1987 Commodore 1901 CRT Monitor Repair, Modification, & Restoration

I was recently given a 1980s Commodore 1901 CRT display by a friend, the third Commodore display for my collection (following a 1701 and 1084S-D2). It was in quite good condition physically, though it was missing its front control cover (a common problem with displays of this era) – it also came with a non-standard mains plug, so was untested and in unknown operating condition.

The Commodore 1901 monitor was a PAL-only colour monitor produced between 1986 and 1988, and was designed for use with the Commodore 8-bit line of computers (primarily, the C128). It supports a luma/chroma (S-Video) input with audio via three RCA jacks, and a DB9 RGBi input.

These were often bundled together with Commodore 128 computers in electronics stores, so sold reasonably well in the UK. However, CRTs were often the first to be binned and they are notoriously difficult to ship, so it can be a bit tricky to get hold of a 1901.

On its arrival, I noticed that the power switch was very sticky, and often got stuck in the on position – a common problem with displays of this era.

I fitted a standard 3A-fused UK plug, and tried the monitor out with a luma/chroma/audio output from a Commodore 64 – the power LED would light up immediately and I could hear high-voltage kick in immediately, however it would take about twenty seconds before an image was displayed, and when it was the only image was a very wobbly black rectangle.

The display was therefore in need of repairs (my favourite part).

Monitor initial testing.

The first step was to disassemble the monitor and check over everything inside.

CRT Display Safety

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 for colour displays, less for monochrome), 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 – keep one arm behind your back so as to minimise shock across you torso; if possible, work on the CRT when it is unplugged and powered down, and safely discharge the CRT anode cap before starting work (preferably using a high-impedance lead so as not to damage the CRT interior coating).

Due to a phenomenon known as dielectric absorption, capacitors (including the CRT itself) can self-charge again even after being discharged – if you’re worried about this, keep any potential hazards shorted out while you’re working.

If you do need to work on the CRT while it’s powered up (for example, whilst measuring voltages or making control adjustments), use insulated tools and an isolation transformer.

Disassembling the 1901

The 1901 is relatively easy to dismantle – the rear plastic case piece is held in place with four large screws, one in each corner, and two smaller screws around the input panel.

SHOCK RISK: the neck board can then be pulled (gently) from the CRT neck – both the PCB and CRT neck are very delicate, so take care.

SHOCK RISK: the red HT lead and anode cap can be discharged and removed from the CRT.

The mains input cable is clamped to a plastic holder, which is screwed to the underside of the plastic monitor case – this can be undone and the holder removed.

The flyback transformer is supported by a plastic holder, which is screwed with two screws, one on each side – these can be undone and the holder removed.

The mainboard is held in place using a plastic bracket to the front of the monitor, in a rectangular hole in the PCB – this simply unclips from the underside and comes out.

SHOCK RISK: the mainboard can then be slid out of the case along its two mounting rails. Various connectors for stuff connected to the case and CRT (power LED, dag ground, degaussing coil, speaker) can be disconnected – be sure to take lots of before pictures to ensure that everything goes back in using the correct orientation!

Assessing the 1901

Any repair first begins with a visual inspection, which can give you a lot of useful information – this includes looking for obvious damage, such as PCB cracks (a common problem with single-layer boards), cold solder joints (a common problem with displays of this era), burnt/damaged components, burned/damaged traces, corrosion, electrolytic capacitor leakage/bulging, etc.

The eagle-eyed amongst you may already have noticed a couple of obvious problems.

The first obvious problem was a blown mains filter capacitor, marked CP01 (100nF Class X). RIFA-brand mains filter capacitors of this era are known to be problematic – they crack with age and commonly fail catastrophically, releasing lovely clouds of acrid smoke. If you don’t want to ruin your day, remove these types of capacitors before powering up any form of equipment that has these original parts. They aren’t strictly required for operation on the 1901. In this case, the capacitor explosion had also left a large amount of black soot across the mainboard and power resistor RP12 (10k 3W).

The second obvious problem was damaged electrolytic capacitors, primarily CP08 (1uF 50V) – this appeared to be deformed, bulging, corroded, and leaking electrolyte, seemingly because of its close proximity to a blob of adhesive holding the line filter capacitor in place. Some vintage glues are either hydrophilic (absorb water) or corrosive, a common problem with some 1980s electronics – in this case, the glue appears to have wrecked the electrolytic capacitor at CP08.

The third obvious problem was damaged power resistors, primarily RV52 (47k 1W) – this appeared to corroded, seemingly because of its close proximity to a blob of adhesive holding a ribbon cable in place, similar to CP08 (see above). RV52 tested bad out-of-circuit.

Repairing the 1901

The repair would comprise the following elements:

  • Adhesive removal.
  • Mains filter capacitor replacement.
  • Power resistor replacement.
  • 12V regulator replacement.
  • Electrolytic capacitor replacement.
  • Power switch replacement.
  • IEC mains input modification.

Before working on the boards, I cleaned off all of the major bits of dust using an electric air duster, and cleaned the cabling and major parts using a microfibre cloth and degreaser. The monitor seemed to be quite a high-use set due to the concentration of dust and grime.

Adhesive Removal

To prevent further damage in the future, I removed as much of the corrosive adhesive as possible on all parts of the board using a combination of mechanical force (i.e. picking the bulk off with a pair of needle-nose pliers, scraping the remains off with a flat-head screwdriver), then cleaning the board with acetone and cotton buds.

This also meant removing the mains transformer (SO62) to clean underneath.

Mains Filter Capacitor Replacement

I pulled the damaged mains filter capacitor CP01 (100nF Class X) out using my desoldering station – you can clearly see the cracking and soot.

With this capacitor removed, it was quite clear just how much soot had been left on the mainboard following its failure.

I removed power resistor RP12 to clean underneath.

I then installed a suitable replacement part (0.1uF 275Vac X2 safety capacitor).

Power Resistor Replacement

As previously discussed, power resistor RV52 (47kOhm 1W) tested intermittently both in- and out-of-circuit, so needed to be replaced; RV50 (1.8kOhm 0.25W) and RP12 (10kOhm 3W) were also corroded, so I decided to replace these too.

Removing these resistors was simple using my desoldering station; I installed suitable replacement parts (47kOhm 1W, 1.8kOhm 0.25W, and 10kOhm 3W respectively).

12V Regulator Replacement

The 1901 has a 7812 linear regulator to supply a +12Vdc power rail – these linear regulators run hot and the original was not attached to its heatsink, so I decided to replace it out of caution in case it was worn out.

Removing the 7812 regulator was simple using my desoldering station; I installed a suitable replacement part (L7812CV +12Vdc 1.5A linear regulator); I also put some thermal paste on the back of the new regulator and bolted it to the heatsink behind it, to help keep it cool. NOTE: before attaching a TO-220 package like this to a metal mount, ensure that the common and mount are supposed to be at the same potential! In this case, both are grounded, so it’s safe to attach them – not all circuits are like this.

Electrolytic Capacitor Replacement

As discussed above, the aluminium electrolytic capacitor at CP08 (1uF 50V) had been destroyed by corrosive adhesive.

Aluminium electrolytic capacitors are commonly used for filtering, smoothing, and decoupling in both high- and low-voltage electronics. They typically comprise aluminium windings which are coated with a liquid electrolyte, which can dry out over time (negatively affecting the performance of the capacitor, often causing them to fail dead-short), or leak out and cause corrosion to the PCB and surrounding components.

The 1901 was manufactured using through-hole electrolytic capacitors – being an established technology for the time, these capacitors are generally quite reliable. However, one had catastrophically failed and several others tested bad in-circuit using my ESR meter, so I replaced them all out of caution – including those on the mainboard, the CRT neckboard , and the RGBI/PAL daughtercard.

I usually remove each capacitor one-by-one using my desoldering station (a Duratool D00672) and immediately install a replacement, taking particular care to ensure that the value, voltage rating, and orientation of the new capacitor are correct – electrolytic capacitors are polarised, so must be installed the correct way around, else they’ll cause problems later (potentially even blowing up during use). I then clean up all the remaining flux residue or heat marks using 99.9% IPA.

You can’t always trust the markings on the PCB silkscreen, as sometimes mistakes were made in the design from the factory (take the PCB layout of the audio circuit on the Commodore CD32, for example), so care must be taken to match the orientation of the new capacitor with the original. Take lots of “before” pictures for reference.

I couldn’t find any commercially available capacitor packs for the 1901, so I just made up my own by noting the specifications of all of the electrolytic capacitors on the board, and ordering a set of high-quality known-brand (i.e. Panasonic, Nichicon, etc) 105C-rated parts.

This was a total of:

A very useful list with all of the capacitor locations is available here.

When substituting electrolytic capacitors, the capacitance needs to be the same, and the voltage rating can be the same or higher (within reason) – when you’re going through all this effort to recap something, be sure to use high-quality replacements.

Please note that this list will likely change between mainboard revisions, so can’t necessarily be relied upon for your own 1901 – if in doubt, check.

Power Switch Replacement

The original power switch was very notchy and kept sticking into place – these type of switches were used extensively in CRT displays of the era, and are a common failure.

I compared the original part with a similar new-old-stock part which I had available to ensure that they were compatible, and it appeared that the only functional difference was its metal collar, which would therefore need to be swapped between the two parts.

The metal collar is simply held into place by two locking tabs on the top, which can be bent out of the way using a pair of needle-nose pliers, then the collar slides out.

I removed the metal collar entirely from the replacement switch, however I left the original in place on the mainboard and just unclipped it from the switch, as there was no point in desoldering this unnecessarily.

Removing the original power switch was a relatively simple process, as the switch itself only has four pins. I first reflowed these with fresh solder, removed all the solder using my desoldering station (a Duratool D00672), then gently rocked the power switch until it fell out of the board. I then cleaned the board underneath using IPA.

With the original switch removed, you can see how similar this is to the replacement part – the replacement also features a secondary switch circuit which is not used in this application, so I simply bent the two unused pins out of the way underneath the switch.

Installing the new switch was the reverse of removing the original – the new switch was fitted into the original collar then soldered into place.

IEC Mains Input Modification

The Commodore 1901 has a lot of design features which seem to be either optional from the factory or not fitted, including space on the mainboard and a cut-out in the rear case for a SCART connector. Another cut-out is also available for an IEC C13 mains input, which allows the use of a flying power cable.

I wanted to make use of this, as I find that a permanently-attached mains cable has a habit of getting in the way, and I prefer monitors with a kettle-lead input as it makes them much easier to move around and to store on shelves.

I bought a few different IEC C13 sockets to test-fit into the chassis, and one panel-mount socket in particular seemed to fit perfectly – it simply clips into place in the chassis, and I tied it in place with a cable-tie to stop it from moving from side-to-side.

I then cut down the original mains cable to size, stripped the outer insulation, cut the three cores down to size, stripped the inner insulation, and cut some heatshrink to size.

It was then just a case of soldering each of the cores to the appropriate pin on the back of the IEC connector (which is usually clearly marked), and fitting the heatshrink.

The case then needs to be modified to allow the socket to be used – there is already a hole marked into the case with a weakened outline, so all I did was drill some pilot holes into the centre, then remove the excess plastic using a pair of pliers.

With the case installed, the modification looks almost factory.

Reassembly & Testing

The reassembly of the CRT is the opposite procedure to its disassembly.

After all of this work, I tested the CRT on my isolation transformer without a video input connected – the power switch worked properly now, the power LED lit up, I could hear high voltage kick in immediately, and the CRT seemed to display an image – when turned off, the video flashed away as expected.

I connected up a luma/chroma/audio input from a working Commodore 64, selected the PAL input using the switch on the front of the display, and turned it on – I originally got a black-and-white image with vertical rolling. After adjusting some of the settings on the front of the monitor (primarily vertical sync and colour), the image looked great!

Testing the CRT following restoration.
  • Display works correctly on all video inputs (PAL/RGB).
  • Audio output (mono) works OK.
  • All controls work OK.
  • Power LED works OK.
  • Power switch works OK.

Published by themightymadman

My name is Adam Wilson - I'm an electronics engineer based in the North East of England, UK, and I like tinkering with old junk. In my spare time, I collect, repair, refurbish, and (sometimes) sell vintage computer systems and peripherals, typically from the 1980s (the likes of Commodore, Sinclair, Acorn, Apple, Amstrad, and Atari).

7 thoughts on “1987 Commodore 1901 CRT Monitor Repair, Modification, & Restoration

  1. Thank you for this post – I own a 1901 which has a faulty power switch, i.e. it doesn’t remain in place. I resolved using a toothpick so for now: I feared my electronics knowledge is not enough to repair, but maybe with your post I shall reconsider.

    Like

  2. Thank you for this post – I own a 1901 which has a faulty power switch, i.e. it doesn’t remain in place. I resolved using a toothpick so for now: I feared my electronics knowledge is not enough to repair, but maybe with your post I shall reconsider.

    Liked by 1 person

  3. I used to repair a lot of the old tubeway monitors as well as the later lcd ones I remember tatung monitors had lots of problems with melted fit a blue capacitors in the fly back tuning circuit S well as dry joints cracked print etch the monitors made by Nokia badged as Dell used to catch fire conductive corrosive glue was a menace and manufacturers put electrolytics next to heatsink so they fail Aztec did that in their psi used in bbc computers

    Liked by 1 person

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: