Telequipment DM64 'scope help with missing X scan

[boater sam]

I was glad to acquire a DM64 scope, thanks Keith, with no horizontal scan.
Everything else works, power supplies seem to all be there.
I have tested the X plates output and driver transistors to no avail. Access to the X amps is very difficult, even refitting transistors in their holders is very tricky.
Any help gratefully received.

[Refugee]

Telequipment PCB often have poor via holes and it is not easy to get at them without removing the PCB from the chassis.
I have had it on a battery/mains portable with the Y amp.

[boater sam]

I looked at removing the pcb but it looks horrendous. All the front panel controls would need to be removed, some are mounted on the pcb, some aren't. The timebase speed switch is on a sub pcb mounted on the switch over the main pcb.
To get soldering access to the rear would seem to be by removing the crt!
I really need to isolate the fault first, it will be impossible to power the pcb after access is possible.
It took me about 4 hours to unplug and replace 4 transistors! Extra long nose pliers and firing transistors off in all directions whilst trying to get all 3 leads back into the holders using a magnifying glass and high powered light.

[Mark Hennessy]

About 15 years back, I was offered a DM64 once - IIRC, the gent wanted just a tenner for it. I looked inside, and declined the offer

I've been spoilt, having had a DM63 for 25 years. It's still my default 'scope - I like the form factor, which occupies less bench space than a convention "horizontal" 'scope. These days I have 2 or 3 of them in various states. Despite the small difference in model number, there is a massive difference in construction. You can really see the Tek influence on the DM63 (and D83, which is related).

Do you have access to another 'scope? With that, you should be able to break it down before diving in there. I assume you've downloaded the manual?

Is it a lack of sweep, or is it a faulty X-amp? Or is it a fault in the mode switching (external X or X-Y modes)? It might be that the trigger circuit is locked out, indicating a fault with the comparator or bistable. Can you get anything from the Gate and Sweep outputs? From what you say, I assume you have 1 or 2 spots? Are these at normal brightness, and where on the screen are they? Do they move when you alter the X-pos controls, the X5 magnifier, or press Beam Locate?

Usual generic Telequipment faults:

- Resistors failing high, especially in the power supply or EHT sections.
- Transistors failing - often MPS6518, but others as well. Especially if they are in a plastic TO92 package and run hot, but the MPS6518s fail for fun in any position (I replace them with BC327-25 as I have hundreds of them)
- Transistor sockets needing to be cleaned.
- Capacitors drying up. Not surprisingly... Probably not the main issue here, but expect to replace a few later on.
- EHT transformers failing. Luckily not seen that personally, but it happens.

All the best,

Mark

[AlanBeckett]

According to the net the DM64 is an Analogue Storage 'scope. Not fore the faint-hearted I would suspect

http://drkfs.net/REPAIROFDM64.htm

Alan

[Mark Hennessy]

Yes, that's correct. It uses the inferior Tek CRT - the DM63 uses one by English Electric which is a lot better.

Not that I'm an expert on storage CRTs - far from it. I'm just quoting what I've read in several other reliable/trusted sources, and it sticks in my mind because it's unusual for Tek to be the inferior player in any game That said, we do have a 7000-series 'scope at work with analogue storage, and that really is lovely. If the size of a small house!

Still, the storage stuff can be safely ignored for now. It's not a facility that I use all that often personally. Perhaps the most useful option is the variable persistence mode when sweep speeds are slow, or when you're using it in X-Y mode for IF lineups and similar...

[boater sam]

Thanks for the reply one and all, I know I face a challenge with this box of bits.

The scope will show 2 stable spots in the vertical centre of the screen at adjustable intensity and focus. one for each channel. Astigmatism  works ok
The Vertical positions can be adjusted at will and input to either channel will produce a vertical trace, a line only.
There is no horizontal sweep at all and no X positional adjustment will move either dot. X mag does nothing, Beam locate will switch dots on and off. Trigger is to internal and makes no change when put to external.
I have no other working scope so I cannot check gate or sweep output directly.
I have done a close inspection, nothing disconnected or distressed. An all  device wiggle gives nothing. The 2 X drive and 2 X output transistors have been removed and tested ok, what a job getting them back in.
+300 , -14 and +14 volts rails are to the X amp though +14 is at 11.88v and 300 is at 287v.
Disconnecting the +14 from the X amp circuit does not significantly increase the voltage.

Both dots are immovable horizontally no matter what the controls are set to.

Reading through the service guide suggests that the X amp is at fault but its a complicated circuit and taking pot luck and guessing is not a way to proceed. The circuit diagram is sadly lacking in voltage readings apart from supply rails.

I considered checking every transistor on that pcb but access is terrible, it takes a lot of very careful fiddling to get them back in.
It would appear that the only access to the back of the pcb is after tube removal which rules out dynamic testing.

[Mark Hennessy]

OK, so if the dots can't be moved horizontally, that suggest a problem with the X output stage. The fact they are in the centre of the screen tells us that the X plates have the same voltage across them. Good...

The fact the dots are of normal brightness is good - that means the timebase bistable has told the Z-amp to brighten up the trace. You might find (with a DVM) that the Gate output is high. I would have thought that by fiddling with the timebase and trigger controls you should be able to make the dots disappear and/or flash...

It's perhaps safe to assume that only one component is at fault (famous last words, but you have to start somewhere!). So, looking at the schematic, is there a single component that could cause this?

Before going further, it would help to know what the voltages are on the X plates. If they are high, for example, then that tells us that the output transistors aren't conducting at all, and that might point us to TR82, Or perhaps R137 and/or R152, which supplies base current. In my experience, any resistor that has more than about 50V across it is immediately suspect - even though the resistor might not be running obviously hot, film resistors have a failure mode that sends them high and ultimately O/C.

The fact the X-shift controls do nothing is a big clue here. Despite the apparent complexity, the X-amp is pretty simple when you break it down. TR76 is a common-emitter amplifier with shunt feedback via R122. The base of the transistor is a summing point, taking current from R121 (from the X-shift controls) and R103 (from the sweep generator or Ext X). The output is fed into the long-tail pair made of TR78 and TR79. The output transistors TR77 and TR81 are cascoded from the collectors of the LTP. There is some complexity caused by all those diodes, but those are switches used to tell the long-tail pair to take their input from a Y amplifier when in X-Y mode.

Anyway, a few voltage checks would be useful. I appreciate what you say about the difficulty of doing this, but you should be able to reach the nodes from the component-side of the PCB? You might need to go to a resistor lead rather than a transistor, of course. At the risk of stating the obvious, it's useful to remember that the metal cans of transistors are usually connected to collector - if that helps?

If nothing else, what's the voltage across R136?

[boater sam]

Great! Thanks Mark for taking the time to look it up.
I will spend a few hours today reading off voltages around the X amp.
I have checked all those transistors already, time to dig a little deeper.
Good to see we are both singing off the same sheet, avoids mass confusion!
Sam.

[boater sam]

OK, many voltages taken. What a job finding components on this pcb.
Mains in is 239v set to 240v tapping.

Voltages on pin outs.
110/25      4.8v   R121
110/34      108v  spec is  110v
110/35      -14.57  spec is -14v
110/36     11.06v
110/37      11.08v spec is 14v
110/38      191v
110/41      7.2v     base of TR79
110/42      6.67v    x5 off, emitter TR79
110/43      6.3v   base TR78
110/44      6.43V
110/45      10.8V  R103
110/46      0.06V collector TR76
110/47      7.81v  R142/R143/anode D81

TR77 and TR81 collectors at 192v
R140 base TR81    11.2v
R139  base TR77   11.2v
Base TR 76    0.7v
Basr  TR82   -13.6v
R152 jnc D69       75v
R144  6.3 & 6.2 opp ends

Tested on magic Chinese box TR5, TR76, TR77, TR78, TR79, TR81, TR 82

beam locate does nothing

Trig level and stability randomly switches dots on/off when fully anti clock

Single shot and reset appears to switch dots on/off

Gate out socket is at 0.63v AC
Sweep out is at 11.11v AC
Cal is at 0.244v DC  AND 0.263v AC

Across R124 is 4v
across R123 is 104v
Across R122 is 0.7v
Across R136 is 0.1v
Across R138 is 0.9v

X plates are at 286v & 290V  There is 100v across both plate zeners.


Hope that all adds up to something. The 14v rail being low is a bit strange, disconnecting the feed at 110/37 does not change it.All measured with DVM

Thanks for your patience,
Sam.