R2001A High Voltage supply issue

[cahighwayman22]

Got an R2001A on the bench that has an issue with the High Voltage supply. 3 high ohmage resistors burned themselves up in the Z axis modulator circuit. The resistors are basically 2 1.2 meg ohm resistors in series with a 1 meg ohm resistor. One side of the resistor chain goes to ground and the other side splits off to a set of opto isolators using for the intensity, and focus circuits. So far I haven't been able to track down the issue.

Any suggestions?


Thank you

-Trevor

[cahighwayman22]

Does anyone happen to have a junk R2001A chassis with a good HV supply?

[DJP126]

Got an R2001A on the bench that has an issue with the High Voltage supply. 3 high ohmage resistors burned themselves up in the Z axis modulator circuit. The resistors are basically 2 1.2 meg ohm resistors in series with a 1 meg ohm resistor. One side of the resistor chain goes to ground and the other side splits off to a set of opto isolators using for the intensity, and focus circuits. So far I haven't been able to track down the issue.

Any suggestions?


Thank you

-Trevor

This is from the R2001A Maint. Manual.....

16-5. Intensity and Focus Control. An 87V zener diode and resistive divider provide the intensity and focus
voltages. The modulator circuits provide variable output voltages within their bias ranges under the control of
the low voltage INTENSITY/FOCUS TV inputs.
16-6. DC control loops are utilized to stabilize the gird and focus voltages. For the intensity circuit the
INTENSITY SAMPLE and HV REF signals are compared, on the Scope Amplifier Module, with the control
input. The result of the comparison is the INTENSITY TV drive signal to the modulator. The focus voltage is
controlled in the same way, except the HV REF signal is not used in the comparison.

Bad zener?

[cahighwayman22]

I tried that, however now I've got another issue. The unit is in a constant reboot, as if something is shorted. I think it's the HV supply, but I have no way of verifying it since the unit doesn't power up with out the HV supply plugged in. IS there a way to jump the connection to simulate that the HV supply is in place?

[DJP126]

Sounds like a short on one of the LVPS outputs. With an ohmmeter, check for a short to chassis ground on the A2 connector at pins 5 & 6 (+130 V circuit), 9 &10 (-139 V circuit), 33 & 34 (-12 V), 35 & 36 (+12 V), 37 & 38 (+33 V), 39 & 40 (-5 V) and 41 & 42 (+5 V circuit). If you find a short to ground, remove a module (one at a time) until the short is cleared. That will bring you to the defective module for further troubleshooting.

[cahighwayman22]

I found the problem. Got a shorted MC14514 IC on the scope/DVM board. With the A3 unplugged, the unit boots up normally now.

With that being said I changed out the Zener diode, and the 3 resistors still have too much voltage across them. Only thing I can see being defective are the optoisolators. Could those be the culprit?

[DJP126]

Probable cause, may be hard to find nowadays. Part is CNY17-1.

[cahighwayman22]

I'm now thinking that the shorted IC (U34) on the A3 board maybe why the Z axis circuit isn't working. I found a place that has replacement MC14514 IC's.

I will replace it and see what happens.

[cahighwayman22]

So replacing the IC on the A3 board, and Optoisolators did not fix the resistor overheating problem. All of the components on the Z axis board are okay. I'm beginning to think that the problem is now on the A2 board. The intensity circuit maybe the problem as the designated test point that suppose to measure the Intensity Tracking Voltage doesn't do anything when adjusting the level on the front panel, it stays at 9V. The Focus Tracking Voltage works, and varies between 0V and 8V when the Focus pot on the front panel is adjusted.

[cahighwayman22]

Ok. No problem with the A2 board. So I just discovered that those three resistors act as one bleeder resistor. 1.2M + 1M + 1.2M = 3.4M ohms. I think it's normal for them to get hot cause of the voltage that is sent through them. Motorola should of used 2W resistors in that case. So now with that said, looks like i'm going to need a new CRT. This one is somewhat weak.

-Trevor