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IBM 5154 EGA monitor fried... is it a goner ?

I would focus on the small electrolytic capacitors which tend to dry out more quickly than the larger ones. On my well used 5154, nearly all of the small caps in the power supply and video compartment were bad. Most of the larger caps were fine. The residue under C29 could just be flux, but it can't hurt to replace it. An ESR meter would be very helpful here.
 
Thank you for your input, Andy!

I do have an ESR meter. Did you also have a 'black screen' monitor or what was the fault in your unit?

Picture did not come back just by tweaking RT1 on the power supply. I still have good(ish at least) power values
from the power supply, so I think I will next open up the video box and the video board and see what is going on in there.
 
I opened the case for video board yesterday and decided to pull all of the 10uF caps to test them out.
5 of them showed 1.2 - 1.3ohm ESR value. One was 1.4ohm and the worst 1.5ohm.

I guess a good one should show just under one ohm ESR, right ?

I'm making shopping list to replace at least all of the small caps.
 

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VintageVic said:
I guess a good one should show just under one ohm ESR, right ?
Click to expand...
Refer to an 'ESR chart'. Search the internet for those. A 'bad' cap is expected to have an ESR that is very much larger than the chart-indicated ESR value, e.g. chart indicates 0.012 ohms and I measure 0.7 ohms.

There are many ESR charts on the Internet, and the figures in them vary from chart to chart. That subject is discussed at [here].
 
Thank you Modem7 for referring to this!

Based on a chart, it appears that the 10uF caps I pulled may actually have been healthy.
I have understood that it is better to pull the cap from pcb rather than trying to measure it
when it is still connected to the pcb. Is this correct ? It's not too big deal to pull them out though.

One thing I noticed on the variable resistors, they all have some oxidy or corrosion in them.
These variable resistors have three legs. How to determine, if a variable resistor is good?

I've tried those out. Between one pair of legs they have some 330...730 ohm resistance. Same
with another pair of legs and the final pair of legs seem to be open. Since they all behave the same,
I'm assuming they're fine.
 
On old equipment, ESR is often an ignis fatuus. Before going off willy-nilly and replacing every aluminum electrolytic in sight, one should examine the function of each. In the case of small signal (e.g. coupling) capacitors, the primary check should be capacitance and then a check for shorting. The ESR, unless it's huge, hardly matters in non-power applications.
Understanding the circuit and the function of each component is key to troubleshooting. After that has been determined, one needs to ascertain what the symptom would be if a particular component failed.
"Shotgunning" component replacement often results in more damage. If it ain't broke, don't fix it.

The SAMS Computerfact is great because half the work has already been done for you, with 'scope traces and voltage readings.

My paltry .02 at any rate.
 
VintageVic said:
Thank you for your input, Andy!

I do have an ESR meter. Did you also have a 'black screen' monitor or what was the fault in your unit?

Picture did not come back just by tweaking RT1 on the power supply. I still have good(ish at least) power values
from the power supply, so I think I will next open up the video box and the video board and see what is going on in there.
Click to expand...

My monitor had lots of problems, including previous repair attempts and a bad HV multiplier.

Bad capacitors probably didn't cause your sudden loss of video, but might be causing the unstable picture you had before. The ESRs for the ones you checked don't look bad enough to cause a problem.

For the no video problem, I would start by checking all of the voltages to the CRT. Make sure the CRT heaters are lit, and check the cathode and G1 voltages. Is there any HV?
 
If the CRT goes utterly blank, then you need to ask yourself what is needed for an image to form. You obviously need power for the CRT heater, which can be verified visually. The electrodes on the CRT need the proper voltages and there needs to be a video signal. If deflection is messed up, you'll still see something.

Divide and conquer. Think through the problem.
 
Thanks again for your input.
Quite right, unwise to blindly replace components.

I've studied the video board schematics and pcb for hours now.
Did that so I could take correct measurements as SAMS manuals let me check
voltages of IC's and Transistors.

I may be onto something now. I'm not seeing 7.4V source at the video board, it's
somewhere around 0,6 V. Unless I'm measuring it wrong (e.g to a wrong ground again),
this issue needs solving.

I pulled video board out of the main board and I'm not seeing 7.4V (B) source at the main board either.
I think at least components T120, C120 (22uF electrolytic cap), D120 and R114 (118k) are related to
that 7.4V (B) source. But also the HV tripler is involved, like in Andy's case.

Gosh, I hope that HV tripler or any transformer is not defective, as they might not be easy to find.
Also, I have limited tools - I've got no high voltage meter, yet at least.

Andy, if I may ask, how to check if heaters are lit? I'm not that familiar with crt monitors.
But, before someone gets worried, I am aware of both importance and safe way of discharging
that 25kV before working on the monitor.
 
Looking at the PSU schematic, the 7.4V (3) derives from the 56V line. So if you've still got 56V on 4, then check R114 (180K). If that checks out, then the suspicion is T120 (BC237B) has gone shorted. We can discount small signal diodes D121 and D501, whose purpose is to make sure that the 7.4V doesn't rise above the 10V feeding the 5V regulator (7805).

So, the procedure is to check to see that you have 5.0V from the sub-regulator board and that you also have 56V on terminal 4, but nothing on terminal 3 (7.4V)
 
Yes,
there is 4,9V at vcc pin on IC501, IC502, IC503 and IC505.
Diodes D121 and D120 checked out ok. Did not check out D510 yesterday, but I will do that today.

However, D510 is on the video board, so I should have 7.4V present at A1 (15 or 16 pins) regardless if D510 is good or bad.
Now, it is not there. I will also check T120 for shorts and R114. C120 was not shorted, but I have not yet measured its ESR value.
 
Chuck(G) said:
Looking at the PSU schematic, the 7.4V (3) derives from the 56V line. So if you've still got 56V on 4, then check R114 (180K). If that checks out, then the suspicion is T120 (BC237B) has gone shorted. We can discount small signal diodes D121 and D501, whose purpose is to make sure that the 7.4V doesn't rise above the 10V feeding the 5V regulator (7805).

So, the procedure is to check to see that you have 5.0V from the sub-regulator board and that you also have 56V on terminal 4, but nothing on terminal 3 (7.4V)
Click to expand...

I've been a few days out of this project, but I can now confirm that
- 56V exist at the main board (other side of R114)
- 0.04V exist at terminal 3, where should be 7.4V. (measured from the other side of R114)
- R114 is 178k - close enough so I count it OK
- Also checked C120 (22uF/35V), ESR value is 0.64ohm -> I also presume this good
- R120 99ohm (ok), R121 was also ok.

As suggested, T120 is a suspect, although not shorted. I've ordered a few transistors
and waiting for them to arrive, first thing to do is replace T120 and see, if we've got 7.4V back.
If not, I need to go forward in 7.4V rail to see what might be shorted to ground.
But one step at the time.
 
With the monitor part unplugged or the 7.3V rail isolated, what happens when you lift one leg (emitter or collector) of T120? The 1n4148 diodes should clamp it to about 10-11V.
 
Speak of the devil...

Spare parts I had ordered were waiting for me at my mailbox!
So, I attacked the T120. I pulled it away from the mainboard and replaced
it with BC547.

And what do you know, we have power again at the 7.4V rail!
Furthermore, we got image back :). I also notice improvement in the image,
it is no longer jumping up/down at the screen! There is definitely some vibration,
like too low refresh rate or something, but this is improvement to the image quality that
I had before!

I think next steps are; doublecheck the voltages that are coming from the power supply
and tweak accordingly. Then, I will consult SAMS documentation and try to adjust
image alignment to the center of the screen.

Edit:
Voltages are good.

Adjusting Vertical centering control RT302 does not appear to be having any effect.
A bad variable resistor or... short somewhere or a bad capacitor. Back to study the schematics
what RT302 is related with.
 

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Back to troubleshooting the EGA monitor.

I noticed that actually the horizontal and vertical centering DO work.
The problem(s) really are that horizontal width is too wide. And not only that, but
there is also horizontal foldover. The image attached tells all about it. I cleared the
screen (CLS command) and typed dir/w after it. The image is from the result.

I already tried to adjust horizontal width from RT401 (for EGA monitor) but it had no effect.


Sams documentation suggest to check voltages from T400 to tackle horizontal foldover issue.
I have 56.6V on all legs, Emitter, Collector and Base. This is not correct as Collector should
have 41.7V. When I power off the monitor, Base seems to be shorted on one leg and having
about 100ohm to the other leg.

IC401 should have 5.7V at pin 14, but it has 8.9V instead. But higher voltage at IC401 might
be caused by shorted T400, right ?

I should check waveforms also but unfortunately, I have no scope. What I'm going to do is
pull that T400 and see if it is still shorted when it is removed from the pcb. if so, it's probably
a safe bet to replace the T400 and see what happens next.


Edit:
I have pulled T400 and can now confirm that pins 2 and 3 (collector and emitter) are
indeed shorted. I have to order this part.
 

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It looks to me like both the horizontal and vertical are too big. One thing that can cause that is if the high voltage is too low. It's worth checking on the HV regulator circuit. Feedback for the HV regulation comes from the HV multiplier block. It was bad on mine, which caused the HV to fluctuate and drop as it warmed up. Amazingly, I was able to find a site that had the part in stock.
 
I tried to take pics of the yoke. Not exactly sure is it correctly in place,
but it looks like it is where it should be.

I've located replacement part for T400. Perhaps the HV tripler is also
gone, since there has already been many different parts broken. But, I will
take it one step at a time. I want to see 41.7V or very near that at the collector of T400.
 

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