Hitachi CRT Monitor Was working fine now Bowed. Where to start?

[Hoagtech]

I have this awesome Monitor I bought from a group sale in this forum and its been a joy to own until this issue popped up.

It was working fine and then one day. I powered it on and the screen shrank and bowed to an extreme.

I was wondering which section of the chassis or yoke would cause this to happen and a good place to start diagnosis and repair?

This particular set is 480p VGA (dsub) and was known to have a bad quality solder from the factory. A part of it was reflowed by Josh as it was known to cause screen issues.

Thanks for checking it out and I look forward to your feedback

[MKL]

If it's the model in this topic, we can use the service manual as a reference:

viewtopic.php?p=1465551#p1465551

The relevant parts under "circuits description" are:

Paragraph 9: "vertical and horiz. distortion correction". It's the Side Pin PWB on p.128 of the manual, with IC602 (TA8859CP) and IC621 (NJM2903D).

Paragraph 11: "HV regulation/Raster centering circuit operation", point 3: "Side Pin Output". It's the H. Size PWB on p.127 of the manual, with Q754 (2SK2382).

The basic principle of a pincushion correction circuit is to take the sawtooth signal of the vertical deflection (waveform 45 in the manual), convert it into a parabola of the same frequency (waveform 48) and inject it into the horizontal deflection yoke through the diode modulator (upper and lower damper diodes D712 and D714 on p.116).

And obviously, this parabola signal is not making it to the diode modulator. There can be several reasons for this. Bad solder joints is one. As the circuit is on two vertical modules, their connections to the main board need to be checked. Same goes for the soldering of coils L775 and L753 which are in series with the correction signal. Component failure is another reason (more likely in my view). Q754 could be bad. The schematics use a NPN symbol but this is in fact a FET so a bit harder to test but still easy. D754 and D755 too. Q603 and Q604 could be bad (easily tested with meter in diode check). And of course IC621 and IC603 could be bad too. If you replace the ICs, it would be a good idea to socket them. Missing supply voltages is a third reason (which could have to do with reason 1). Check for 12V on the sub-boards.

[Hoagtech]

Thank you MKL. That will give me something to work with

[Josh128]

I had a similar issue that I was able to repair on at least one of them. I think it was a cracked solder joint as well but I dont remember where. I also had to replace the flyback on another and it may have had something to do with this. Sorry I dont remember, but MKL is on the right track of at least where to look on the board to get started.

[Hoagtech]

If it's the model in this topic, we can use the service manual as a reference:

viewtopic.php?p=1465551#p1465551

The relevant parts under "circuits description" are:

Paragraph 9: "vertical and horiz. distortion correction". It's the Side Pin PWB on p.128 of the manual, with IC602 (TA8859CP) and IC621 (NJM2903D).

Paragraph 11: "HV regulation/Raster centering circuit operation", point 3: "Side Pin Output". It's the H. Size PWB on p.127 of the manual, with Q754 (2SK2382).

The basic principle of a pincushion correction circuit is to take the sawtooth signal of the vertical deflection (waveform 45 in the manual), convert it into a parabola of the same frequency (waveform 48) and inject it into the horizontal deflection yoke through the diode modulator (upper and lower damper diodes D712 and D714 on p.116).

And obviously, this parabola signal is not making it to the diode modulator. There can be several reasons for this. Bad solder joints is one. As the circuit is on two vertical modules, their connections to the main board need to be checked. Same goes for the soldering of coils L775 and L753 which are in series with the correction signal. Component failure is another reason (more likely in my view). Q754 could be bad. The schematics use a NPN symbol but this is in fact a FET so a bit harder to test but still easy. D754 and D755 too. Q603 and Q604 could be bad (easily tested with meter in diode check). And of course IC621 and IC603 could be bad too. If you replace the ICs, it would be a good idea to socket them. Missing supply voltages is a third reason (which could have to do with reason 1). Check for 12V on the sub-boards.

I did exactly as you suggested and started by removing the SIDE PIN PWB. I reflowed the 2 IC and ran ESR and CAP on the capacitors. One was low (900 uf instead of 1000) and ran a touch high on ESR but not by much. I noted the cap replacements and moved on



I removed the SIZE PWB and reflowed Q754. (pictured)



I buttoned it back up and same issue I started with unfortunately



I wanted to rule out cold solder and component failure so I grabbed a duplicate Hitachi Set and removed the SIDE PIN PWB and SIZE PWB and swapped them out on the problem set.

There seems to be nothing wrong those PWB's as it did not solve the issue. Same pincushion problem.



At least I ruled out those PCB but I would love to hear a suggestion on where to diagnose next.

I had a similar issue that I was able to repair on at least one of them. I think it was a cracked solder joint as well but I dont remember where. I also had to replace the flyback on another and it may have had something to do with this. Sorry I don't remember, but MKL is on the right track of at least where to look on the board to get started.

You wouldn't happen the area where the cold solder was most frequent would you? Can you confirm if you saw bowing and the flyback corrected it? I wouldn't opposed to swapping out the flyback but would prefer not to try unless inclined to.

Appreciate the insight please let me know if you have any suggestions

[Josh128]

Go back to the Hitachi Heist thread and look for the pictures I posted showing some of the problem areas where the transistors frequently developed the broken solder joints. Im pretty sure I reflowed all the known problem areas before shipping out though (after the first 3 or 4 sets shipped, when I discovered the issue), so those might not work.

I too remember attempting to fix the bowing problem by swapping the boards mentioned above, with no success. I definitely did get the problem fixed on a couple of them though, I just dont remember where. I did change a flyback and it fixed a similar problem, but I want to say it was intermittent and had a clicking/popping sound accompanying it.

Be sure to thoroughly read the other thread though, maybe I talked about it there.

[MKL]

You need to see if the pincushion correction signal is being sent out of the SIDE PIN PWB. That's waveform 47, pin 10 (H SIZE). This signal is then sent to the H SIZE PWB through the main board where it could be lost. I see R631 (between brackets??) in series with it. If the signal makes it to the H SIZE PWB and to the base of Q754, you need to see if the correction signal in its final form (a parabola, waveform 48, pin 5 of the H SIZE PWB (HFB) is being sent to the main board. On the main board it goes through coils L775, L753 and to the center point of the diode modulator (cathode of D714, anode of D712).