Converting a monitor from 625 to 405

[ppppenguin]

I'm doing some experiments to convert Melford DU1-12 professional monitors to 405. It would be useful to have some high quality modern 405 monitors and I happen to have a number of these 12" monitors available. The service manual is here: https://elektrotanya.com/melford_du-1_se...nload.html but I also have a paper copy. I've scanned the circuit at much higher quality than the Elektrotanya pdf. Doesn't do anything about the drafting which is horrible and hard to follow.

There's a fair bit of interaction between the line scan, power supply and EHT. Also several auxiliary rails are taken from the LOPT. These include 110V for the video amplifier and the CRT heater supply. This was common practice in many later CRT TVs. While I'm playing with the line scan the heater voltage will be all over the place so I've hooked up an external 6.3V supply. Before I did this I measured the AC supply to the heater. It's 15625Hz and nowhere near sinusoidal. My Fluke 87 true RMS DMM read about 5.8V. I have no idea how accurate this is.

At some point I'm going to need to power the heater internally again and check that it's getting the right voltage. Any ideas for a quick and easy way to do this? I really don't think my Fluke is up to the job. I suppose a couple of 6V 150mA bulbs, one from the heater supply, the other from a DC supply and optical comparison is the easiest way I can think of. Anyone got any better ideas?

I've attached part of the schematic. The heater is fed by a capacitor dropper, C15, which if the waveform were a sinusoid would drop about 4.6V at 15625Hz. The 2 taps on the LOPT are for alternative CRTs with 12.6V and 6.3V heaters.

[Amie]

Jeffrey.

Find a valve, any valve with a heater rating similar to the tube, connect a dmm set on say the 2mA range, between cathode and control grid.
Power the heater at a verified 6.3Vac and note the current on the dmm, for argument it may be 500uA, (you are measuring the current driven by the space charge). When you arange your monitors internal heater supply, test with the same valve and aim to be as close to the current that you noted in the first test as you can, the result will be absolute.

Thanks,
Amie.

[ppppenguin]

I find it hard to believe that 2 very different valves will have similar characteristics. But doing it with HF heating and DC heating both on the CRT and comparing the emission ought to work.

[Amie]

Hello Jeffrey

I wasn't suggesting you should compare two valves, that wouldn't tell you anything! just use a donor valve to do the test rather than potentially damage the tube. Test your verified supply and get the reading, then test the monitor tube heater supply with the same valve. It will give a qualative but accurate comparison of the two supplies: same space charge current = same heater power delivered. QED.

I only suggest a similar power rating for the valve and tube to negate loading effects in the monitor supply.

Of course the greatest accuracy would be by doing it directly with the tube.

Feel lucky?

Amie  

[ppppenguin]

Sorry, misunderstood your original post. It's a very good idea. Best to do it with the tube itself as the heater voltage will be sensitive to loading - it's hardly a low impedance supply. Little risk of damaging the tube. Short term overvoltage won't do any harm and I'd check it with the DMM before doing the accurate measurement. I already suspect that my DMM reads about 10% low.

[Mark Hennessy]

I like Amie's thinking. A lot. And even if it doesn't give the expected results in practice (it might, of course), it certainly demonstrates a high level of insight and ingenuity that is to be encouraged IMHO. I wish I'd thought of it!

My suggestions are embarrassingly obvious, so I hesitate to offer them.

However, I have a Fluke 8920A thermal RMS meter which copes well with high crest factors like this. Alternatively, didn't you buy a Rigol some time back? That does RMS pretty well - probably better than the 87 will in this context. Obviously the result is a calculation based on what's displayed on the screen.

We still have a few DU1s here. I spent the first year of my career fixing them, and am in no rush to see the insides of one again. At least they don't hurt so much now that I'm 25 years older. But I could be persuaded to measure the heater volts with my 8920A if that helps.

[pwdrive]

21 ohm resistor and aim for the same.

Lawrence.

[ppppenguin]

Lawrence, perhaps I 'm being thick but not quite sure what you're getting at. 21 ohms would be the resistance of a 6.3V 300mA heater. This CRT is actaully 6.3V 150mA but I still don't understand.

Mark, I've never explored the measurement capabilities of my Rigol 1054Z scope so thanks for the reminder. Just had a quick look and it's got RMS capability. Just need to be careful because the heater supply is sitting at about +30V so I'll have to disconenct that to make the meaurement. I'd rather not float the scope though it wouldn't be the first time that somebody had disconnected a safety earth to make a measurement.

Thanks also for the offer of the 8290A which would give a true RMS reading until hell freezes over. I think this is overkill.

Incidentally, the DU1-12 minitors were acquired by the BECG for the planned refit of the ABC-Thames truck. https://becg.org.uk/projects/abc-thames/ They are not quite authentic but close enough. It's not often that we would have the opportunity to acquire a decent set of identical small monochrome monitors. We also got some very nice black alloy framework from the same installation which we can use to make the stack. We need 18 for the truck but I think we've got nearer 30 so I took a few for experiments. All have been in an area that wasn't quite dry. A few in a different area have been downright damp.

Of the 6 I've now got, 2 work well, 1 emits magic smoke, 2 work really badly with nasty high voltage noises and one did work until something went pop and the picture went funny. I think it's in the 110V supply which is derived from the LOPT. There seem to be at least 2 versions, an earlier one with 4 knobs and a later 1 with 3. The basic circuit seems to be identical, it's mainly mechanical and cosmetic changes.

For my 405 experiments I've decided on one of the workers, which has a slightly less good CRT than the other.

Assumning my experiments go well, I'll keep 1 or 2 here as lab 405 monitors. Another will go to Dulwich to replace the unreliable Prowest PM14/1A. I've promised 1 to Frank Cuffe who, astonishingly, did all his Hedghog converter work without a 405 monitor. There may be 1 or 2 more available for a donation to the BECG.

[pwdrive]

I got the heater resistance by using the data for the M31-184 as I couldn't find the data on the W version.

Basically I was saying figure out the heater resistance as per CRT data and use a dummy when converted to run at 405 line frequency and aim to get the same voltage reading across it as was measured when the crt was connected and running at 625 line frequency.

Just out of interest, the data book I used:

https://archive.org/details/BrimarIndust...ch/m31-184



Lawrence.

[Mark Hennessy]

We have 4. Two of them have D65 CRTs and the others are standard "domestic" phosphors. They are used several times a year in the monitor line-up exercises.

I remember buying a couple brand new about 20 years back. These would have been 20 years newer than the newest examples we already had. There was surprisingly little difference between them - the PCBs had been "respun" so looked a bit more modern, but the electronics was practically the same (and just as painful!). It it ain't broke...

The Rigol has more measurements than I knew existed. Some aspects of the UI could be improved in this area, as you're probably discovering. If you don't want to float the 'scope (or the monitor and whatever is feeding it, which no-doubt is much easier said than done), then a differential measurement is probably good enough. I'd have the same problem if I wanted to get the 8920A involved, as that is earthed. I have got some decent audio transformers that would work well here - must get around to mounting some in boxes for this sort of job...