Marconi 2955R Shrunken Text/Linearity Woes - LOPT or H drive issues?

[TheBay]

The circuit diagram has 15kV at the CRT anode. I agree it does sound like a lot for a small tube.

Yes it is quite high for a small CRT, I can only assume these are driven a bit higher as there is a fine metal mesh filter in front of the CRT which deceases brightness.

Apart from the low EHT the 475V and 50V output from the LOPT is lower than it should be.

[TheBay]

Had it up on the bench again today and tested the EHT again, definitely 8.25KV and the 475V output from the LOPT is only 445V

I turned the brightness up to full and it doesn't really get really bright and the focus goes quite soft.

I tried to take a video of the raster changing size as it warms up and with any slight change on the 12V rail but it doesn't seem to show up on the video for some reason.

Here is the YouTube video, but doesn't really show much. My camera seems to make the CRT a lot brighter than it is, I'm no photographer and tried to put some bright lighting on to balance it out a bit. Around 25 seconds the raster will change size, this can happen a few times as it warms up or if the 12V rail changes very slightly.
https://youtu.be/Lbhb5DPggis

[james_s]

It might be interesting to try a ring tested on the flyback, the one I have is a discontinued Dick Smith kit but I think the schematic is floating around somewhere. It really does seem to be a problem with the transformer, sure would be nice to find another identical monitor to try swapping.

[TheBay]

It might be interesting to try a ring tested on the flyback, the one I have is a discontinued Dick Smith kit but I think the schematic is floating around somewhere. It really does seem to be a problem with the transformer, sure would be nice to find another identical monitor to try swapping.

I have a EVB Blue2 Ring Tester here which is a Bob Parker design, when I get chance I'll take it out and put the ring tester on it.
I haven't taken the flyback out so far as I was really hoping it was something else causing these issues but my gut feeling is it is the flyback.
And it is a bit of a pain to take the AC1 board out, there is a big metal shield bolted to it which covers half of the solder side of the pcb.

I did move it to on side today to swap a 5V6 zener that I was hoping wasn't working properly but it tested fine, when this migraine passes I'll get it back on the bench.

[james_s]

You should be able to ring test it in circuit, I remember the instructions with my tested said to probe it via the collector of the HOT. Out of circuit is the most reliable test though.

If it does turn out to be the problem, it's possible you could wind a new one or wind new low voltage windings on the core of a similar small flyback. Usually the low voltage windings are just a few turns.

[TheBay]

Just unplugged the scan coils and put my ring tester on the following.

LOPT Primary (Across HOT Collector + GND) 85 Rings
Lin/width coil. 45 Rings

[TheBay]

Apologies for all the updates...

Just thought I'd try something that I haven't already.
I removed the fuse and ran the AC1 board 12V rail off one of my lab PSU's (TTI TSX3510)

Linearity issue still there and the picture resizes slightly as it warms up. Though I'm wondering if that's the load from the OXCO oven on startup or the PSU locking to the frequency.
Current draw seems to be around 1.20A then drops off a little and then finally down to 0.85A (Oven off?)

Boring video here:
https://youtu.be/Vz3i7umHwUg

[james_s]

From my experience playing with OCXOs the oven never turns off, it doesn't cycle, it is a linear circuit and the draw varies according to heat required. It will draw a good bit of current when cold, slowly tapering off to a much lower value which then drifts around a bit with ambient temperature and drafts and such.

[TheBay]

From my experience playing with OCXOs the oven never turns off, it doesn't cycle, it is a linear circuit and the draw varies according to heat required. It will draw a good bit of current when cold, slowly tapering off to a much lower value which then drifts around a bit with ambient temperature and drafts and such.

That makes a lot of sense thinking about it, I've not had any experience with the insides of an OCXO.

[james_s]

I've never taken one apart myself either though I did xray one once. I've only messed with them externally and by watching the current draw it was not too hard to figure out how they worked. I did see somewhere someone had disassembled a couple of them and reverse engineered the circuit, typically it's pretty simple, just some op amps and passives, often a power transistor is used as the heater itself rather than controlling a heater.

[TheBay]

A big update today.

A member of the Marconi group sent out a replacement LOPT tuning capacitor and LOPT to test.

These arrived with me today and I couldn't wait to get it on the bench.

I tried the capacitor first and there was no change. then I removed my old LOPT and fitted the one that arrived today.
Did a quick sanity check and checked it all over and powered it up.

On power up the H/V was smaller, so I knew straight away the EHT voltage must be higher.
Adjusted the picture up and down with the H position pot, everything looked perfect.

Adjusted the height by adjusting the core in L1, then did the width.
Image is still perfect.

Then adjusted the intensity and WOW it's so much brighter, it's that bright if I turn up it too far I see the whole raster and retrace lines.
Image is so crisp and perfect, no linearity issues at all and I gave the EHT a quick check and I'm getting a reading of 11KV which is
certainly higher than what I saw before.

So it is definitely the LOPT that was at fault, this is great news as I now know exactly what the issue is and that AC1 and the CRT are good.
The only bad news is trying to find a replacement LOPT, that is probably the hardest part.

[CJay]

I *may* have one.

[TheBay]

I think I can see the fault with this LOPT, the core wasn't installed properly at the factory, it is skewed and too close to one side so the air gap is wrong. So the seller was right saying it has done this from new.
Fortunately this has had a nice side effect, the EHT was too low and the brightness wasn't great so the CRT has been driven a bit less than most.

Picture of the bottom of the LOPT attached, I will try and get some better pictures or get it under the microscope.

I *may* have one.

That sounds very interesting

[james_s]

That's crazy, I wonder how it got through QA? One nice thing about that though, the low EHT and corresponding dim picture likely preserved the CRT so it should have plenty of life left in it.

[TheBay]

That's crazy, I wonder how it got through QA? One nice thing about that though, the low EHT and corresponding dim picture likely preserved the CRT so it should have plenty of life left in it.

It is crazy given what this must have cost new.

The owner from new said he just got used to it and lived with it, what's even more crazy is I have folders upon folders of it going in to Marconi, then IFR etc for servicing and calibration each year. Yet no one bothered to sort this issue. And around that time Marconi did an upgraded LOPT with an adapter board which this would have benefited from.

[CJay]

If I get time I'll dig out the chassis and take some pics, I was going to see if I could get it running but TBH, I will never get the time.

[TheBay]

If I get time I'll dig out the chassis and take some pics, I was going to see if I could get it running but TBH, I will never get the time.

That's such a shame, what is wrong with your unit, it may not need much to get it going?
If you do decide to part it out or sell it I'd be interested.

[james_s]

It is crazy given what this must have cost new.

The owner from new said he just got used to it and lived with it, what's even more crazy is I have folders upon folders of it going in to Marconi, then IFR etc for servicing and calibration each year. Yet no one bothered to sort this issue. And around that time Marconi did an upgraded LOPT with an adapter board which this would have benefited from.

I'm surprised Marconi didn't fix it but it doesn't surprise me at all that the calibration place ignored it. At my old job we had a Tek VM700 that went out to calibration several times and the monitor in it had an obvious fault resulting in a washed out picture with retrace lines. The calibration place said they couldn't fix it, I could have easily but it would have meant breaking the calibration seal