Tektronix 224 display issues (connecting scan lines visible)

[MrQuan]

Hello!

I recently picked up an old Tektronix 224 portable scope. It's fairly dirty so I need to give it a good clean outside, but all the features seems to work.

The issue I'm having is there are lines on the display that shouldn't appear, as if the intensity isn't turning off properly as the ray is scanned around the screen. It's hard to describe, so I've attached a couple of images. The first image is the scope upon start up for a typical display. The second image is the alignment pattern (all the diagonal lines should be invisible).

Before I go poking around inside, I was hoping for some advice or guidance. Does anyone have any ideas on what could cause this intensity issue and where I should be looking first?

Thanks

[Andy Watson]

I picked up a 222A with a similar problem (it had other problems too). The fault was with the grid connection to the CRT. The pin in the CRT socket had worked free over time and retracted back into the socket housing - hence only capacitive coupling to the grid pin. Does your brightness control work? If it does the fault is probably elsewhere in the Z-control circuit.

[MrQuan]

Thanks for the reply

Yes, the brightness control does work, as does the focus. I will check the CRT grid connection anyway, just in case.

By the way, I purchased a user manual (PDF) from Qservice, but as far as I can tell no service manuals or schematics are available anywhere for the 224.

[Andy Watson]

By the way, I purchased a user manual (PDF) from Qservice, but as far as I can tell no service manuals or schematics are available anywhere for the 224.

Yes, the schematics are hard to find. There are plenty of sources for manuals but the only one that I found for manual with schematic for my 222A was:
http://artekmanuals.com/

Might be worth dropping Dave an email at Manuals@ArtekManuals.com

[MrQuan]

Will do! Thanks

Update: Ok email sent and fingers crossed.

I also found a great Tek CRT troubleshooting guide in the first post in this sticky: https://www.eevblog.com/forum/repair/repair-documents-and-links-sticky-me-please-mods/

From about page 35 seems relevant, so I'll go through this tomorrow and post my findings here.

[MrQuan]

Dave at Artek Manuals didn't have anything for the 224, but *might* be able to find one to scan. Seems like pretty slim chances though.

I found a service manual for the 222 here: http://www.ko4bb.com/getsimple/index.php?id=manuals&dir=Tektronix/Tektronix_-_222.
It looks like a very similar unit, so I think this should be helpful.

I found some promising info. This will be my first oscilloscope or CRT repair, but it looks like I need to check the Z-amplifier and CRT blanking.
There was a good block diagram of the display system (see attached).
The manual said this about the Z-amplifier.

The Z-amplifier is a class B stage that is non-linear, since it only needs two output states: blanked and unblanked.
When the output amplifier is about 0.2 volt; when off, its output goes to 119.3 volts.
The pulse is coupled via a dc restorer to the grid voltage of the crt to turn the electron beam on and off for blanking.
The grid voltage is about -1550 V and the dc restorer circuit allows the Z-amplifier to take the grid 119 V below that level to blank.

I don't have a HV probe (been meaning to buy or build one actually), but I think I should be ok testing the signal into the Z-amp and the output out of the Z-amp directly with a DMM as it should only be a maximum of 120Vdc (I'm assuming this is floating at the 1550V? But I could take a relative measurement?).

...is this correct? 

[siggi]

I expect there'll be a grid bias pot someplace in the DC restorer circuit. Try and tweak it. This is adjusted to the CRTs cutoff, and if the bias is too hot, you can't blank the CRT.

The service manual is sure to have a CRT adjustment section, read through it and see whether you can adjust this out before suspecting a fault.


Sent from my iPad using Tapatalk

[Gyro]

Isn't there a separate setting for readout brightness? That could be set too high.

It would be nice to make the fix switchable, I've always been fascinated by vector screen drawing. 

[tggzzz]

I don't have a HV probe (been meaning to buy or build one actually), but I think I should be ok testing the signal into the Z-amp and the output out of the Z-amp directly with a DMM as it should only be a maximum of 120Vdc (I'm assuming this is floating at the 1550V? But I could take a relative measurement?).
...is this correct? 

Based on experience with other scopes (i.e. there might be significant differences!)...

The "Z-amp" in that block diagram will be in two sections: low voltage (i.e. a few hundred volts from ground) and high voltage (i.e. a few kV from ground). A "blanking waveform" will be created in the low-voltage section, and then its AC component "copied" up to the high voltage section by a "DC restorer" circuit. The grid-cathode waveform should be the same as the blanking waveform, offset from zero by the cathode voltage.

You can debug the low "voltage blanking" waveform using standard tools and techniques that can withstand a few hundred volts. You cannot do that directly at high voltage. What you can do, with care, is use a handheld DMM to measure the AC voltages in the low voltage section for various combinations of brightness settings, and then see the same voltages are visible in the high voltage section. If not, then the DC restorer is faulty.

Personally I would suspect that your DC restorer is working fine, but that the blanking voltages created in the low voltage section are faulty. There maybe potentiometers in the low voltage section that allow the voltages to be tweaked.

[TERRA Operative]

Epic thread bump , but now it's my turn to try to fix this thing.... (MrQuan is my brother and I commandeered the scope from him, at least temporarily. )

I have the 222A Schematics which should help, and I can confirm that all the connections to the CRT are good and correct.

I have included the two relevant pages below, it seems the signal comes from pin 2 on U261, through a 40V, 1A schottky diode (Part number MBRS140T3), weaves it's way around to the next schematic shown (via about 4 other schematics with nothing of interest) and through the Z Amplifier then to the CRT.

Seems like a simple enough circuit to troubleshoot... Does anyone have any pointers of things I should look for?

[TERRA Operative]

Ok, so the schottkey diode from the CPU tests good, as well as most of the stuff in the Z Amplifier for a quick poke in-circuit (No obvious shorts or open circuits).

The 120V supply is reading 116.4V, so that isn't toooo bad.
The two 5V rails are also pretty much spot on too.

The easy things to test are drying up quick.... Gonna have to start digging in deeper to figure this one out...


One other strange thing, it seems that after running for a while, there is a 'snap' sound as I get a single arc over in the voltage multiplier section (Center left of the second schematic above), I'll have to clean the board to make sure there isn't any contamination causing it.
I wonder if it is related somehow?

[TERRA Operative]

Hmm, might go and replace those transistors for fun.

The first picture shows what is coming into the base of Q407, the first transistor in the chain of the Z Amplifier.
Second picture is what is coming out and heading to Q405, the second transistor.
Third picture is what is coming out of Q405... Not much happening there it seems.

Transistors are cheap, so it can't hurt to swap them out, they are SOT-23 versions of 2N5551 transistors (MMBT5551LT1G), so easy and cheap enough to get.

[TERRA Operative]

WINNER WINNER, CHICKEN DINNER! 

I replaced the two transistors and it's working.
I'm not sure exactly which one was bad, but I replaced both as they are cheap and easy to do. I didn't want to leave a possibly marginal part in and have to open it up again to replace it later.

Some of the electrolytic caps also tested bad for ESR and capacitance, so I replaced them all (all 4 of them).


Now I got the scope working, time to replace batteries with lipo. I reckon this one by electronic fox might be a good one to try:
http://kitsune-denshi.net/projects:tek222bat

[Martin.M]

be happy the CRT is allready. They are extremly sensitive for mechnical bumps, this can destroy the deflection plates inside.
So be always carefully with that fine little scope. Here is a picture from the "bumped CRT problem" ( X-Ray )

Martin

[TERRA Operative]

Yeah, this one will be looked after. No field work, I think it'll be mostly used on the bench when a fully isolated scope is needed.

I wonder if that CRT could be fixed by dropping it on the other side? Nothing to loose!