Yes will do. I sent away for some new trimmer pots which were pretty bad.
I will let you know when they are in and installed.

I have a question. I don't need the beam control to use the scope, yes the points are very bright
but at least I have beams, right? It will wear out the phosphor over time, but the scope is on
death row anyhow.

Secondly, the only possible things that could be wrong producing no beam, is the grid to
cathode voltage is too negative, thus repelling electrons back, or the cathode to front anode is not
positive enough resulting in not enough pull for the electrons coming through the grid.

I have no easy way to test the high voltage tension unit, output is supposed to be about 8x00Kv. My volt
meter only goes to 1Kv. Is there an easy answer to this?

Schematic on its way.

Homer

 

Can you confirm that this 'scope has no XY mode?

 

Can you confirm that this 'scope has no XY mode?

In XY mode the trace is never blanked so there needs to be a way of doing that but I can't see any such system in the circuits you hace posted.

If there is no XY mode then the timebase controls blanking so there is always a pulse controlling the brightness. If that pulse fails to arrive then the trace will remain permanently blanked - the symptom which you have.

Check for a waveform at points 6, 11, and 12.

 

View attachment 157621 View attachment 157622 View attachment 157621 View attachment 157622

I apologize for the haitus, llife service calls sometimes

I have replaced loose and old parts and there seem to be no more bad solder joints.

I still have a stable set of two bright spots, mostly focusable, with grid/cathode shorted.

1.) Yes there is an XY mode, I will verify that it still works. That is how I made my logo on the left there.

2.) The beam FADED over time, and finally one night just slowly faded away right before my eyes over a period of 30 minutes, so blanking is probably not the problem right?

3.) I will check the waveforms as you suggest, I trust the scope works well enough to test its own waveforms

Homer

 

I apologize for the haitus, llife service calls sometimes

I have replaced loose and old parts and there seem to be no more bad solder joints.

I still have a stable set of two bright spots, mostly focusable, with grid/cathode shorted.

1.) Yes there is an XY mode, I will verify that it still works. That is how I made my logo on the left there.

2.) The beam FADED over time, and finally one night just slowly faded away right before my eyes over a period of 30 minutes, so blanking is probably not the problem right?

3.) I will check the waveforms as you suggest, I trust the scope works well enough to test its own waveforms

Homer

 

The points 6, 11 and 12 show some activity, its hard to interpret, certainly does not look like the diagrams, however I am not sure if it is logically meaningful for a scope to be able to trace and sync with its own blanking circuits

Anyhow the sine waves show up fine except for an up and down on the far left which looks lilke it is waiting
for a horizontal go signal, all the while it is still going up and down.

Here's the picture

 

If you still have the grid cathide short then the vertical line on the left of the sine wave is to be expected.
I don't think you can get any sensible results trying to look at a 'scopes own blanking waveforms.
I can't see from the circuit how it can unblank for XY mode.

 

If you still have the grid cathide short then the vertical line on the left of the sine wave is to be expected.
I don't think you can get any sensible results trying to look at a 'scopes own blanking waveforms.
I can't see from the circuit how it can unblank for XY mode.

A quick repeat of the question above, it would seem that if it isn't a blanking problem per se, then it has to be a voltage differential problem between cathode and grid, such as grid voltage is too negative wrt to the cathode and thus not letting electrons flow at all. Or the anode voltage is failing and not pulling hard enough. I have no way to measure the 8KV of the anode, is there some other way I can test it?

 

A quick repeat of the question above, it would seem that if it isn't a blanking problem per se, then it has to be a voltage differential problem between cathode and grid, such as grid voltage is too negative wrt to the cathode and thus not letting electrons flow at all. Or the anode voltage is failing and not pulling hard enough. I have no way to measure the 8KV of the anode, is there some other way I can test it?

Can you actually measure the 1500V safely?

 

Can you actually measure the 1500V safely?

Well there's a fire truck outside. I have a multimeter that says it goes to 2000V DC, and that meter says
grid is above 1500 and so is cathode, and different between them is about 190 to 120 depending on settings.

Let me get this straight.

The beam intensity is controlled by the cathode voltage, but the beam blanking is controlled
by the grid. The grid is conenected to the blanking circuit which then changes the grid voltage as
needed. Thus it is conceivable that something in the blanking circuit is no longer delivering the drop in voltage necessary to allow the unblanking to take place.

Homer

 

Here is the circuit description, not sure it will help at all. The first is the block diagram, and the second
is quick description of the beam blanking circuit.

 

It is possible that the schematics do not include the XY circuitry, I have found other minor inconsistencies
between the schems that came with the scope and the actual scope.

 

It is possible that the schematics do not include the XY circuitry, I have found other minor inconsistencies
between the schems that came with the scope and the actual scope.

The description does help. I hadn't noticed that TS702 and TS703 form an oscillator and provide the DC blanking path that is needed for XY mode.
You need (access to) an oscilloscope to look at those waveforms. It may be that the oscillator is not running.

 

Let me get this straight.

The beam intensity is controlled by the cathode voltage, but the beam blanking is controlled
by the grid. The grid is conenected to the blanking circuit which then changes the grid voltage as
needed. Thus it is conceivable that something in the blanking circuit is no longer delivering the drop in voltage necessary to allow the unblanking to take place.

Yes, exactly.

 

Yes, exactly.

OK, I have ordered a PC USB scope on ebay, that should show me those blanking pulses.

Homer