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Old 3rd Nov 2019, 1:57 pm   #1
Sean Williams
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Default Repair of industrial open frame monitor

Yet another teleprinter related thread from me, but this relates to the 9" monitor that is attached to one that was working up till yesterday....

Symptoms

All was working well, just receiving a page of weather information, when for no reason, and with no warning, the picture just shrank away to nothing.

Tube heater is still lit (but probably powered from the 12v rail anyway.)

I have taken the monitor out, and powered it up on the bench - Zero activity from the LOPT - have checked the drive transistor, tests as working (BU408)

Have performed a "Ring Test" on the LOPT and it certainly appears to be ringing well.

There is a TDA1170s that appears to be responsible for most of the heavy work in the monitor.

Any ideas as to where I should be looking - as I say the line stage appears to be completely inoperative.
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Old 3rd Nov 2019, 3:16 pm   #2
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Default Re: Repair of industrial open frame monitor

Pic of ring trace.

If I grab the LOPT and hold it tight, I can damp the oscillations, so I think it is ok.
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Old 3rd Nov 2019, 5:53 pm   #3
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Default Re: Repair of industrial open frame monitor

The TDA1170 is a vertical deflection IC (only) I think. It does not include the line oscillator.

Is there another IC in there? The TDA1180 was often used for the line oscillator, etc. Or it might be discrete components. Is the line drive transformer-coupled to the base of the BU408? I've had those drive transformers fail open-circuit (which obviously stops the line output stage from doing anything), they are not hard to rewind though.
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Old 3rd Nov 2019, 6:05 pm   #4
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Default Re: Repair of industrial open frame monitor

So you have a likely good LOPT and good H output transistor but its not working.

Check the drive at the base of the transistor, this includes therefore testing the driver transformer and the IC/circuitry driving that. If the drive to the base is normal then check the power supply feed to the primary side of the Lopty, are there any open solder joins or fusible resistors.

Generally the driver transformer has a ratio of about 10:1. If you do find an issue with it, and you attempt to rewind and replace it, be cautious of the winding polarity.

It pays to be aware how that works, it is often glossed over in the textbooks. When the driver transistor conducts or switches ON, it switches OFF the horizontal output transistor for flyback and during this time energy is stored in the magnetic field of the driver transformer. When the driver transistor switches OFF, it is the stored energy in the driver transformer's field which switches on the the H output transistor.

It sounds after a few simple tests you should be able to find the issue, most likely the H drive has gone missing and the transformers are ok.
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Old 3rd Nov 2019, 6:30 pm   #5
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Default Re: Repair of industrial open frame monitor

Thanks both.

Steep learning curves on all this modern stuff!

The description of the H output system makes a lot more sense now - I was surprised to find the output transistor conducting even though nothing was happening.

More checking and testing to do.

Should the drive circuit work with the LOPT and output transistor removed? I think it will be safer this way?
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Old 3rd Nov 2019, 8:35 pm   #6
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Default Re: Repair of industrial open frame monitor

Okay, I have checked the driver circuit - there appears to be no voltage getting to it. The oscillator coil is giving continuity through it, so I guess I need to look deeper.

As luck would have it there is no schematic available for the machine, and it is a double sided board
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Old 3rd Nov 2019, 9:38 pm   #7
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Default Re: Repair of industrial open frame monitor

Been having a look at some online schematics for similar monitors.

I am now wondering if this will actually fire up on the bench - it looks like the H sync line is needed to set the driver transistor going, or am I missing something here?
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Old 3rd Nov 2019, 11:02 pm   #8
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Default Re: Repair of industrial open frame monitor

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The description of the H output system makes a lot more sense now - I was surprised to find the output transistor conducting even though nothing was happening.
The way it is configured out course with the driver transformer when the H scan osc stops (for any reason) the H output transistor remains in the OFF state. If it stayed on it would be a disaster as the low primary resistance of the H output transformer would be switched across the power supply and something would be destroyed.This is the beauty of driver transformer coupling.

There are some computer VDU's (probably not this one but possible) that do not have their own H osc, they use the incoming H sync. Fortunately this is not common. IBM did it though in some early VDU's.
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Old 4th Nov 2019, 6:14 am   #9
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Default Re: Repair of industrial open frame monitor

Yes, the IBM5151 (MDA monitor for the PC family) had no horizontal oscillator, it fed the HSync (more correctly called Horizontal Drive in the technical reference manuals) signal to the base of the line driver.

If the little monitor disussed here takes a composite input then it will have a horizontal oscillator. If it takes separate syncs and video (often on a 10 pin 0.156" pitch edge connector) then it may not. You may have to trace out the appropriate bit of the circuit to work out what is going on.
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Old 4th Nov 2019, 2:46 pm   #10
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Default Re: Repair of industrial open frame monitor

Have a look at the base of the driver transformer and see where it comes from. If it's from an IC, it's likely the oscillator is in it.
Your clue is that there's no voltage getting to the driver - there should be HT. Two ways this can be achieved. One is a permanent feed from the PSU - possibly 12v, though it may be higher. The second is to feed a kick from the PSU to get things going which is then taken over by a rectified winding from the LOPT with an isolation diode in the PSU feed. As this is a small screen unit I'd suspect the former method.
Put a meter on the primary of the oscillator transformer and switch on - you may see a kick. Of course this all assumes the driver transistor is OK. If it's short, then the feed resistor will take the current, but might not appear distressed.
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Old 5th Nov 2019, 8:35 am   #11
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Default Re: Repair of industrial open frame monitor

I've hooked the drive transistor out and tested it - it's an NPN device, and has some gain, so I think all is well with it.

If I get time this evening I will continue trying to trace (and draw) what is there - I'm getting lost continually flipping the board from one side to the other trying to follow traces.
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Old 5th Nov 2019, 11:16 am   #12
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Default Re: Repair of industrial open frame monitor

I'd leave the transistor out and see if a voltage magically appears at the collector pin. It could be the lack of a sync signal is turning the transistor hard on.
I'd disconnect the line output transistor's collector feed while fault finding in case things spring to life unexpectedly.
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Old 5th Nov 2019, 10:23 pm   #13
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Default Re: Repair of industrial open frame monitor

what sort of connector does it use? Have you got a pic of the thing?
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Old 5th Nov 2019, 10:28 pm   #14
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Default Re: Repair of industrial open frame monitor

Its got a 10 pin DIL connector - I could get a pic - have closed up for the night.

The equipment is a German/English project, and manufactured by ITT Creed, or ITT Lorenz.

Water is somewhat muddied by the Italian CRT

Ive had a closer look at the drive transistor circuit - If my tracing is correct the base if the driver transistor is R/C coupled to the output pin of an NE556 - this makes me thing the H Oscillator is on board, and it just gets sync pulses from the video card - please correct my shoddy understanding if I am wrong.

I will put a pic up later this week - Work is going to get in the way again now
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Old 5th Nov 2019, 11:04 pm   #15
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Default Re: Repair of industrial open frame monitor

Quote:
Originally Posted by Sean Williams View Post
Ive had a closer look at the drive transistor circuit - If my tracing is correct the base if the driver transistor is R/C coupled to the output pin of an NE556 - this makes me thing the H Oscillator is on board, and it just gets sync pulses from the video card - please correct my shoddy understanding if I am wrong.
It may be that the 556 is just a monostable to make the line drive pulses the right length. As Tony and Argus said earlier, it's very common for this sort of monitor to have no line timebase and just to rely on the horizontal output from the video circuitry to have the right characteristics to drive the line output stage more or less directly. I've recently worked on an Apple Macintosh SE/30, one of the classic ones in the upright beige case with a monochrome display, and that works this way. In this case there's no horizontal activity or EHT without a video input.

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Old 5th Nov 2019, 11:13 pm   #16
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Default Re: Repair of industrial open frame monitor

Hmm, this does rather complicate bench testing Gonna need a bigger bench - the machine it lives in isn't exactly small.

However, this might explain the almost complete "deadness" of the unit when only shown 12volts.

Okay, will rearrange the workshop to allow for the TP to supply everything.

Thanks All, this is just too modern for me.
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Old 6th Nov 2019, 6:12 am   #17
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Default Re: Repair of industrial open frame monitor

One thing to quickly check is if the halves of that 556 are used in conventional monostable or astable circuits.

A monostable has discharge linked to threshold. An astable has threshold and trigger linked.

If you have an astable section it is likely that is the line oscillator here. Two monostables could be cross-connected as an oscillator, but I think that is unlikely.
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Old 9th Nov 2019, 8:12 pm   #18
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Default Re: Repair of industrial open frame monitor

I have managed to get the monitor (and rest of the machine) on to the desk.

With H and V syncs the board is a lot more lively (Yes I know, several of you have pointed this out)

The drive circuit is certainly switching well, there is a feeble, and I mean feeble spark of EHT.

Second picture is off the driver transformer, First is the drive to the base of the line transistor.

Wondering if it could just be a failed Boost cap (or tuning cap)
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Old 11th Nov 2019, 2:12 am   #19
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Default Re: Repair of industrial open frame monitor

Quote:
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there is a feeble, and I mean feeble spark of EHT.
Gulp ! Did you attempt to short out the EHT or get it to jump across a gap to ground ? Not a great idea in a semiconductor based monitor/TV. It can destroy or risk destroying the EHT rectifier that is integral to the LOPY (usually). Still, even if this does fail & short or go leaky the problem can be rectified (no pun intended) by adding a series diode. If it goes open is game over for the Lopty assembly.

The drive looks ok.

You need to scope the collector of the line output transistor now. The peak voltage there varies, it depends on the monitor size and other factors, likely its under 400V peak, so "probably" a x10 scope probe will be ok, but if you have a x100 probe handy use that. (This voltage though in color sets can be over 1kV, but in a 9" to 12" monochrome monitor its around a two hundred to 300 volts peak, or thereabouts)

Assuming the LOPTY is ok, there might be something loading one of its auxiliary windings, shorted diode, or capacitor.

The form of the collector voltage will give the game away as to where the problem is and also indicate if the output transistor's collector voltage is doing what it should with the normal drive voltage a its its B-E junction. The tuning capacitor on the primary might also be faulty. Unlikely the energy recovery diode shorted, as that shorts the supply across the Lopty's primary, if that goes open, it likely destroys the HOT because the collector voltage goes very negative with respect to its emitter voltage.

Generally these sorts of monitors do not have a "boost capacitor" as such. When the energy recovery diode conducts, magnetic energy of the yoke and output transformer is returned to the power supply directly and the damped current scans the left side of the raster. In valve sets the recovered energy charges a boost capacitor. In transistor sets though there are often a few lopty secondary windings with rectifiers to get negative and positive voltages to run things like the CRT's electrodes and the video output amplifier.

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Old 11th Nov 2019, 1:40 pm   #20
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Default Re: Repair of industrial open frame monitor

As usual, Argus has given a detailed explanation of how it all works. A couple of things. Does the line output transistor get hot? If not, then it's more likely an open circuit somewhere, and the favourite point on a small monitor is the high-value non-polarised coupling capacitor to the scan coils. If it does get hot, then it's an overload, possibly a secondary derived supply or the LOPT itself.
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