Tek 2440 scope EHT measurement

[Electronpusher0]

I have just bought a non working Tek 2440 scope for a very good price and have started work on it.

The fault described by the seller was that it had been in storage for some time and on switch on there was a smell of burning.

As I expected the Rifa X class suppression caps had let out the magic smoke so out they came.

During testing two separate Tantalum capacitors also went short and let out the smoke, they were on the 15V supplies.
After replacement the supplies are all in tolerance.

The fault I now have is that the screen is only displaying an extremely unfocused illuminated patch on the extreme right hand side. The focus control does have a very marginal effect but certainly does not focus.

I have checked the Cathode and Grid voltages and they are both as expected about -1.9kV

I want to check the EHT.
My question is - Is it safe to measure the EHT with the cap off of the tube?

I know about discharging the tube and its tendency to still bite unless discharged several times. I have a 40KV probe so can measure it safely, I am just concerned what the effect of running the eht multiplier without a load (and final capacitor) would be.

The expected EHT is 14kV


Peter

[MotorBikeLes]

If your cathode voltages are OK, it is rather unlikely that you are missing EHT. I would start by finding why the display is off to one end. Disconnected scan connection on the tube? Faulty scan circuit? Trigger problem?
Les.

[Diabolical Artificer]

Check the X plates first Peter, measure voltage on each with respect to ground or you could try what Tek says on a lot of it's scopes,try shorting the two X plates together with I think a 1k resistor between them. Perhaps best check your service manual on that one, a thorough read of the TB section would be appropriate.

Andy.

[RogerEvans]

The Tek 2440 is almost totally driven by the digital signals from the processor and the 'soft' front panel controls (except for the focus and astigmatism presets which are electrically connected to the CRT circuitry) so the first question is 'is the processor working?'. On power up the three GPIB status lights and the TRIG'D light should be lit, then the status lights and the lights below TRIG'D should flash and you should hear the attenuator relays clicking. After self-test you should be able to toggle the trigger slope +/- lights by pressing the SLOPE button.

There are warnings in the manual about operating the 2440 without the case in place. The cooling of the CCD chips depends on the fan airflow being ducted past the CCDs and if operating the 2440 for more than a couple of minutes with the case off you should direct an external fan at the CCDs.

Do you have another scope available to help trouble-shoot the 2440? It would help to know if the ripple on the low voltage supplies (particularly +5V) is within spec and what voltages and waveforms are appearing on the X and Y deflection plates.

Regards,

Roger

[RogerEvans]

I should have added that if the only fault is the lack of the 14kV PDA voltage then you should see a slightly dim and blurred image on the screen so I would not worry about the PDA voltage at this stage.

Roger

[Electronpusher0]

Originally Posted by RogerEvans

The Tek 2440 is almost totally driven by the digital signals from the processor and the 'soft' front panel controls (except for the focus and astigmatism presets which are electrically connected to the CRT circuitry) so the first question is 'is the processor working?'. On power up the three GPIB status lights and the TRIG'D light should be lit, then the status lights and the lights below TRIG'D should flash and you should hear the attenuator relays clicking. After self-test you should be able to toggle the trigger slope +/- lights by pressing the SLOPE button.

There are warnings in the manual about operating the 2440 without the case in place. The cooling of the CCD chips depends on the fan airflow being ducted past the CCDs and if operating the 2440 for more than a couple of minutes with the case off you should direct an external fan at the CCDs.

Do you have another scope available to help trouble-shoot the 2440? It would help to know if the ripple on the low voltage supplies (particularly +5V) is within spec and what voltages and waveforms are appearing on the X and Y deflection plates.

Regards,

Roger

Thanks Roger and Les,
I will ignore the EHT for the moment. The EHT and the -1.9kV all come from the same potted block and the -1.9kV supplies are OK.

I am getting rather worried about the two tantalum capacitors that burned up, they were both on the +15V supply on the Display output board (drives the defection plates).
The +15V supply is in spec spec now but something must have taken out the caps, I just put it down to the fact that they are old tantalum ones but if the +15V supply went sufficiently out to blow caps it may have damaged an awful lot of circuitry.

Thanks for the warning about the CCDs, I had seen that elsewhere and have only had it switched on for brief periods, I will use a fan if I need to test for extended periods.

The Power up notes are useful, the GPIB lamps do light and the trigger lamps do flash during start up.

"Have I got another 'scope"......three more I am afraid, somehow I just can't resist them.

I will continue testing the Vertical output board and report back.

Peter

[RogerEvans]

Peter,

The usual failure mode of the tantalum capacitors is that they go short circuit due to ageing rather than any other provocation. In most cases the over current protection of the low voltage PSU will prevent further damage. One notable exception is that Tek had a habit of providing local decoupling via a low value series resistor, maybe 10R or a carbon resistor with an inductor wound around it, and the short circuit tantalum causes the resistor to go open circuit.

For some reason I have multiple 2440 manuals in my PDF collection which suggest the first attempts did not include schematics, if you have difficulty in finding a suitable manual let me know.

Regards,

Roger

[RogerEvans]

Peter,

I had a look at schematic <18> and all four low voltage supplies to the X and Y output drivers are decoupled with series inductors L801 to L803. Taking out either of the +/-15V supplies to the output op-amps will result in the beam being deflected way off screen and you probably see just the reflected electrons from something inside the CRT. If your failed tantalums were on this board then this must be a prime suspect.

Roger

[Electronpusher0]

Thanks Roger, the capacitors that failed were C182 and C284, both on the +15V rails adjacent to driver transistors (see attached)

However they both failed at separate times during testing and the display fault was already present.
When they failed they took the rail down, I replaced them with ordinary electrolytics.
I did have the display board hinged up, ie flexing the ribbon cables, when the caps failed.

Your description of the image is spot on (excuse the pun).

I am going to monitor the +15V rail while testing in case it is spiking up or something.

Peter

[Electronpusher0]

I have done more investigation, its not the EHT.
The analogue parts of the H & V output circuits are all ok, the beam is being driven hard right because thats what the output of the DAC is telling it to do.

I suspect that this is the fail safe position to prevent spot burn.

The inputs to both H & V DACs are at an indeterminate state, looks like the tristate data outputs feeding them are diasbled.
Looks like the display uP is not working.
There are 3 uPs in the system, 2 are on the processor board, the main processor (6809) and the video one. They share some memory and there is a bus contention handshake to sort out who has access.
There is also a force "Bustake" line that the main uP can assert to force a take over of the bus, this is currently asserted.

Looks like I am into sorting out uP faults, oh goody.

Some of the memory is backed up by a 3.6V lithium battery, this will certainly need changing after this time.

It would help tremendously if I could access the GPIB interface, looks like its time to get out the GPIB to usb interface I built and test it.

Does anyone know where I could get the program used to interface to the Tek? I guess it would be XP but I have an XP machine I can use.

Peter

[Chris55000]

Hi!

This any help?

https://github.com/Tek-User/Tektronix-GPIB-Download

Chris Williams

[Electronpusher0]

Thanks Chris, thats very tempting, I could certainly use that if I ever get it going again.
At this stage I just want to know if the processor is alive and executing code.

Peter

[RogerEvans]

Peter,

If you are planning to use the GPIB interface then have you discovered the 2440 programmers manual, it is on TekWiki? It has useful info about the power on sequence and error codes. I built a similar GPIB to USB adapter a couple of years ago but had the advantage that the 2440 was working at the time! The SRQ light on the front panel will only be illuminated after self tests have completed and control is passed to the user, so if you don't get that far then don't expect any response from the GPIB port.

Happy hunting,

Roger

[Electronpusher0]

Thanks Roger, I have downloaded the programmes guide.

I have just checked the GPIB lights again, on switch on all 3 LEDs light up, after a few seconds there is a brief flurry of activity on all 3 and they then stay lit.

What program did you use to talk to your GPIB to USB adaptor?

Peter

[RogerEvans]

Peter,

I am a Linux fan so variously I used the terminal program inside the Arduino IDE, 'picocom' and the serial interface from C (very clunky) which I used to write somethng approximating to a screen dump. All you have to do for a functionality check is type "ID?" into the terminal emulator, then you can ferret for most recent error messages but I can't remember the details - it should be in the programmers maual.

Regards,

Roger