Microvitec CUB 1431-MS CRT Monitor Restoration

I recently won a charity auction for a BBC Model B bundle on eBay (see my original blog post here), which included a Microvitec CUB 1431-MS CRT colour monitor, a common sight in British schools during the 1980s thanks to the BBC’s Computer Literacy Project.

The monitor was rather worse for wear: it was covered in grime, missing its front badge, had a wonky power LED, had various scratches, and had a large dent on the underside where it seems to have been dropped.

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It seemed to be a good candidate for restoration, though, as the tube itself is in good condition, and the previous owner said that it worked OK. I wanted to service the monitor before powering it on, however, due to its age and unknown history.

Electronic components generally don’t “age” as such, except for electrolytic capacitors, which are commonly used for filtering, smoothing, and decoupling in both high- and low-voltage electronics. These capacitors typically comprise aluminium windings insulated by a liquid electrolyte, which can dry out over time and negatively affect performance (even failing dead short), or leak out and cause corrosion to the PCB and components – as such, these should be replaced as a part of preventative maintenance.

Apparently, the brand of electrolytic capacitor used at C225 (the gold capacitor at the back of the PCB) is known to suffer from manufacturing defects and commonly fails short-circuit during operation, causing damage to its surrounding components. As such, I was advised to replace this capacitor (at least) before powering the monitor on.

Given the amount of effort involved in safely removing the mainboard and neck-board from the chassis to service it, I figured I’d just replace all the electrolytic capacitors whilst it was out. Based on advice from RetroClinic, I also decided to change the tripler unit, which degrades with heavy use and can affect the picture quality.

The first step was to dismantle the monitor, and remove the mainboard.

Before we start, a word of warning: like all other mains-voltage electronics, CRTs can be dangerous to work on, so it is important to take care and use the proper precautions – if you don’t know what you’re doing, it’s probably best to leave it to someone who does.

The high voltage side of CRTs can contain extremely high voltages (up to 30 KV), and the “low” voltage side has mains voltage and large line filter capacitors, all of which can give you a nasty shock – most CRTs contain safety features to automatically discharge any stored charge (i.e. bleeder resistors), but even if these exist they can fail, so it’s always best to err on the side of caution.

If you work on a CRT, you do so at your own risk – only work on the CRT when it is powered down, keep one arm behind your back so as to minimise shock across you torso, and ensure that you safely discharge the CRT anode cap before starting work (preferably using a high-impedance lead so as not to damage the CRT coating).

The back panel is held in place with six screws, and is connected to the mainboard by several multi-cables, which can be carefully pulled from their headers; the top panel is held in place with two screws, on the monitor bezel; the neck-board is connected to the front bezel via the CRT dag cable, which can be pulled from its header, then the neck-board can be pulled (carefully) from the CRT neck; the mainboard is connected to the front bezel, tube, and rear panel by several multi-cables, and to the case via a ground lead, which need to be removed; the mainboard is held in place by a set of plastic clips and standoffs, which can be popped off using a flat-head screwdriver and the boards removed; the bottom panel is held in place by four screws, on the monitor bezel.

Before dismantling, it’s always a good idea to take lots of pictures, to ensure that everything goes back together correctly afterwards, especially the cabling.

Once the mainboard was removed, I put together a list of all of the electrolytic capacitors required (see below), then ordered good-quality replacements (low-ESR, good brand, 105C-rated capacitors, generally with a higher voltage rating than the factory parts):

C11 – 100uF 400V (radial)
C12 – 4.7uF 35V (radial)
C18 – 1uF 50V (axial)
C23 – 4.7uF 35V (radial)
C26 – 33uF 250V 105C (radial)
C27 – 47uF 250V 105C (radial)
C28 – 47uF 250V 105C (radial)
C31 – 470 uF 25V (radial)
C34 – 220uF 25V (radial)
C101 – 10uF 35V (radial)
C102 – 10uF 35V (radial)
C105 – 10uF 35V (radial)
C206 – 0.68uF 50V (axial)
C207 – 4.7uF 35V (radial)
C212 – 220uF 25V (radial)
C224 – 1000uF 40V (radial)
C304 – 100uF 35V (radial)
C308 – 10uF 35V (radial)
C311 – 470uF 35V (radial)
C312 – 220uF 25V (radial)

This is a total of:
1 x 100uF 400V (radial)
1 x 1uF 50V (axial)
3 x 4.7uF 35V (radial)
1 x 33uF 250V 105C (radial)
2 x 47uF 250V 105C (radial)
2 x 470 uF 35V (radial)
3 x 220uF 25V (radial)
4 x 10uF 35V (radial)
1 x 0.68uF 50V (axial)
1 x 1000uF 40V (radial)
1 x 100uF 35V (radial)

Please note that this list will likely change between monitor revisions, so can’t necessarily be relied upon for your own monitor – if in doubt, check.

I also ordered a replacement tripler unit and focus potentiometer from Donberg Electronics, based on advice available online.

Once all the new components had arrived, it was just a case of fitting them. This monitor has a single-sided PCB, commonly used in CRTs of the era, which is easy to solder on.

The new tripler unit came with a separate focus potentiometer, which was easy enough to install – on the new part (HRT 240BS), the focus potentiometer clips onto one side, and needs to be connected between the green cable and ground. The ground cable was not insulated on the new potentiometer, so I after I cut it to length I covered it in heat-shrink.

The original tripler unit was held onto the PCB with two bolts – once these were removed, the tripler was free from the board. I cut the cable to the neck-board (white) where it meets the tripler as this is re-used, then I desoldered the LOPT cable (red) and the ground cable (black). After this, the original part could be removed.

The new part has the same physical footprint, but it has an extra cable and all the wiring colours are different. First, I fixed the new tripler to the board; then, I soldered the white cable from the neck-board to the exposed tab on the top of the tripler, and covered the joint with heat-shrink; then, I soldered the cable from the focus potentiometer to the original ground point; then, I soldered the white cable from the new tripler unit to the same point on the LOPT as the original red cable; then, I soldered the blue cable and yellow cable from the new tripler unit to the two ground points next to the tripler.

When it comes to electrolytic capacitors, I remove each using my desoldering station (a Duratool D00672) and replace it one by one, taking particular care to ensure that the value, voltage rating, and orientation are correct – electrolytic capacitors are polarised so must be installed the correct way around, else they’ll go pop when powered up.

You can’t always trust the markings on the PCB silkscreen, as mistakes may have been made in the design from the factory (take the Commodore CD32, for example), so care must be taken to match the orientation of the new capacitor with the original.

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A new front badge came courtesy of a very helpful guy called Henryk at Microvitec (yes, they’re still in business!), who happened to have a spare lying around, and very kindly sent it to me to complete the restoration.

I also gave the board, case, and tube a thorough clean whilst the monitor was still apart, and straightened out the dent in the underside – the case metal is thin enough to manipulate into its original shape by hand, carefully.

Then, I put everything back together for testing…

Success! No explosions yet, just a crisp, stable image. I’m looking forward to using this a lot more – coupled with an original metal stand, everything fits together nicely.

I also did some tweaking to various internal adjustments to get the stability, focus, and size of the image just right. After using the monitor for some time, I noticed an intermittent “screeching” sound which seemed to be coming from transducer T202 (near the flyback transformer) and which changed pitch when the PCB in the area was flexed. I pulled the board again and applied some hot glue to the transducer frame and coils – this seems to have damped any resonance and stopped the screeching.

Next step, upgrading the Model B!

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