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Repairing Oliveti M24 monitor

That should not be necessary. The most straightforward approach is to find the failing component and replace it. My thought about the pot was that it had become dirty and "noisy", much like old audio volume controls. Clearly that's not the problem. For what it's worth, simply spraying cleaner on a pot won't really clean it. Mark the position of the knob, spray the pot and then work the knob back and forth several times, then reset it to the marked position.
 
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Well, I had to try.
 
DeltaDon said:
Would replacing the two diodes be a shot in the dark attempt at a fix for someone not skills in troubleshooting this problem?
Click to expand...

I never recommend shotgunning a repair. The chances of doing more damage is always there. If you add a new problem, by breaking an unseen trace or putting the part in wrong will only make the effort more difficult. Trouble shooting it about locating the failing circuit.
Still, with your skill level, what do you have to lose. You are not likely to repair the monitor by replacing the diodes. It might work, it might not. Do carefully document the polarity of the diodes before replacing them.
I give it about a 15% chance of fixing anything.
Dwight
 
Thanks DeltaDon for the question. But the answer was discouraging :)

Actually that is why I suggested adding a circuit! Please answer me if a custom circuit or even small factor arduino will be able to correct the signal. I can feed the sync signal into the circuit and control this very same pot that corrects the problem now (but only for a few seconds). Please give it a thought ... it is not how it is done usually, but given the circumstances and my low electronics skills... it is actually a good idea. Also nowadays it is so easy to make a new circuit. It is not a circuit that replaces the electronics inside, but one that circumvents the problem.
 
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I think you should try to identify components which could be sebsible for heat in the area of the sync signals. And you should have a look on cold solder points.
 
It is more likely from his description that the failing part is completely dead and completely blocking the sync signal. I can't think adding a circuit would be any easier than finding a dead transistor. I doubt he'd even know where to attach it. The pot is only the frequency adjust of part of the oscillator. It is likely several components away from the phase detector diodes.
The voltages used in a small uP are not likely compatible with the voltages in a horizontal oscillator. He neither knows what voltage swing is needed or where to attach it.
The best attack is to follow the horizontal sync input and check each active device with an ohm meter.
Dwight
 
So I managed to disassemble my Olivetti M24 monitor. Basically you need to pull the tray motherboard card out (or rather back) leaving the power supply in. I can write detailed instructions later.

View attachment 66255

So here is a photo with the board. On top you have the two pots responsible for the horizontal adjustment. HOLD is the basic one and SHIFT is the fine tuning.
You can also see the horizontal sync cable that comes from the video connector.
There are also 7 diodes that are connected to the output leg of the SHIFT pot and they go to pin 3 and 7 of the TA7643AP.

Questions:
1) What is the TA7643AP. I can not find the datasheet. Somebody says that it is a "wave shaping and oscillations" IC, but just a guess.
2) Do you see the multivibrator ? Which diodes are the most likely to be at fault? Wild guess?
3) Do you see a "ramp capacitor"?
 
So I managed to disassemble my Olivetti M24 monitor. Basically you need to pull the tray motherboard card out (or rather back) leaving the power supply in. I can write detailed instructions later.

olivetti_card2.png

So here is a photo with the board. On top you have the two pots responsible for the horizontal adjustment. HOLD is the basic one and SHIFT is the fine tuning.
You can also see the horizontal sync cable that comes from the video connector.
There are also 7 diodes that are connected to the output leg of the SHIFT pot and they go to pin 3 and 7 of the TA7643AP.
Basically the left side is dedicated to the horizontal sync.

Questions:
1) What is the TA7643AP. I can not find the datasheet. Somebody says that it is a "wave shaping and oscillations" IC, but just a guess.
2) Do you see the multivibrator ? Which diodes are the most likely to be at fault? Wild guess?
3) Do you see a "ramp capacitor"?
 
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The TA7634 looks to be an IC that is the horizontal and vertical oscillator chip.
I see that ebay has one that you can buy but you still are not doing what is needed. You need to trace the circuit from the horizontal input to this circuit. Make a schematic drawing. ( ignore the pots for now ) It is likely along that path that there is something that has failed.
Dwight
 
Actually it is there. On my photo you can see it. I have labeled it "Horizontal Sync cable". It is pin1 of the video connector that is soldered to this hole in the card. So basically the left side of my photo shows how the signal from the sync cable goes through several diodes (around 7) and it ends in TA7643AP. On the photo there are markings for D diode and R for resistance and C for capacitor. Can you see it now?
 
I don't see a picture with the cable. One of your post has an invalid pointer to a picture attachments. The diodes on this line may be related to the phase compare but may also be limiters. You need to make a schematic drawing. Trying to figure from pictures of a PC board is not a useful way of showing things.
Dwight
 
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Please have a look.

Here is the photo again: olivetti_card4.png

From what I see horizontal and vertical sync are connected here and then after a series of diodes and resistance they go into TA7643AP. The multivibrator should be before the TA7643AP?
 
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The H osc (multivibrator) is inside that IC. Ideally if you cannot get the schematic for your set, the data sheet on the IC will help a lot, the components around that IC will be part of a standard circuit.

If you have a video monitor where the picture is locking, but then drifting over time with horizontal shifting, there is a way to narrow down where the problem likely is and even a partial schematic will help.

The first move is to find out if the H scan oscillator frequency is stable. If its not, then in the locked condition, any frequency shift will manifest itself as a horizontal movement shift.

Firstly the H sync is disconnected, usually by disconnecting its feed into the IC or sync discriminator circuit by lifting one leg of a coupling capacitor The H osc will then "free run". You can adjust the H hold until the picture is floating by horizontally. And see how stable that creep rate is, or if it intermittently changes, suggesting the H osc frequency is intermittently changing.

If its not the H osc frequency deviating, it could be a problem with the incoming sync level (this results in a phase shift) or the comparator stage where the H sync is compared with the H scan pulse, possibly in a diode discriminator circuit there, where the incoming sync is compared to a pulse which is derived from the H osc, but usually picked off downstream from the actual osc.

In cases of intermittent behavior in a circuit that amounts to a phased locked loop, it is often necessary to break the loop like this because any change inside the loop, by an intermittent component, will be partly counteracted by the loop behavior.

Don't start shot gun replacing parts without any reasonable evidence that a particular part is defective (I agree with Dwight 100% )
 
Thank you!

The problem is that I can not find the datasheet for the TA7634AP ...
So no documentation for the monitor and no datasheet for this IC.
 
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tonata said:
Thank you!

The problem is that I can not find the datasheet for the TA7634AP ...
So no documentation for the monitor and no datasheet for this IC.
Click to expand...

Toshiba made a lot of custom IC's for TV and VDU applications. They didn't always offer up the data sheets, not on the internet anyway.

But, it does not matter for two reasons:

The good news is that the IC is still easy to get if you do have to replace it.

Also, the schematic is right there in front of you on the pcb, you can copy it out from what is there.
 
Only if you have already the experience. If you have already seen a similar IC.
Maybe I should inspect some the components: diodes, resistance and possible capacitors first before changing the TA7634AP?

There is no single one that look damaged, leaked, etc. I tried to check the normal diodes ... but I get a connection in both directions on many of them. That is because they are connected somewhere else, right? The others are Zener diodes and I do not know how to measure them, they allow voltages in both directions?
 
Solder out the diodes on one side and then you can measure them, also the zener diodes should measure like that if the voltage of your device is low enough (lower than the zener voltage of the diode).
 
As was mentioned, one can measure diodes by lifting one lead. The zener is likely a greater back voltage than the meter you are using. Things like the 1K or 10K scale usually show open if over 2 volts for most meters. You should be able to see it conduct in one direction but not in the other.
It would really help if you drew a schematic and post it.
Also, are you sure the machine you are connecting it to is generating a H-Sync signal? You could be trying to fix the wrong thing.
Dwight
 
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