CRT Hitachi turns off

[jobsam21]

After turning on Hitachi model:C2128FS, it turns on with horizontal lines on the screen then after 5-10 seconds it turns off!

 

[betwixt]

Can you post a picture of the horizontal lines or at least describe them please.
Are they overlayed on a normal picture or are they against a bright background?

Brian.

 

[jobsam21]

Can you post a picture of the horizontal lines or at least describe them please.
Are they overlayed on a normal picture or are they against a bright background?

Brian.

here is the picture!

 

[betwixt]

Those a 'flyback lines', it's the dots that normally make the picture as they scan back to the top of the screen ready for the next visible raster. They are spaced out because the sweep back to the top is much faster than the normal sweep to the bottom.

They are normally supressed so you don't see them by cutting the voltage off to stop the electron flow to the screen but the fault you see indicates that isn't happening. In addition, the bright background tells me that not only is that voltage missing but the rest of the picture isn't having its brightness controlled either. It cuts off because the CRT and driver circuits are overloading the power supply and causing it to shut down.

The most likely cause of the fault is the supply voltage to the video driver stages is missing. Please do not attempt to adjust anything until the fault is found as the set-up procedure can be quite complicated. I will search for the service manual and get back to you with some likely culprits to test.

Brian.

 

[betwixt]

I've traced the schematic. My first guess is C756 is faulty.
Do you have a voltmeter if I ask you to take measurements?

Brian.

 

[jobsam21]

I've traced the schematic. My first guess is C756 is faulty.
Do you have a voltmeter if I ask you to take measurements?

Brian.

Yes, i have it!

 

[betwixt]

Sorry for the delay, I've spent the last day in hospital so I couldn't write back!

I suspect the problem is the +200V supply to the video amplifier stages is missing or has dropped very low. Please do the following and tell me what your meter reads:

CAUTION - There are very high voltages near to where I need the measurements, be very careful not to directly touch anything!
Measure on the small circuit board at the back of the neck of the CRT. It might be marked "CPT PWB".

1. use a DC voltage range suitable for measuring up to 1000V.
2. locate the connector (E805?) that has 7 wires going to the main board.
3. Find pin 1 of the connector, if they are not numbered, it is the one that links to R811, R812 and R813.
4. Connect the red meter probe to that pin.
5. Connect the black meter probe to pin 3, they are numbered sequentially. If it isn't marked, pin 3 connects to R804, R805 and R806 as well as several other components.
6. Switch on.

The voltage should quickly rise to about 200V before it shuts down. Please let me know what it actually reads.

Brian.

 

[jobsam21]

Sorry for the delay, I've spent the last day in hospital so I couldn't write back!

I suspect the problem is the +200V supply to the video amplifier stages is missing or has dropped very low. Please do the following and tell me what your meter reads:

CAUTION - There are very high voltages near to where I need the measurements, be very careful not to directly touch anything!
Measure on the small circuit board at the back of the neck of the CRT. It might be marked "CPT PWB".

1. use a DC voltage range suitable for measuring up to 1000V.
2. locate the connector (E805?) that has 7 wires going to the main board.
3. Find pin 1 of the connector, if they are not numbered, it is the one that links to R811, R812 and R813.
4. Connect the red meter probe to that pin.
5. Connect the black meter probe to pin 3, they are numbered sequentially. If it isn't marked, pin 3 connects to R804, R805 and R806 as well as several other components.
6. Switch on.

The voltage should quickly rise to about 200V before it shuts down. Please let me know what it actually reads.

Brian.

So sorry for the challenges! The maximum voltage value before it shuts down is 27.7V

 

[betwixt]

That is a good sign, for the scan to be full width but that voltage so low indicates a problem in generating the 200V line. The lower that voltage is, the brighter the screen will be and the less the likely it would be for the real video to show through.

I've attached the service manual, it isn't very clear I'm afraid but the best I could find without buying it from Hitachi.

The 200V comes from (page numbered 13 at the bottom) pin 6 of T701. I think it is 6, it's the top left pin on the schematic, then through D751 to C756. T701 must be working to produce any picture at all so it is almost certainly something between it and the CRT base panel. Before changing any parts it would be worthwhile looking under the main circuit board at the solder joints on T701, D751 and C756. T701 is a fairly large component and it is quite common for the solder joints to break because of how much it expands and contracts with temperature changes. It would be a good idea to resolder its pins just to be sure.

Brian.

 

[jobsam21]

Okay!Thank u. Let me try then i'll give feedback!

 

[jobsam21]

But also after checking on the circuit board i have found a ceramic capacitor(B 102k) connected in parallel with D751. I dont know what's the reason for such arrangement and i'm sure someone attached it purposely!But also i guess that's where the problem arises from!!

 

[jobsam21]

That is a good sign, for the scan to be full width but that voltage so low indicates a problem in generating the 200V line. The lower that voltage is, the brighter the screen will be and the less the likely it would be for the real video to show through.

I've attached the service manual, it isn't very clear I'm afraid but the best I could find without buying it from Hitachi.

The 200V comes from (page numbered 13 at the bottom) pin 6 of T701. I think it is 6, it's the top left pin on the schematic, then through D751 to C756. T701 must be working to produce any picture at all so it is almost certainly something between it and the CRT base panel. Before changing any parts it would be worthwhile looking under the main circuit board at the solder joints on T701, D751 and C756. T701 is a fairly large component and it is quite common for the solder joints to break because of how much it expands and contracts with temperature changes. It would be a good idea to resolder its pins just to be sure.

Brian.

I tried to resolder all the joints and even to change the components, D751 and C756 but the problem still there!!

 

[betwixt]

I hope you appreciate it is difficult to diagnose faults remotely but I'll do my best :grin:

I still suspect a problem in that area. Is it possible for you to temporarily disconnect the 200V feed wire to the CRT base (pin 1 of the connector) and check the voltage on the wire again? Something is making the 200V line voltage too low which in turn makes the CRT conduct too much current, I am trying to see whether the problem is on the main board or the CRT base board.

Note that there appears to be an error in the schematics of the CRT base board, it shows the 9V and 0V lines shorted together and I'm sure they are not in reality.

Brian.