Author Topic: Tektronix 2235 repair thread  (Read 23891 times)

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Offline tautech

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Re: Tektronix 2235 repair thread
« Reply #50 on: March 07, 2019, 11:16:12 am »
If that doesn't work I'll see if I can find a suitable focus pot. Seem to be unobtainum. It's a 5M AB modpot which has to withstand 2KV.
Not at all easy these days so maybe a strip and clean might be an option. 5M will most likely be a carbon track so could just be real dirty and wiper needs a tweak.

In the D83 I had a HV zener in the CRT socket went leaky and needed replacement, check for that. IIRC it was 150 or 175V rated and set up the grid bias or something. Might be on the PCB in your Tek.
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Offline bd139Topic starter

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Re: Tektronix 2235 repair thread
« Reply #51 on: March 07, 2019, 12:09:20 pm »
Nothing in tube base at all. Straight wired. Have checked bias everywhere for jumps - nothing :( The tube bias and blanking is totally stable.

I just disconnected the deflection plates and anode and powered it up again and it's 100% stable as well, just dull obviously. No jumping or anything. I have already replaced the multiplier with another one and the issue persists exactly the same.

I just found my HV probe after digging around in the cupboard for nearly an hour as well. The PDA voltage is 7.1KV (about right) and not jumping.

Totally stumped!
 

Offline tautech

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Re: Tektronix 2235 repair thread
« Reply #52 on: March 07, 2019, 12:52:50 pm »
Time to pull out the isolated push stick and give things a poke ?
Dry joint perhaps ?
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Offline bd139Topic starter

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Re: Tektronix 2235 repair thread
« Reply #53 on: March 07, 2019, 12:53:53 pm »
Possible. I might take the tube out and inspect it as well. Look for flashover. Wonder if it got gassy
 

Offline med6753

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Re: Tektronix 2235 repair thread
« Reply #54 on: March 07, 2019, 12:55:18 pm »
Nothing in tube base at all. Straight wired. Have checked bias everywhere for jumps - nothing :( The tube bias and blanking is totally stable.

I just disconnected the deflection plates and anode and powered it up again and it's 100% stable as well, just dull obviously. No jumping or anything. I have already replaced the multiplier with another one and the issue persists exactly the same.

I just found my HV probe after digging around in the cupboard for nearly an hour as well. The PDA voltage is 7.1KV (about right) and not jumping.

Totally stumped!

Gee...almost sounds like it's the CRT itself. Perhaps a loose internal element? Try carefully tapping the neck of the CRT while powered up and see what happens.
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Offline tautech

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Re: Tektronix 2235 repair thread
« Reply #55 on: March 07, 2019, 12:58:41 pm »
Possible. I might take the tube out and inspect it as well. Look for flashover. Wonder if it got gassy
This looks mint to me:



Gotta be control circuitry but maybe CRT socket pins could be cleaned, if for nought than to eliminate them.
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Offline bd139Topic starter

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Re: Tektronix 2235 repair thread
« Reply #56 on: March 07, 2019, 01:09:07 pm »
I did a quick video earlier of it.

This is in X-Y mode, deflection plates disconnected (to eliminate amps).

Brightness unstable, deflection is vertical (scope was on its edge)

https://youtu.be/Lo-WzwbXh1Q
 

Offline David Hess

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Re: Tektronix 2235 repair thread
« Reply #57 on: March 07, 2019, 07:47:38 pm »
A change in the acceleration voltage causes a change in deflection.

A dirty insulator or high voltage breakdown might cause it but would be difficult to track down.  Still, try examining the high voltage sections in a dark room and maybe it will be visible.  Or just scrub down and clean the high voltage parts of the board with distilled water.

Easier to test are the acceleration voltages and especially the primary side of the switching regulator since this design does not rely on secondary side regulation of the acceleration voltages.  I would make a careful measurement between the collector of Q939 and ground to see if it is jumping.  Maybe a part in the regulator circuit has developed flicker noise.
 

Offline bd139Topic starter

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Re: Tektronix 2235 repair thread
« Reply #58 on: May 28, 2019, 07:54:43 am »
Just a final update on this one. I managed to get hold of a later fully working 2235A so this 2235 is now relegated to parts mule status I'm afraid.

The 2235A does however have a terribly whiny power supply. This goes away after it has warmed up but it's a bit irritating for the first 5 minutes.
 

Offline wasyoungonce

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Re: Tektronix 2235 repair thread
« Reply #59 on: May 28, 2019, 11:27:45 am »
Sounds like it needs new psi caps
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Offline bd139Topic starter

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Re: Tektronix 2235 repair thread
« Reply #60 on: May 28, 2019, 11:32:20 am »
Have tested them all and they are good on both ESR and capacitance.
 

Offline wasyoungonce

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Re: Tektronix 2235 repair thread
« Reply #61 on: May 29, 2019, 03:06:42 am »
I have a 2236 and its a little whiny as well
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Offline bd139Topic starter

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Re: Tektronix 2235 repair thread
« Reply #62 on: May 29, 2019, 07:22:36 am »
Thanks for the reply. I'm probably just going to leave it TBH. If anything goes wrong I'll fix it. I don't fancy a fight after that original 2235 that started this thread :)
 

Offline paronaram

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Re: Tektronix 2235 repair thread
« Reply #63 on: December 19, 2019, 05:07:52 am »
Hello Everybody, Just joined this forum. I am learning electronics, reading here there, watching videos.. Trying to fix faulty audio equipment; troubleshooting. I start purchasing some test equipment for this task and that included Tek 2235 oscilloscope.
Scope was cheap because it it has ONLY one problem (as per guy who sold me). It was listed no horizontal sweep on left side. I was hopping maybe something with PS, but no.. And after troubleshooting more I find out that I have no Ch1 as well ...  :'( 
Can someone help me where is this problems maybe coming from?
I checked all voltages on TP they are fine, was trying to do reverse by adding signal on the Ch1 and see where it gets lost, but fist stage U10 is buried under 3 PCBs, I don't even know how to check TP there ...

 

Offline bd139Topic starter

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Re: Tektronix 2235 repair thread
« Reply #64 on: December 19, 2019, 08:07:08 am »
If it’s not sweeping on the left side only check the horizontal wires attached to the tube. Sometimes these fall off. Might be as simple as that. If not it’s going to require a lot more effort to find.
 

Offline paronaram

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Re: Tektronix 2235 repair thread
« Reply #65 on: December 19, 2019, 09:23:58 pm »
If it’s not sweeping on the left side only check the horizontal wires attached to the tube. Sometimes these fall off. Might be as simple as that. If not it’s going to require a lot more effort to find.

Thanks will check that later today
 

Offline paronaram

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Re: Tektronix 2235 repair thread
« Reply #66 on: December 20, 2019, 05:24:13 am »
Yes, it does.
 

Offline paronaram

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Re: Tektronix 2235 repair thread
« Reply #67 on: December 20, 2019, 05:50:11 am »
bd139 - you are correct. H cable is cut, but it's not just cut, it's cut from glass  :palm:
I honestly not sure, because I can't see there, clearance is about 1/2"

Now I got another 2235, and I think tube is working but case is bent badly (what the heck are they doing with this equipment???)  on the sides, and can not remove cover, over the weekend I will cut it with Dremel

Is it allot involved in tube replacement?
Any major calibration that I will need to do ? (aka buy another scope to calibrate this one  :D)

Thank you

« Last Edit: December 20, 2019, 06:01:47 am by paronaram »
 

Offline bd139Topic starter

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Re: Tektronix 2235 repair thread
« Reply #68 on: December 20, 2019, 08:55:57 am »
Oh that’s not good. See if you can feel around in the hole. Sometimes the connection is just bent.

I’ve had that problem getting the case off a 2235 as well. Looking at the front it’s usually jammed on the right hand side next to the power supply section. When some inevitable idiot drops it, it bends the rails slightly. I had to hammer a metal ruler down the sized to get it off.

Tube replacement isn’t terribly difficult. I think there’s instructions in the manual. It will need recalibration afterwards though. Make sure you work on top of a towel or something and wear safety glasses. There is an implosion risk. I haven’t imploded one yet accidentally but I have disposed of one on purpose and it’s quite loud and glad goes everywhere :)
« Last Edit: December 20, 2019, 08:58:31 am by bd139 »
 

Offline paronaram

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Re: Tektronix 2235 repair thread
« Reply #69 on: December 21, 2019, 06:38:40 am »
Removed scope tube (very easy BTW), and guess what, pin is there, it's just cable was not connected. Looks like someone was trying to troubleshoot already.
So I reconnected both cable from PCB to Horizontal pins, start the scope, and same issue... Well something should start working, cable was completely disconnected from that pin.
On the internet I saw a video were guy was troubleshooting unrelated issue on the same Tek, and he was talking about pots on the top PCB (see picture), I move this pots around about 20 times, and turn the scope back On. See last two pictures ... very unstable but I see A and B now!

But Horizontal sweep still not there 
When I press beam finder dot gets in the area when I don't see signal at all, so tube is capable to display something there
 

Offline Shock

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Re: Tektronix 2235 repair thread
« Reply #70 on: December 21, 2019, 08:18:47 am »
Regarding power supply noise, doesn't say specific to 2235a but there is this service note.

http://www.hakanh.com/dl/docs/hardtofind/2200_ps_noise.pdf
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Offline bd139Topic starter

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Re: Tektronix 2235 repair thread
« Reply #71 on: December 21, 2019, 08:28:27 am »
Power supply noise is normal on these.  Usually goes away after a few minutes. Or the power supply explodes  :-DD

Removed scope tube (very easy BTW), and guess what, pin is there, it's just cable was not connected. Looks like someone was trying to troubleshoot already.
So I reconnected both cable from PCB to Horizontal pins, start the scope, and same issue... Well something should start working, cable was completely disconnected from that pin.
On the internet I saw a video were guy was troubleshooting unrelated issue on the same Tek, and he was talking about pots on the top PCB (see picture), I move this pots around about 20 times, and turn the scope back On. See last two pictures ... very unstable but I see A and B now!

But Horizontal sweep still not there 
When I press beam finder dot gets in the area when I don't see signal at all, so tube is capable to display something there

That’s good news then.

Next steps are:

1. Check power supply voltages
2. Stick it in X-Y mode and inject a signal into X and see if that works.
3. If it doesn’t then signal trace that through the amplifier stages until you find the bit that stops making sense.
 

Offline H713

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Re: Tektronix 2235 repair thread
« Reply #72 on: December 22, 2019, 07:17:17 pm »
Hello, I just purchased a Tek 2235 off university surplus with what seems like a similar issue. No trace, however when I push the beam find button, I get some part of a trace that sweeps appropriately with the time base setting. I can also move it up/down and left/right with the X and Y position controls (though with limited movement). When I feed a signal (random sine wave in this case), there is still no trace, however I can see some part of the signal displayed when I push the beam find button. I measured the power supply rails, (or at least the test points on the board) and they all seem to be in spec, so I'm inclined to say that it is not a power supply issue.

Both channels behave exactly the same, so I don't necessarily expect it to be the vertical amplifiers (they'd have to have both failed in the same way). In my case, I've got other scopes that I can use to troubleshoot this one. Some direction on where to start looking, however, would be welcome.

 

Offline paronaram

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Re: Tektronix 2235 repair thread
« Reply #73 on: December 27, 2019, 04:53:53 am »
That beam finder button maybe faulty ... ?
If its affecting both channels
 

Offline floobydust

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Re: Tektronix 2235 repair thread
« Reply #74 on: December 27, 2019, 05:18:54 am »
I can't see your trigger section buttons. In Auto sweep, a trigger is not needed and the sweep generator does the work.
It might be triggering/sweeping fine but the position is way off (beam finder reigns it in).  Beam finder also cranks the intensity.
Looking at the Tek service procedure, check first for a Z-axis unblanking signal at TP482, then sweep sawtooth at R707, then H-deflection plates having 40Vpp sawtooth.
 


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