VOGONS


First post, by VileR

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Finally managed to source a CGA RGB monitor locally. Not the 5153 I've been after, but beggars can't be choosers.

Meet the patient:

samtron1.jpg
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samtron2.jpg
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samtron3.jpg
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It doesn't give the best picture I could hope for - there's hardly a difference between red and brown (colors #4 and #6). And dark grey (#8) is just missing - it's completely black regardless of any brightness/contrast knob twiddling.

But the main problem is a deficiency in uh, stamina. When you power it up, the image looks alright at first:

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However, pretty quickly you start noticing that the RED component is fading away, and some of the brightness with it.
Here's what the above screen looks like after three minutes... it only goes south from there:

samtron5.jpg
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Obviously it hasn't aged well.
Now, in my neck of the woods, color CGA monitors are every bit as common as unicorns that shit weapons-grade plutonium bricks. I don't know why that is, but my best bet is to try and get this one fixed.

One diagnosis I got is that the caps must be bad, and presumably recapping the thing would solve the issue.

- If so, which caps should I be looking for? I assume something related to the driving of the emitters, but that's the limit of my guessing power.
- Any second opinion on that? Could it be something completely different?

I've never been on a first-name basis with the innards of a CRT, so getting my bearings here is a bit difficult.
It might be simpler if I had the service manual, but it's only "available" (for a price) on shady websites that all look like scams... so if anyone knows a legit source, that could help too. EDIT: found it.

Last edited by VileR on 2020-09-11, 13:07. Edited 1 time in total.

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Reply 1 of 18, by darry

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VileR wrote on 2020-09-09, 23:20:
Finally managed to source a CGA RGB monitor locally. Not the 5153 I've been after, but beggars can't be choosers. […]
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Finally managed to source a CGA RGB monitor locally. Not the 5153 I've been after, but beggars can't be choosers.

Meet the patient:

samtron1.jpg samtron2.jpg samtron3.jpg

It doesn't give the best picture I could hope for - there's hardly a difference between red and brown (colors #4 and #6). And dark grey (#8) is just missing - it's completely black regardless of any brightness/contrast knob twiddling.

But the main problem is a deficiency in uh, stamina. When you power it up, the image looks alright at first:

samtron4.jpg

However, pretty quickly you start noticing that the RED component is fading away, and some of the brightness with it.
Here's what the above screen looks like after three minutes... it only goes south from there:

samtron5.jpg

Obviously it hasn't aged well.
Now, in my neck of the woods, color CGA monitors are every bit as common as unicorns that shit weapons-grade plutonium bricks. I don't know why that is, but my best bet is to try and get this one fixed.

One diagnosis I got is that the caps must be bad, and presumably recapping the thing would solve the issue.

- If so, which caps should I be looking for? I assume something related to the driving of the emitters, but that's the limit of my guessing power.
- Any second opinion on that? Could it be something completely different?

I've never been on a first-name basis with the innards of a CRT, so getting my bearings here is a bit difficult.
It might be simpler if I had the service manual, but it's only "available" (for a price) on shady websites that all look like scams... so if anyone knows a legit source, that could help too.

If you don't feel up to repairing it yourself, you may want to try your luck finding a TV repair shop in your area that has been around for a while and where they still remember how to handle CRT monitors . I don't want to discourage you, but a CRT and associated high voltage electronics may be life-threatening to a newbie . Additionally, since these things are so rare in your area, learning on one may not be a gamble you would want to take .

Reply 2 of 18, by Jo22

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I think the same. But if you decide to repair it on your own nevertheless, make sure another person is ar0und, at least.
To pull the power cord, for first aid, to call an ambulance etc. Never do this when you're alone. High voltage is something that accompanied the whole radio tube history.
Oldtimers with experience in the radio scene often used to use batteries for the higher voltages (anode batteries, say 90V ?) rather than AC from the power outlet.
For the heating (6v, power hungry), they used a simple transfomer for the outlet, though. Or 6V lantern batteries / 4,5v batteries (LR12 etc).
Of course, there are/were also "harmless" low-voltage tubes around meant for 12v systems (6v heating), such as cars (car radio) or headphone amps.
But the CRT is usually a high-voltage device still. I still remember when I got zapped by that little VGA/EGA CRT in an old Siemens 386 portable.
However, I was lucky and rather young (say, 12 to 14 ?) so my heart wasn't exactly "weak" at this point. 😀

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In what to one race is no time at all, another race can rise and fall..." - The Minstrel

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Reply 3 of 18, by keropi

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nice monitor! glad you found one in the end 😀
well jm2c here...
It is not uncommon for older monitors to have a warm-up time: some start with low brightness and stabilize after a couple of mins some do the opposite and start with high brightness.
I would not not go replacing caps just yet - these tend to fix geometry issues not color ones.
What I would do is allow the stabilization time and re-adjust the R G B pots on the neckboard (these adjust the intensity of each color) and also locate and adjust the contrast/brightness knobs.
It might also be needed to adjust the SCREEN pot that is usually located on the LOPT (the thing that has that suction cup that end up to the tube) this adjust how bright the screen is in simple words.
So if after those adjustments you still can't get a descent picture it could mean that the screen has worked countless hours and can't really show anything better.
If you get a good image and you can do it yourself then also do some electrolytic recaping, it won't hurt but first mess with the knobs and pots and see what you get

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Reply 4 of 18, by Quadrachewski

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I'd start with your original plan: recapping. It's very easy and soldering these boards is a dream compared to modern multilayer mainboards that transfer the heat away. Just follow standard safety precautions. Take it apart, check all caps and write down their values, order the appropriate good brands from a reputable source (Panasonic, Rubycon, Nichicon, Hitachi, Sanyo, Nippon,Vishay, ELNA), desolder the old ones and solder in the new ones 1 at a time. Check polarity.
Put it back together and test it. If it's still bad, report back.

Reply 5 of 18, by VileR

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Thanks for the tips. Yeah, I don't really feel like getting zapped, although the process of safely discharging these things doesn't look *that* complicated. 😀
On the other hand, adjusting the pots inside and testing means I'd have to do that "live", so that may not apply...

I do have a TV repair shop right down the street that does CRTs, that guy sold me my 14" Brother TV which I currently use as a composite monitor (and also did a quick repair on it a couple weeks ago).
I'll consult with him as well, but I know that he tends not to order new/quality parts for the repairs he does - he simply has a big gallery space full of old salvaged parts, and he just recycles whatever he's got there. If I can figure out the parts that need replacement I could try to source them myself and bring them over.

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Reply 6 of 18, by keropi

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^ do the pots live, there is no danger if you only touch them don't worry

🎵Link to buy a PCMIDI mpu
🎧Orpheus soundcard project
💻WTB Amstrad PC7486SLC-33 system

Reply 7 of 18, by Benedikt

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If that guy down the street is running a CRT TV repair shop, he might have access to one of these babies: https://youtu.be/YikOY8WTnLU.
(Title: Rejuvenating a Macintosh CRT with equipment from 1969)

Reply 8 of 18, by VileR

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Oh, BTW:

keropi wrote on 2020-09-10, 06:28:

What I would do is allow the stabilization time and re-adjust the R G B pots on the neckboard (these adjust the intensity of each color) and also locate and adjust the contrast/brightness knobs.
It might also be needed to adjust the SCREEN pot that is usually located on the LOPT (the thing that has that suction cup that end up to the tube) this adjust how bright the screen is in simple words.

Even if those pots can be adjusted to correct the image after it's stabilized, I guess that means it will be way *too* bright for the first few minutes... no danger of overloading something there?

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Reply 9 of 18, by keropi

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VileR wrote on 2020-09-10, 19:08:

Even if those pots can be adjusted to correct the image after it's stabilized, I guess that means it will be way *too* bright for the first few minutes... no danger of overloading something there?

no do not worry about it , old monitors are like this
it's actually common practice to allow "stabilization" time before you do any adjustments
stabilization times might improve if you recap but that's not a sure thing in my experience - the most important thing is the tube state (meaning how worked out it is)
ofcourse there are faults that can drive colors off but they would be really visible like missing a color or having a dominant one

🎵Link to buy a PCMIDI mpu
🎧Orpheus soundcard project
💻WTB Amstrad PC7486SLC-33 system

Reply 10 of 18, by pentiumspeed

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That is not bad CRT since it does not have the characteristic raster scan lines. You have degraded solder joints (cracked) and old capacitors in the power circuits and in video paths all the way to the CRT cathodes.

I worked with CRT monitor and TV repair for a living while ago.

Cheers,

Great Northern aka Canada.

Reply 11 of 18, by VileR

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A little news - very little, since I haven't had much time to mess with this. I'm not thrilled about performing invasive procedures, due to the local unobtainium status of these monitors... and in any case, we're in the middle of another lockdown, so no there's no getting parts/tools for another week or two.

But it turns out that this monitor CAN produce some decent browns and dark greys if you put a gun to its head. 😉 I did some deliberately rough manhandling of the brightness knob, thinking that the contacts may just be dirty, and now I can actually get the following colors (for a frame or two until the contacts start sucking again):

samtron6.jpg
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A bit of deoxit or such might take care of the contacts. But the fading red issue will probably need some deeper treatment.

For the heck of it, I'm thinking of putting together something like this: https://www.diericx.net/post/drive-cga-screen-with-arduino/. Needs a couple of slight modifications, but this could make for a very cheap 16-color "test pattern generator" for CGA, so that someone like my local TV guy could actually have a look at what's going on.

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Reply 12 of 18, by pentiumspeed

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Monitors will *need* capacitors checked with ESR meter and replaced any that is exhausted (tired). Electronic repair shop who remembers fixing CRT monitors apply.

Cheers,

Great Northern aka Canada.

Reply 13 of 18, by digger

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I know others have said it here already as well, but please, please, please, be **careful** when you open up those things, due to high voltage. 🙏

CRT monitors may be rare, but they're not worth risking your life over.

Special care must be taken, including discharging them in a safe and proper manner, etc. If you're not 100% sure what you're doing, wait for someone more qualified to help you, lockdown or not. Also, use the right tools for the job.

Reply 14 of 18, by rmay635703

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Albeit off topic color monitors made before 1988 were uncommon in 1988, most seemed to be B&W or RCA composite (or both)

Most CGA compatible screens I’ve encountered weren’t dedicated CGA and instead a Magnavox or similar with selectable ttl/analog was easier to locate.

Reply 15 of 18, by Jo22

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pentiumspeed wrote on 2020-10-04, 21:40:

Monitors will *need* capacitors checked with ESR meter and replaced any that is exhausted (tired). Electronic repair shop who remembers fixing CRT monitors apply.

Cheers,

I'm using an L/C meter here to check capacity.
It's not ideal, but it works for a quick check.
Testing its RF usability also might need an oscillator and a scope, I guess.

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In what to one race is no time at all, another race can rise and fall..." - The Minstrel

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Reply 16 of 18, by mkarcher

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pentiumspeed wrote on 2020-10-04, 21:40:

Monitors will *need* capacitors checked with ESR meter and replaced any that is exhausted (tired). Electronic repair shop who remembers fixing CRT monitors apply.

This is good advice in general, but ESR of old capacitors usually improves with warmup, so the fading red is most likely not ESR related. On the other hand, leakage current on semiconductors (and possibly capacitors too) gets worse when they warm up. I thus suspect the problem with red going away is leakage current related, not ESR related.

Reply 17 of 18, by Hanamichi

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Disclaimer: everything is done at your own risk, I am not liable 😜

I think caps are unlikely to be the issue in this case but would benefit geometry and sync issues later on as keropi said.

I suspect the real issue like on arcarde / Japanese multisync monitors is that these are older shadow mask tubes with innefficient phosphors (but still a great image) leading to a lot of heat building up in the chassis.

Over time that causes:
Dry solder joints. (my bet)
Transistors to go bad.

Some good info here:
https://www.youtube.com/watch?v=XhrhLcCsyho
http://www.electronicrepairguide.com/monitor- … or-problem.html
Quoting this blog:
"In fact this problem contributed about 80% of Monitor color problems. Applying fresh solder would usually solve the color problem."

I bought electricians gloves (not made in China ones) for working on CRTs, might give some peace of mind but like Jo22 said don't be alone in the house when you work on CRTs (if possible), as you never know.
That retro tech youtuber has some good general info too. If you need to work on the PCB, it might be recommended to learn how to discharge components safely.
One issue is the chassis on these monitors is more cramped and harder to work with then broadcast and arcade monitors that they show on youtube quite often.

Reply 18 of 18, by VileR

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Appreciate the tips. As mentioned, for now I'm not planning to work on it myself, that's why I mentioned the Arduino gadget in order to provide the TV repair dude with some way to test the colors. 😉

BTW, forgot to mention -- this monitor has a seemingly-pointless feature: if you push that little button with the RGB stripes, it imitates a monochrome monitor by turning the output green.

I've seen this in other RGB monitors before, and some of them add greyscale and amber "modes", too. But why is it "seemingly pointless", you ask? - as far as I could tell, the monochromatic green is achieved by simply *cutting out* red and blue completely, rather than scaling the brightness properly by doing luma summing. So whenever you're actually dealing with color text or graphics, it's next to useless. You basically get the worst of both worlds: color is missing, but unlike a real MDA-frequency monitor, you get to keep the low resolution and the shadow-mask with its huge 0.52mm dot pitch. Yay!

However, I now think that this is also related to the bad contacts - if I wiggle the brightness knob forcefully enough, I get fleeting glimpses of more shades of green. So a proper appraisal of this 'feature' will have to wait til later. 😄

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