Your unstable picture looks like the horizontal oscillator is out of sync. The nominal horizontal scan rate for the 5175 is 30.5 kHz, whereas VGA uses 31.5 kHz. You likely need to adjust the horizontal oscillator in the monitor to be able to capture the 31.5kHz signal using the "horizontal hold" control, which is possibly only accessible when you open the monitor. CRT monitors have a lot of dangerous voltages inside, so better have someone around when you work on it, or have some with TV/monitor service experience doing the adjustment.

mkarcher wrote on 2022-07-26, 17:16:

Your unstable picture looks like the horizontal oscillator is out of sync. The nominal horizontal scan rate for the 5175 is 30.5 kHz, whereas VGA uses 31.5 kHz. You likely need to adjust the horizontal oscillator in the monitor to be able to capture the 31.5kHz signal using the "horizontal hold" control, which is possibly only accessible when you open the monitor. CRT monitors have a lot of dangerous voltages inside, so better have someone around when you work on it, or have some with TV/monitor service experience doing the adjustment.

This

+1

Also,don't use a metal Philips screwdriver to adjust pots/trim caps. Use an adjustment set made of plastic.
Or, at least, a modified ice cream stick made from wood. 😉

It doesn't really matter, strictly speaking.
It just must not be made from metal.
The metal type will cause hand capacity or break the pot. Especially if it's made of ceramic or just old..
So please use something soft for adjustment first.

Good luck! 🙂🤞

mkarcher wrote on 2022-07-26, 17:16:

Your unstable picture looks like the horizontal oscillator is out of sync. The nominal horizontal scan rate for the 5175 is 30.5 kHz, whereas VGA uses 31.5 kHz. You likely need to adjust the horizontal oscillator in the monitor to be able to capture the 31.5kHz signal using the "horizontal hold" control, which is possibly only accessible when you open the monitor. CRT monitors have a lot of dangerous voltages inside, so better have someone around when you work on it, or have some with TV/monitor service experience doing the adjustment.

wouldnt it be easier to change clock on the PC side with something like powerstrip?

rasz_pl wrote on 2022-07-27, 02:34:

mkarcher wrote on 2022-07-26, 17:16:

Your unstable picture looks like the horizontal oscillator is out of sync. The nominal horizontal scan rate for the 5175 is 30.5 kHz, whereas VGA uses 31.5 kHz. You likely need to adjust the horizontal oscillator in the monitor to be able to capture the 31.5kHz signal using the "horizontal hold" control, which is possibly only accessible when you open the monitor. CRT monitors have a lot of dangerous voltages inside, so better have someone around when you work on it, or have some with TV/monitor service experience doing the adjustment.

wouldnt it be easier to change clock on the PC side with something like powerstrip?

Not if he wants a vga monitor for dos.

Measure twice, adjust once never have to go inside again once it’s set

MMaximus wrote on 2022-07-26, 14:12:

I have a Pentium machine connected to the HD15 input of the Extron with a standard VGA cable. I have the R,G,B,S output connectors of the Extron connected to a BNC to HD15 converter, that is itself connected to a VGA 15-pin to VGA 9-pin converter through a gender changer, that is connected to the 9-pin signal cable of the 5175 😅

I'm not sure there was ever a standard pinout for "9-pin VGA" - have you checked with a multimeter that the BNCs are actually making it to the correct pins on the monitor's input cable?

You can use a shielded/insulated metal shafted fat bladed or phillips screw driver to adjust the pots inside a CRT housing with no problems at all.

The pros that resold PGA screens as VGA would actually wire the missing pins inside the chassis somewhere and then use a vga end.

I had one back in the day from Computer Reset that was modded, got sold as a cheap old vga monitor back in the day. 🙁

mkarcher wrote on 2022-07-26, 17:16:

Your unstable picture looks like the horizontal oscillator is out of sync. The nominal horizontal scan rate for the 5175 is 30.5 kHz, whereas VGA uses 31.5 kHz. You likely need to adjust the horizontal oscillator in the monitor to be able to capture the 31.5kHz signal using the "horizontal hold" control, which is possibly only accessible when you open the monitor. CRT monitors have a lot of dangerous voltages inside, so better have someone around when you work on it, or have some with TV/monitor service experience doing the adjustment.

Thank you - somehow I assumed the scan rates would be equivalent! I'm certainly not a CRT maintenance expert but I've been adjusting the trim pots in some of my other CRTs mainly to correct white balance issues so I'm not afraid to open them up. I of course follow the safety guidance and try to stay away from the dangerous parts 😀

Jo22 wrote on 2022-07-26, 18:51:

+1 […]
Show full quote

+1

Also,don't use a metal Philips screwdriver to adjust pots/trim caps. Use an adjustment set made of plastic.
Or, at least, a modified ice cream stick made from wood. 😉

It doesn't really matter, strictly speaking.
It just must not be made from metal.
The metal type will cause hand capacity or break the pot. Especially if it's made of ceramic or just old..
So please use something soft for adjustment first.

Good luck! 🙂🤞

Thanks - I have bought a set of CRT adjustment tools for this purpose a few years ago, they're a bit brittle but they've been doing the job so far

maxtherabbit wrote on 2022-07-27, 02:44:

I'm not sure there was ever a standard pinout for "9-pin VGA" - have you checked with a multimeter that the BNCs are actually making it to the correct pins on the monitor's input cable?

Good suggestion - I haven't done this yet and will try to adjust the H-Hold pot first and will double check to pinout with a multimeter if it doesn't work.

I've just popped open the 5175 again, first there is some kind of metal mesh that needs to be moved out of the way

Then on the left side I can see these labels for the trim pots (from left to right and top to bottom):

- V. LIN
- V. FREQ
- V. CENT (larger pot)

and on the right side these labels:

- H. CENT (larger pot)
- H. LIN
- E/W AMPL.
- V. SIZE 2
- V. SIZE 1
- H. WIDTH (not visible on the picture as it's hidden below the red and black wires)

It's a bit more complicated than I thought it would be 😆 - I assume I should try to adjust the H.LIN pot (which has a weird shape BTW 🤔) but I might wait for your input before adjusting anything 😃

UPDATE:

I found some more pots:

On the right side of the monitor when facing the front

- 22KV ADJ. (not gonna mess with this one 😆)

On the left side of the monitor when facing the front

- H. PHASE
- H. FREQ

I believe H.LIN that I mentioned in my previous message is related to geometry so I should probably adjust H. FREQ instead... stay tuned 😀

That's correct - don't mess with H. Lin(earity) you will just warp the picture. H. Frequency is what you want.

The linearity coil is a ferrite core adjustable inductor which is typically set and glued in position. They are easy to damage

maxtherabbit wrote on 2022-07-28, 14:29:

That's correct - don't mess with H. Lin(earity) you will just warp the picture. H. Frequency is what you want.

I agree on that. I expected it to be called "Horizonzal hold", because that term is commonly used in TVs / monitor circuits, but "Horizontal Frequency" is definitely the adjustment I recommended you to tweak.

THANKS GUYS! 🍾 😹

It was indeed just a matter of fiddling with the H. FREQ pot to get a stable picture. You don't know how happy I was to finally see the DOS prompt on screen 🤣

I also tweaked H. PHASE, H. WIDTH and V. SIZE 2. (V. SIZE 1 doesn't seem to have any effect for some reason.) Somehow I can't find any pots to adjust the R,G,B intensity but the white balance is actually acceptable so I'm not too fussed. The picture quality is actually pretty decent for such an old monitor that predates VGA by a few years 👍

MMaximus wrote on 2022-07-29, 17:34:

I also tweaked H. PHASE, H. WIDTH and V. SIZE 2. (V. SIZE 1 doesn't seem to have any effect for some reason.) Somehow I can't find any pots to adjust the R,G,B intensity but the white balance is actually acceptable so I'm not too fussed. The picture quality is actually pretty decent for such an old monitor that predates VGA by a few years 👍

The reason V. SIZE 1 doesn't do anything is most likely that this is a dual-mode monitor (remember it has a "mode" input on the 9-pin connector), possibly to toggle between a 400-line mode and a 480-line mode. Each V-SIZE pot is for one of the two modes.