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Old 3rd Feb 2019, 11:20 pm   #1
TheSquirrel
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Default Microvitec Cub monitor model identification and repair

Hello,

I'm new here and this is a place of gurus. Forgive me for troubling you.

I have a Microvitec Cub monitor, acquired several years ago. The Microvitec label on the back has a serial number but where the model number should be, it's blank. If it helps, there's also a 'National Power' stock label on it.

A hand-written sticky label on the side says "1441MS4". The obvious answer is that it's a 1441 M series 4. It's definitely an M series but I can't be certain whether it's a 1441 or a 1431 and whether it's a series 3 or 4. From circuit diagrams obtained for the series 3 and 4, it looks more like a series 3 but I'm uncertain and likely need to look harder. Did the tripler change from the series 3 to the 4 and is this the easiest way to tell the difference? How does the 1441 differ from the 1431 internally? Any help would be gratefully received. Apart from a Microvitec label on the edge of the main board, there is no other labelling to help. From the series 3 circuit diagrams, the tube base board appears to match the hires rather than standard res diagram and so my inexpert conclusion is that it's a 1441.

It worked nicely for several minutes when it arrived and then displayed a single thin line in the middle of the screen without even a phut to accompany the change. 2 years later, I've finally got around to thinking about it. I've been looking at the circuit diagrams and trying to determine the cause of the frame collapse and I've been looking in the area of the frame coils circuits, fusible resistors, electrolytic capacitors and the vertical control IC, IC301, a TDA1170s. No damaged components leap out on a visual inspection but what does that really tell me as there could be dry joints and cracked traces: 1983 was a long time ago and no doubt it was stored in a loft for a long time before it travelled in the back of a courier's van to me.

I'm trying to drag my electronics knowledge back through a 25 year void and it wasn't the greatest then anyway: much textbook study but very basic practical and little in the way of diagnostic repair. Any help in guiding my thinking through this would be great. I'd like to work it out rather than throw components at it and the hope is to understand it, repair it and then use it with the BBC Micro that I've fettled. The problem has already gotten under my skin.

I've been looking at building a safe discharging method for the capacitors and CRT. Buying resistors of sufficient wattage to cope is proving more of a challenge than I'd expected but I clearly don't know enough. Any help there would also be appreciated. I was concerned that the instantaneous wattage of the CRT discharge through 10 M Ohm would be initially in the kWs for a fraction of a second before diminishing.

I've attached a top and back view of the monitor's internals. The tube base board is loose and the main board is off the stand-offs with most connections removed as I had dismantled it to look under the main pcb - just to explain the photos.

https://jmp.sh/TC8HQ1U
https://jmp.sh/D1eECTb
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Old 4th Feb 2019, 1:32 pm   #2
mhennessy
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Default Re: Microvitec Cub monitor model identification and repair

Welcome to the forum

Series 4 didn't have a tripler, so that looks like an earlier model. I'd say Series 3, but will also add that I've no idea about Series 1 or 2. Did they exist? Perhaps they did, but pre-dated the Cub?

I must say that I'm no expert on these monitors specifically, but we've discussed them many times before, and what we've all learnt from these conversations is that there were many, many variations made over the years. Sometimes model numbers (when present) don't always make sense. Perhaps innards got swapped around to keep them alive - they were bought in large quantities by many organisations - or perhaps different innards were installed from new to keep them flying out the factory doors? Certainly, I have one at home which I know is original - the chassis is definitely Series 3, but the model number implies Series 4! My ones have a beige PCB with a black silkscreen that includes more clues about the series - if your board has similar markings, they'll be at the back, towards the middle.

I think I've seen high-res tube bases in standard-res monitors. Maybe they decided that having 2 separate assemblies was more costly than have just one (even if it was perhaps slightly more expensive?). Or again, maybe it was just about keeping the production lines going?

Broadly, the high-res tubes had a dark background, and the standard-res tubes were light grey. I say "broadly" because there are many variants, and as many pedants waiting to correct me. If in doubt, look up the datasheet for the CRT fitted. The best ones had a dot pitch of 0.31mm, which is very respectable for a BBC micro. I suspect those CRTs were intended to VGA monitors back in the day...

OT: I did once make a lovely BBC Micro monitor from an CGA/EGA monitor - I just had to build a simple sync separator. It was the clearest, sharpest display I think I've ever seen on one of these. Unfortunately, I couldn't bear the line whistle, and while I managed to damp down some of the inductors, I couldn't eliminate it completely. That was 20 years ago - I wish I'd kept it, as I wouldn't be bothered by it now! I can still hear 15.625kHz with no problem, but I'm definitely much less sensitive than I once was

Anyway, I agree that dry joints is the most likely thing, but it is very possible that components have failed with no visible signs, unfortunately.

Most people discharge the CRT by simply shorting it. Years ago I made an insulated probe that incorporated a 1k wire-wound resistor that made the crack a bit less scary - more "because I could" rather than any actual concrete reasons. Whatever method you use, it's important to understand that a CRT will recharge itself if left unconnected (read up on "dielectric absorption"), so leave a croc-clip lead attached while it's being worked on. When you discharge, make sure you're discharging to the outer "dag" via the braid rather than the monitor's chassis.

It is possible to work with a charged EHT providing you do not disconnect the earthing between PCBs and dag. However, this is risky, so I mention it for completeness only - not recommended if you're not sure about what you're doing.

Cheers,

Mark
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Old 4th Feb 2019, 5:30 pm   #3
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Default Re: Microvitec Cub monitor model identification and repair

A prime suspect for frame collapse is the reservoir capacitor for the frame output flyback supply, possibly 1000uF/40V or similar and which might be that orange ROE cap visible near the edge of the board in the pic. Even 25 years ago they were failing so regularly that I used to change them on sight, working or not. I can't recall if it takes out a fusible resistor with it. The frame output IC and other stuff did fail too but that was the most common cause.
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Old 5th Feb 2019, 8:55 pm   #4
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Default Re: Microvitec Cub monitor model identification and repair

Thanks to both of you for swift and very helpful replies.

I've made a search for the tube data but can't find anything. The label states, "NEC 370KTB22 - TC09, No. 30704288". The white plastic surround at the end of the neck tube has "JVC" embossed on it.

The ROE 1000uF 40V capacitor has failed with no visible signs. Given what was said about having to change these often, does the circuit design lead to frequent failures of this capacitor? Should I stock up? I appreciate that a loft stored monitor from 1983 powered up in 2019 will present very different failures from its day.

Fusible resistors aren't something that I've seen before. Is it possible that they can fuse without any visible sign? I'm currently determining which are fusible from the circuit diagram and parts list.
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Old 5th Feb 2019, 10:42 pm   #5
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Default Re: Microvitec Cub monitor model identification and repair

You shouldn't need to change that cap again - the originals were just unreliable. I don't have a manual but follow the power rail back from that reservoir cap towards the LOPT, and you should find a diode and the fusible resistor. They can indeed fail without visible signs of heating. The diode might be bad instead / as well. Before changing, check that a short to ground is not still present on the rail due to a failed frame output IC.
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Old 6th Feb 2019, 12:43 am   #6
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Default Re: Microvitec Cub monitor model identification and repair

I've had a quick look in the manual, and the resistor you need to check is R235. It should be 10 ohms. Perhaps this would be a suitable replacement: https://cpc.farnell.com/neohm-te-con...-5w/dp/RE07246

I also looked for the CRT details, but unless I've missed it, the manual doesn't seem to give details about CRT types. Certainly, 370KTB22 is the model number (370mm meaning 14", of course), but I also failed to find anything useful about it online. I'm afraid my examples are up in the attic, so it won't be easy to check the part numbers of those. But as I say, look at how dark the screen looks. The hi-res ones are almost black - lovely
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Old 6th Feb 2019, 8:52 am   #7
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Default Re: Microvitec Cub monitor model identification and repair

Quote:
Originally Posted by TheSquirrel View Post
... trying to determine the cause of the frame collapse and I've been looking in the area of the frame coils circuits, fusible resistors, electrolytic capacitors and the vertical control IC, IC301, a TDA1170s.
I've been looking at building a safe discharging method for the capacitors and CRT.
Generally you are on the right track. For this sort of fault (Frame scan collapse) you don't need to worry about discharging power supply caps, or the CRT for that matter (as there is no need to take the crt's eht connector off or pull off its base socket). The power supply caps will self discharge due to leakage if you wait 20 minutes, likely there will be bleeder resistance anyway, but its really academic for this fault, because the tests you need to do, ideally, are done with the unit powered up anyway.

The common faults for zero frame deflection current in the yoke are failed power supply to your frame oscillator or frame output stage, all of that is in your TDA1170 IC (perhaps a fusible resistor powering it is open). The TDA1170 itself could have failed, they can run hot. Even if the power supply filter capacitor on the TDA1170's power input supply had failed unlikely it would stop the IC from working completely and there would be some frame deflection.

(The frame output stage is very much like its power audio output stage counterpart, its a linear amplifier primarily. The frame deflection output amplifier (in your IC) is designed to apply a trapezoidal shaped deflection voltage to the yoke, which produces a linearly increasing ramp of current with a rapid crt beam flyback, back to the start of vertical scan)

So to fix this fault the best move is to power the monitor with the brightness set to near zero (you don't wan't to burn a horizontal line into the screen) then test the power supply to the TDA 1170. If the supply is there, I would suspect the IC, or one of the components immediately associated with it. If its power supply feed has failed, find the reason why.

If you can't get the exact schematic for your particular monitor, just look up the data sheet for the TDA1170 instead, the circuit will be nearly identical to the example circuit there.

Here is a useful data sheet:

https://datasheetspdf.com/pdf-file/1.../SGS/TDA1170/1
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Old 6th Feb 2019, 2:14 pm   #8
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Default Re: Microvitec Cub monitor model identification and repair

Quote:
Even if the power supply filter capacitor on the TDA1170's power input supply had failed unlikely it would stop the IC from working completely
Not on this chassis! The original capacitors made by ROE were disastrously unreliable and routinely failed dead short and grounded the flyback supply. Any monitor coming into the workshop, for whatever reason, with the orange ROE cap still fitted, would have it changed as otherwise it would soon be back with no frame scan.

The OP mentions his capacitor had also failed - I would have been pretty surprised if he had said it was OK! I expect R235 is O/C.
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Old 6th Feb 2019, 3:36 pm   #9
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Default Re: Microvitec Cub monitor model identification and repair

Thanks again to all for your help. The circuit diagram had pointed me to R235 and possibly D201, but confirmation of those helped enormously.

R235 has gone open circuit. The accompanying diode, D201, appears to function correctly but that conclusion comes only though multimeter resistance and diode checking. The plan is to replace the faulty components and then check around the TDA1170 IC.

The tube is black, as far as I can determine, and the main board includes C225 which is annotated as only being fitted to med and hi res models.
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Old 6th Feb 2019, 10:23 pm   #10
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Default Re: Microvitec Cub monitor model identification and repair

Quote:
Originally Posted by Lucien Nunes View Post
Not on this chassis! The original capacitors made by ROE were disastrously unreliable and routinely failed dead short .
That sounds nasty. I have never seen a ROE capacitor. Though I once had a ERO brand electrolytic short out. I disassembled it to find out why, it looked like there was probably a piece of metallic particulate matter embedded in the insulation between the foils, probably contamination at the factory.
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Old 6th Feb 2019, 11:33 pm   #11
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Default Re: Microvitec Cub monitor model identification and repair

Sounds like you have a nice one

Sadly, a lot of the Cubs where I work were low-res. They were bought to be used as composite monitors in edit suits and similar, so they had the PAL decoder option. Much cheaper than "proper" broadcast monitors, and in a nice, convenient cube shape, of course - but the picture quality was nowhere near as good. But for where they were used, it didn't really matter - it was a fairly pragmatic decision for an organisation (or at least, a part of the organisation) that has nothing like the amount of cash than some like to believe.

While looking at the diagram for C225, I was able to confirm something that I was fairly sure about when I replied earlier. As I didn't have the diagrams to hand, I decided it was safest to not mention it. However, here goes:

Inside the tripler is a chain of resistors - including the focus pot. These resistors will discharge the CRT. I don't know how long it takes, but certainly that CRT will hold its charge for a matter of minutes rather than weeks.

When I eventually get around to sorting out my two examples, I will test for this and report back. In fact, given the amount of questions about these, plus the rather scattered nature of the information available out there, I'll probably put some effort into making a feature on my website about them. At the same time, I'll scan the diagrams. One day...

Something else I double-checked:

As is usually the case, the dag is not connected directly to chassis - it goes via a resistor, and this is used to sense the EHT current and control the beam limiting circuitry. So my earlier advice to discharge to the dag, not the chassis, was good advice. OK, it's standard practice, but occasionally you meet sets where the dag is connected directly to ground.

Oh, and 25kV into 10Mohm is 62.5 watts. I didn't give it a thought when I read your first post the other day, but upon re-reading it, it caught my eye - perhaps as a result of thinking about the resistors fitted inside the tripler (which will be higher than 10M, but won't be G-ohms). If there was 100M total in series, then that's about 6 watts - my guess is they'll be perhaps double that? Something else to attempt to measure when I get them on the bench. I'm also tempted to measure the capacitance of the CRT - those two measurements will enable me to figure out how quickly the EHT discharges.

So many thoughts, such little time.

Forgive the ramble!
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Old 7th Feb 2019, 11:12 pm   #12
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Default Re: Microvitec Cub monitor model identification and repair

Thanks again for all of the very friendly and generous help. This thread is a great read and looking around at other threads, the knowledge herein is remarkable. My interest lies in 80s computers and their various monitors and there is so much to read and learn here.

I've been looking at suitable replacements for the ROE 1000uF 40V capacitor and hope that I can ask another question. Would a Panasonic 1000uF 50V +/- 20%, such as:

https://cpc.farnell.com/panasonic-el...7437?st=1000uF

be suitable, or am I on the wrong track and missing something from the specification? If I can't find a 40V capacitor, is it acceptable to go for the next higher voltage? Is there any problem with doing this? Is there a sensible limit of difference in voltage when replacing capacitors?

The parts list states:
"C224 1000uF 40V ALUM ELECT CAPACITOR - 20+50% RADIAL"

Is the " - 20+50%" a tolerance of -20% and +50%? Hence is +/- 20% the most appropriate as a replacement?

I was considering replacing all of the electrolytic capacitors on the main board. Is this wise? Is there anything else that I should be looking at replacing, in preventative paranoia, or that will improve things: assuming that the replacement of the known faulty capacitor, fusible resistor and diode leads to success?
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Old 7th Feb 2019, 11:26 pm   #13
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Default Re: Microvitec Cub monitor model identification and repair

Quote:
Originally Posted by mhennessy View Post
Sounds like you have a nice one

As is usually the case, the dag is not connected directly to chassis - it goes via a resistor, and this is used to sense the EHT current and control the beam limiting circuitry. So my earlier advice to discharge to the dag, not the chassis, was good advice. OK, it's standard practice, but occasionally you meet sets where the dag is connected directly to ground.

Oh, and 25kV into 10Mohm is 62.5 watts.
Since the glass bulb of the tube with its internal and external dag forms a capacitor, there is only current flowing in the external dag circuit when the capacitance is charging and discharging, so I cannot see a way that the current in the external dag circuit can be used to measure the average EHT current, the EHT current is the beam current and that can be measured at the cathode of the CRT. Most sets I have seen the external dag is grounded.Though there might be some value of measuring dynamic changes.

Also here are my thoughts on discharging the CRT:

There is really no need to discharge it most of the time. The charge is built up in the glass bulb between the internal and external aquadag, the internal dag is connected to the final anode clip. The internal aquadag is also connected to the final anode structure in the CRT's gun assembly, but that is insulated (by a vacuum) from the pins that connect to the tube socket and the other electron gun metallic elements that are connected to the tube base. I have never received a discharge of any kind unplugging the CRT base and touching its pins, even when the bulb remains charged. After all, if there was any leakage across the gun's final anode to focus electrode or other gun structures, the bulb charge would already have dissipated.

So with the set switched off, you can take off the CRT socket and pcb assembly off the CRT without being exposed to the charge stored in the CRT's bulb. You only require to discharge the bulb if you plan to take off the anode connection (EHT clip) which for many fault finding exercises is not required.

There is only one common reason to discharge the CRT, that is if you are removing the final anode cap and might touch that lifting out the CRT or carrying the CRT across a room and get a zap and drop the CRT on the floor as a result.

The total charge stored in the CRT could give an an unpleasant zap, but not too risky, but it might make a person drop the CRT.

Some people short the anode cap out to the external aquadag or ground, it is not really a great idea actually, without current limiting, as it is possible the dag on the internal connection to the anode cap could be damaged by very high peak currents. It would be better to do it with a 100M Ohm high voltage rated resistor, if you really want to do it.

Therefore, the best tool to do it with, is a standard EHT voltage probe that has this resistor already built into its body and you can slip its tip under the plastic EHT cap and its flying wire is grounded to the sets chassis or external CRT aquadag. So with the set switched on you measure the EHT voltage, with the set off, the probe gently discharges the CRT.

Using wires, screwdrivers etc etc, you might accidentally discharge it to a pcb component and create a new fault.
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Old 8th Feb 2019, 12:15 am   #14
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Default Re: Microvitec Cub monitor model identification and repair

ROE and ERO capacitors (Roederstein) are generally pretty well regarded in industry but in the cub they seemed to be a failure point and when they've gone they seem to be rather old already. I know the cubs at our school (showing my age) were switched on first thing in the morning and only turned off at night by a key operated contactor on the wall. Most of those yellowy gold caps seem to go short, ISTR when you took them off the board they were disconcertingly lightweight. Any cub these days probably needs all the electrolytics checking by now, a good resolder of the power resistors, smpsu and lop transformers etc, but we've been here in another thread.
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Old 8th Feb 2019, 1:31 am   #15
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Default Re: Microvitec Cub monitor model identification and repair

Quote:
Originally Posted by Argus25 View Post
Since the glass bulb of the tube with its internal and external dag forms a capacitor, there is only current flowing in the external dag circuit when the capacitance is charging and discharging, so I cannot see a way that the current in the external dag circuit can be used to measure the average EHT current, the EHT current is the beam current and that can be measured at the cathode of the CRT. Most sets I have seen the external dag is grounded.Though there might be some value of measuring dynamic changes.
That is how beam limiting is done in the vast majority of sets that I've encountered. Only a small number of those had the dag grounded directly. The way it works is really simple: the dag (and negative end of the EHT generator) goes to chassis via a resistor. There is 25kV nominally between dag and EHT. Beam current - via the cathodes - passes from EHT through to chassis. Hence, beam current passes through the resistor that connects dag to chassis.

If you look at the Microvitec schematic, it's trivial to see how the potential of the dag, relative to chassis, is used to effect the beam limiting. In many other sets, the same signal is used to vary width and height to reduce "breathing".

If you'd like more details, there are countless explanations of this in manufacturers' manuals and in articles in Television magazine. Of course, there will be variations on the theme, but this is how it's usually done, I promise

The sets that did have earthed dags (often Sony) took the negative output of the EHT system to ground via a resistor. The effect is the same, give or take the time constant formed by the capacitance of the CRT. The Sony schematics can be a pain to follow, especially on a screen, but usually the width and height control elements can be fitted on the screen together with the line output stage, so it's easy to see how they work, and the signal from this that goes off to the signal processing parts of the set is usually labelled "ABL" (auto beam limiting) - knowing that helps you to follow a Sony schematic.



Quote:
Originally Posted by Argus25 View Post
Also here are my thoughts on discharging the CRT:

There is really no need to discharge it most of the time. The charge is built up in the glass bulb between the internal and external aquadag, the internal dag is connected to the final anode clip. The internal aquadag is also connected to the final anode structure in the CRT's gun assembly, but that is insulated (by a vacuum) from the pins that connect to the tube socket and the other electron gun metallic elements that are connected to the tube base. I have never received a discharge of any kind unplugging the CRT base and touching its pins, even when the bulb remains charged. After all, if there was any leakage across the gun's final anode to focus electrode or other gun structures, the bulb charge would already have dissipated.

So with the set switched off, you can take off the CRT socket and pcb assembly off the CRT without being exposed to the charge stored in the CRT's bulb. You only require to discharge the bulb if you plan to take off the anode connection (EHT clip) which for many fault finding exercises is not required.
I agree that you don't always need to discharge the CRT - indeed, I said so in my first post in this thread, albeit with a suitable caution/disclaimer. Yes, you probably* won't get a shock from the tube base pins.

However, what you didn't say: you must keep the lead from the dag connected to the chassis (usually connects to the tube base PCB at least - for flashover protection - but there might be other connections in some cases).

If you don't do this, you have a 25kV battery connected to the chassis via the EHT cable, meaning there is 25kV between the chassis's PCB and the dag. You usually get away with this because the EHT generator usually contains no reverse path, but in this case, there is a chain of resistors joining EHT to chassis, and so you could end up with -25kV at the dag, relative to the chassis. It won't kill you - especially as it comes from a high source impedance - but there's a risk of damaging static-sensitive components, etc. Far easier to just discharge the CRT to eliminate all doubt IMHO.


* Be aware that some Sony CRTs contain internal resistors from EHT to various connections on the tube base pins. See attached...



Quote:
Originally Posted by Argus25 View Post
Some people short the anode cap out to the external aquadag or ground, it is not really a great idea actually, without current limiting, as it is possible the dag on the internal connection to the anode cap could be damaged by very high peak currents. It would be better to do it with a 100M Ohm high voltage rated resistor, if you really want to do it.
Earlier I mentioned a homemade CRT discharge probe. The bulk of that actually came from a Philips TV set (a K12 IIRC). It was attached to the same braided wire that was sprung onto the rear of the CRT - the standard way to connect to the dag. An extra length of braid was then crimped onto this probe, provided by the manufacturer for convenience. The set was sadly scrap, but I spotted this and rescued it. Anyway, the point of relating all this is to say that it seems to be fine to short directly to dag - certainly, I believe that we can safely trust Philips in these matters - they probably know a thing or two about CRTs. See also the attached extract from the K30 service manual.

I also mentioned the dielectric absorption point - easily overlooked.



Quote:
Originally Posted by Argus25 View Post
Therefore, the best tool to do it with, is a standard EHT voltage probe that has this resistor already built into its body and you can slip its tip under the plastic EHT cap and its flying wire is grounded to the sets chassis or external CRT aquadag. So with the set switched on you measure the EHT voltage, with the set off, the probe gently discharges the CRT.
Yes, I agree that an EHT probe is an excellent way to do it. I have a Fluke 80K-40, which is basically 1GOhm. It's a relatively recent acquisition, and I haven't had the chance to try it with a CRT monitor yet, but I've used it to set up oscilloscopes. It's comedically huge. Inspires confidence though

Hope this helps,

Mark
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Old 8th Feb 2019, 1:45 am   #16
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Default Re: Microvitec Cub monitor model identification and repair

Quote:
Originally Posted by mhennessy View Post
Earlier I mentioned a homemade CRT discharge probe. The bulk of that actually came from a Philips TV set (a K12 IIRC). It was attached to the same braided wire that was sprung onto the rear of the CRT - the standard way to connect to the dag. An extra length of braid was then crimped onto this probe, provided by the manufacturer for convenience.
Confirmed - just found the K12 service manual. Not in English, but Google to the rescue

I haven't seen it for a while - really must look for it.
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Old 8th Feb 2019, 1:48 am   #17
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Default Re: Microvitec Cub monitor model identification and repair

Quote:
Originally Posted by mhennessy View Post
That is how beam limiting is done in the vast majority of sets that I've encountered. Only a small number of those had the dag grounded directly. The way it works is really simple: the dag (and negative end of the EHT generator) goes to chassis via a resistor. There is 25kV nominally between dag and EHT. Beam current - via the cathodes - passes from EHT through to chassis. Hence, beam current passes through the resistor that connects dag to chassis.

Mark
Hi Mark, thanks,

That , makes sense if the negative of the EHT generator is connected to the external dag, then through the resistor to ground the beam current could be sensed there. I have never owned a set that did this, the only ones I have seen have the external dag grounded, but mostly the ones I worked on were early models from the 1970's and early 80's.Though they might have has current sense resistors in the EHT negative.

Also, I guess for the CRT types where there is a resistive connection between the internal dag and the gun electrode structures, the stored bulb charge should self dissipate as there is a complete discharge pathway with the power off.

PS: What is a typical value current sense resistor for the application, also are they a physically large part ?

Last edited by Argus25; 8th Feb 2019 at 1:57 am.
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Old 8th Feb 2019, 3:42 am   #18
Argus25
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Default Re: Microvitec Cub monitor model identification and repair

I just had a look at the last color monitor I worked on, the IBM5153, it has a current sensing resistor network in the ground return of the EHT supply, which they use to modify the CRT grid 1 voltage for brightness compensation, but the aquadag is grounded (not returned to the EHT negative).

Also, of note, there would be no need in this monitor to discharge the CRT, because the entire EHT supply has a voltage divider across it that supplies the focus pot and G2 voltage.So it self discharges.
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Old 8th Feb 2019, 3:04 pm   #19
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Default Re: Microvitec Cub monitor model identification and repair

Quote:
Originally Posted by Argus25 View Post
Also, I guess for the CRT types where there is a resistive connection between the internal dag and the gun electrode structures, the stored bulb charge should self dissipate as there is a complete discharge pathway with the power off.
That'll depend on the time constant formed by the CRT capacitance and internal resistors - I've no data on that, but they will be using large value resistances to minimise dissipation. So if you unplug the tube base socket very soon after switching off, then the CRT might not have discharged - leaving "CV" (see Sony extract) at a high potential, potentially

As I say, I much prefer to not take chances - it's practically zero effort to discharge (then strap a lead there to stop it charging up again), so why waste time analysing it when there's probably more pressing matters to think about?



Quote:
Originally Posted by Argus25 View Post
PS: What is a typical value current sense resistor for the application, also are they a physically large part ?
They vary hugely depending on the design of the set - I'd hate to generalise. Beam currents are usually limited to a couple of milliamps or thereabouts, so you pick the resistor according to the voltage swing you need elsewhere. You might choose to tie the resistor to a rail rather than ground - it'll still do the same job. Or you might incorporate potential dividers and perhaps zener diodes. My knowledge on these things mostly come from Television Magazine - as a kid I used to love reading all the in-depth reports on new sets, even though I didn't understand most of it at first! I definitely recommend dipping into some back-issues if you're interested in the sorts of sets we saw other here (which might well have been very similar to what was sold over there - I don't know?): https://www.americanradiohistory.com...n_Magazine.htm

As to the size of them, see earlier comment: they're only handling a few milliamps...
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