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Old 24th May 2022, 6:36 pm   #1
omegaman1969
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Default Help fixing Hitachi B & W monitor needed.

Hi Chaps,

I have an old black and white monitor from my Space invaders arcade machine that I just can't figure out what is wrong with it.

I have checked every component, wire, connection. Unfortunately there are no schematics available for it.

I started a thread over on KLOV https://forums.arcade-museum.com/thr...onitor.506109/ Which has come to a dead end.

I'm sure there are some experts on here that could figure out the problem. I have HV, brightness and contrast controls don't do anything and I have no picture. I accidently managed to make a picture appear by having my meter set on ohms and touching Pin 7 ( Cathode on the CRT) this brought up a picture as seen here

https://youtu.be/X8vD7Ks6rhU

Chassis can be seen here

https://youtu.be/4w7D0_O16oQ

I have an impedance/resistance meter, a semiconductor tester and an ESR meter. I am pretty sure I have check every component.

Any help would be much appreciated. I have never seen the monitor work, I purchased the game as a non worker.
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Old 24th May 2022, 6:55 pm   #2
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Default Re: Help fixing Hitachi B & W monitor needed.

where's the child on the bike come into it?
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Old 24th May 2022, 6:58 pm   #3
omegaman1969
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Default Re: Help fixing Hitachi B & W monitor needed.

That was an video tape in a VHS player I was using for composite input
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Old 24th May 2022, 7:04 pm   #4
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Default Re: Help fixing Hitachi B & W monitor needed.

Looks like the tube is cut off, can you tweak the A1 up? I assume the monitor is actually designed for 1V video rather than modded to accept TTL (I've seen them before you ask).
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Old 24th May 2022, 7:09 pm   #5
omegaman1969
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Default Re: Help fixing Hitachi B & W monitor needed.

A1, would that be G1? It's tied to ground via a 33k resistor.
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Old 24th May 2022, 7:11 pm   #6
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Default Re: Help fixing Hitachi B & W monitor needed.

I get 130v to 164 on pin 7 Cathode depending on brightness pot position. 1 and 8 are heater. Pin 2 G1 tied to ground. Pin 3 43V, Pin 4 focus 0 to 366 volts depending on pot position.
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Old 24th May 2022, 7:13 pm   #7
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Default Re: Help fixing Hitachi B & W monitor needed.

The monitor model is an MCL-461F. I have searched everywhere for a schematic.
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Old 25th May 2022, 7:17 am   #8
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Default Re: Help fixing Hitachi B & W monitor needed.

a hunt round the web found circuit HITACHI CHASSIS MCL-437F MCL437F MCL 437 437F

which may be close and help google search for download
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Old 25th May 2022, 7:39 am   #9
omegaman1969
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Default Re: Help fixing Hitachi B & W monitor needed.

Thanks, that has been discussed before on another forum and unfortunately it's not similar.

I do know someone with a working chassis, I'll ask him to take some voltage readings at the neck.
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Old 25th May 2022, 10:28 am   #10
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Default Re: Help fixing Hitachi B & W monitor needed.

There may be some value in looking for the tube data sheet.
The resistance range experiment is already telling us it is a static voltage problem in the tube biasing circuit.
At one time when they were worth repairing I used to repair computer screens with circuit diagrams being rarer than rocking horse manure.
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Old 25th May 2022, 1:14 pm   #11
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Default Re: Help fixing Hitachi B & W monitor needed.

Looks like the cathode volts may be a little high, especially as shunting it to chassis with your meter (poor meter) brings a raster up. ie it's cut-off. As per previous post I suspect a standing voltage, some of the resistors might be quite high values that are good at going open as they age. Could also be the video transistor (usually a pair, a big one and a little one) as I'm guessing its cathode drive. Examine all the joints around the heat-producing parts, tube socket pins, etc
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Old 25th May 2022, 1:29 pm   #12
omegaman1969
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Default Re: Help fixing Hitachi B & W monitor needed.

Quote:
Originally Posted by Refugee View Post
There may be some value in looking for the tube data sheet.
The resistance range experiment is already telling us it is a static voltage problem in the tube biasing circuit.
At one time when they were worth repairing I used to repair computer screens with circuit diagrams being rarer than rocking horse manure.
Yes I think you’re correct, what voltage would be expected on pin 3 (g2) ? I was thinking maybe one of the high voltage caps is bad causing a lower than expected voltage there.
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Old 25th May 2022, 1:32 pm   #13
omegaman1969
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Default Re: Help fixing Hitachi B & W monitor needed.

Quote:
Originally Posted by McMurdo View Post
Looks like the cathode volts may be a little high, especially as shunting it to chassis with your meter (poor meter) brings a raster up. ie it's cut-off. As per previous post I suspect a standing voltage, some of the resistors might be quite high values that are good at going open as they age. Could also be the video transistor (usually a pair, a big one and a little one) as I'm guessing its cathode drive. Examine all the joints around the heat-producing parts, tube socket pins, etc
I removed all the transistors and they tested good apart from one in the vertical size section.
Yes it’s a poor meter but I like that it uses voice to read out the measurements, so you can keep your eye on the probes

I’ll reflow any solder joints that may heat up and see if that makes a different.
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Old 25th May 2022, 7:08 pm   #14
omegaman1969
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Default Re: Help fixing Hitachi B & W monitor needed.

Quote:
Originally Posted by Refugee View Post
There may be some value in looking for the tube data sheet.
The resistance range experiment is already telling us it is a static voltage problem in the tube biasing circuit.
At one time when they were worth repairing I used to repair computer screens with circuit diagrams being rarer than rocking horse manure.

Tube is a 500 ZB4, I'll see if I can find something on it.

This Tube is the same as the ZB4
https://tubular.atomized.org/tube/500NB4
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Old 25th May 2022, 7:14 pm   #15
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Default Re: Help fixing Hitachi B & W monitor needed.

The voltages you are measuring around the tube base are not what I would expect for a typical CRT.

You have measured:

Cathode (Pin 7) 130-164V depending on the brightness control setting. The 130V should correspond with the fully clockwise brightness adjustment.

G1 (pin 2) = 0 V - tied to ground via a 33k resistor

G2 (pin 3) = 43V

Focus (Pin 4 ) 0 to 366V depending on setting of the focus control.

I have looked for a datasheet for the tube that is used in the monitor, without any success. Toshiba has a range of tubes of the same size, which may give some idea as to what the voltages should be.
They quote typical values as follows:

Focus 0-400V

G2 400V

Raster cut off voltage ( g1 voltage - cathode voltage) -36 to -94V.

Based on this, your G2 voltage is far too low. It will be fed via a high value resistor from a high voltage point, The resistor may have risen in value (or your meter really is terrible and is loading the voltage!).

Either your cathode voltage is too high, or your g1 voltage is too low. It's not unusual for g1 to be biased to a few 10s of volts, so have a look for any other resistors connected to g1.

On another forum you reported some voltages from transistors in the video amplifier stages. It would help others here if you could report the voltages here.

Paula
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Old 25th May 2022, 7:22 pm   #16
omegaman1969
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Default Re: Help fixing Hitachi B & W monitor needed.

Cathode-Ray Tubes B4-71.pdf

5th line up from the bottom is ZB4
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Old 25th May 2022, 7:27 pm   #17
omegaman1969
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Default Re: Help fixing Hitachi B & W monitor needed.

Quote:
Originally Posted by frsimen View Post
The voltages you are measuring around the tube base are not what I would expect for a typical CRT.

You have measured:

Cathode (Pin 7) 130-164V depending on the brightness control setting. The 130V should correspond with the fully clockwise brightness adjustment.

G1 (pin 2) = 0 V - tied to ground via a 33k resistor

G2 (pin 3) = 43V

Focus (Pin 4 ) 0 to 366V depending on setting of the focus control.

I have looked for a datasheet for the tube that is used in the monitor, without any success. Toshiba has a range of tubes of the same size, which may give some idea as to what the voltages should be.
They quote typical values as follows:

Focus 0-400V

G2 400V

Raster cut off voltage ( g1 voltage - cathode voltage) -36 to -94V.

Based on this, your G2 voltage is far too low. It will be fed via a high value resistor from a high voltage point, The resistor may have risen in value (or your meter really is terrible and is loading the voltage!).

Either your cathode voltage is too high, or your g1 voltage is too low. It's not unusual for g1 to be biased to a few 10s of volts, so have a look for any other resistors connected to g1.

On another forum you reported some voltages from transistors in the video amplifier stages. It would help others here if you could report the voltages here.

Paula
Thanks for your input

G1 is tied to ground via a 33k inline resistor. G2 does look low though, I'll trace it back from the cable and double check resistors/capacitors.

I'll paste some info from the other thread on here.
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Old 25th May 2022, 7:30 pm   #18
omegaman1969
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Default Re: Help fixing Hitachi B & W monitor needed.

Funnily enough the 1 bad electrolytic I found was on the G2 circuit (315v 1uf) Maybe that fried something further back.
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Old 25th May 2022, 7:39 pm   #19
omegaman1969
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Default Re: Help fixing Hitachi B & W monitor needed.

https://www.trutechtools.com/iDVM510 My meter
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Old 25th May 2022, 9:43 pm   #20
omegaman1969
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Default Re: Help fixing Hitachi B & W monitor needed.

Well I checked every resistor, every cap and transistor in that section and they all check out ok. damn thing. Flyback issue?
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