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Help with Elston DM30-09BO VDU board

Thanks Hugo, very informative - I'm learning a lot here.

There is usually a tag on the yoke where the two coils join up.

I expect so, but there's a cover glued to the back of the coils that the wires go into and I don't want to remove it. I'm betting the tag is under there.

Sorry I did not see you had swapped out the IC already.

Well spotted. It was a pull from the other board. There is a chance that it, too, is damaged but I thinnk it's highly unlikely to be the same damage as we are surmising the original chip may have suffered. Meaning by that, given swapping it out had no effect, it seems neither of them are bad.

I'll do the other checks this morning and report back.
 
Assuming a fairly constant current for the H scan stage (its not constant and will drop a little with slightly reduced supply voltage), the power dissipated in the resistor is I^2 x R. So if the resistor value is increased it will run hotter. For example if you replaced that resistor with a link wire of near zero Ohms, you would not feel that link wire warm up at all. So the 2.6 Ohm resistor would likely run twice as hot as a 1.2 Ohm resistor. The actual temperature the resistor gets to though, for any power dissipation, depends on its its physical size as the resistor body has a thermal resistance (much like a heat sink) so the bigger the physical size of a resistor the cooler it will run for the same dissipated power.

A scenario like this can be a little confusing because obviously the resistor value, if increased large enough, would run cooler again as the current would drop very low.

In fact the maximum heat that can be generated in a resistor occurs when the resistor value is equal to the internal resistance of the supply, or that of another load in series with the supply, if the supply resistance itself is very low . It is a similar scenario in AC circuits, audio amps for example, where say the maximum power that can be delivered to a load occurs when the load impedance matches the amplifier's output impedance.

You can check this out just a simple model with a 10V DC power supply and a series 10 Ohm resistor. You have created a 10V supply with an internal resistance (or output resistance) of 10 Ohms.

If you play around with different resistor values loading this supply, the maximum power you can get into the load resistor you add occurs when it is also 10 Ohms, equal to the internal resistance of the supply you have created. Obviously when it is zero Ohms you can't get any power dissipated in the load resistor you add, and when it is infinity Ohms, no power either.

About this. The specification for this resistor is 1.2R @ 2W. Given that wattage is the product of voltage and amperage, doesn't that imply that the circuit shouldn't draw more than ~166mA given a 12v power supply? Or am I being a bit thick?
 
Interesting to see the variations of the schematics with 3 x 555 monostables and the Vert deflection IC running of 24V rather than 12v. I wonder if these versions had larger CRT's than the one Jon is working on, probably by the look of the EHT voltage. The document is quite good as far as they go, giving a reasonable description of the function of the H scan stages.

I think the DM40 schematic is for a 12" tube and the DM30 schematic is for a 9" tube like mine (though as we can see it is not quite the same, and in fact, my board isn't quite the same as the photographed schematic that Larry sent me as it is missing Q298). As you may know I have Superbrains with Elston video modules in them and I've had to find other Elston VDU schematics in the past (notably becasue there were unreadable or missing values on some of the other schematics). So, both Northstar Advantage and VT100 (terminal) also used Elston, and the schematics are subtly different but all based on the same design with the same component labelling conventions and values. In all I have gathered six different schematics for this design, and I'll bet there are quite a few others out there.
 
Before replacing (the IC) just disconnect the yoke and make sure that there is no continuity between the H & V yoke coils, just in case. Also make sure to check the diode CR301. Also, a very important resistor to check, though you have probably done it, is the 1.5 Ohm current sensing resistor in series with the V yoke coils as this provides feedback to the IC to help control the scan linearity, but likely it is ok.

There's no continuity between the H and V coils, thankfully. CR301 checks out OK on my diode tester. R305, the 1.5R you mentioned, is reading 1.5R on my meter. Both components tested with one leg disconnected from the board.

Interestingly, the R305 on the other board is reading 1.6R. I'm a bit confused as the one taken from the Kaypro board looks like a 1R - Brown,Black,Gold - whereas the spare is Brown,Green,Gold. Anyway, I replaced it.

Can't say it has improved much after this (and I've adjusted V-Size and V-Lin to try and improve it):

IMG_9301.JPG

But TP3 seems to be cleaner, albeit with the same shape to the down ramp (this captured with the test image displayed as before):

TP3 waveform 2.png

My conclusion, then, is that CR301 / R305 aren't the cause of the problem.
 
About this. The specification for this resistor is 1.2R @ 2W. Given that wattage is the product of voltage and amperage, doesn't that imply that the circuit shouldn't draw more than ~166mA given a 12v power supply? Or am I being a bit thick?

The resistor, being a two terminal device, has no knowledge of the potential with respect to common (or ground) that it is operating in, unless one of its terminals is connected to ground, which in this case it is not, so a value of 12V with respect to ground has no meaning in a calculation relating to the resistor's power dissipation.

Think of the resistor, in series with the power supply feed to the H scan stage, as being a device which converts a current through it, into a voltage drop across it with the relation V=IR.

So lets say the current is one Amp via the resistor. And say its a 1.2R resistor. , from V= IR, the voltage drop across the resistor terminals is 1 Amp x 1.2 Ohms = 1.2 Volts.

However, the power dissipated in the resistor, you can calculate as V x A = 1 Amp x 1.2 V = 1.2 Watts, or you could calculate it as I^2 x R = 1 x1 x 1.2 = 1.2 Watts, or you could calculate it as V^2/R = (1.2 x1.2)/1.2 = 1.2 Watts, the same in all cases.

Going back to the problem we have with the vertical output stage. The problem here must be simple, but for some reason we cannot put our finger on it.

Lets assume for a moment, that all the resistors are normal, all the capacitors are normal, the IC is normal and the power supply is normal.

That would lead to the inescapable conclusion that the yoke is defective, perhaps shorted turns in one of the two coils. This sort of thing though, doesn't often happen, but not impossible.

If we state that the V yoke coils are normal, that forces us back into the position that we have missed something. That the capacitors are not normal (unlikely as you have replaced every single one around the vertical stage) or that one of the resistors are out of spec. The IC itself was eliminated by substitution. So double check everything. So use the model Police Detectives use investigating a Crime; Assume nothing, trust nobody and check everything.

Playing devils advocate, the thing that we have not checked properly yet is the yoke. If you cannot get access to the yokes internal connections, instead, disconnect the vertical yoke coils, substitute in a plan resistor for it , say 10 to 18 Ohms at least 1W rated, and check the waveform across that resistor on the scope to see if it still has the same defect in the sawtooth voltage profile, or not.
 
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This last test also occurred to me just now as I was driving home (minus the load resistor). I probed at TP3 and got this:

DS1Z_QuickPrint3.png

It's not exactly the same as before (no spire at the start) but we still see the level + rampdown that is indicative of the on-screen artefacts. Just to be clear, the screen at this point had a single hirizontal line showing, dead centre, so I can confirm I did disconnect the vertical yoke coils (I've been knowwn to make rookie mistakes before!)
 
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Ok, the yoke is fine most likely.

What next ?

Lets make the following bold statements:

All the resistors are normal (tested)
The capacitors are normal (replaced)
The diodes are normal (tested)
The IC is normal (replaced with no effect)
The power supply is normal (tested)
Yoke is normal (presence or absence doesn't affect the problem)
There are no pcb track breaks or bad connections anywhere (you have probably inspected over & over).

Then the only thing left I can think of is that the V sync input signal is abnormal at the IC pin 8, and upsetting the IC's operating conditions, either its pulse duration or its DC axis.

Plan:

1) Re- connect the V yoke coils. Try disconnecting the V sync input and adjusting the V hold until the picture is just slowly rolling by, is the Linearity defect still in the raster lines ?

2) Connect one channel of the scope to the IC's vertical output again and trigger/sync lock to that.
Connect the scope's other channel to the collector of Q298 and we will see what the sync signal looks like there.
 
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Ok, the yoke is fine most likely.

What next ?

Lets make the following bold statements:

All the resistors are normal (tested)
The capacitors are normal (replaced)
The diodes are normal (tested)
The IC is normal (replaced with no effect)
The power supply is normal (tested)
Yoke is normal (presence or absence doesn't affect the problem)
There are no pcb track breaks or bad connections anywhere (you have probably inspected over & over).

Then the only thing left I can think of is that the V sync input signal is abnormal at the IC pin 8, and upsetting the IC's operating conditions, either its pulse duration or its DC axis.

Plan:

1) Re- connect the V yoke coils. Try disconnecting the V sync input and adjusting the V hold until the picture is just slowly rolling by, is the Linearity defect still in the raster lines ?

2) Connect one channel of the scope to the IC's vertical output again and trigger/sync lock to that.
Connect the scope's other channel to the collector of Q298 and we will see what the sync signal looks like there.

I need to amend those statements somewhat.

  • Resistors haven't been checked yet.
  • Only those caps I mentioned are changed.
  • Only one diode has been tested.
  • PSU is an AT-PC unit which seems to work. Has no more noise than any other, at least.
  • PCB has been looked at but happy to do it again.

Regarding the compoments I haven't touched (tested / repaced), is there an order of preference? I asked this before, as I didn't want to just blindly do them all. I'm guessing the vertical section. But the down ramp at TP3 is generated by the TDA1170 [I assume], in accordance with its inputs. But which inputs? Meaning by that, which part of the circuit provides these inputs?

I did focus on the V-LIN section capacitors but I didn't check the resistors. In fact, the only ones checked / replaced is R305 and of course the smoker, R478.

As to 1) and 2) I will perfom the tests and upload the traces.

Cheers
JonB
 
All diodes checked out OK, with the exception of CR407 as it is inline (part of the HT lead which I'm not messing with).

Found a burned looking resistor R318 but it checks out OK - measures 8.6R, should be 8.2R. Replaced it, has not helped.
 
Jon,
I have an Electronics book that explains how to do a "Ringing Test" of a Transformers Windings.
It will tell you if there are shorted turns. It might be a good way for you to test the Vertical
and Horizontal Windings. It starts on Page 178 of the PDF at this URL.
https://u.pcloud.link/publink/show?code=XZp6a5VZu6V2IO6BgNbLijbSF5o5xy5ptpvy

It might be an easy way to verify the Windings.

Larry
 
The H yoke coils are normal, otherwise the H scan would be abnormal and it is ok.
The v yoke coils are likely fine as when they were replaced with a dummy load resistor, an identical scan current defect remained present and on top of that, the v flyback pulse is normal with the v yoke present, indicating the v yoke inductance is normal.
 
Well.. I tried to read the technical docs for the TDA1170S vertical defection IC but it made very little sense to me. What I'm looking for is "how do the discreet components around the chip generate the vertical yoke waveform?" - but I can't follow it. During today's trial and error session, I had a look at the V-LIN circuit again. Specifically R314 (labelled 1M but is actually 470k), R306 (labelled 47K but actually 470K), R309 (470K), R310 (56k). These all check out, although I replaced R307 and R309. I kept R306 as 470k on the grounds that the monitor worked properly before it failed.

I have to say, resistor failure seems very unlikely without some sort of witness marks on the body, such as burns.

Whilst poking around, I happened to notice that the H-Width choke L403 seems to affect the vertical pinch slightly when a screwdriver gets near its core. This also affects horizontal width as expected, but I wasn't expecting any effect on the vertical pinch.

I wonder if the horizontal circuit is causing the vertical pinch by virtue of its excessive current pull which is still heating up the fat resistor R478. The way to check this, perhaps, is to see if there is any correlation between the waveforms at TP7 and TP3 other than what you'd expect from the H and V sync pulses. Sounds like a plan?
 
Two traces. In each picture the magenta trace is TP3 that we have seen before. They are shown with TP8 and TP7 of the horizontal circuit, and there is no obvious correlation between them and TP3's weird plateu. So, scratch that idea then!

TP3 vs TP7

TP7 TP3.png

TP3 vs TP8

TP8 TP3.png

(Click to enlarge.)
 
Right, I'll stick my neck out and say the deflection issue is sorted. On a hunch I decided to try and power the VDU with the Kaypro PSU (hopefully you'll recall that I had it running with an AT-PC unit with the Kaypro itself on its own PSU - with the grounds connected on both supplies). And the result was a reasonably straight picture that needs some adjustment, but is, nevertheless, straight.

TP3 with Kaypro PSU.png

After some adjustment:

Working, sort of.JPG

As you can imagine, this is a huge relief, but I still have issues. Overheating, presumably in the horizontal stage, and the Kaypro PSU is glitching with the VDU connected (which I guess is down to the amount of current it's pulling being over spec - recall I said it was ~1.2A and I know that is deemed "normal" it still seems to be upsetting the PSU).

The PSU has only had its main two caps replaced. Racapping it fully would seem a good idea, but the resistor on the VDU, R478, is still getting very hot so there must still be a problem there that needs to be addressed.

Now what it is doing is clicking repeatedly, starting slow then getting faster by the second. Each click is accompanied by a screen flicker, and the clicks are coming from the video board, not the Kaypro's PSU. But when we look at the trace of the 12v rail during these events, we can see something...

DS1Z_QuickPrint2.png

Each one of those voltage drops is a click.. so I might assume the PSU is weak and needs some attention. I'm surprised I cannot hear any noises coming from it, they normally tweet when this sort of thing happens.

So, I think we need to return to the Horizontal Section of the VDU and see if we can find out what's pulling the excessive current, or debug the PSU.
 
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There must have been a defect in the supply voltage to the vertical output IC, though I thought that we had checked that on the scope.

In any case if you run the VDU from a stable 12V supply, that is not being overloaded, I suspect the VDU is working fine. Make sure the 1.2R resistor is replaced with a 1.2R 2W rated resistor and double check the voltage across it and calculate the power, it should be well under 2W.

When SMPS's are overloaded they go into a shut-down mode and recover and you get an interrupted output, as they shut down & recover and clicking sounds.
 
There must have been a defect in the supply voltage to the vertical output IC, though I thought that we had checked that on the scope.

We did. The 12v rail from the PC-AT PSU looked OK on the scope, but I did try another PSU and it gave the same result. The picture only straightens with the original PSU connected, how odd..!

In any case if you run the VDU from a stable 12V supply, that is not being overloaded, I suspect the VDU is working fine. Make sure the 1.2R resistor is replaced with a 1.2R 2W rated resistor and double check the voltage across it and calculate the power, it should be well under 2W.

Resistor is still overheating although it now reads ~4 Ohms, I suspect due to it getting hot in the first place. There's a bulge in its case now... I'll replace it and see if it behaves. Meanwhile the whole unit is pulling 1.277A steadily, from the PC-AT PSU. Resistor is hot to the touch but not burning and the picture is stabe (though pinched due to being connected to the AT PSU).

When SMPS's are overloaded they go into a shut-down mode and recover and you get an interrupted output, as they shut down & recover and clicking sounds.

Agreed, but the clicking is coming from the VDU board, which I think is a bit odd, but is undoubtedly caused by the PSU dropping out. I am going to recap it and see if it improves.
 
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Jon,
In the past we used the OHMITE Series 93 - 3.25 Watt Resistors that were Vitreous Enamel Molded.
I forget the specific application but they worked really well. We likely purchased them from Newark
Electronics or another larger Electronics supplier. If you could locate a few of these, I'm sure they
would do the job.

Larry
 

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In this case, as noted before, the resistor will run hotter as its value increases, up to around 8 or 10R I suspect and values higher than that it would start to cool down again. By then though the H output stage would only be getting about 6V.

Often, as carbon composition resistors get heated their value goes up. Bad news in this case, as the extra heat causes more degradation of the resistor. Best replaced with a 2W metal film resistor, 1.2 Ohms and it will be just fine. The current via that resistor will be around 1A or a little below possibly, perhaps 800 to 900mA, perhaps even a tad lower, as the theoretical current is in the order of 700mA to the H output stage. The rest of the current is supplying the V output stage that the resistor does not supply.

Probably the reason the power supply rail defect was missed on the scope was the scope's time-base rate wasn't suitable, this is especially so with a digital sampling scope, vs an analog one and things can get missed. The trick to have avoided that trap would have been to put one channel of the scope on the vertical IC's output, locked to that then adjusted the sweep rate until the linearity defect was visible as it was in some of your recordings, then put the other channel of the scope on the IC's power supply feed, and it would have been pretty obvious that the power supply rail was defective.
 
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Regarding the resistor, I bought a wire wound part that I think should do the trick. It's a bit high rated (7A) which I may come to regret, but it'll do for a test. Not expensive, but I will seek a metal film resistor anyway.

The Kaypro PSU was defective before - it was tweeting when the machine accessed its drives - so replaced its main caps and put the VDU on a separate PSU to test. Yes, the VDU had failed in some way (maybe the PSU dropping out then coming back repeatedly broke it) and something I changed got it running again. Recall I had the brightness on zero so I'm not sure which component it was, but likely a capacitor in the H section. All the vertical pinch stuff was a red herring as it wasn't the VDU but the PC PSU I'd connected it to. However I did learn some useful things, which to me is a big bonus.

Anyway.. roll on delivery day when all the new caps turn up!
 
In this case, as noted before, the resistor will run hotter as its value increases, up to around 8 or 10R I suspect and values higher than that it would start to cool down again. By then though the H output stage would only be getting about 6V.

Often, as carbon composition resistors get heated their value goes up. Bad news in this case, as the extra heat causes more degradation of the resistor. Best replaced with a 2W metal film resistor, 1.2 Ohms and it will be just fine. The current via that resistor will be around 1A or a little below possibly, perhaps 800 to 900mA, perhaps even a tad lower, as the theoretical current is in the order of 700mA to the H output stage. The rest of the current is supplying the V output stage that the resistor does not supply.

I bought a wire wound part that I think should do the trick. It's a bit high rated (7A) which I may come to regret, but it'll do for a test. Not expensive, but I will seek a metal film resistor anyway.

Probably the reason the power supply rail defect was missed on the scope was the scope's time-base rate wasn't suitable, this is especially so with a digital sampling scope, vs an analog one and things can get missed. The trick to have avoided that trap would have been to put one channel of the scope on the vertical IC's output, locked to that then adjusted the sweep rate until the linearity defect was visible as it was in some of your recordings, then put the other channel of the scope on the IC's power supply feed, and it would have been pretty obvious that the power supply rail was defective.

I'm pretty sure I did that but found no correlation (I posted the traces). Of course, I can't profess to being an expert with the Rigol as it took me so long to switch the signal inversion off.. :rolleyes: But I do love it. One of the most useful tools I have apart from the Hakko.
 
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