Tek 468 repairability

[Aldobrandi]

Looking to buy a first cheap analogue scope and have been looking at Teks for a while now. This one http://www.ebay.com/itm/Tektronix-468-Digital-Storage-Oscilloscope-/280979489160?pt=BI_Oscilloscopes&hash=item416baec188 poped-up on fleabay for about $100 including shipping. Since it is listed as "For parts or not working" I'm curious to know if this series is generally repairable (i.e. how much unobtainium parts does it have?). Of course it could be working just fine. As a follow-up question, generally what are good robust and "fixable" analog scopes to purchase ? I know Jim Williams recommends the Tek 547 or 556 but I couldn't find one at a decent price.

Thanks for the heads up!

Aldo.

[Paul Moir]

The fact that it's showing a trace suggests the CRT is good.  That series uses some tunnel diodes in the analog section (triggering) which are hard to find, but most are fine.  My 465 had some bad solder joints in the input attenuators which made some vertical ranges not work, but that was an easy fix.  That's a bit of a common problem; they're ceramic modules and use some sort of special solder for their pins.  Other than that, I can't think of any irreplaceable unobtainium in there.

Since there's nothing demoed on the digital section, I would presume it to be non functional.  There are some proms and stuff in there.  But mostly fairly standard parts throughout.  If you just want it for the analog, it may be fine for that.

The TekScope group on yahoo groups (unfortante, I know) is a *great* resource.  I'd join up if I were you and search it for 468 specific problems.  There's (at least) a 465 service manual out there:  google boat anchor manual archive.

These tek scopes are works of art. 

(PS - I got my 465 for about $125 locally but mostly working with a few Tek probes.  Sometimes these are coming from techs that just want them to go to good homes.  That's the best way to buy them.  Personally I think you could possibly do a bit better, but you may have to hunt or wait more.)

[Aldobrandi]

This is great news, thanks a lot Paul. I think I'm going to make the jump and get it. I am in it just for the analogue end for now (planning on getting a new DSO in a few months), and if worse comes to worse looking for faults and trying to repair them will be a great teaching experience, so really I don't think there's much to loose for this price. Thanks a lot also for mentioning the Yahoo group, I'll definitely go there when this fine beast arrives.

Aldo.

[tekfan]

The scope is probably fully functional and only needs minor calibration (I hope I'm not being too optimistic here). The 468 is a combined digital/analog scope. The thing that can fail is the AD converter. However that is very unlikely. And even if it does you can still use the scope in the all analog non storage mode which completely overrides the ADC and connects the inputs directly to the vertical output amp.

For that price you can't beat it.

[Aldobrandi]

Alright the deed is done, I just bought it! I guess I only needed a minimal push from the forum wise men, and the endorsement from you Tekfan was more than I needed 

I will try to remember updating this post with pictures and comments when I receive it.

Thanks again for the help!

[Paul Moir]

More stuff for you:
http://www.ko4bb.com/manuals/index.php?dir=Tektronix/Tektronix_-_468_Digital_Storage_Oscilloscope

[Aldobrandi]

Thanks a lot for the links, it is much appreciated. This is going to be some nice bedside reading while I wait for my scope!

[true]

Sorry to resurrect, but I got a 468 recently, and it has a problem Well it had a couple, I fixed them, this is the one that's left...

After startup when cold, the screen will sometimes go crazy - usually gets bright, traces going all over the place, sometimes it turns into a dot, then brightness beings to fade. The indicators will dim, the fan will change pitch, and basically the scope won't work, digital or analog. But a quick tap to the side of the scope after it has its fit generally brings it back, and it will continue working until it is turned off and cools down again.

I've been trying to find out what might be loose or otherwise causing this by tapping on various parts of the board when on while the problem manifests, making sure all contacts are good, etc. but just can't seem to fix it. No knobs or buttons bring it back or change it. When it comes back, everything functions 100%. I've tried tapping on components and on the boards but that usually won't bring it back, tapping on the side will though. Don't have to tap very hard either. All obvious connections seem to be OK (have tried unplugging/cleaning/replugging everything I can get to). All socketed components seem to be OK. Basically I'm at a loss now. Hard to troubleshoot because even if I do not tap the chassis, it will fix itself in a couple minutes.

Sometimes if the problem doesn't occur I can induce it by pushing on the left side of the front panel.

Any ideas on what this is or where to even look? Should I ask TekScopes?

Also the scope is camera shy, it would do it if the camera wasn't on but would refuse when being recorded. I eventually got it:

[PaulAm]

Sounds like fun.

Try monitoring the low voltage supplies and see if you can spot anything when it's acting weird.   The fan slowing down and light dimming are a clue.  If the lv ps values are off, these scopes behave very badly. 

[PaulAm]

OK, I watched the video.  Take a close look at the ceramic resistor network that's half under the HV shield on the interface board.  I had a scope (also a 468) that acted a bit like that and I had a microscopic crack in one of the leads on that network.  Drove me nuts finding it.  Looks like there's something going on in the HV area.

[true]

I imagine the LV supplies will likely vary but actually I didn't measure them when it freaks out....I can try that to see what is affected. When operating normally they are well within spec.

I did remove the HV shield previously and did probe around that resistor network, but could not induce nor repair anything happening. Problem is, this problem doesn't always happen, it tends to happen when cameras are not around, but I can't get it to happen now to troubleshoot. But it does seem like some little crack or something somewhere. Will have to wait

EDIT: During failure 55V goes down to around 51V steady, changes only when the machine decides to act up differently. Other voltages similarly off as expected. Probed at the resistor network, its pins and connections, physically pushed it and other HV parts with no change. I don't know HV / CRT stuff so I'm at a loss around here. Lamps DS323 and DS324 glow and oscillate during most of the failure mode, I never see them glow when operating normally.

[PaulAm]

There might be a problem in the HV supply which is loading down the lv supplies.  The output of the multiplier is 14-16 KV though, but that's where I would start poking around.  There's probably a cracked lead or bad solder joint somewhere.   If that happens inside the body of a part, it is very difficult to track down.  Might want to get a can of freeze spray and see if that turns up anything.

You may want to bounce this out to the tekscopes list.