wbahnassi wrote on 2025-08-29, 13:21:

I was told it only turns on randomly. So I took it and tried it, but I couldn't get it to turn on at all. You flick the power switch and the power LED remains unlit.. so I believe it's a basic power issue.

[...]

How do you suppose I start diagnosing this?

Test the most simple thing first: Probe the power switch for continuity. That's what failed in my Schneider MM-12. I already started to disassemble the transformer to check for broken windings before I noticed I messed up the initial measurements and the power switch is the actual culprit...

Probed the switch and it works fine. Mains goes into a small board hovering over what I assume is the transformer. Mains black wire goes through the fuse (which is fine and continuous) and into the left most metal post of the 4-post array at the far end of the small board. Mains white line goes directly into the metal post right next to the left-most one.

Now, a 4-pin connector sits on these 4 metal posts. It has brown and pink wires. The connector routes through the monitor's power switch.. so when the switch is on, each color pair get connected.

So the right-most two metal posts are the ones that take mains after being controlled by the power switch. Interestingly, if I measure resistance between the two right-most pins I get 17.2 Ohms. Does this sound right? The the left metal posts are OL..

Looks very similar to my green monochrome monitor with the Japanese Orion tube. What you are describing sounds like a contact issue on the AC input side, I'd check voltages to see where the issue is. Maybe a broken mains cable.

wbahnassi wrote on 2025-09-01, 18:17:

So the right-most two metal posts are the ones that take mains after being controlled by the power switch. Interestingly, if I measure resistance between the two right-most pins I get 17.2 Ohms. Does this sound right? The the left metal posts are OL..

17.2 Ohms sounds quite low. If this were a color monitor, I would assume the low resistance is due to the degaussing circuit. As your monitor is a monochrome monitor, there is no degaussing circuit and the only thing connected to the right-most pins is supposed to be the main transformer. A resistance of just 17 ohms is quite low for the primary winding of a transformer handling around 30 watts, at least that's my intuition coming a 230V country. It might be about fine for a 120V device. I suggest you perform a end-to-end measurement of the mains power side: Measure resistance between the two prongs of the power cable. If the fuse, the switch, the connectors and the transformer are working fine, you should get "OL" with the power switch turned off and 17.5 ohms (or something like that, slightly more than 17.2 Ohms) with the power switch turned on. If this is the case, the mains circuit is fully working, and the actual fault is on the secondary side.

Right. I get 18.1 Ohms on the prongs when the switch is on, and OL when it is off. So perhaps this is fine after all?
I guess the next step would involve probing voltages on the main board? I'll be frank I'm not very excited about this step at all 😰

wbahnassi wrote on 2025-09-03, 19:19:

Right. I get 18.1 Ohms on the prongs when the switch is on, and OL when it is off. So perhaps this is fine after all?

Mains side looks perfectly fine. So something is wrong on the secondary side. The primary operating voltage of 8"-14" monochrome CRTs often is around 10V to 18V, obtained from rectifying the output of the main transformer and regulating it with some transistor pass circuit. There might be a fuse as well on the secondary side. As you don't get the power LED lit, the issue is likely in the generation of the primary DC voltage - well, unless the power LED is coupled to some circuit powered from the flyback transformer. If the power LED is powered via the flyback transformer, it might not light without a proper video signal arriving at the monitor.

The IBM 5151 monochrome monitor does not have an internal oscillator, but drives the flyback transformer directly from the HSYNC signal, so everything powered via the flyback transformer is dead if there is no periodic HSYNC. Maybe your monitor works the same way.

wbahnassi wrote on 2025-08-29, 13:21:

Hi, I got one of those monochtome TTL 12" monitors from 1989. It's small and lightweight compared to a regular color monitor. I was told it only turns on randomly.

If it's working fine one day but refusing to work the next then it's 99% a cold solder joint, broken traces, cracked board, bad switches, bad cables, corroded pots or leads, sometimes dried caps but I've yet to find one in a mono. These are like 40 years old though so anything's possible.
I'd say you should discharge the tube and filter caps first, pull the boards out completely (make photos of all the connections first!), check visually for obvious fails, check and resolder all solder points, apply contact cleaner to pots, connectors and switches, check continuity on ALL cables then put it all back together (compare to photos you took earlier). Test.

I disassembled the board out and took a few photos.
A couple of things:

* I measured 22.8V AC coming out of the transformer when I flick the power switch on. I think this is good?

* I ran the power LED connector via the multimeter diode test and the LED didn't light up, but it measured a 1.2V voltage drop. Is it possible that the LED requires more voltage? Typically the multimeter lights up the small LEDs I have.

I took a close look at the components on the board, I don't think I saw any cold solder joints. They're all shiny and smooth without any "rings" that typically appear around cold solder joints. Capacitors look very healthy as well.
There are a couple voltage regulators. I measured continuity on their legs and they have no continuity between any pair of their three legs.

What did the previous owner mean by "turns on randomly"? Turns on like it was generating an image or just the led lighting up sometimes?
Hard to judge but I don't like the look of those Chinotaiwanosium solder points with barely any solder used.

What does that 3-pin yellow/pink/white connector plug into?

wbahnassi wrote on 2025-09-07, 14:51:

* I measured 22.8V AC coming out of the transformer when I flick the power switch on. I think this is good?

That sounds sensible, although I expected a slightly lower voltage like 18V AC. Nevertheless, there are monitors running at 11 VDC main voltage, and others running at 18 VDC, and for 16-18 VDC after regulation, 22 VAC seems like a good start

wbahnassi wrote on 2025-09-07, 14:51:

* I ran the power LED connector via the multimeter diode test and the LED didn't light up, but it measured a 1.2V voltage drop. Is it possible that the LED requires more voltage? Typically the multimeter lights up the small LEDs I have.

1.2V is not enough to light up an LED. You require about 1.8V for red LEDs, 2V for yellow LEDs and 2.2V for green LEDs. Your meter is indicating that its test current (typically a couple of milliamperes) flows through something that is connected to the test probes. The power LED is not supposed to conduct a significant amount of current at 1.2V, expect something way below 10 microamperes, if any current at all. But if you measure the LED in circuit, the current may flow a different way through components connected to the power LED. Yet 1.2V is a very low voltage for the circuit I am expecting. I would expect the power LED to be connected via a dropper resistor to the main DC voltage (it actually is, I checked the circuit board photo. R801 is the dropper resistor). Let's assume it's 16V, and the LED current is 20mA. 2V gets dropped at the LED, so the dropper resistor is supposed to drop 14V. dropping 14V at 20mA requires a 700 Ohm resistor, so if these assumptions hold, the dropper resistor is likely 680 ohms or 810 ohms (two stanard resistor values).

Now assume the meter does its diode test at 1.5mA, and furthermore assume that it's not a the power LED itself thats broken, and concting the current (both assumptions seem well-founded in my oppinion), this means the 1.5mA test current first has to pass the dropper resistor before it can reach other components. 1.5mA over a dropper resistor of 680 ohms would be 1.02V, and at 810 ohms 1.23V. As you see, this comes quite close to what you measure when testing the power LED. While it is possible that I tweaked some numbers in this discussion to hit the 1.2V value in the end, I didn't need much tweaking to get there. So in essence, this means: The dropper resistor seems to be the component dropping most of the 1.2V displayed by your meter, and the remaining circuit does not drop a significant voltage at all. Or, in to be more to the point: If your measurement was performed in-circuit, 1.2V drop at the power LED likely indicates a dead short on the primary DC circuit in the monitor. My main suspects would be the primary DC smoothing capacitor and the horizontal output transistor (aka flyback transistor).

On the other hand, after checking the circuit board, I also noticed that the power LED is not mounted to the circuit board, and you likely tested the power LED out-of-circuit, at the the connector that plugs into CN805. If that's the case, either the power LED is broken, or the power LED has a parallel resistor. A broken LED would not explain why the monitor fails, though.

wbahnassi wrote on 2025-09-07, 14:51:

There are a couple voltage regulators. I measured continuity on their legs and they have no continuity between any pair of their three legs.

Well, this might contradict my conclusion that likely the main DC voltage is shorted. If there are indeed voltage regulators, it would be likely that they either have the main DC voltage as input or output. If the regulators are adjustable (LM317-type) regulators, they don't have a direct ground connection, which might save my theory, as the short would be between ground and the main DC voltage supply.

jmarsh wrote on 2025-09-07, 15:29:

What does that 3-pin yellow/pink/white connector plug into?

If that's not the primary DC voltage regulator, that's mounted to some metal plate as heatsink, I would be extremely surprised.

The circuit seems to be AC comes in at CN804, gets rectified by D801-D804, which are paralleled by capacitors C804-C807 to eliminate switching spikes, the raw DC voltage is stabilized by C809 and C808 (interesting ground pattern on that PCB, though...), regulated down by IC801 which is connected to that 3-pin wire jmarsh asked about, and the regulated DC is finally stabilized by C810. I suggest to check both C808 and C810 for short circuit. If either of these capacitors were shorted (or something connected to them), this would explain a complete failure of the monitor.

When on, the heater should be powered by the low voltage DC. Meaning, you should be able to see the heater come on inside of the tube. That would tell you that the DC side is at least generating power even if your LED is bad.

You also have ICs on the board. Datasheets are likely available. All of them are going to have power inputs that you could measure to ensure your DC side is working.

My suspicion is that you are not generating the 12-15V DC voltage which would be used to power the LED as well.

Get yourself familiar with CRT circuits so that you know how to avoid making contact with mains and high voltages. Once you feel confident, there are so many places on this board that you can check voltages which you absolutely need to do while it is live. You just aren't going to be able to troubleshoot this powered off.

Adrian Black (Adrian's digital basement) has many GREAT youtube videos on troubleshooting and repairing CRTs. Much easier to refer you to those than try to direct you in a forum. He covers a lot of the safety too which works far better visually than verbally.

mkarcher wrote on 2025-09-07, 17:31:

If that's not the primary DC voltage regulator, that's mounted to some metal plate as heatsink, I would be extremely surprised.

That's what I assumed, the power LED is basically connected across the output so if it's not lit the regulator can't be doing its job. The plug also looks a bit scorched, but that could be normal wear.

I also thought the ground plane looked a bit strange; would it be right to say this board won't operate correctly unless it's screwed to its chassis?

Alright, first thanks a lot for the explanation. And thanks for mentioning Adrian's channel. Before I opened the monitor, I took a look at a few CRT repairs by the 8-bit guy since he opens a few and shows the precautions he follows to discharge the dangerous parts. Ironically, Dave actually refers to Adrian for help on a couple of cases. But yeah, I don't intend to bore you guys with a step-by-step investigation. I just wanted to ask for some typical routines you'd recommend for common power issues on old CRTs. You've been already generous enough with all the details and the time spent looking at the board photos.

I'll answer a few questions that were asked:

The three-pin connector indeed hooks up to the three legs of a voltage regulator that's screwed in the middle of a very big heatsink that is just next to the transformer block (photo attached).

The power LED indeed has a wire coming out of it that connects to the board. When I tried to power the LED, I was doing it on that wire while it's disconnected from the board.. I just hooked up the LED wire to a 3.3v power source, and the LED came to life brightly.. so now we know there is a problem with power getting to CN805 and of course other parts as well.

The previous owner said "it has a mind of its own" and "it powers on when it wants to"... When I tested it, I couldn't get it to power on at all, and I did tap it on the back, switched it on/off several times, waited on it for several minutes.. no luck.

I'll check to see if there are any shorts on caps. I should have done that before.

I found two caps that are shorted. The rest are fine.
The two shorted ones are C810 (electrolytic, 25V 470uf), and C206 (ceramic, it reads B 272K).. The two aren't connected on the board, so I believe I'll have to replace both.

BTW both look good and don't show any physical evidence of failure. C810 has a brown blob under it that I believe is some sort of glue, not fluid from the cap.

The electrolytic one should be easy to replace, but I'm not sure what to order for the ceramic one. It's not polarized I assume, and looking online it seems to be 2700pf with 10% tolerance and 50V rating... Does that look right?

Before you replace C206 have a good look at the traces it is soldered to on the bottom...

jmarsh wrote on 2025-09-07, 20:05:

I also thought the ground plane looked a bit strange; would it be right to say this board won't operate correctly unless it's screwed to its chassis?

I didn't look past the supply section (components 8xx), so I didn't notice the split ground plane, but you seem to be correct that the chassis is used as first layer in a star grounding scheme. As the ground planes for the different sections get next to each other, this seems like a deliberate design, and not a consequence of routing difficulties.

So, if you try to do tests on the monitor while it is opened, expect it not to work unless the PCB is screwed to the chassis with all of its screws!

jmarsh wrote on Yesterday, 03:30:

Before you replace C206 have a good look at the traces it is soldered to on the bottom...

Good advice. WTF? I can't think of any good reason for that design. If they need HF coupling between the connections of C206 at that point, why don't they couple DC as well and use a "component" labelled Jnumber instead of a capacitor

wbahnassi wrote on Yesterday, 00:08:

I found two caps that are shorted. The rest are fine.
The two shorted ones are C810 (electrolytic, 25V 470uf), ...

BTW both look good and don't show any physical evidence of failure. C810 has a brown blob under it that I believe is some sort of glue, not fluid from the cap.?

If C810 is short (verify after desoldering!), the voltage regulator on the heatsink might be damaged as well. The monitor PCB will work up to C810 even if not installed in the chassis, but it will not generate high voltage in this case - if jmarsh and I didn't miss something. You thus should be safe to measure the voltage at C810 after replacing the cap and compare it to the nominal voltage of the regulator.

Adrian says that the glue to mount the capacitors in some old monitors is hygroscopic and gets conductive over time. I wouldn't expect a dead short to develop, but you should keep in mind that the culprit might be the glue instead of the capacitor.

Thanks for the help mkarcher and jmarsh!
I replaced the electrolytic cap only, as the ceramic one was shorted "by design" as you guys noticed.
While I was desoldering the cap I did notice a "fishy" smell coming out with it, which confirms it being bad.
I replaced it and put back everything together, and what do you know... it worked perfectly 👍

Turned out it's an amber phosphor CRT, which is really nice. Here are a couple of photos of it connected to a Hercules MDA card...

It's a fast refresh monitor.. the phosphors don't "ghost" and playing Xenon 2 on it was quite nice and responsive without any ghosting.

Thanks again for the help guys!

Oooo, pretty. I would love to have that orange glow in my home too ~