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IBM 5154 Troubleshooting

All kinds of possibilities. You can try flexing the board with a plastic stick or wiggling small wires and see if the interference "moves". If so, could be ground connection
 
retrogear said:
All kinds of possibilities. You can try flexing the board with a plastic stick or wiggling small wires and see if the interference "moves". If so, could be ground connection
Click to expand...

Well, I haven't had a lot of time. But, I measured the noise on the on the driver. The red channel has 13Vpp of noise at 244ns period....~4Mhz. The green/blue channel have about 20Vpp of noise at the same frequency.

So, anything clocked around 4Mhz in the monitor? This noise is very large compared to the actual signal; it simply has to be a large component of the problem.

Both the 154V rail and the 12V rail had 900mv and 500mv of noise. I'll be tracing into this more this weekend. I didn't get pictures yet.

Overall, moving the driver board does not change the signal. However, changing the brightness / contrast does change the ripple of the noise visible on the screen.

Noise on driver
Noise and signal on driver

More to come.

Chris V.
 
By the waveforms in the SAMS schematic, the power supply oscillator is running at around 400KHz. There is nothing in the monitor that clocks at 4MHz. Your computer probably does. How close is your monitor to the computer?
Was your computer off when you took the screenshot of no signal? Your ripple looks like a full sine wave so almost picked up like it's radiating through the air. That can get picked up by poor grounding somewhere.

>However, changing the brightness / contrast does change the ripple of the noise visible on the screen.

That is a very big clue. You are varying the load on the power supply by increasing the brightness. The power supply regulates by varying the frequency (on time) of it's oscillation so the ripple on the offending supply will change.
One problem chasing high frequency power supply ripple with your scope is using a very good ground. Usually chassis ground is not good enough. When you are looking at the ripple on your scope, move your probe over
and ground it to your scope alligator clip and see if ripple is present. I've chased ripple problems before and realized it was just a poor ground of my scope. You would then need to clip to a ground on the circuit board itself.
Of course, use secondary ground and not "hot" ground on the primary side of power supply. That would be catastrophic.
If the ripple is from poor filtering in the power supply, I would think you'd be seeing half wave ripple, not full sine wave - baffling. I still think it's a problem in the power supply filtering or grounding. All the waveforms off the
secondary diodes in the power supply are shown in the SAMS. Compare those. Also notice there are small value RF suppression caps across the electrolytics. Those suppress the RF noise generated by harmonics which would include 4MHz.
I will be out of touch from now until after this weekend - good luck.

Larry G
 
Well, I have great news. The monitor is working now! It turns out that the SAM's Photofact that I bought has some bad information in it. The output transistors are NEC C1507. SAMs has the replacement listed as a NTE198; which I bought and replaced all of the C1507's with NTE198.

When looking at the system, I couldn't find any place where the 4Mhz noise was coming from; but, as you stated, the noise was sinusoidal. Then, it dawned on me that the transistor could be creating the oscillations. I then started looking at the datasheet. The NTE198 has a minimum bandwidth of 10mhz, while the true C1507 has a minimum of 40Mhz. So, with this difference in mind, I took some of the older working C1507's and replaced the NTE198's and turned off the red channel with the NTE198's. Brilliant clear picture on those channels.

I've ordered some C1507's to replace the two that were bad. I'll report next week when they come in, but the difference is spectacular.

I'll report back next week; but, it looks like one more 5154 is back in the world.... ;-)

Chris V.
 
Back when I did electronics service, I remember Philips ECG substitutions being fairly accurate but problems with NTE subs. Yes, lack of bandwidth creates harmonic distortion with increase in fundamental frequencies which are sine waves.
That explains why that symptom was something I never remember encountering in a CRT display except back in the tube TV days when the video IF bandwidth would get mushed by a weak tube :)
Funny, that old memory was the first thing that popped in my mind when I saw your horrid screenshot.
Larry G
 
Slightly off topic, but I too recently repaired an IBM 5154. Apart from the usual bad caps (I recommend replacing ALL the caps in the video cage), it had a problem that caused the image brightness to flicker. It got worse the more I ran it, and was accompanied by fluctuations in the HV. After ruling out everything else, I came to the conclusion it was a bad HV multiplier. I was expecting that to be the end of the road since it's a custom part, but I found a cheap source for the exact part. Just wanted to mention it in case it helps someone else.

https://www.tedss.com/2023000565
 
Good job. There is a circuit called an ABL which stands for Automatic Brightness Limiter or Automatic Beam Limiter. It tries to set a max current level for the tube to prevent "blooming" which will cause the raster size to change. With weak HV, the ABL correction is unsteady causing the fluttering. As the tube warms up, it draws more current which lowers the HV even more making the regulation worse. Color TV Training 101 :p

Larry G
 
andy said:
I was expecting that to be the end of the road since it's a custom part, but I found a cheap source for the exact part. Just wanted to mention it in case it helps someone else.
Click to expand...
Thanks. I had been keeping my eye open for a spare.

I notice that the seller indicates part number BG2097-642-501. The unit in my 5154 is instead BG2097-642-505. Was your original unit a -501 or a -505 ?
 
modem7 said:
Thanks. I had been keeping my eye open for a spare.

I notice that the seller indicates part number BG2097-642-501. The unit in my 5154 is instead BG2097-642-505. Was your original unit a -501 or a -505 ?
Click to expand...

The original in mine was a BG2097-642-501, just like the replacement. I'm not sure what the difference could be.
 
retrogear said:
Good job. There is a circuit called an ABL which stands for Automatic Brightness Limiter or Automatic Beam Limiter. It tries to set a max current level for the tube to prevent "blooming" which will cause the raster size to change. With weak HV, the ABL correction is unsteady causing the fluttering. As the tube warms up, it draws more current which lowers the HV even more making the regulation worse. Color TV Training 101 :p

Larry G
Click to expand...

I think in this case the problem was that the HV multiplier provides feedback to the HV regulator. This type of direct feedback allows for very good HV regulation. I suspect part of the feedback resistor was breaking down and causing the HV to fluctuate. What ever was happening, it would get worse with the HV turned up, and better with it turned down. It was probably arcing internally.
 
Hello,

I know that this is an old thread but am wondering if any of you could help me fix my IBM 5154 monitor. It repeats the text display at multiple positions on the screen, so if it starts DOS and asks the user to input the date and time, the prompt will appear multiple times at different locations on the screen in different sizes. Some of the text is fuzzy and some is inverted (appears upside-down, not colours)

Any help would be greatly appreciated.

Thanks,
Max
 
Hello,

Yes the prompt is always on the left hand side and in different positions vertically. Sometimes it is orientated differently, like a reflection, and is different sizes.
As for the card I have no idea unfortunately. But the card is working fine as my other 5154 works with no problems on that computer.

Thanks,
 
My first guess is you've lost vertical sync on pin 9 of the video cable?
There is a vertical frequency control RT301 internally on the board but that shouldn't have changed?

PS - you could try flexing the cable around the plug end to see if it corrects



Larry G
 
Last edited:
IMG_0631.jpgHello,
Thanks for the suggestion. I will try that.
Is there anything I can do to fix this?
I would open it up but I cant find any screws on the unit at all?? At a guess they are probably located under the square pieces of plastic on the top of the unit but I don't want to force them off.


EDIT: unfortunately wiggling the cable had no effect.
I have a picture to upload
Thanks,
 
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Hmmm - as they say, a picture is worth a thousand words ...
Is the bright narrow band in the middle of your photo what you are seeing with your eyes? Are there usually two bright bands on the screen? Are they slowly moving up the screen?
Your picture looks more like an open main filter capacitor. Maybe like a large black one. Looking at SAMS I see two large black ones in the middle of the power supply board where the fuse is.
220mfd 250V. Good possibility.

PS - yes, I think the screws to remove back are under plastic covers. Pry them out. It's been a long time since I serviced these ...

PSS - those big caps can hold a charge !! Be careful and discharge with a screwdriver. If there is no spark when you discharge, then it could be bad ...

Larry G
 
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Hello,

I had a look inside the machine and nothing immediately jumped out, like bulging capacitors (but I know that that doesn't mean much)
I don't feel like my understanding of CRT's is good enough to discharge it etc.
The bright bands are there in real life. Its really strange. This monitor has had a variety of issues. It use to exhibit fold-over, that went away and then this came along. This also went away for a while and then came back.
I own another 5154 which has also had issues. A capacitor in the power supply blew up and release a lot of very smelly smoke. The monitor still works fine though, it just smells a little. The one we are discussing has never shown anything like that. Does anyone service CRT's anymore? I'd imagine It would be an easy fix for someone with the know how.

Thanks,
 
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