Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems.

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Glynn R.
 
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Default Lower half vertical of one interlaced field creeping up GDM-1952

Hi;
I'd almost bet this is another dried-up electrolytic, I just can't seem
to find it! The perpetrator is an HP 98751a, aka Sony GDM-1952, (aka
Rasterops 19, Supermac etc), and it is a 5-BNC RGB fixed-frequency
monitor that does 1024x768 INTERLACED only. Given that, it has been a
great display anyway and I'd hate to throw it out.

Problem started a few weeks ago at turn-on. The screen seemed to be
traced fully and normally by one field, while the other scrunched up
away from the bottom-- so that I had two mouse-pointers spaced about an
inch apart when at the lower half of the screen, but moving the mouse
up the screen, the pointer images melded back together. As the monitor
warmed, the scrunched field stretched back to normal over about 5
minutes. I operated it "as-is" a couple of weeks, just waiting for it
to settle out.

Well, it is worse now-- the whole lowest quarter of the screen is
affected, and no getting back to normal after even 30 minutes of
on-time. I downloaded a zip of .pcx files of the GDM-1952 Service
manual, which has the schemata but they neglected to scan in usual
boilerplate like Theory of Operation or anything. So I'm guessing what
I need to look for. But I am not seeing electrolytics in area which
might affect one field but not the other!

Has anyone else battled this symptom? Am I making stupid assumptions or
have I missed something? My current web method of posting does not
allow me to send a file along, but for those who already have access to
the schematics, I am suspicious of C1, C2, and C8 (in the
SyncSep/PulseGen section). My reasons for looking at these a 1) they
are electrolytic; and 2) they are roughly around an area that looks
like it might have something to say about one fields vertical as
opposed to anothers. But I may be all wet.

TIA-- Glynn.

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Franc Zabkar
 
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On 18 Mar 2005 13:52:28 -0800, "Glynn R."
put finger to keyboard and composed:

Hi;
I'd almost bet this is another dried-up electrolytic, I just can't seem
to find it! The perpetrator is an HP 98751a, aka Sony GDM-1952, (aka
Rasterops 19, Supermac etc), and it is a 5-BNC RGB fixed-frequency
monitor that does 1024x768 INTERLACED only. Given that, it has been a
great display anyway and I'd hate to throw it out.

Problem started a few weeks ago at turn-on. The screen seemed to be
traced fully and normally by one field, while the other scrunched up
away from the bottom-- so that I had two mouse-pointers spaced about an
inch apart ...


I saw problems like this in fixed frequency 19"/20" RGB Sony monitors
attached to IBM 5080/5085 CADCAM workstations. The problem was dried
out caps on a little daughter PCB hanging off the middle of one of the
main PCBs. Sorry, I can't be any more specific than that. BTW, when
was your monitor manufactured?


- Franc Zabkar
--
Please remove one 's' from my address when replying by email.
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Glynn R.
 
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I saw problems like this in fixed frequency 19"/20" RGB Sony monitors
attached to IBM 5080/5085 CADCAM
workstations. The problem was dried
out caps on a little daughter PCB hanging off the middle of one of the
main PCBs. Sorry, I can't be any more specific than that.
BTW, when

was your monitor manufactured?

Feb 1990. I would imagine your IBM workstations were probably in the
same timeframe-- we may be talking about the same chassis.

There are several "daughterboards" on this mainboard. They are all
soldered on heavy wires-- for robustness I suppose. You said in the
middle-- that does narrow it down and I think I could get to that one's
caps without having to unsolder the whole board-- well maybe not!!

That board, about 3x3.5", is likely to be what the schematics show as
Board "Dc", subtitled "T/B correction". That makes sense, doesn't it?

I think I'll see if I can pull that board without destroying anything--
thanks much, Franc, for the tip!!

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Franc Zabkar
 
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On 19 Mar 2005 20:48:23 -0800, "Glynn R."
put finger to keyboard and composed:

I saw problems like this in fixed frequency 19"/20" RGB Sony monitors
attached to IBM 5080/5085 CADCAM
workstations. The problem was dried
out caps on a little daughter PCB hanging off the middle of one of the
main PCBs. Sorry, I can't be any more specific than that.
BTW, when

was your monitor manufactured?

Feb 1990. I would imagine your IBM workstations were probably in the
same timeframe-- we may be talking about the same chassis.


I think so.

There are several "daughterboards" on this mainboard. They are all
soldered on heavy wires-- for robustness I suppose. You said in the
middle-- that does narrow it down and I think I could get to that one's
caps without having to unsolder the whole board-- well maybe not!!

That board, about 3x3.5", is likely to be what the schematics show as
Board "Dc", subtitled "T/B correction". That makes sense, doesn't it?

I think I'll see if I can pull that board without destroying anything--
thanks much, Franc, for the tip!!


The size sounds right. IIRC, the PCB had a few aluminium electrolytics
and a few tantalums. I changed them all, but I suspect the electros
were the guilty ones.

I didn't have a circuit diagram at the time (1991?), but I saw the
vertical ghosting on a CRO. IIRC, the image was a jittery parabola.


- Franc Zabkar
--
Please remove one 's' from my address when replying by email.
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Franc Zabkar
 
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On Sun, 20 Mar 2005 20:18:53 +1100, Franc Zabkar
put finger to keyboard and composed:

On 19 Mar 2005 20:48:23 -0800, "Glynn R."
put finger to keyboard and composed:


There are several "daughterboards" on this mainboard. They are all
soldered on heavy wires-- for robustness I suppose. You said in the
middle-- that does narrow it down and I think I could get to that one's
caps without having to unsolder the whole board-- well maybe not!!


I desoldered the PCB to gain access to the caps. I can't remember if
really needed to, though.


- Franc Zabkar
--
Please remove one 's' from my address when replying by email.
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