Cub 1431APMS4 fault finding

[astrovark]

Hi all,

I have a Cub 1431APMS4 (which appears to be impossible, I can find 1451 with APMS4 designation, but not 1431) which suddenly decided to not display a picture any more. Does appear to have EHT (get the static crackle on power on). Tried two different BBC's on the TTL input, no response (TTL appears to go straight to the main board, it also has other inputs that go to a decoder? board first then onto the mainboard).

Looking for ideas - I do have services manuals for series 3,4,5,6 and the C suffix variations, but having trouble matching up this board with the service manual information (I think it's a series 3 ... maybe?)

Was working absolutely fine until it didn't tonight.

Many thanks

Ricki

[SiriusHardware]

Hi Ricki - welcome to the forum - as a new forum member your posts will be held for moderation for a while, but after a few posts that will wear off and you will be able to post without any delay.

Obviously your monitor was intended to be used with a computer so this is ostensibly the right section to post your problem in, but technically speaking it is really more of a CRT display problem - there are as many CRT experts on this forum as can be found anywhere in the world but not all of them are interested in computers - see how you do here first, but you may ultimately get a better / wider response if you post about this in the 'vintage television and video' section (these days, anything with a CRT is legitimately classed as 'vintage').

[SiriusHardware]

Afterthought: This particular variant doesn't also have analogue RGB input does it? - just wondering if it could have been accidentally switched to the wrong input mode.

[ortek_service]

I did once fix a Microvitec Monitor once around 35yrs ago - It could have been a 1431, but I don't think the design varied too much between these (mostly the CRT dot-pitch). It was the most-common metal cased one, that lets you do an E.T. impression with spark from your finger (and quite a bit of a static shock!) if you touch the screen then the earthed metal case!

IIRC, there were some around 150k 2W (carbon film?) resistors that (at least one) had gone high - possibly one to the 'A1' / 'Screen' grids on the CRT.
I can't recall if these were on the 'CRT base panel' (On the back of the CRT's neck) or the main PCB.
However by now there may be a few Electrolytics failing (Although these may have had their hardest life in the early days of the monitors being on all day in classrooms etc that some museums have now recreated).

I do recall there being an article on fixing these in the (UK) Television (Servicing) magazine, that it may be possible to find a copy of online.

There are some past threads on this forum, such as: https://www.vintage-radio.net/forum/showthread.php?t=145002
And maybe: https://www.vintage-radio.net/forum/showthread.php?t=201981

And as these monitors were mainly used with Acorn 'BBC' computers in the UK, these quite a bit of info on the Stardot Acorn forum, such as: https://stardot.org.uk/forums/viewtopic.php?t=24584
https://stardot.org.uk/forums/viewtopic.php?t=19673
- Where they mention:
“Servicing the Microvitec Cub Series 3” - Arthur Rumbelow G3KKC. Television August 1992, page 718.
“Servicing the Microvitec Games Monitor” - Peter Hubbard. Television October 1994, page 870.

I've found the first of these on this webpage (along with an explanation of the model numbers): https://www.radios-tv.co.uk/microvitec-monitor-1431ms/

[astrovark]

Hi,

This variant is switchable between TTL and Pal Video, and has the audio in. I think this is where the AP designations come from in the model, I just couldn't find that variant in the service manuals I have (covering series 3-6, and the 3000, and C variants - inherited from a past (RIP) member here, Lucien Nunes, who was a very good friend of mine since school days). I'm more certain now that this is a series 3.

This particular monitor was from the collection he and I made a long time ago, along with the BBC B and BBC Master that are used with it. It sat for many years (nearly 20) before traveling to me here in Aus. All are from schools originally, in Barnet (London).

So, TTL switch is in the right configuration, always good to check

Currently recapping the mainboard, did find some references to the SMPSU being unhappy if certain caps give up, and I see no reason not to do all the electrolytics.

Thanks for the thoughts and ideas so far.

[astrovark]

No luck with the recap

Time to probe the volts

[ortek_service]

Sorry about the loss of your friend, Lucien Nunes - I recall seeing him featured on the News, showing some of his collection, with a thread on here about this, a short while before he sadly passed-away.


If your Microvitec is based on their series-3, then hopefully that 'Television' servicing mag article on repairing these at the end of here is useful: https://www.radios-tv.co.uk/microvitec-monitor-1431ms/
And I think I referred to that, when fixing one.
Plus of course many on Stardot have these (usually ex-school) so eventually require fixings after many hours use.

IIRC, the power resistor(s) that had failed was fairly-obvious that it had a fairly-stressed life, as was discoloured / surface flaking-off?
I don't recall changing any capacitors, but it wasn't that old back then.

And I believe there was the HT-supply (a bit over 100V?) at one end and should have been similar on the other end (or at least >>0V). So some voltage checks on either side of the power / high-value resistors or around the pins of the CRT should hopefully find the problem.

[cmjones01]

If you've got an EHT crackle at switch on then most of the difficult bits must be working, which is a good start. In your situation I'd start by trying to trace the RGB input signals from the socket, via the decoder board (I'm guessing the decoder board uses a standard decoder IC like a TDA3561 or, as I've seen in some other Microvitec products, a TDA3300) which probably uses the "teletext" RGB inputs of the decoder chip to serve the RGB input. Check that the decoder chip power supply and its RGB/composite switching voltage, contrast and brightness control voltages. Even if we haven't got the Microvitec circuit diagram for the decoder board, the data sheet for the decoder chip will give you most of it.

Also check the power supplies to the RGB video output stages. I can't remember whether they're on the CRT base PCB or the motherboard in these monitors. I have a 1451 in very frequent use here, mostly by my children with our BBC Micro, so could make comparative measurements if it helps.

Cheers
Chris

[astrovark]

Hi

Ref Lucien - yes, the BBC did an interview, but unfortunately time beat him and the collection was split.
Most found good homes, the sheer amount of stuff wax impressive tho!

RGB goes straight onto the mainboard, PAL to a decoder. Waiting on some IC's (TDA1170, TDA1180) to arrive, and tracking down the transistors, as am sure I will be needing them eventually.

Read through that article, thanks all for the links. Plan to start checking the various power rails later this week.

[astrovark]

Well, whilst some voltages are OK I have some really screwy voltages on D10, D21 and D7. I'm no SMPSU specialist, so time to pull out components and test them one by one I guess all around that circuit - oh, and track down some old transistors.

[ortek_service]

Originally Posted by astrovark

Hi

>>

RGB goes straight onto the msinboard, PAL to a decoder. Waiting on some ic's (tda1170, tda1180) to arrive, and tracking down the transistors, as am sure I will be needing them eventually.

Read through that article, thanks all for the links. Plan to start checking the various power rails later this week.

If those are the RGB CRT Cathodes driver-transistors, then probably unlikely for all 3 to have failed. And much-more likely to be a common supply to these / a power resistor or diode that gets quite warm that might have failed.
- Although (as recommended in that Television Servicing article):
It is worth checking that TR907 Blanking Transistor hasn't gone short-circuit. But should be able to easily check that, by measuring resistance across collector & emitter in-circuit. Or just remove it temporarily, as should still get a picture (just with flyback lines visible).
And check voltages on TR106 Beam-Limiter - In particular that there's approx 12V on the emitter - to ensure that it's working OK and not inhibiting the voltage supply to the video circuitry when it shouldn't.


The TDA1180 TV Horizontal 'Processor' is also probably going to be OK, as a lot of that needs to be working in order to generate the EHT.
And problems with the vertical processing parts, would most-likely show-up as reduced height / 'frame collapse - a single bright white horizontal line across the centre of the screen.
The TDA1170 'TV Vertical Deflection System' is maybe also unlikely to have failed, as that too would result in 'frame-collapse' etc.
It is possible that the picture is being deliberately-blanked, if there is frame-collapse, in order to prevent screen-burn. But briefly turning up the 'A1' / 'Screen' first-anode voltage potentiometer (Noting where it is set to first, in order to return it to that) - Often co-located with the focus on most modern Line Output Transformers - will often allow this to be seen.

If you've got an oscilloscope, then you can also check the drive waveform on the vertical-deflection coils, to see if this is a problem .


So measuring the voltages on the CRT's neck pins on the trackside of the 'CRT base panel' is likely to be a lot quicker in finding where the fault lies, than swapping the IC's or transistors.

[ortek_service]

Originally Posted by astrovark

Well, whilst some voltages are OK I have some really screwy voltages on D10, D21 and D7. I'm no SMPSU specialist, so time to pull out components and test them one by one I guess all around that circuit - oh, and track down some old transistors.

Well looking at the SMPSU circuitry D7, D10 & D21 are on the primary-side, and are likely to have quite-high switching waveforms on them, when this is running.
So it may be difficult to get sensible DC voltage readings on these.

I'd be inclined to measure the DC voltages on the cathodes of the secondary-side rectifiers, first, as the voltages here are what really matters.

D22(k): = +200Vdc
D23(k): >= +124Vdc
D24(k): >= +18Vdc

- If these three outputs all measure about right, then it's most-likely that the SMPSU is all OK, and the fault is elsewhere.

Plus then check that the 78M12 & 78L05 regulators are producing the right voltage outputs.

[astrovark]

Hi, thanks for the input.

I was measuring the voltages from the test points marked on the Series 3 schematic (actual one, not one downloaded). They were all quite a lot higher than indicated, but I'll give the points mentioned a try as well. Some of the other voltages were low compared to the schematic.

When I sit down in front of it again I'll record and relay the voltages that I'm seeing.

In terms of semiconductors, I'm working on the basis that irrespective of the fault present right now, having spares of a resource that is getting harder to find might be wise if I want this monitor running for years to come May my tripler never die

[McMurdo]

Have you checked for dry joints and cracked print? There are several components on the main board (and crt base) that run hot and have a tendency to get poor joints which can stop the picture appearing for various reasons.

Take a look at the resistors to see if any have discoloured indicated they've maybe drifted. There are a few fusible resistors too and it only takes one to go open, usually as a result of a weak capacitor.

EHT crackle means most of it is working anyhow so it's not going to be a massive fault.

Attacking component changes shotgun style is likely to increase the chance of user-induced faults and and then we'll be going around in circles. Do treat the main PCB carefully especially when lifting it on and off the pcb pillars where it goes under the front of the tube as the components are easily knocked and the print can punch-through almost invisibly.

[astrovark]

So far I've been over the main board both by eye and tracing voltages. When I recapped (which I decided was worthwhile anyway) I also reflowed any joints that looked slightly suspicious. I haven't done the CRT base board yet, it's on the list probably for this weekend.

Failure mode was very sudden tho - it worked one day, I had to move it to put a new shelf underneath it, when I powered it on afterwards I had red neon and EHT crackle, no picture. Tap'n'test didn't find anything loose (so far). No bang, no magic smoke, no smells - just no picture. Tested with 2 different leads, 2 different BBC micros.

Not looking to do random component replacements, I'm being guided by here, the articles, and the Series 3 fault tracing charts atm! I am however building some stock of relevant parts for future use even if they are not the cause right now.

My series 3 service manual(s) also includes, usefully, comments in the margins from people that fixed these regularly - Lucien Nunes included

I'm sure Mr Murphy is hiding the simplest possible fault just for fun!

[ortek_service]

Hopefully the IC's in these are usually fairly-reliable, and that spares won't actually be required / are fairly-common ones, so can still obtain working ones.
These may be a bit too modern to have a separate Triple, and this may be efectively integrated into a 'Split-Diode' LOPT - Which could definitely be more difficult to get a replacement for anymore.

I did think about dry-joints etc, but as it hadn't been mentioned that tapping etc made it work, then thought it may be a bit more-permanent.

I think once the appropriate places voltages are checked, then they will reveal a lot more as to what is going-on - I'm still thinking it is most likely a power resistor that has eventually burnt out / a ? 10k resistor that has gone very-high due to high-voltages across it eventually causing this.
But voltage checks - particular around the CRT base, should hopefully find this fairly quickly and show whether it is a too-high resistor or a (blanking etc.) transistor that has gone s/c.

[McMurdo]

the 1431 does use a tripler (usually a green Roderstein one) but they're pretty reliable, it also sources the focus but good EHT is a good indicator the tripler is ok.
I think the only IC I ever changed was the 1170 but again it does not result in a blank screen as already mentioned.

[astrovark]

Voltage update: some higher voltage rails seem a little low - e.g D22 cathode is 188V instead of 200, R225 is 61v (should be 68), R137 is 122.7 (should be 124 so prob within tolerance). 5V, 12V and 18V volts all appear to be good. All fusible resistors are closed, and those I can get to at the moment (its still in the case) read good.

Some progress - I appear to be missing the 200v-675v feed onto the CRT tube base (white wire). That's probably important

Further progress - looks like R326 is o/c (150K/2W) - now to rummage through my components!

[ortek_service]

Originally Posted by astrovark

Voltage update: some higher voltage rails seem a little low - e.g D22 cathode is 188V instead of 200, R225 is 61v (should be 68), R137 is 122.7 (should be 124 so prob within tolerance). 5V, 12V and 18V volts all appear to be good. All fusible resistors are closed, and those I can get to at the moment (its still in the case) read good.

Some progress - I appear to be missing the 200v-675v feed onto the CRT tube base (white wire). That's probably important

Further progress - looks like R326 is o/c (150K/2W) - now to rummage through my components!

Good to hear you may have now found the problem.
I wouldn't be too concerned about a 5 or 10% drop in those HT rails, as they may be dependent upon loading, mains voltage and maybe an HT voltage setting adjustment - Although also possibly an indication that their smoothing Electrolytics may not be working too well, so might be worth checking, but probably not going to stop it showing something on the CRT.
If the lower-voltage regulated voltage rails are all OK, then those should be more-important.

But the missing 200-675V feed to the CRT is much-more significant.

And a 150K 2W resistor does sound very-familar, as IIRC that's what had failed in the one I fixed many decades ago (Having to go to a Maplin shop - when they existed! to get a replacement, back then). So I'd suggested this could be the problem, in my first post reply near the start of this thread! - as below

Originally Posted by ortek_service

I did once fix a Microvitec Monitor once around 35yrs ago - It could have been a 1431, but I don't think the design varied too much between these (mostly the CRT dot-pitch).
>>
IIRC, there were some around 150k 2W (carbon film?) resistors that (at least one) had gone high - possibly one to the 'A1' / 'Screen' grids on the CRT.
I can't recall if these were on the 'CRT base panel' (On the back of the CRT's neck) or the main PCB.
However by now there may be a few Electrolytics failing (Although these may have had their hardest life in the early days of the monitors being on all day in classrooms etc that some museums have now recreated).
>>
>>

[astrovark]

@ortek-service - Yes. you did - only when tracing the circuit for the missing voltage rail did I notice that I did have a quick look for it at the time but didn't spot it. Thanks for all the input.

All the electrolytics have been replaced anyway, I'll recheck the voltages once I've replaced that resistor. Looking at it really closely there is only the slightest indication of failure, a darker ring at the middle of the resistor. Fortunately I've found somewhere relatively local I can go get a replacement on Monday, given the lack of "high street" component shops nowadays!