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Old 4th Feb 2019, 1:32 pm   #2
mhennessy
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Join Date: Jul 2007
Location: Evesham, Worcestershire, UK.
Posts: 4,241
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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