Moderators: Dave Moll, Steve Anderson
Steve Anderson wrote:This isn't going to be as easy as the previous 'scope fix due to it's age and the very tight space that everything is crammed into...it is some 56 years old!
I have downloaded the manuals/circuits but I haven't yet looked at them. I note you've already powered it up, you were lucky that nothing went 'bang' or no clouds of smoke. For the while don't power it up again, let me go though the circuits etc...
Steve A.
Steve Anderson wrote:Just to confirm the model No., is the nameplate on the back of the case the same as below? The serial number of course will be different, but hopefully it's a GM5655/02.
What valves/tubes are fitted? This will help date it and find the correct manual for it. The manuals you posted cover three different models.
You're correct, that push-button wouldn't have been there originally but an input connector.
[/quote]You'll note that in the video there's a horizontal burn across the tube, it must have been left on for a long time with no input signal. Thankfully you can still get the DG7-32 and they're not that expensive, well, as far as CRTs go. The last two I bought about 10 years ago were GBP 25.00 each.
Steve A.
Steve Anderson wrote:OK, armed with that new data the most applicable manual is your upload GM5655_4.pdf. I have no problem with the diagrams, they're not pin-sharp but quite legible.
The valve/tube compliment should be/is from what you've posted:-
2 x ECC81/12AT7 (a slight typo there Harry) Dual Triode.
2 x EF80/6BX6 (hard to read so an 'X' looks like a 'V') RF Pentode
1 x ECH81/6AJ8 Triode-Heptode
2 x EZ80/6V4 Full-wave rectifiers
The European and US equivalents are exactly the same above. All of these are still easy to get these days should it be necessary.
Let me think about where to go and what to do next...
[/quote]This will make a very useful NBTV display where you can check out other circuits/cameras/whatever without having to tend to a mechanical device. Once whatever you're working on is OK replace this with a genuine mechanical device. The screen is clear, no graticule and at 3" diameter large enough for 32 lines. But it's useless for SSTV due to the phosphor type. The only other thing that would need be to added is some form of grid modulation, but that's down the road for now...
Steve A.
Steve Anderson wrote:It seems that there's going to be quite a bit to do on this.
My plan of attack would be simply getting the thing to work first, then remove the modifications performed by someone else.
If possible trace out and draw the mods. and have a guess as to why they were done. If the thing performs as it should with the mods removed and put back to original then all is well.
This is an oscilloscope, but compared to later models it's not completely useless but nothing is calibrated and there is no graticule so you can't measure voltages or time/frequency only observe waveshapes - you can have a very rough guess at volts/time though. So with that it would make a good NBTV display, but some gentle modifications would be required.
The internal timebase will run at the NBTV 12.5Hz frame rate but this would require a swap of the deflection plates to get it to go right-to-left - a minor mod.
You would then need to make an external timebase of 400Hz to feed into the vertical channel (easy).
The biggest mod is actually an addition so putting the instrument back to original would be easy. This is for the grid modulation arrangement which would be quite simple using an opto-isolator and a few other parts.
Now as far as SSTV goes you would need to replace the DG7-32 CRT with a DP7-32 which has a P7-like phosphor, and although they were made they are very rare, and I mean very rare... = very expensive if you can find one. The vertical channel would need extensive work to be able to provide the 8-second vertical scan. So I wouldn't even contemplate any of this.
[/quote]The link you provided has sound advice and I suggest you follow this guys method and gradually weed out all the duff components.
Steve A.
Steve Anderson wrote:It seems that the graticule was for go/no-go testing of some form of device with its central rectangle, we can only guess. Perhaps the mods were somehow tied into this and was a customization for a production line. A rather poorly carried out customization though. At the radar company I used to work for they also sometimes had to customize instruments for production purposes but it was done properly with assistance from the instrument manufacturer. It would appear not so in this case - I can't imagine Philips condoning this sort of bodging.
Steve A.
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