Telequipment DM64 'scope help with missing X scan

[Mark Hennessy]

Firstly, I wouldn't worry too much about the transistor substitutions at this stage. Manufacturers frequently made changes - mostly to save costs, or perhaps improve reliabilty in light of experience gained. Often, seemingly unusual choices might have been made because it meant using fewer different types overall. And every designer has their favourites.

Now we have more faults to explore. The various symptoms are interesting, to say the least, and it's difficult to know which to tackle next.

OK, so still have dots, so no sweep. Are you sure the controls are set correctly for a sweep? The fact that channel 2's Y-shift control moves the dots diagonally suggests that you might be in X-Y mode, but of course there might be a fault. So, can we set the controls as follows:

Ensure the 'scope is in non-storage mode, which I think should be just a case of making sure the "STORE" button is out.

Press "REP" so that the timebase is not in single-shot mode.

Ensure all 7 of the "TRIG MODE" buttons are out. This should put the 'scope into auto-trigger mode, meaning it free-runs in the absence of a valid trigger signal.

Although this 'scope has a "STABILITY" control, and if this is turned fully anti-clockwise, the sweep will stop. Turn it fully clockwise.

Set the timebase "TIME/DIV" to somewhere in the middle. 1ms/div should be fine.

Make sure it's not in X-Y mode. It looks like you have to pull out the red "VARIABLE" timebase knob on this one.

Press in the red knob on the X position control so it's not in X5 magnification mode...

As for the Y channels, just set those to something like 1V/DIV, position in the middle, and to be honest, and set to ALT if you want both traces on.

Having done all that, have you got a sweep?

How experienced are you with 'scopes generally? The trouble is, even with a fully working unit, it can be quite hard to get a stable and sensible display. When fault-finding a 'scope, you really do need to draw on operational experience. If you tell me your level, then hopefully I can occupy the safe ground between "teaching you to such eggs" and "losing you completely"

Cheers,

Mark

[boater sam]

Had a day of hospital visiting so tomorrow back to the bench.
I will check exactly what controls settings I have, I think that I will be using as you suggest as I followed Tek's "troubleshooting your 'scope"

I was a Ferranti trainee many years ago (Is it really nearly 50 years?!) using BIG Tek valve scopes, can't remember the #s, so I do know or knew my way around a scope. But it has been a long time since I had to use one in anger. And my transistor fault finding is poor really.
I have spent many years fault finding and fixing but I am finding the ergonomics of working on this scope unbelievably poor.

Thanks for the guidance,
Sam.

[boater sam]

I have it set up as you suggest. Have limited X shift control, dependant on X-Y on channel 2 setting.
No sweep, there is no sign of trigger, int or ext, single shot is lit permanently.
Link 110/46 goes from 0.04v to 9.8v and link 110/43 goes from 9.2v to 3.5v, with X posn from end to end.
Tested TR73, 74, 64, 65, 66, 68 69. all seem ok, gain of TR64 (FRB749) is 95 others are in the 400 to 600 range but most are BC 239C
D85 is on the reverse of the pcb, no access. I could not establish whether it was OK so I have lifted the transistor leg and bypassed with another diode. Made no change.
Found a few more voltages. 
D68 cathode 2.8v which seems correct pre trigger.
TR73 base  -7.8v  emitter  -7.5v
TR74 base -6.8v Collector +6v
TR75 base 16.6v emitter 6.28v collector 16.2v
TR76 base 0.58v collector 9.8v
D73 anode 6v

It would look like the TR73, TR74 bistable is not switched either way?

Mark, I know why you passed on one of these, its terrible to work on.

[Refugee]

From what you say it sounds like the switch is faulty leaving it stuck in single shot.
Have a go at measuring the wiring from the board to the switch and the switch contacts.

[AlanBeckett]

That must indeed be a possibility. On one of my Tek Scopes there was a piece of O/C Wire between the Mains Socket and the Switch. That fooled me for quite some time before I cracked it.

Alan

[boater sam]

Helpful comments from Refugee and Alan, but sadly not that simple.
It seems to have a response in X-Y mode all the time, but not as fully as it is when actually in X-Y mode which then appears to work properly.
Checking down this route I was looking for the trigger circuit from off th Y amps. Eventually found board 119, it is buried underneath  between 2 other boards, almost totally inaccessible.
 On this board I could just see TR757, squashed flat in the holder, leads shorted. 
Someone has been here before.
Spent a half hour  removing it, straightening it out and testing it, an another hour getting it back in. Sadly it made no difference!!!!
Patience exhausted, its lunch time. I will find something else to do for a while or I can see this scope going into the farm tip, which woukd be a shame.

[Mark Hennessy]

Hi Sam,

This is a puzzle! Still, it's easy to forget that we have made progress, and there's no reason to believe that more can't be had. I have dug out my paper copy of the manual, which is obviously easier to read than the PDF, and I've been wondering where best to go next. Of course, a big stumbling point for me is the lack of second oscilloscope. Your DM63 will be much easier, BTW, but let's persevere with this one for a while.

You have given a lot of information in previous posts, but it's too much to take on board and put in to some sort of structure. Let's start at the beginning, taking small steps.

Of course, while you don't have a fully-working second 'scope, the one you do have will be of use. It's not sweeping, but the Y-channels are working sufficiently well to tell us more information than just a DMM. We will take advantage of this.

So, we know the X-amp is working to some extent. But there is no sweep. So we need to answer this question:

Is the sweep generator working, but not getting to the X plates?

And of course, we have a bunch of diode switches that are used to switch between the sweep generator and the vertical channel when the 'scope is in X-Y mode, so it is indeed possible that the sweep is running.

When you are next at the bench, here are two things I need you to investigate:

1. When the 'scope is NOT in X-Y mode, what effect does Channel 2's Y-shift control have on the horizontal position of the dots?

2. With all the controls set as suggested in my earlier post, turn off Channel 2 - that should leave a single dot. Set Channel 1 to 1V/div, and select DC input coupling. Connect Channel 1's input to the sweep terminal on the front panel. What happens to the dot? Does it move to a new position on the screen? Or, does it change into a vertical line?

Let me know what you find, and I'll tell you where to go next

Mark

[pwdrive]

"Connect Channel 1's input to the sweep terminal on the front panel. What happens to the dot? Does it move to a new position on the screen? Or, does it change into a vertical line?"


Damn...I was about to suggest that

Sweep out to ext X in ?

Edit: Since writing the last sentence above I've just found the book of words...not a lot to be gained from that test...anyways how's about an external signal to the X in, might give some sort of indication/confirmation/idea etc.
Lawrence.

[boater sam]

Thanks Lawrence, and Mark. Your great minds are thinking alike. Why did I not think of that. 
I'm going down to collect some stuff in Worcs from Steve at Radiocraft tomorrow but will have a go when I get back.

 To answer your question Mark, the X-Y even when off is having an effect on the horizontal of both channels. If I get both dots on screen, trying to move channel 2 up the screen with the #2 posn control moves the dot sort of diagonally to the top right, shifting the channel 1 dot to the right horizontally at the same time.
With X-Y selected it works properly giving a full across screen capability.
Despite TR757 being squashed, it still works. I have a  voltage changeable with the Y posn. controls at both 110/2 and 110/3 collectors of TR757 and 755 . But can't see any change at the base of TR2.
I was checking out S4 and S2 when it got too cold in the workshop! 

PC119 is unbelievably buried!

Thank again for your brain work.
Sam.

[Mark Hennessy]

Steve lives 2 miles from me. Do you want to bring the DM64 to me?