm at the right place on the board.
Many thanks
Philip
Back on the job
Hi All. I'm picking up where Tezza left off with this monitor. Firstly I should say my 'monitor theory' is little better than his, but my electronics understanding maybe more advanced.
To recap symptoms: please revisit Pg 1 of this thread. Please note I have an identical, (but working) monitor here for comparative measurements, but we don't have a schematic for these things.
I have since verified power supply operation with a scope to the good unit - it measures the same, ie good.
Ok, looking at the Video amp: Tracing back from the CRT neck there is one wire labelled "K" on the PCB. I assume this is the Cathode going to Video amp. The others from the neck go to the HOT. However I have noticed G2 (as labelled on PCB) is 250VAC on bad monitor, but 306VAC on good. Is that a concern or perhaps a side effect of possible Video amp trouble?
2nd observation is "K" wire at PCB measures 31VAC verses 206VAC on good monitor. These are measured with no signal input & raster just showing. Before I start the Herculean task (for me anyway) of sketching up the video amp circuit, I just want to make sure I'm at the right place on the board.
Many thanks
Philip
pavery
Experienced Member
All fixed!
All fixed!
Faultfinding on a: Dick Smith Video Monitor (Model X-1198)
Just an update to close off this thread. The monitor is now fixed. It had two faults.
Fault 1:
I had indeed correctly identified the wire from the video amp & found the amplifier transistor (Q302
2SD415). Rather ominously the collector voltage was only 15V whereas the good monitor showed 95V. However the base & emitter were 3V higher (8.6V vs 5.6) on the bad unit - was the transistor being driven too hard? Also I considered whether the the B+ (is this correct term for the 95V supply?) was good, or possibly a bad transistor dragging the 95V down. An ohms check of the transistor in-circuit against the good unit (also in-circuit) revealed same readings. Not content I removed the two transistors and rechecked - same readings. I concluded the video amp transistor was fine & reinstalled.
Over the course of 10 hours (spread over 7 days) I pursued the theory that the video amp was being driven too hard as the base signal (on scope) was higher than it should be. This lead me all over the board trying to find the source of this increased signal. Eventually I found myself back at the video amp.
I then suspected perhaps B+ was weak at its source, so went through that circuit. Then removing the video amp transistor, B+ lifted to over 100V. Finally suspecting this transistor maybe internally
shorting, I touched the C & E pins to their connections and B+ fell. I then offered up C & E pins of a
known good video amp transistor - B+ stayed high. Soldering in the new transistor - Bingo - good
picture.
How nasty - the transistor was breaking down only in its high-voltage environment. Jeepers, if only I had swapped the two transistors when I first had it out of the board...
Fault 2:
While the picture was good, I did notice a jitter occasionally (every 5-10 secs or so) and knowing Tezza would like his monitor to be 100%,
I thought I better have a look at that.
With no signal going into the monitor I scoped about. I could see the jitter in the 'base?' signal
everywhere I tested. (I don't know the proper term for 'base' signal - monitor noobie here). I
suspected another transistor breaking down so removed a couple. While these checked out fine, it
enabled me to isolate what area was generating the problem. I found myself at the Input transistor
(Q301 2SC184) with a resistor-capacitor circuit. The capacitor connected to earth, but scoping it on
the earth side produced a strange signal compared to that of the identical (good) monitor. I'd expect a flat line, but the good monitor on that part of the board produced a steady base signal.(Perhaps it was inducing RF into the probe or something). On the other side of this cap (on the bad monitor) was the jitter. I removed the cap & held a substitute on the solder-pads. Rock-steady signal - Bingo! Soldered the new cap (C133) in & perfect picture.
Philip
All fixed!
Faultfinding on a: Dick Smith Video Monitor (Model X-1198)
Just an update to close off this thread. The monitor is now fixed. It had two faults.
Fault 1:
I had indeed correctly identified the wire from the video amp & found the amplifier transistor (Q302
2SD415). Rather ominously the collector voltage was only 15V whereas the good monitor showed 95V. However the base & emitter were 3V higher (8.6V vs 5.6) on the bad unit - was the transistor being driven too hard? Also I considered whether the the B+ (is this correct term for the 95V supply?) was good, or possibly a bad transistor dragging the 95V down. An ohms check of the transistor in-circuit against the good unit (also in-circuit) revealed same readings. Not content I removed the two transistors and rechecked - same readings. I concluded the video amp transistor was fine & reinstalled.
Over the course of 10 hours (spread over 7 days) I pursued the theory that the video amp was being driven too hard as the base signal (on scope) was higher than it should be. This lead me all over the board trying to find the source of this increased signal. Eventually I found myself back at the video amp.
I then suspected perhaps B+ was weak at its source, so went through that circuit. Then removing the video amp transistor, B+ lifted to over 100V. Finally suspecting this transistor maybe internally
shorting, I touched the C & E pins to their connections and B+ fell. I then offered up C & E pins of a
known good video amp transistor - B+ stayed high. Soldering in the new transistor - Bingo - good
picture.
How nasty - the transistor was breaking down only in its high-voltage environment. Jeepers, if only I had swapped the two transistors when I first had it out of the board...
Fault 2:
While the picture was good, I did notice a jitter occasionally (every 5-10 secs or so) and knowing Tezza would like his monitor to be 100%,
I thought I better have a look at that.With no signal going into the monitor I scoped about. I could see the jitter in the 'base?' signal
everywhere I tested. (I don't know the proper term for 'base' signal - monitor noobie here). I
suspected another transistor breaking down so removed a couple. While these checked out fine, it
enabled me to isolate what area was generating the problem. I found myself at the Input transistor
(Q301 2SC184) with a resistor-capacitor circuit. The capacitor connected to earth, but scoping it on
the earth side produced a strange signal compared to that of the identical (good) monitor. I'd expect a flat line, but the good monitor on that part of the board produced a steady base signal.(Perhaps it was inducing RF into the probe or something). On the other side of this cap (on the bad monitor) was the jitter. I removed the cap & held a substitute on the solder-pads. Rock-steady signal - Bingo! Soldered the new cap (C133) in & perfect picture.
Philip
Last edited:
MikeS
Veteran Member
Well done! Between the two of you there's not much left that you guys can't fix, not to mention the great detailed writeups.