Telequipment DM64 Storage Scope.

[winnem1]

Hi all.

This is my first post so forgive me if I get it wrong.

Seeking some knowledge from anyone out there that knows about a old Telequipment DM64 storage scope. It was working but died suddenly, the transistor in the EHT module went s/c. Having replaced that the EHT came back, but was well over 4K when it should only be 3200V and the set EHT control has no effect. Further checks revealed that the transistor? on the small sub panel on the PC111 board was open circuit. The only number on this device is M151 012400 and looks like a standard TO5 transistor, but I can find no reference to it anywhere.

Hoping there is a wiz kid out there that has worked on one of these scopes and knows what this transistor is.

Regards.

Mike

[peteratnith]

Here's the DM64 CCT. You don't say if you have it!
I've got a DM64 lying unused here if you want readings etc.

www.drkfs.net/DM64.pdf

Regards
Peter

[winnem1]

Hi Peter
I have the manual for the scope, but this a mod and is not shown in the manual.
The transistor data that I'm after is on a small pcb on stand-offs on the PC111 board. The only other components on this pcb are a 470R 5W resistor and a 7R5 0.5W resistor.
Any help much appreciated.
Regards
Mike

[peteratnith]

Hi Mike,

My DM64 doesn't seem to have the sub board yours has.
Heres a pic of my PC111 board. The S/no of mine is DM64 is 459258




Regards Peter

[winnem1]

Hi Peter
Thanks for the picture, as you say your scope does not have the Sub PC. On mine the resistor top left is removed and is on the Sub PC with the unknown transistor.
Anyway thanks very much for your help.
Regards
Mike

[winnem1]

Hi All
Just an undate to my faultfinding efforts. The unadjustable EHT was due the the current amplifier which controls the conduction of the eht oscillator. This was measured before but appeared to be ok, but on substitution proved to be the culprit. The eht is now the correct value. The unknown transistor on the sub-panel on PC111 is still a mystory as well as what it is for? If anyone knows what this transistor is there for perhaps you could let me know.
Regards
Mike

[WME_bill]

Could you sketch out the circuit around this mysterious board on PC111, and give us a few actual circuit references so that we can study the circuit.
Your 151-0124 is given as a 2N3501 by Tek. Selected in later models, probably for gain. I suspect a BF259 or MJE340 would serve perfectly well, as the main design point is probably high voltage rating. Even take a failed economy light bulb. The tube itself usually goes, so pinch the transistors, usually a pair of MJE13003 series or similar, small T0126 and very high voltage rating. I have used them on several occasions as a source of spares. Technology has moved on hugely since this scope was designed, and what was difficult to achieve with the transistors of 37 years ago is now common-place. It is strange how often the failure is of a high voltage transistor. Some surge peak proves too much. Very common with Bright-up circuits and the EHT controllers. S you have done well to locate and remedy it. Bill

[winnem1]

As asked for the modification to PC111.
R415 removed and placed on sub-board, the two holes that it occupied are now labelled 70 (-ve end of C412) & 69 (-ve end of C416). R302 changed from 47R to 470R, R304 changed from 10K to 12K.
Connections to sub-board as follows:-
111/70 connects to emitter of transistor, collector connects to 111/31. The 470R (R415) connects from base to 111/69 and a new resistor of 7R5 0.5W is connected between base and emitter.
This circuit appears to be some form of protection circuit for the eht oscillator, the current flow through the 7R5 controls the conduction of the transistor. When this transistor conducts it pulls down the base of the eht oscillator TR302.
Scope works with or without this transistor in circuit.
Only thing left to do now is sort out the focus circuit, I think the phototransistor has past it's sell by date?
Mike.

[WME_bill]

Thank you. Very interesting. Your analysis seems right. So almost any high voltage transistor which will do - 150V/100ma should be fine. A video driver type from the junk box will be enough to see if that is what the trouble is.
Focus control. Photo transistors. The led diode traditionally fails, rather than the photo-transistor. Some replacement types have a switch effect, which is not suitable for this circuit. The actual circuit is a curious arrangement which I do not recall having seen elsewhere, presumably to keep the focus control at low potential, near the chassis, instead of the usual -500 to 700v. I suppose makes it easier to mount the control. I have raided old computer power supplies to get something to use for the opto isolator. The Maplin catalogue for instance gives the usual connections, but check with the actual PC Board. Incidentally, have you checked the two zener diodes which hold 300v across the photodiode+R313 3M resistor. High voltage zeners I have often found troublesome, and a failure here could effect the focus control . Bill

[winnem1]

Hi Bill
Thanks for your information. Found a replacement for the photo transistor on ebay, fixed in place with the mes lamp in the plastice tube and all works fine.
Just need to check the calibration etc. and will be finished.
I would just like to thank averyone for their help.
Regards
Mike

[m0cemdave]

Originally Posted by winnem1

Just need to check the calibration etc. and will be finished.

When I serviced one of these, a long time ago, I remember I had to replace the string of high value resistors (R309-R314) in the EHT compartment before the EHT voltage would adjust to the correct value. They had all drifted high and the preset pot R308 had no effect on the voltage.
The EHT needs to be correct (-3200V) before starting calibration because it affects everything else.

HTH
Dave.

[ianian_c]

I have just repaired a DM64 and also only just seen the Post by Dave of Bletchley. Dave is right on the money and unless the EHT is set to 3kV2 everything else will be weird.
The resistor chain Dave refers to consists of 3 X 3M3 resistors and I suspect it will be hard to replace them with some which have the required Voltage rating. I used a string of 14 X 470k to replace 2 of the 3M3 resistors.
Another point on the power supply Capacitors of this model: I found at least 3 dry/fatigued joints.
Next time I will read the posts first.
Ian