2465 Display problem


NigelP
 

I've recently acquired a 2465 for repair. It has a strange horizontal display phenomenom such that the picture (regardless of which channels are in use), and also the readout characters, appears jittered or smeared horizontally.

If I put a sine wave into CH1 for example (in this case about 4KHz) the timebase synchonises and displays a recognisable sinewave but the whole display is jittered horizontally. If I reduce the timebase rate to say 50mS/div you can observe the amplitude of the sinewave but it's travelling across the screen as a horizontal "block" of signal rather than a simple vertical line. Something is horizontally modulating the display and also the readout.

The A-gate output and also the ramp from the A-sweep U700, and also into and out of U735, are perfectly clean and exactly what I would expect but the U800 H-drive to the CRT contains a lot of rubbish in the flyback period which is not apparent in the U735 signal. So far I have not identified any perturbations on any of the U800 pins, although it is not always easy to find direct access to the pins to check.

All supply rails to U800 are clean. As far as I can see at the moment the remainder of the scope seems to function as expected.

Any suggestions as to where to look for this problem?

Maybe it is one of the digital signals heading into the U800?


Jean-Paul
 

Please post photos, difficult to imagine what you describe.

Besides U800, CRT deflection pins get loose in shipment and the CRTs occasionally fail, especially if shipped with bad packing.

Jon


NigelP
 

Big steps forward today! I found some oddities while testing in XY mode but the most significant is that the H-output chip U800 is unstable and oscillating. Placing a scope probe on W918 (one of the H-out tube terminals) kills the oscillation completely. Touching the terminal with a metal tool and finger only partially kills it; optimum "kill" is touching pin-9 of the U800. With this "modification" all traces and readout on screen are clear and readable.

Now interestingly enough U800 has been mounted in an IC socket for some unkown reason (probably for trouble-shooting by a previous owner). Removing the chip from the socket reveals the originally soldered pins. Maybe I should return the device to its original mounting and that might fix the problem completely. I note some capacitors specifically on this output terminal are not also applied on the other output terminal.... does anyone have the internal schematic and / or device operating mode for the U800 (155-0241-02)?

Is there meant to be some kind of heatsink on U800? There are two screw studs/nuts but frankly they are not really doing anything right now other than making sure the chip can't fall out of the socket!


Paul Flinders
 

Another headache 'scope from my collection - I'm glad Nigel has managed to get to grips with it!

I see that you note the U800 is a -02

Looking through the group the other day I happened upon a link to a site with various Tek documents, one of which was describing a field upgrade for the U800 to the -02 rev - that mentioned changing a couple of passive components as well.

Presumably the original U800 in this scope (a plain 2645, not A or B) would have been the -01

Perhaps the upgrade was done without changing the passives.

Search the list for U800

--
Paul

On 26/01/2022 12:31, NigelP wrote:
Big steps forward today! I found some oddities while testing in XY mode but the most significant is that the H-output chip U800 is unstable and oscillating. Placing a scope probe on W918 (one of the H-out tube terminals) kills the oscillation completely. Touching the terminal with a metal tool and finger only partially kills it; optimum "kill" is touching pin-9 of the U800. With this "modification" all traces and readout on screen are clear and readable.

Now interestingly enough U800 has been mounted in an IC socket for some unkown reason (probably for trouble-shooting by a previous owner). Removing the chip from the socket reveals the originally soldered pins. Maybe I should return the device to its original mounting and that might fix the problem completely. I note some capacitors specifically on this output terminal are not also applied on the other output terminal.... does anyone have the internal schematic and / or device operating mode for the U800 (155-0241-02)?

Is there meant to be some kind of heatsink on U800? There are two screw studs/nuts but frankly they are not really doing anything right now other than making sure the chip can't fall out of the socket!




Jean-Paul
 

The original TEK ICs were made in house. Later on TEK outsourced some to Maxim.

If you seek another U800 beware of pulls, and Chinese junk. The best sources of the special TEK ICs like the hybrids is Qservice in Greece and Walter at Sphere Research in Canada.

The IC that has discussion of mods for a heatsink is the horizontal.

I have replaced at least one U800 for similar issues.

Bon Chance

Jon


Paul Flinders
 

On 26/01/2022 15:02, Jean-Paul wrote:
If you seek another U800 beware of pulls, and Chinese junk. The best sources of the special TEK ICs like the hybrids is Qservice in Greece and Walter at Sphere Research in Canada.
True but neither have any stock.


satbeginner
 

This man is still improving his drop in replacement he is developing.
If I remember correctly just recently he confirmed this, and it is looking very promising.

https://youtu.be/nfc_z2FNeb8

Leo


Ozan
 

On Wed, Jan 26, 2022 at 04:31 AM, NigelP wrote:


Big steps forward today! I found some oddities while testing in XY mode but
the most significant is that the H-output chip U800 is unstable and
oscillating. Placing a scope probe on W918 (one of the H-out tube terminals)
kills the oscillation completely. Touching the terminal with a metal tool and
finger only partially kills it; optimum "kill" is touching pin-9 of the U800.
With this "modification" all traces and readout on screen are clear and
readable.
------
It would be useful to observe the frequency and shape of the noise/oscillation/coupling on a scope without killing it. Since there is a large DC voltage it will be tricky to use an active probe even if you had one. Inserting ~1-2pF cap in series with the scope tip may me good enough if the noise is large. You will get some attenuation because of the series cap but loading will be lighter (less than the cap you inserted) so noise may still be there while observing.
Ozan


Jean-Paul
 

Easy to swap U800 Hybrid between 2465x if you have any others. Be careful on the nuts and seating of the Hybrid heatsink.

When swapping, I serialize the ICs eg blue painters tape.....

Jon


NigelP
 

I shall dig into this a bit more shortly. I would imagine that mounting the IC directly on the PCB will improve the oscillation. There are two, what I would imagine to be, compensation capacitors (not uncommon for power ICs..... I'm an ex-silicon App Engineer!!!) namely C808, just 10pF to a "compensation" pin-8, plus C809 22pF across the series 160 ohm feeding the H-plate from H+out pin-9. I suspect with such low values the simple act of mounting the IC direct might fix the problem. There is no similar circuitry on the H-out terminal. An IC internal schematic would be handy for understanding this.

Of course one solution, even if not elegant, is to add a bit of extra capacitance to equate what the scope probe is doing. I guess this would only be an issue at the fastest sweep speeds. It's even possible that C808 is actually defective.

Regarding scope probes, I do have a 100x probe so I'll look at the capacitance that adds versus the 10x probes.

Before I transfer U800 back to the PCB there is one other odd characteristic of the scope......

In XY mode where CH1 becomes the X channel, CH1 shift produces a diagonal spot movement (rather than simply horizontal), meaning that somewhere the V-shift is coupling into the H-shift system. The movement is from left to right with associated bottom to top movement. Hopefully this will be something obvious but its another area for investigation. For what it is worth, CH2-4 shifts when switched on only produce a vertical movement, as expected; this fact should help to pin down where the problem is ocurring.


Ozan
 

On Thu, Jan 27, 2022 at 05:32 AM, NigelP wrote:

I shall dig into this a bit more shortly. I would imagine that mounting the IC
directly on the PCB will improve the oscillation.
I agree extra inductance and capacitance is not good. I think others mounted the IC in a socket without an issue but if my understanding is right on your scope socket is soldered on remaining pins of the old IC instead of cleaning out the old pins.

There are two, what I would
imagine to be, compensation capacitors (not uncommon for power ICs..... I'm an
ex-silicon App Engineer!!!)
The scope is in good hands.

namely C808, just 10pF to a "compensation" pin-8,
That is what I would have thought too but another schematic (Tek 11301) marks pin 8 as "Bootstrap". Perhaps this pin goes to the cascode of the driver.

plus C809 22pF across the series 160 ohm feeding the H-plate from H+out pin-9.
Also one unexpected feature is "+" and "-" side are not completely symmetric. This is because a horizontal amp only needs to do a good job in one direction (direction of sweep).

I suspect with such low values the simple act of mounting the IC direct might
fix the problem. There is no similar circuitry on the H-out terminal. An IC
internal schematic would be handy for understanding this.
https://w140.com/tekwiki/images/e/e0/Tek_11301_11302_prelim_service.pdf
shows a little more on page 230 but I haven't seen a full schematic.


Before I transfer U800 back to the PCB there is one other odd characteristic
of the scope......

In XY mode where CH1 becomes the X channel, CH1 shift produces a diagonal spot
movement (rather than simply horizontal), meaning that somewhere the V-shift
is coupling into the H-shift system.
This is normal, I just checked a on a 2467B again. Since Ch1 becomes X axis in XY mode, displaying it as Y signal as well shows as a diagonally moving spot.

Ozan


NigelP
 

Now fitted a 27pF from pin 9 to GND; pretty much kills the horizontal jitter, which now incidently is actually controllable to some degree by the jitter pot (it wasn't really before).

Now to sort out the diagonality issue in XY mode.................


Ozan
 

Please see my previous message. Diagonal Ch1 in XY mode is normal.
Ozan

On Thu, Jan 27, 2022 at 09:16 AM, NigelP wrote:


Now fitted a 27pF from pin 9 to GND; pretty much kills the horizontal jitter,
which now incidently is actually controllable to some degree by the jitter pot
(it wasn't really before).

Now to sort out the diagonality issue in XY mode.................


satbeginner
 

XY on a 2465 works different then we are used to:

On a 'normsl' scope, like a 475, you enable and use the chi and ch2.

On a 2465 you only select one channel, ch2 for vertical, and without enabling ch1, it will do horizontal.

Leo


NigelP
 

Mounting; no in fact the socket has been soldered directly into the PCB but not quite as prettily as I would have done it I must admit! It's flush to the PCB on the pin-24 side and not very flush on the pin-1 side. Sorry if someone listening actually did this mod on this particular scope but sometimes I get a bit picky about such things!

Bootstrap; so that's what BTST means.... doh, I should have known that one; I've worked with plenty of devices that use bootstrap techniques!!!!

XY; so actually CH1 should never be "displayed" when using XY mode? And if not then the scope is essentially a 3-channel beast in this mode of operation? I don't have any reason to need a four channel XY scope but it's something that never even ocurred to me.

The great tome looks very useful, even if for the wrong scope.

I think I'll look at adding a heatsink just as a precaution having read some of the blurb on this subject. Does the U800 normally have some kind of bushing on the threaded studs below the heatsink line so that over-tightening can't occur? I've not found any pictures showing this aspect.

You wouldn't believe some of the horrors I've seen in my silicon days with over-zealous mounting techniques. I was often the first recipient of customer's devices that had failed in the manufacturing line. I well recall the use of one of our TO220 voltage regulators in the dashboard application of a very well-known (American) motor manufacturer!!! The tab had been rivetted to a metal piece in the dashboard with such force that the die inside was disconnected from the tab internally! I actually watched the process being done.... quite frightening actually. It took a bit of explaining that they were not just working with a piece of metal with a bit of plastic stuck to it!

In answer to a previous query, I did check with a 100X probe and it was sufficient to still display the ramp with oscillation on it and it runs at about 30MHz. If I recall, it was a burst in the flyback and hold-off period, not the actual linear ramp period, but this of course is effectively adding some extra scan time; this was particularly noticeable with the readout since the characters were supposed to terminate somewhere on-screen rather than in the overscan area.