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B&W TV Advice sought on repair of a monochrome computer monitor

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Anonymous
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HI

 

First post here.. I'm trying to repair a monochrome video monitor module from a Kaypro-16 computer. It's a typical built in VDU unit made by Elston, having a 9" CRT and TDA1170S vertical scan chip. I've been soliciting advice on the VCF forum (a prominent retrocomputing site in the US) and I posted a schematic here:

https://forum.vcfed.org/index.php?threads/help-with-elston-dm30-09bo-vdu-board.1238123/#post-1250145

The schematic can't be uploaded to this forum as it's too large. As can be seen, it takes a 12v DC supply for power, and I am running this off a PC-AT PSU at the moment. Test signals are provided by the Kaypro, which also generates a good composite video signal so I have a second monitor plugged in, so I can see what the image should look like.

The current state of play is that after some work on the horizontal section, I have a proper horizontal scan, but the vertical has a sort of compressed area about 1/3 of the way down the screen:

IMG 9295

As well as that, there's a resistor - R478, a 1.2 Ohm 2W that sits between the power supply and the board - which is overheating, but the unit is pulling less thatn 1.5A and the resistor is at least a 2A part, so really ought to be able to cope.

The immediate question is, what might be the cause of the picture distortion? I've replaced capacitors C301, C311, C302, C309 in the vertical circuit but nonne have made any difference to the image. The V-LIN control will alter it as expected, but only in the "normal " part of the display (the bottom 2/3 of the screen). I'm wondering if the TDA1170S (U300) could be faulty..

Any suggestions?

 
Posted : 03/04/2022 12:15 pm
crustytv
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Posted by: @jonb

The schematic can't be uploaded to this forum as it's too large.

How large is the image, as this forum can process up to 8MB image files.

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Posted : 03/04/2022 12:23 pm
Anonymous
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Hi crustytv (nice forum handle by the way!)

I noted the 8MB limit when I uploaded it and assumed it was too big, as it is a stitched together composite of four different photos of the same big sheet of paper. However looking at it on disk, it is only 268kb so i can only assume the forum software didn't want it for some other reason, or I made a mistake when i uploaded it. You seem to have uncommon forum software so it is quite likely I got it wrong. Let's try again..

Elston DM30 09 schematic

 

Success! It wouldn't upload using the drag & drop feature, but worked with the My Media -> Add Files... button.

I'm keen to get the forum's thoughts on likely candidates of the cause of the image distortion... and what waveforms I should expect to see at the three test points on the Vertical circuit.

 

Cheers

JonB

 
Posted : 03/04/2022 2:19 pm
crustytv
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Posted by: @jonb

... and what waveforms I should expect to see at the three test points on the Vertical circuit.

Looking at the datasheet for the TDA1170S, I would think you should see sync, ramp and flyback waveforms for 1,2 & 3.  Along the lines of this below. Have you taken a voltage map of all the pins on the IC, and compared it to the datasheet? Are they with spec for monitor application?

waveforms

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Posted : 03/04/2022 4:04 pm
Marconi_MPT4
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Posted by: @jonb

As well as that, there's a resistor - R478, a 1.2 Ohm 2W that sits between the power supply and the board - which is overheating, but the unit is pulling less than 1.5A and the resistor is at least a 2A part, so really ought to be able to cope.

R478 overheating suggests the horizontal output stage is drawing more current than necessary and if up to 1.5A that would be under 3W dissipated. I would suggest checking C438 and C441 very carefully for correct values and dielectric type as these are critical for third/fifth harmonic tuning. Also look at horizontal drive waveform on Q413 collector for correct mark space ratio. Is Q414 overheating?

One final thought, is this monitor being driven with a non compatible video format?

Rich

 
Posted : 03/04/2022 6:35 pm
Anonymous
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@crustytv Here are the waveforms at the three test points.

TP1

TP1 waveform

TP1 (zoomed to show peak)

TP1 waveform zoomed

TP2

TP2 waveform

TP3

TP3 waveform

TP1 appears to be a bit odd? And there's a fair bit of noise. One thing I did notice was that the ramp in TP2 changed appearance according to what is on the screen, as if it was connnected to the video (luminance) signal in some way. I wasn't expecting that but maybe it is normal?

Regarding voltage map, do you mean "measure the voltage at each pin"?

 
Posted : 04/04/2022 8:20 am
Anonymous
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Posted by: @marconi_mpt4
Posted by: @jonb

As well as that, there's a resistor - R478, a 1.2 Ohm 2W that sits between the power supply and the board - which is overheating, but the unit is pulling less than 1.5A and the resistor is at least a 2A part, so really ought to be able to cope.

R478 overheating suggests the horizontal output stage is drawing more current than necessary and if up to 1.5A that would be under 3W dissipated. I would suggest checking C438 and C441 very carefully for correct values and dielectric type as these are critical for third/fifth harmonic tuning. Also look at horizontal drive waveform on Q413 collector for correct mark space ratio. Is Q414 overheating?

One final thought, is this monitor being driven with a non compatible video format?

Rich

Hi Rich

The overheating resistor was replaced with another high wattage 1.2R which subsequently overheated and is now reading 2.6R. At some point after replacement I got the horizontal circuit up and running again - not entirely sure which component was bad as I'd replaced a few caps by then, but as I'd managed to knock the brightness down to zero I couldn't see any effects of the replacements. Rookie mistake! But I was quite happy when I reset it and saw the image posted earlier. I since checked the current drain and it is 1.18A without the filament heater connected, which seems reasonable. So now I think the reason why R478 is still getting hot is because its value has doubled due to heat stress. I expect that it will be OK once replaced with another 1.2R 2W.

About the video format - it's a VDU module from a Kaypro computer, and I'm feeding it the Kaypro's video signals, so there won't be any incompatibility as they were designed to work together.

Cheers

JonB

 

 
Posted : 04/04/2022 8:31 am
Anonymous
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OK, I've been having a think and I wrote a quick program in GWBASIC to draw a test pattern on the screen:

test pattern

They are just rows of dashes in specific places on the display:

  • Row 1 at the top, where the horizontal lines are spread out
  • Row 2 about 1/3 of the way down, where horisontal compression is at its strongest
  • Row 3 is the GWBASIC menu at the bottom.

So.. looking at the waveform of TP2, I believe it is a representation of the raster vertical position. I can see where the three rows are, and the vertical lines that look to me like noise on the ramps must correspond to each of the visible scan lines; then, at the top of the ramp we have the frame flyback (big vertical line dropping, ready for the next frame). Is my interpretation correct?

raster frame structure

If so, I would expect to see the scan lines well spaced out at the start of the frame, getting closer together around the position of the second line, then spreading out to normal thereafter, but it doesn't seem to be the case. I've zoomed in and I can't see any tightening of the timings between the lines. Which in turn implies the TDA1170 is doing its job properly, but something else isn't.

There are some shorter scan lines at the start of the frame (highlighted in red), is this normal? Front porch?

It looks like a bad cap somewhere, but I have replaced many of them: C310, C301, C311, C302, C307, C309, C303, C306, C300. Especially those around the V-LIN control.

Looking closer at TP3's waveform, I think there is a correlation between the ramp down and the image, just like TP2, only this time the gradient is changing in a way that seems to mirror the compression of the image.

TP3 zoom

Would anyone care to comment on this? Am I off base with these musings?

 

 
Posted : 04/04/2022 3:48 pm
Anonymous
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One more thing, I replaced C308 and C312. C312 on the board differed from the schematic (1nF instead of 300pF) so I used a 300pF. These replacements made no difference to the picture. So, I seem to have replaced every cap in the vertical section to no avail.

 
Posted : 04/04/2022 4:27 pm
Boater Sam
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A resistor going high in value will not increase its dissipation.

 

The line stage is drawing too much current.

Boater Sam

 
Posted : 04/04/2022 4:57 pm
Anonymous
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@boater-sam OK, thanks. So I will be returning to the Horizontal stage in due course! 🙂

 
Posted : 05/04/2022 8:47 am
Anonymous
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Quick update - I worked out what was wrong with the vertical deflection stage. It was caused by PSU interference, in that I was using one PSU to power the Kaypro itself, and another to power the display (albeit with both PSU grounds connected together). Once I reconnected the VDU to the Kaypro PSU the vertical kink disappeared. However I still have an issue with the horizontal stage as boater-same pointed out, which is causing the Kaypro PSU to glitch. The VDU is clicking and each click corresponds to a large negative spike in the 12v rail, so either the Kaypro PSU is weak and needs attention, or the VDU is pulling too much and the PSU is responding by collapsing the voltage. Note the VDU does work on the other (higher rated) PSU without clicking, but the resistor in line with the horizontal stage is still getting too hot.

 
Posted : 08/04/2022 2:51 pm
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