Converting a monitor from 625 to 405

[`FRANK.C]

Well done Jeffrey!
You made it look too easy.
Not something I would like to tackle especially with all those silicon fuses waiting to go pop with the least provocation

Frank

[ppppenguin]

Thank you Frank.

These monitors are actually pretty tough. My only casualty so far is a diode as mentioned in post #18. I'm not convinced this failed due to my experiments. It's hard to see how it could have been run outside its ratings. To minimise risk to silicon and wound components I've kept the supply voltages down as far as possible during most of the work. The worst that's happened has been a bit of discharge from the EHT probe to the dag on the CRT. Just a matter of careful positioning.

There have been times when the line scan has been going crazy but it's not killed anything.

The only thing that's worrying me slightly is how hot the back panel is getting. It's the heatsink for all the power transistors incuding my additional darlington. I've just run up an unmodified 625 version to see how hot that gets. Might end up moving the darlington to the bottom panel.

[ppppenguin]

I have a total of 6 of these monitors at present. Had a look at the only 2 other monitors that worked when initially switched on. One is excellent, not looked inside yet. The other worked well for a few minutes, then gave a bad smell and some smoke. Picture still present but poor contrast. Looking inside revealed an obvious culprit. C12, the smoothing cap for the +110V rail, looked lumpy and iffy. The 110V rail itself was down at about 25V. Probably being held up by the main HT via some route that's not immediately obvious. The 110V rail is rectified off the LOPT, as are several other supplies.

As I was removing the duff cap I saw that R10 (22R) a surge limiter in series with the 110V rectifier was badly burnt. It was open circuit. The rectifier itself tested OK. The cure was obvious and that monitor now works again. The height control preset pot is very intermittent, probably due to damp storage. Probably need to replace. Need to look at all the pots.

I might review the other 3, all in various stages of non-workingness.

[Murphyv310]

Great work Jeffrey.
To be honest I'd snap up a monitor........... a year and a half ago but not now.
I have lost interest in the hobby and have parted company with a few things. I will of course keep my prized items like the TSC30A, TC268, V330, V4, V659, Perdio Etc Etc and of course some radios.
My life now is full of other rewarding things and I am happy in what I do.

[ppppenguin]

Tried another 2 of them. The first of these had iffy C10 (it was getting warm) but was still working. Also needs cleanup round EHT as there are odd crackles. The other one had failed a smoke test and had blown its mains fuse. Shorted diode in bridge. Now works well.

The main series pass regulator gets very hot. Now i know why. I used a variac to determine that hum appeared at 193V mains. My mains is about 235V. Unless you have chronically low mains voltage that means a lot of heat in the regulator. The transformer doesn't have any alternative taps so for my own monitor I may fit a small buck transformer. I'll have a dig around my toroids. Even a tiny one will be enough. A 12+12 one will knock 24V off the mains which feels about right.

Just 1 more of these monitor lurking here. i may have a go at it tomorrow.

[ppppenguin]

The remaining monitor, one of the 2 that had been in a very damp place, was a ragbag of problems. Got a picture on it for long enough to see that the CRT was good. I'm treating it as a parts donor.

Got a buck transformer running on the bench now. An RS potted 12+12 20VA toroid. Takes my 238V mains down to about 213V. Monitor runs much cooler. Toroid just dumped in bottom of case, quite close to CRT, but no visible positional hum.

[ppppenguin]

I wrote up the instructions for conversion to 405. Then converted another monitor by following them. Made a neater job of the darlington dropper for the line output. All works OK but the edge focus seems worse than the first one. As is usual on this sort of CRT the focus control has little effect. This monitor has dynamic focus so I might have a look at that. In any case the picture is more than OK, I'm being fussy because I want top class professional monitor quality.

[ppppenguin]

Actually it's a bit weirder than I thought. I thought it was defocusing but on more careful examination it looks like the 2 fields are being displaced sideways towards the top and bottom. What's more, the higher the brightness the worse the effect. This feels like a fault rather than a design or 405 conversion problem. I'll need to look very carefully at the sync sep and line oscillator. Just seeing the problem on a 'scope won't be easy.

Since I've more than one monitor I can swap PCBs to find out whether it's on the main or EHT board.

[`FRANK.C]

At the NVCF in May 2022 I picked one of these monitors from Jeffrey with the intentions of converting it to 405 lines. Jeffrey supplied me with step by step instructions on how to do the conversion and also a setup procedure. It went straight to the top of the high priority pile but it is only now I am getting round to it.

Before I did any modifications I thought it best to check that it was still working OK on 625 lines.  After switching on and waiting for sometime a dim unlocked picture appeared it was only one third of its height and width. So it obviously didn't enjoy the sheep it had for the last two years.
The 30V rail  was down to 12V. Z1 a 4.7V Zener diode that provides a reference voltage for the power supply had just 1.8 V across it. C2 470uF which is in parallel Z1 was gone very leaky. Replacing it brought back normal operation.
I had to remove the main PCB to replace it. While I had it out I spot checked some of the other electrolytic capacitors. Some were OK, some were high ESR, and I found another low capacitance/leaky. So for reliability I decided to change all the original blue philips electrolytics. Some had been replaced already.

I followed Jeffrey's instructions to convert it to 405 and now I have a working 405 line monitor.
Everything went to plan. After doing the mod I couldn't set the 27V supply with P1 . I could only get to 25V with P1 at one end of its travel. R27 which is across P1 and is a AOT part was 6k8. It was marked as 15K in the diagram. I replaced it with 12K and this gave good range to P1.
As a final step I did fit a buck transformer as in the instructions to reduce the mains voltage.

The picture it displays is really superb. I have nothing that comes anywhere near it.
It is a great addition to the workshop and I am sure it will get plenty of use.
The photo does not do it justice.

One thing I am curious about is the red lamp on the front panel. What is it for? The wires from it goes to the rear panel. Possibly to a 3 pin connector.

A big thank you to Jeffrey for the monitor and also for sharing how to convert it.

Frank

[Mike Watterson]

One thing I am curious about is the red lamp on the front panel. What is it for? The wires from it goes to the rear panel. Possibly to a 3 pin connector.

To show which screen/camera has been selected in a Gallery, possibly same signal as the light on the camera, but I think sometimes the person in shot was told to look at the lit camera light, which cunningly might not have been the active one.