Not familiar with this particular monitor but I have a few hints you might find useful:
- Preferably run all the tests you do now with minimal brightness level you can work with. Collapsing deflection can quickly put a lot of gun energy into one spot and make a visible burn-in, so lowering the gun energy will help prevent that.
- Clean/spray the pots. Dirty pots can easily cause such issues.
- Some electrolytic caps in the monitor are more imporant than others. Obviously anything in the PSU section, but also the flyback transformer secondary voltages. There might be a boost voltage capacitor, that one is usually the first to go bad, and anything connected more-or-less directly to yoke coils. The rest going bad would usually affect linearity of the scan, or make the picture too small, but should not cause the issue you have.
- HOT rarely goes bad in a quirky way. It's not impossible but typically it goes short, and possibly then open when it blows. I'd consider replacing it only if all other things where checked and tested good, or replaced, and the problem persisted.
- Resolder the flyback transformer. Even if the solder points look good. If you have issues it's a simple job and might save you a lot of time so don't even bother looking for cracks. Unless the cause is obviously something else resoldering the flyback when horizontal sweep has issues is the #1 job.

I did not yet get around to hunt for the service manual and take a peak at schematics, but you should know that some monitors at that time used to cut off the horizontal deflection circuit as emergency shutdown in case something goes "terribly bad", is specially if the circuit senses something that can cause X-ray emissions above the permitted level. Both excessive anode voltage and excessive beam current can trigger the X-ray protection circuit, depending on monitor design.

If that monitor has an emergency shutdown circuit, for example for X-ray protection, and it does cut out the base drive to the HOT, you should check electrolytic caps in that circuit. If they go bad (high ESR/low capacity), it may cause false triggering of that circuit.

As both the anode voltage and the picture width are close to the flyback transformer, there may be interaction between the picture width and the anode voltage, triggering an HV-overvoltage circuit if the picture gets wider. Wider pictures need higher voltages at the deflection coil, which might couple to higher voltages at the anode.

Thanks for the suggestions so far , ill look into them , if it helps here is the service manual if anyone wants to give them a look.

https://ia903403.us.archive.org/28/items/nec- … 0JC-1401P3A.pdf

mkarcher wrote on 2024-12-17, 16:53:

I did not yet get around to hunt for the service manual and take a peak at schematics, but you should know that some monitors at that time used to cut off the horizontal deflection circuit as emergency shutdown in case something goes "terribly bad", is specially if the circuit senses something that can cause X-ray emissions above the permitted level. Both excessive anode voltage and excessive beam current can trigger the X-ray protection circuit, depending on monitor design.

If that monitor has an emergency shutdown circuit, for example for X-ray protection, and it does cut out the base drive to the HOT, you should check electrolytic caps in that circuit. If they go bad (high ESR/low capacity), it may cause false triggering of that circuit.

As both the anode voltage and the picture width are close to the flyback transformer, there may be interaction between the picture width and the anode voltage, triggering an HV-overvoltage circuit if the picture gets wider. Wider pictures need higher voltages at the deflection coil, which might couple to higher voltages at the anode.

Hey So a bit of an update , I did a full recap but that didn't solve the issue , it just made the issue more pronounced and so it does the shutoff every time now , I'm guessing it since it might be delivering a better current or signal to whatever component is faulty. I measured the B+ voltage and found that if I adjusted the horizontal the voltage would drop from 96.7 @ 31khz when its supposed to be 93 in the manual , down to 80ish then would start rising again to the 90s , 100s , 110s , then when it hits 122 the shutdown would happen to be the B+ Limit voltage.

At 25khz it would start at 74 volts when it should be 64 and have a similar issue with the spike to 122 then shut off.

Only at 15khz would the voltage stay at 51.8 , and the recommended voltage is 51.
15 khz has no issues with the shut off , but the screen does get slightly brighter when you move it to center with the horizontal adjustment.

Once in a blue moon 31khz would let me adjust it without the voltage rising but after restarting the crt it had the same issue.

There's also a list of potential faulty components in the manual.

AppleSauce wrote on 2025-04-27, 20:15:

I measured the B+ voltage and found that if I adjusted the horizontal the voltage would drop from 96.7 @ 31khz when its supposed to be 93 in the manual , down to 80ish then would start rising again to the 90s , 100s , 110s , then when it hits 122 the shutdown would happen to be the B+ Limit voltage.

At 25khz it would start at 74 volts when it should be 64 and have a similar issue with the spike to 122 then shut off.

Only at 15khz would the voltage stay at 51.8 , and the recommended voltage is 51.

So you clearly have an issue regarding B+ voltage regulation, and a working safety circuit that kicks in once the monitor notices B+ got severely out of hand. The rising B+ voltage, if measured correctly (i.e. with the correct ground reference) should also affect the horizontal geometry, most likely the picture getting wider.

The schematics show that this monitor actually has two switch-mode power supplies: One that outputs constant voltages (85V, 24V), called K1/K2, and a second supply that outputs a voltage dependent on the horizontal frequency (called C1), controlled by feedback on a contact called C2. The theory of operation seems to be like this: The higher the horizontal frequency, the less magnetic charge is deposited per scanline into the core of the fly-back transformer at a given B+ voltage. The consequence is that during flyback, at a given B+ voltage there is less energy available the higher the horizontal frequency, so B+ needs to go up to have enough power per scanline in the FBT. The regulation occurs using the waveform at pin 6 of the FBT (one of the secondary windings). According to the scope picture displayed next to the FBT, you should get pulses of around 29V there, quite independent of the horizontal frequency. The waveform of that pin is rectified via D2001 and buffered in C2001. The expected voltage at that point is given as "Nr. 12 for TP2002A", and is 25.x volts.

For troubleshooting, you should monitor what happens at TP2002A in case of the B+ run-up. If TP2002A stays at around 25.x volts, or even sinks below that, something in the monitor draws excess power, which is being compensated until B+ hits the safety limit of 122V. On the other hand, if TP2002A rises when B+ runs away, the feedback path from TP2002A into the power supply doesn't work as intended.

mkarcher wrote on 2025-04-27, 22:06:

So you clearly have an issue regarding B+ voltage regulation, and a working safety circuit that kicks in once the monitor notice […]
Show full quote

AppleSauce wrote on 2025-04-27, 20:15:

I measured the B+ voltage and found that if I adjusted the horizontal the voltage would drop from 96.7 @ 31khz when its supposed to be 93 in the manual , down to 80ish then would start rising again to the 90s , 100s , 110s , then when it hits 122 the shutdown would happen to be the B+ Limit voltage.

At 25khz it would start at 74 volts when it should be 64 and have a similar issue with the spike to 122 then shut off.

Only at 15khz would the voltage stay at 51.8 , and the recommended voltage is 51.

So you clearly have an issue regarding B+ voltage regulation, and a working safety circuit that kicks in once the monitor notices B+ got severely out of hand. The rising B+ voltage, if measured correctly (i.e. with the correct ground reference) should also affect the horizontal geometry, most likely the picture getting wider.

The schematics show that this monitor actually has two switch-mode power supplies: One that outputs constant voltages (85V, 24V), called K1/K2, and a second supply that outputs a voltage dependent on the horizontal frequency (called C1), controlled by feedback on a contact called C2. The theory of operation seems to be like this: The higher the horizontal frequency, the less magnetic charge is deposited per scanline into the core of the fly-back transformer at a given B+ voltage. The consequence is that during flyback, at a given B+ voltage there is less energy available the higher the horizontal frequency, so B+ needs to go up to have enough power per scanline in the FBT. The regulation occurs using the waveform at pin 6 of the FBT (one of the secondary windings). According to the scope picture displayed next to the FBT, you should get pulses of around 29V there, quite independent of the horizontal frequency. The waveform of that pin is rectified via D2001 and buffered in C2001. The expected voltage at that point is given as "Nr. 12 for TP2002A", and is 25.x volts.

For troubleshooting, you should monitor what happens at TP2002A in case of the B+ run-up. If TP2002A stays at around 25.x volts, or even sinks below that, something in the monitor draws excess power, which is being compensated until B+ hits the safety limit of 122V. On the other hand, if TP2002A rises when B+ runs away, the feedback path from TP2002A into the power supply doesn't work as intended.

I just checked the voltage and it more or less maintains itself at 25ish , maybe dropping to 24.8 when adjusting the pot. When it goes into shutdown it rapidly drops down to 3 volts then down to 0.

So what could be drawing excess power?

Also it might have changed amplitude and frequency when adjusting and measuring

Also i found the B r rail goes to this big voltage switching chopper.

Suspiciously it has no heat sink.
Maybe it doesnt need one though?

AppleSauce wrote on 2025-04-28, 13:14:

Also it might have changed amplitude and frequency when adjusting and measuring

The attachment amp.gif is no longer available

The oscilloscope shows a fixed frequency on the graph, but the display in the lower left corner changes between 31.5 and 15.7. Am I correct in assuming that you were inputting VGA all along, and the 15.7 is a mis-reading on the scope, because some pulses are not high enough, so every second pulse is ignored?

AppleSauce wrote on 2025-04-28, 13:14:

Also it might have changed amplitude and frequency when adjusting and measuring

Are you actually measuring 3V peak-to-peak waveforms at pin 2 of IC554? I can make absolutely no sense of that. That pin is connected to

• ground via R583, which can not introduce peaks.
• stabilized +16V via R581, which can not introduce peaks.
• Op-Amp output (IC554 pin 1) via R582. This could introduce peaks. The Op-Amp tries to keep pins 2 and 3 at the same level, so 3V peaks at pin 3 might cause similar peaks at pin 2 (by generating way higher peaks on the output.

But pin 3 of IC 554 is connected to pin 2 of IC552 and TP552. This output pin is intended to follow input pin 3 of that IC, which has a 2.2µF tantalum cap on the same line. You can't generate that sharp peaks without driving the capacitor with very high currents, so if there are peaks on TP552, you likely have an issue with C552.

AppleSauce wrote on 2025-04-28, 07:37:

I just checked the voltage and it more or less maintains itself at 25ish , maybe dropping to 24.8 when adjusting the pot. When it goes into shutdown it rapidly drops down to 3 volts then down to 0.

This shows that the B+ power supply is working as intended, keeping those 25ish volts. So the question is: Why does the supply need to increase B+ that much to keep this level. It is unlikely that the problem is on the low voltage output side of the flyback transformer (pins 4/5/6/3/2 if I read those numbers correctly). If TP2002A is kept at 25V independent of the horizontal frequency, all the other voltages in that region also don't depend on the scan frequency. As you have no issues at 15kHz, it's very unlikely for a frequency-dependent issue to be rooted there.

So why do we need increasing B+ to keep the flyback transformer output stable? There might be multiple reasons. Either excessive load on the secondary side of the flyback transformer (my first guess), or you have an issue with the horizontal drive. To get the desired voltages to the secondary of the flyback, you do not only need sufficient B+, but also that voltage needs to be applied to the transformer. If you can measure the base of TR503 without risking to touch the collector, verifying that signal as displayed in the schematic can make sense. To be more safe, you might grab that signal at R521 instead of TR503. And just to make sure: Please confirm that you did not replace TR503 (the horizontal output transistor). If I understand the base drive circuit of TR503 correctly, it depends on negative base-emitter breakdown of that transistor. If you replace the transistor with a "similar" model, this aspect can fail to work as intended.

You can also verify the horizontal drive output at IC401 pin 12. You expect a square wave (as indicated in the schematic). While the duty cycle isn't exactly 50%, it should not be extreme (like below 5% or above 95%).

mkarcher wrote on 2025-04-28, 21:30:

This shows that the B+ power supply is working as intended, keeping those 25ish volts. So the question is: Why does the supply n […]
Show full quote

AppleSauce wrote on 2025-04-28, 07:37:

I just checked the voltage and it more or less maintains itself at 25ish , maybe dropping to 24.8 when adjusting the pot. When it goes into shutdown it rapidly drops down to 3 volts then down to 0.

This shows that the B+ power supply is working as intended, keeping those 25ish volts. So the question is: Why does the supply need to increase B+ that much to keep this level. It is unlikely that the problem is on the low voltage output side of the flyback transformer (pins 4/5/6/3/2 if I read those numbers correctly). If TP2002A is kept at 25V independent of the horizontal frequency, all the other voltages in that region also don't depend on the scan frequency. As you have no issues at 15kHz, it's very unlikely for a frequency-dependent issue to be rooted there.

So why do we need increasing B+ to keep the flyback transformer output stable? There might be multiple reasons. Either excessive load on the secondary side of the flyback transformer (my first guess), or you have an issue with the horizontal drive. To get the desired voltages to the secondary of the flyback, you do not only need sufficient B+, but also that voltage needs to be applied to the transformer. If you can measure the base of TR503 without risking to touch the collector, verifying that signal as displayed in the schematic can make sense. To be more safe, you might grab that signal at R521 instead of TR503. And just to make sure: Please confirm that you did not replace TR503 (the horizontal output transistor). If I understand the base drive circuit of TR503 correctly, it depends on negative base-emitter breakdown of that transistor. If you replace the transistor with a "similar" model, this aspect can fail to work as intended.

You can also verify the horizontal drive output at IC401 pin 12. You expect a square wave (as indicated in the schematic). While the duty cycle isn't exactly 50%, it should not be extreme (like below 5% or above 95%).

I can confirm that I haven't replaced the HOT , its the original.

I also just remembered that I did measure pin 12 of IC401 a few days ago , because I was suspicious of the horizontal oscillator.

I think this is the right measurement

The second one is what happens when moving the horizontal control.

This is it during shutdown.

AppleSauce wrote on 2025-04-28, 23:57:

This is it during shutdown.

The attachment 20250423_183544_7.gif is no longer available

This one is very helpful. It shows that before shutdown, the frequency of the horizontal oscillator rises above 35kHz. If the frequency of the horizontal oscillator rises, B+ is supposed to rise. The upper limit of supported horizontal frequency is 35kHz, so it is OK if at 37 or 38kHz, B+ hits the emergency shutdown limit of 122V. So you do not need to troubleshoot the B+ generation, which seems to be working just fine, but instead you need to find out why the oscillator starts to oscillate too fast.

I looked at your image in the first post, and I notice you do not loose horizontal sync before the collapse! If that collapse happened in the same way as the one you scoped in this video, not only the horizonal oscillator was too fast, but the pixels were too fast at the same rate, so in combination, these two pictures look like the computer is running away and starting to generate video at overly high frequencies - and the monitor just does everything to keep up with it until the frequency exceeds the allowed range.

mkarcher wrote on 2025-04-29, 05:50:

AppleSauce wrote on 2025-04-28, 23:57:

This is it during shutdown.

The attachment 20250423_183544_7.gif is no longer available

This one is very helpful. It shows that before shutdown, the frequency of the horizontal oscillator rises above 35kHz. If the frequency of the horizontal oscillator rises, B+ is supposed to rise. The upper limit of supported horizontal frequency is 35kHz, so it is OK if at 37 or 38kHz, B+ hits the emergency shutdown limit of 122V. So you do not need to troubleshoot the B+ generation, which seems to be working just fine, but instead you need to find out why the oscillator starts to oscillate too fast.

I looked at your image in the first post, and I notice you do not loose horizontal sync before the collapse! If that collapse happened in the same way as the one you scoped in this video, not only the horizonal oscillator was too fast, but the pixels were too fast at the same rate, so in combination, these two pictures look like the computer is running away and starting to generate video at overly high frequencies - and the monitor just does everything to keep up with it until the frequency exceeds the allowed range.

Thanks for all the help so far its been very insightful.

I'm not sure if the sharp X68000 is at fault , (though it might be) because i remember also connecting the screen to a test socket 370 motherboard and having the same issue. So if two different systems have the same issue I'd assume the monitor is at fault.

I'll connect it back up to the socket 370 again just to make sure.

Assuming its the screen at fault , would ic 401 having a faulty horizontal oscillator cause this?
Or IC555 which is a quad bilateral switch connected to the Horizontal oscillator time constant and Phase adjust time constant?
Or maybe the F/V converter?

I just tested connecting the multisync to my DOS PC , had the collapse issue again.

Hold on you mentioned it being something on the computer side , but what if its on the input side , the service manual does suggest checking the sync circuit on the interface board.