Back during my early school days, some of the classrooms had BBC Micros in them along with a beige metal CRT monitor with Cub written on it. These displays were nicely matched to the BBC and were very common at the time.
I happened to log into Facebook at a perfect time last week when a guy I know (Zeb) posted on the Acorn group that he had a Cub monitor free for collection for whomever claimed it first. He doesn’t live that far away from me so I jumped at the offer.
Zeb then messaged me to say, he forgot to mention that it was broken and did I still want it. To most people this would have put them off, but for me it made the offer even better! Now I had a reason to learn more about CRT repair!
I went and collected the monitor and brought it back home and after an initial inspection I plugged it in to see what happened. The monitor fired up, I could hear the high voltage working and then a single horizontal line appeared across the center of the screen so I turned it off before any damage was done to the phosphor.
The schematics for these monitors are available to download so I grabbed them and started to look at the vertical deflection part of the circuit. This seemed to get it’s power from a section of the circuit that contained a nice big capacitor that was connected to ground. This looked like a perfect candidate for the fault so I removed the capacitor and tested it with my ESR meter and sure enough it was shorted. I didn’t have a replacement so I ordered one and decided to do some more research while I waited for it to arrive.
I soon discovered that this fault was actually a fairly common issue with these displays and it often kills one of the fusible resistors at the same time. So I tested this and sure enough that resistor was also dead so I ordered some replacements.
Once those parts arrived I fitted them and turned of the monitor and was greated with full vertical deflection.
I was pretty happy at this stage but that was short lived.
I connected up the BBC and powered the monitor back on. Instead of a nice picture, I got a weird squiggly pattern and then magic smoke started to come from the monitor so I quickly powered it off again. The smoke had come from the resistor I had replaced. So it looked like there was a bigger issue that had possibly caused the capacitor and resistor to die in the first place.
I then spent a couple of hours looking at the schematics and testing capacitors and resistors in the vertical deflection section and everything i looked at tested fine.
The service manual had several images of test points and what the waveforms should look like. So I grabbed my scope and started looking at these signals. Most of them were fine but I had no field output signal. I was pretty convinced now that the TDA1170 IC was internally shorted and was causing a high current draw which killed the resistor.
So, I went back on eBay and found out that these ICs were still available. A couple of days later it arrived and as soon as I finished my day job and went down to my workshop and fitted the new part. Whilst holding my breath, I powered the monitor back on and everything seemed to be fine, full vertical deflection and no smoke.
It was now time to connect the BBC back up and see what happened. This time I got a nice clear image and still no smoke!
This was a pretty satisfying repair and during this I have been watching a bunch of videos about CRT theory and repair. The best ones I have found so far are the CRT workshop videos by “Randy Fromm” on YouTube. I would highly recommend watching these as they contain a wealth of information and real world experience within them.
I also have another CRT here now which I am trying to repair, which is a 28inch Sony Trinitorn that has an over current protection fault. That was is going to be a bit harder to troubleshoot but my understanding of how these CRTs work is much greater now so I am confident that one day I will get it fixed.
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