Telequipment DM64 'scope help with missing X scan

[Mark Hennessy]

It looks like my earlier hunch was promising

With 0.1V across R138, that gives us a tail current of 0.66mA. In other words, nothing at all. So why is this?

Only 0.9V across R138. Hmm... I don't know what the tail current should be, and the manual doesn't seem to give any clues, so let's pretend for a moment that it should be 10mA (it might be double that in practice). In which case, there should be 1.5V across R138, and so 2.1 or 2.2V across R138. As it stands, TR82 is lacking in bias. So that points to R137 or R152.

I'd put money on one (or both) of them being high in value or maybe completely O/C. You could try measuring the resistance in circuit, with the 'scope powered down for a couple of minutes first. If you get a lower than expected reading, then that's in-circuit conditions causing currents to flow thought a different path, but if you get a higher than expected reading, then the resistor is faulty.

If you don't want to remove the PCB or CRT, you might be able to change them from the component side if there is enough space. Just snipe away the old component, leaving enough of the old leads poking through the PCB, and solder the replacements to that. Do it quickly, otherwise the heat will conduct through and melt the original solder on the rear of the PCB. It's a bit of a bodge, but it does work. You can always "make good" the repair should another fault force the complete strip-down at some point in the future...

Regarding the PSU rails, they're not regulated in this model, so the numbers given will always be "nominal". But the low reading might be indicative of another fault - we'll perhaps learn more later.

[boater sam]

That's for tomorrow then, I will check around TR82 and those resistors.

Discovered that it was the 14v rail to 110/36 which was pulling low.
Thats the supply for the trigger and bistables TR2,3,4,5.
So I bit the bullet and removed the time base switch assembly labeling all the wires first as it is the only way to get at this part of the board. There are only 20 odd!
Checked TR1 to 4, all seem OK. So assuming something is drawing too much from this supply I will have to lengthen all the leads and reconnect the timebase switch out of the case so I can dynamically check further.
If all else fails I will measure the voltages around TR1 to 5.
It gets no better!

[Mark Hennessy]

Be careful - don't risk putting additional faults on it just yet

At the risk of stating the obvious, it might not be a case of the rail being pulled down - rather, the power supply might have a fault. As mentioned, it is unregulated, so you'd expect the rails to be only approximately correct anyway. There are a lot of electrolytic capacitors in the power supply that could be faulty, and that would give low output. Once the scope is sweeping, you'll be able to use it to check the power supply rails to see how much ripple is present - right now, a DVM is not the best tool for that job.

[boater sam]

OK, been very careful getting the timebase speed switch assembly out, all 16 wires lengthened and re- attached to the pcb.
Power up and we are back where we started, 2 dots in the middle won't move.
I have checked R152,R137,R136,R138. They are certainly not high but it is impossible to isolate them without cutting them out.
TR82 has 13.6v on the base, but there is only 0.1v across R136 so it is not conducting. That transistor is an FRB749, new to me, ( which has a beta of 77 ) spec says BC109C but the other transistors which should be BC109C too, are BC239C. Swapped a BC239C into TR82 from TR72 and vise versa. No change.
I have checked all diodes on this pcb, all OK.
Beginning to loose patience with this 'scope, its just so difficult to work on. I have a DM63 with a focus problem that may be a better bet!

[pwdrive]

Beg/borrow another scope to see what's what.

Lawrence.

[Mark Hennessy]

OK, been very careful getting the timebase speed switch assembly out, all 16 wires lengthened and re- attached to the pcb.
Power up and we are back where we started, 2 dots in the middle won't move.
I have checked R152,R137,R136,R138. They are certainly not high but it is impossible to isolate them without cutting them out.
TR82 has 13.6v on the base, but there is only 0.1v across R136 so it is not conducting. That transistor is an FRB749, new to me, ( which has a beta of 77 ) spec says BC109C but the other transistors which should be BC109C too, are BC239C. Swapped a BC239C into TR82 from TR72 and vise versa. No change.
I have checked all diodes on this pcb, all OK.
Beginning to loose patience with this 'scope, its just so difficult to work on. I have a DM63 with a focus problem that may be a better bet!

Sam, we need a more focused approach (pun intended!)

Please, let's just look at TR82, as we know that has a problem.

TR82 is a current source. It is required to make the X-amp work correctly. But if there is 0.1V across R136, then there is no current, and this is why the spots are on the middle of the screen. Let's establish why.

You have measured the resistors in-circuit, but I think you need to isolate them to measure them properly. That is just a case of snipping one lead-out on the PCB side and bending it up. Very easy to reconnect afterwards with a blob of solder.

Can you clarify one thing for me? When you say "TR82 has 13.6v on the base", what does that mean? Is it +13.6V with respect to 0V (which is what your sentence implies), or is it -13.6V w.r.t. 0V? Or, is it 13.6V w.r.t. the -14V rail? It matters

A better way of describing the base voltage would be to state the voltage across R138. So, what is it? Earlier it was 0.9V - has that changed?

I see from your parallel thread on UKVRRR that the voltage across R138 ought to be 5.6V, which is quite possible based on the resistor values. If so, the current delivered is 33mA. Right now, there is nothing, and the only way to get current flowing is to have a sensible voltage across R136. And that can only come from R152 and R137. Mostly R137.

Sorry to drone on, but we really need to grip this first. Based on earlier advice I gave, I don't understand why you've done most of what you've done - we just need to conclusively eliminate R152 and R137. Please report back once you've measured them (having snipped one end of each first).

Cheers,

Mark

[boater sam]

The switch had to come out because it is not possible to even see the components behind it never mind get to them!
Just about the whole of the X amp parts are buried under the switch and its sub pcb on top.
To make it worse R152 is stood on end with no lead below it, so it will have to be broken out and replaced otherwise there is nothing to solder to. Sods law, its the only one one the board on its end!
Surgery tomorrow then, I'll send a photo.

[boater sam]

Magic Mark. You have been christened!
R137, 68k histab 1% is O/C.
R152 220k is 248k, ok.

R137 changed for 67.5k

Now have 5.6v across R136, +6.3v on TR82 base. ( voltages all rel to 0v rail i.e. C407 neg  )

Now R405 is running hot after 2-3 mins. Random tick from what I assume is eht box buried in the middle of the steelwork.
BUT- progress--  Horiz shift coarse or fine moves dots 1cm left in 1st 20 degrees of travel. Beam locate does the same.
Shift control to move  channel 2 dot up, it goes diagonally to top right of screen, taking channel 1 dot horizontally with it.
Focus seems to have deteriorated, dots are larger but control still has some effect.
Single shot lamp is lit constantly, reset does nothing.

I assume that the 180v supply is overloaded as R405 is running hot. As the volts on TR82 now seem to be OK, perhaps the overload is on the  the supply to the Y amps. If I could find the connection, 111/57, I would dis it and see.

Your detective work is brilliant, wish I was this good with these cold valve thingies. Thanks, any further ideas welcomed if you have the time and patience.
Sam.

[Mark Hennessy]

Excellent news

I'll have a think about the rest of it later on - I have an urgent job on this afternoon...

Cheers,

Mark

[boater sam]

The heat on R405 would seem to be normal, its a 5% ww on stand off tags with holes in the pcb for ventilation under it. Running for an hour its still only 'hot'.
Could find no excessive current draw on the feed to the Y amps. Voltages around TR751,2,3 seem to be as I would expect, TR751 40v col, 29v base, 28.4v emit,   TR752  113v col, 27v emit,  TR753 105v col, 27v emit. All transistors are OK on test.

Clicking seems to have stopped, maybe it was a wee bit damp around the eht.

I am very confused about the transistors being different when the spec sheet says they should be similar. And being the same when they should be different!
I haven't found any BC109C s on the boards, there should be lots of them.
Example, TR82 should be BC109C, its a FRB749 ( whatever they are ).
But TR75 should be BSX20 but its another FRB749 !

Maybe someone has been here before me.