IBM PS/2 Model 25 CRT - No High Voltage

[Jimmy422]

Hi all! I have an IBM PS/2 Model 25 computer with a built in CRT that is not outputting any high voltage. The computer piece does work, so I know the power board that powers both the computer and the CRT is functioning at least somewhat properly.

When I looked inside the machine I noticed most of the capacitors were leaking, so I replaced all of them out of caution. This didn’t fix the issue but i didn’t really expect it to either.

Here’s what else I’ve checked so far:

All diodes and fusible resistors appear to be OK

All the larger transistors (ones in the packaging where they could screw to a heat sink) tested OK

I don’t see any cold/cracked solder joints on the flyback or any of the power connectors

The 5v, 12v, and 115v power rails coming from the power board are all outputting proper voltages.

There’s a spot on the neck board that is marked 150v, but it only appears to have 115v on it. According to the markings on the power board 150v is not one of its outputs, so my guess is that this voltage is being outputted/boosted by the flyback?

The horizontal output transistor tested OK, I replaced it just to be safe and this didn’t fix the issue.

The horizontal oscillator IC has all the correct voltages at its pins according to its datasheet. However I don’t have a scope to verify the signal it’s generating is OK.

The transformer that works with the horizontal oscillator seems to be OK, and the transistor that works with that transformer tested OK as well.

At this point I’m at a bit of a loss. My guess is that the oscillator chip is bad or the flyback is bad, and I assume that a bad flyback is a death sentence for this monitor, so I’m hoping that’s not the case!

Does anyone have any advice as to what else, if anything, I could check?

Here’s a link to some photos of the boards - if you need any additional photos let me know!

Thanks all!

[zeno]

On a TV that 150 V runs the video output/s. It comes from a FBT winding,
through a fuseable ( typ 1-2 ohm ), A rectifier, then a filter cap ( typ 5 or 10 mfd). On TV's the cap is super high fail rate BUT lowering the voltage
gives a brighter pix. If they tapped the FBT primary you are just reading the unboosted B+
A scope would narrow it down in seconds.

73 Zeno
LFOD !

[zeno]

Oh BTW
Check the CRT G-2. Should be 400 - 500 V range

[Jimmy422]

Silly question - can I measure G2 with a standard multimeter? I’m seeing some stuff on google that says a traditional DMM can’t handle it.

I do have a scope on the way though - what would you recommend checking with it?

[Electronic M]

Originally Posted by Jimmy422

Silly question - can I measure G2 with a standard multimeter? I’m seeing some stuff on google that says a traditional DMM can’t handle it.

I do have a scope on the way though - what would you recommend checking with it?

Depends on the DMM, the TV and how clever you are. Some DMMs are only good to 600V and some G2s can get close to 1000V...Which would exceed what a 600V meter can handle......
.....I encountered that issue once on a friend's TV with their DMM. I needed to measure the 900V boost and the meter on hand was only good to 600V. I had a high IQ moment and realized that if I measured the voltage between the STABLE 400V B+ rail and chassis, and then measured the voltage between the 900V boost and the 400V B+ (a 500V difference) then add the 2 measurements I would effectively get the value of the voltage between the 900V boost rail and chassis ground without exceeding the voltage range/rating of the DMM! It worked like a charm and happened to be a key measurement to the diagnosis and eventual repair of that set.

[Alex KL-1]

Originally Posted by Jimmy422

Hi all! I have an IBM PS/2 Model 25 computer with a built in CRT that is not outputting any high voltage. The computer piece does work, so I know the power board that powers both the computer and the CRT is functioning at least somewhat properly.

When I looked inside the machine I noticed most of the capacitors were leaking, so I replaced all of them out of caution. This didn’t fix the issue but i didn’t really expect it to either.

Here’s what else I’ve checked so far:

All diodes and fusible resistors appear to be OK

All the larger transistors (ones in the packaging where they could screw to a heat sink) tested OK

I don’t see any cold/cracked solder joints on the flyback or any of the power connectors

The 5v, 12v, and 115v power rails coming from the power board are all outputting proper voltages.

There’s a spot on the neck board that is marked 150v, but it only appears to have 115v on it. According to the markings on the power board 150v is not one of its outputs, so my guess is that this voltage is being outputted/boosted by the flyback?

The horizontal output transistor tested OK, I replaced it just to be safe and this didn’t fix the issue.

The horizontal oscillator IC has all the correct voltages at its pins according to its datasheet. However I don’t have a scope to verify the signal it’s generating is OK.

The transformer that works with the horizontal oscillator seems to be OK, and the transistor that works with that transformer tested OK as well.

At this point I’m at a bit of a loss. My guess is that the oscillator chip is bad or the flyback is bad, and I assume that a bad flyback is a death sentence for this monitor, so I’m hoping that’s not the case!

Does anyone have any advice as to what else, if anything, I could check?

Here’s a link to some photos of the boards - if you need any additional photos let me know!

Thanks all!

Your DMM have a frequency meter (some have, on AC scales)? With it, will be possible to read the frequency at oscillator pins or horizontal output pin from IC.

[Jimmy422]

Originally Posted by Electronic M

Depends on the DMM, the TV and how clever you are. Some DMMs are only good to 600V and some G2s can get close to 1000V...Which would exceed what a 600V meter can handle......
.....I encountered that issue once on a friend's TV with their DMM. I needed to measure the 900V boost and the meter on hand was only good to 600V. I had a high IQ moment and realized that if I measured the voltage between the STABLE 400V B+ rail and chassis, and then measured the voltage between the 900V boost and the 400V B+ (a 500V difference) then add the 2 measurements I would effectively get the value of the voltage between the 900V boost rail and chassis ground without exceeding the voltage range/rating of the DMM! It worked like a charm and happened to be a key measurement to the diagnosis and eventual repair of that set.

Good to know! I took my chances and it appears to be outputting a whopping 20v.

[Jimmy422]

I got a scope and did some scope readings from the oscillator IC output, the output of the associated transformer, and the base of the HOT.

I’m assuming that since the output of the oscillator chip is not 15.75khz that it’s not outputting the correct signal? And because the HOT is definitely nowhere near that, that explains the lack of high voltage?

[Electronic M]

I'm not familiar with most early PC monitors, but there were multiple VGA resolutions over the years. If the monitor supports resolutions other than 640x480 it may run at slightly different frequencies than 15.7khz.

[uxwbill]

Originally Posted by Electronic M

If the monitor supports resolutions other than 640x480 it may run at slightly different frequencies than 15.7khz.

All the PS/2 Model 25 display assemblies are VGA only, even in the case of the 25SX, which has SVGA graphics hardware and a cautionary note in the manual not to exceed 640x480 at 60 Hz vertical/31.5 kHz horizontal.

When working and with no signal input connected, all PS/2 displays should show a white raster (provided the brightness and contrast are set somewhat appropriately).

I'll defer to others who have more experience here, but you may want to use great caution poking around directly at the HOT, for the sake of your test equipment.

The built in color monitor is roughly similar to the Taiwanese-made version of the standalone IBM 8513 color monitor, and the monochrome variant to the IBM 8503.