Help on monitor issue with Taxan 770 plus, powers on but video collapses.

[Hugo Holden]

I should be able to get pin 9 grounded and try this here in the next hour or so once I get the kids in bed. As far as a capacitance meter, I don't have anything special. My multimeter has a capacitor setting, then I have the Peak ESR meter. Short of that I don't have any other capacitor tester.

Double check the solder join first.

 

[lowlytech]

Will do..

 

[lowlytech]

Your on to something....

I reflowed that solder joint but it unfortunately didn't change anything. However when I shorted pin 9 of HA11235 to ground I now get 124V steady on the meter ( for the 2 seconds it was on anyway) on the B voltage test point on the neck board. This is the first time I have seen the voltage to be stable with everything connected.

 

[Hugo Holden]

Your on to something....

I reflowed that solder joint but it unfortunately didn't change anything. However when I shorted pin 9 of HA11235 to ground I now get 124V steady on the meter ( for the 2 seconds it was on anyway) on the B voltage test point on the neck board. This is the first time I have seen the voltage to be stable with everything connected.

Is the VDU stable & working normally though if you leave it on longer ?

 

[Hugo Holden]

...PS, if the size of the raster scan looks smaller than it should be, that is an indication that the EHT is running too high.

 

[lowlytech]

I will check. I was scared to leave it on long enough for the tube to warm up.

I also have a HV probe somewhere if we need to look into the HV.

 

[lowlytech]

It looks good to me.

20240204_084217.mp4

drive.google.com

 

[andy]

Now that you can keep it from shutting down, you finally have time to make some measurements to figure out why it's shutting down. It is a good idea to make sure the HV isn't excessive before you run it for too long. If nothing else, you can check that 5.6v feedback output from the flyback. It would be interesting to look at that on the oscilloscope during startup to see if the HV is briefly spiking and shutting things down.

 

[lowlytech]

Hooked a signal to it with brightness all the way down and it still looks pretty overvolted. Took a measurement with HV probe, looks like 34Kv. Gotta be honest, I am somewhat skittish with this. The whole time it is sizzling and popping while taking the HV measurement. I sure don't mind to test things, just need some extra reassurance I have the right connection hooked to the tester. I will check the schematic and see if I can find where to connect for the 5.6v feedback.

 

[lowlytech]

So is it correct that I can just tap on to pin 5 of ic1404 (upc358c) to the scope to get the 5.6v feedback from the flyback or do I need to move closer to the original signal?

 

[andy]

Now that you have confirmed that the HV really is much too high, you need to figure out why. To start with, I'd see if the HV adjustment does anything. Try to not run it too much since excess voltage is bad for the flyback, and CRT.

Since the HV is constant, you can just measure that 5.6v signal with a volt meter. It's just about guaranteed to be high unless your HV meter is way off.

Give the board around the HV regulator circuit as close visual inspection. Something simple like a solder bridge could be the problem.

 

[lowlytech]

Got pin 5 measured @ IC1404. With the HV protection bypassed I get 8.2V and with the HV protection left intact the voltage on pin 5 is 6.25V

I checked the HV adjustment (R1539) as well and it is smack dab in the middle at 12 o'clock. I adjusted it to the far left and right but the voltage on pin 5 stayed @ 8.2V either way I tried it. I didn't test at the anode cap since I feel being 11Kv out of adjustment is a tad much. If I need to measure at the cap I can, otherwise I am gonna tear the monitor apart again and pull the pcb to check the HV area. I will also make sure that R1539 seems to function correctly on an ohms test. If anyone has any probable components in the path to focus on let me know. I am gonna follow Hugo's last schematic attachment where it shows "high voltage regulator section"

 

[Hugo Holden]

Hooked a signal to it with brightness all the way down and it still looks pretty overvolted. Took a measurement with HV probe, looks like 34Kv. Gotta be honest, I am somewhat skittish with this. The whole time it is sizzling and popping while taking the HV measurement. I sure don't mind to test things, just need some extra reassurance I have the right connection hooked to the tester. I will check the schematic and see if I can find where to connect for the 5.6v feedback.

Generally the EHT should be under 30kV, typically in the 25kV to 27kV vicinity. Likely the EHT over-voltage protection circuit is normal and doing its job. Don't power it again until we figure out the cause of the problem, unless we are forced to.

This is where some magnetic deflection scan theory helps.

The appearance of the raster, combined with the high EHT, likely tells us why the EHT is too high.

All else equal in magnetic deflection, the size of the scan, is inversely proportional to the square root of the EHT voltage. If there is an isolated fault affecting the EHT alone to increase it significantly, and nothing else, then the scan width will drop and the raster will be under-scanned. We have an image though, which shows the scan width, if anything is a little over-scanned, but still with high EHT.

But the two systems of scanning and EHT generation are linked, so if we see a combination of excessive EHT not associated with decreased scan width, it gives a clue why. (The raster height behaves the same way too, but we cannot put as much stock in that, because the height control may have been adjusted)

What this means is that the fault is due to actual H over-scan: There is a fault, whereby the stored energy in the H yoke coils and LOPT is higher at the end of scan time prior to flyback than normal. This increases both the scan width and the EHT together. This can happen for a number of reasons I will post again later today on what to do next and explain the determinants of the yoke's and output transformer's peak current at the end of scan.

1) Firstly, did you replace the capacitor C1530 and if you did, can you take a photo of the part you put in there ?

2) assuming no issue with that capacitor:

In the H output system, after the H output transistor is switched on, and from the point where the beam is at the center of the CRT face, the current in both the yoke and the flyback transformer's primary, rises at a rate of V/L amps per second, where V is the power supply voltage to the H output stage an L the inductance of each. The peak current is determined by the time that the HOT is switched on for, which is also related to the H scan frequency.

This affects both the scan width and the EHT, because the width relates to the peak yoke current before flyback and the energy stored in the field of the yoke and flyback transformer determines the EHT, because this energy is transferred to the energy of the electric field of the capacitances half way through flyback, where it peaks.

From this and the increased EHT & width together, can deduce that the voltage supplying the Flyback transformer primary circuit and yoke is too high.

In a multi- standard VDU, to get around the different scan frequencies and different times that the H output transistor is turned on for, they have in this case enclosed the power supply to the Flyback and yoke in a tight feedback loop, as I had highlighted on the previous diagram. Basically the voltage regulator supplying the flyback and H yoke coils is a Buck type, so it can manipulate the power supply voltage below the 125V rail that supplies it.

There is clearly a problem in the feedback loop and the output voltage of the buck regulator will be too high. And the shutdown circuit has detected this.

It is lucky so far that the when the shutdown circuit was disabled that it tolerated the 34kV. At this point, I would recommend release the short on pin 9 to re-enable the shutdown system.

We need to devise a way to power the flyback and yoke from a lower voltage, so that the shutdown does not deploy and the EHT stays normal range or low, so we can fault find the whole feedback loop and the buck-boost regulator controlled by it.

I will work on the method and post later. In the meantime, double check the capacitors and soldering in the buck boost section and the feedback loop.

 

[Hugo Holden]

...also we should check that the buck transistor Q1502 does not have a collector to emitter short and that it is receiving a pulse drive via the coupling transformer to its base-emitter junction.

 

[andy]

The HV is supposed to be 23kV, so 34kV is MUCH too high. Definitely avoid running it unless you absolutely have to. I would also go ahead an re-enable the protection circuit, since it's doing its job. You'll know it's fixed when it doesn't shutdown.

I would start by looking around IC1503. It's a comparator circuit that is responsible for HV regulation. It takes an output voltage from the flyback (which is proportional to the HV) and compares to to a fixed reference voltage. I see several electrolytics in the circuit which you presumably replaced, so be sure to check your work. Did you use a bipolar cap for C1524?

The output from IC1503 is used to control the regulator circuit, which includes power transistor Q1502. This regulates the supply voltage to the flyback to control the HV. If Q1502 was shorted, the HV would be excessive. The schematic is hard to read, but it looks like you should have 54v on the emitter of Q1502. I'd bet it's a lot higher than that, causing excess HV.

 

[Hugo Holden]

Andy and I agree.

It is a matter now though (unless the fault can be found by visual inspection and passive component testing, with the power off) to find a way to reduce the voltage supplying the H output stage, if more powered up tests need to be done to locate the fault.

Can you post a photo of transistor Q1502 , is it on a heatsink ?

And as Andy suggested, if you replaced C1524 check that you installed a bipolar cap.

 

[lowlytech]

Generally the EHT should be under 30kV, typically in the 25kV to 27kV vicinity. Likely the EHT over-voltage protection circuit is normal and doing its job. Don't power it again until we figure out the cause of the problem, unless we are forced to.

This is where some magnetic deflection scan theory helps.

The appearance of the raster, combined with the high EHT, likely tells us why the EHT is too high.

All else equal in magnetic deflection, the size of the scan, is inversely proportional to the square root of the EHT voltage. If there is an isolated fault affecting the EHT alone to increase it significantly, and nothing else, then the scan width will drop and the raster will be under-scanned. We have an image though, which shows the scan width, if anything is a little over-scanned, but still with high EHT.

But the two systems of scanning and EHT generation are linked, so if we see a combination of excessive EHT not associated with decreased scan width, it gives a clue why. (The raster height behaves the same way too, but we cannot put as much stock in that, because the height control may have been adjusted)

What this means is that the fault is due to actual H over-scan: There is a fault, whereby the stored energy in the H yoke coils and LOPT is higher at the end of scan time prior to flyback than normal. This increases both the scan width and the EHT together. This can happen for a number of reasons I will post again later today on what to do next and explain the determinants of the yoke's and output transformer's peak current at the end of scan.

1) Firstly, did you replace the capacitor C1530 and if you did, can you take a photo of the part you put in there ?

2) assuming no issue with that capacitor:

In the H output system, after the H output transistor is switched on, and from the point where the beam is at the center of the CRT face, the current in both the yoke and the flyback transformer's primary, rises at a rate of V/L amps per second, where V is the power supply voltage to the H output stage an L the inductance of each. The peak current is determined by the time that the HOT is switched on for, which is also related to the H scan frequency.

This affects both the scan width and the EHT, because the width relates to the peak yoke current before flyback and the energy stored in the field of the yoke and flyback transformer determines the EHT, because this energy is transferred to the energy of the electric field of the capacitances half way through flyback, where it peaks.

From this and the increased EHT & width together, can deduce that the voltage supplying the Flyback transformer primary circuit and yoke is too high.

In a multi- standard VDU, to get around the different scan frequencies and different times that the H output transistor is turned on for, they have in this case enclosed the power supply to the Flyback and yoke in a tight feedback loop, as I had highlighted on the previous diagram. Basically the voltage regulator supplying the flyback and H yoke coils is a Buck type, so it can manipulate the power supply voltage below the 125V rail that supplies it.

There is clearly a problem in the feedback loop and the output voltage of the buck regulator will be too high. And the shutdown circuit has detected this.

It is lucky so far that the when the shutdown circuit was disabled that it tolerated the 34kV. At this point, I would recommend release the short on pin 9 to re-enable the shutdown system.

We need to devise a way to power the flyback and yoke from a lower voltage, so that the shutdown does not deploy and the EHT stays normal range or low, so we can fault find the whole feedback loop and the buck-boost regulator controlled by it.

I will work on the method and post later. In the meantime, double check the capacitors and soldering in the buck boost section and the feedback loop.

I found C1530 (c530). That cap wasn't an electrolytic, so I didn't touch it and it's original. I took a picture of the front and back for your reference. It is that big blue one. The second group of solder points on the same pads looks to be a direct connect wire right beside the blue cap.

 

[lowlytech]

Andy and I agree.

It is a matter now though (unless the fault can be found by visual inspection and passive component testing, with the power off) to find a way to reduce the voltage supplying the H output stage, if more powered up tests need to be done to locate the fault.

Can you post a photo of transistor Q1502 , is it on a heatsink ?

And as Andy suggested, if you replaced C1524 check that you installed a bipolar cap.

I can confirm C1524 is BP (2.2uf 50V). Q1502 has a gray rubber sleeve over it, however I had taken that transistor off very early on in the troubleshooting process and did a simple check with the multimeter on diode mode and it behaved what I thought was normal. No shorts between legs but I didn't have another C3336 (NPN) for a sanity check to make sure. One thing I do remember is the gray rubber sleeve was somewhat fragile and stuck from being pressed for so long, so a small rip happened at one of the top corners, so I wrapped electrical tape around the metal that pushes it to the heatsink just incase it needed totally insulated from the heatsink. I had made a note of the results in a small notebook I had..

C3336(NPN)

|        |         |

B      C        E      Result

+      -                0.44v

+                -       0.50v

-       +                0L

-                 +      0L

         -        +      0L

         +       -       0L

 

[lowlytech]

If I reenable the protection circuit can I still measure voltage on the emitter of Q1502?

 

[Hugo Holden]

If I reenable the protection circuit can I still measure voltage on the emitter of Q1502?

It won't be there for long, as soon as the shutdown circuit activates it will kill the output pulses from the HA11235, and that will kill the drive to the buck regulator, as you can see the transistor is driven by a transformer, if there are no drive pulses, then simply Q1502 will not be conducting. Running normally the transistor chops up the 125V to a lower voltage around 50V.

oops.......C1524 is supposed to be a 47uF Bipolar (according to the schematic and unless a dot is not visible making it a 4.7uF) , where did the 2.2uF value come from ? This could well be the problem.