Author Topic: Tektronix 465 horizontal issue  (Read 12263 times)

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Online PaulAm

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Re: Tektronix 465 horizontal issue
« Reply #25 on: November 22, 2013, 08:03:48 am »
That stuff looks fine.  L1419 keeps it from the fuse.  The later 465 diagram shows a single 47uF as C1419.  That does look replaced, however.

The problem is on the other side of the transformer since it doesn't show up on the emitter of Q1466.

C1488 is rated at 4KV and C1487 is 6KV.  Be careful around that area.  I would suspect one of those, but your values are close enough.  They may exhibit different characteristics at operating voltage though.

Probably a long shot but check C1481.

Check the resistors as well and it could be one of the diodes.   Something is allowing a little 50KHz ripple through.

If you don't find anything, it will be a good Tekscopes question.
 

Offline bob808

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Re: Tektronix 465 horizontal issue
« Reply #26 on: November 22, 2013, 08:10:54 am »
Well there are 2500V on the board and I'm only measuring that area with the thing unplugged. Not taking any chances on something like that.
Will check the diodes and resistors that are left, and C1481 as you suggested.
Thank you very much for your support!  :-+
 

Offline bob808

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Re: Tektronix 465 horizontal issue
« Reply #27 on: November 22, 2013, 08:23:24 am »
C1481 checks at 0.5uF/7.5ohms ESR so seems ok. I even paralleled it with another 1uF/100V cap and nothing change. There are 10mVpp of 50khz signal on + lead of C1481 thou.
And C1419 is 50uF/0.3ohm ESR.

Update
CR1482/CR1483/CR1487/CR1488 diodes check out ok with 0.6V drop and no reverse conductivity.
« Last Edit: November 22, 2013, 08:35:32 am by bob808 »
 

Online PaulAm

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Re: Tektronix 465 horizontal issue
« Reply #28 on: November 22, 2013, 09:12:24 am »
That's a good puzzle :D

With any luck someone on tekscopes will have had a similar problem.  Sometimes parts look good on a static check but do not work as intended under operating conditions.  That can be a real bear to track down, particularly in a section where lethal voltages are present.
 

Online tautech

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Re: Tektronix 465 horizontal issue
« Reply #29 on: November 22, 2013, 09:43:59 am »
In my experience of CRT's many of the old components subjected to 1Kv+ are under stress and fail or "do not work as intended under operating conditions"
Caps and high value resistors particularly.
I have previously just gone about replacing such BEFORE they fail.
Nothing worse than kit that fails halfway through a project.  >:(
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Offline bob808

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Re: Tektronix 465 horizontal issue
« Reply #30 on: November 22, 2013, 10:46:52 am »
All resistors also check out perfect. I got two 10k that read 9.98k or something like that. And one 22Mohm resistor that neither one of my DMM don't want to measure accurately. I guess it's at their limits, they sometime show me 10-13Mohms or nothing.
I guess I don't have anything more now... I'll have to start again tomorrow and think of something else. Maybe build the voltage divider again.
 

Online tautech

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Re: Tektronix 465 horizontal issue
« Reply #31 on: November 22, 2013, 11:01:29 am »
"And one 22Mohm resistor that neither one of my DMM don't want to measure accurately. I guess it's at their limits, they sometime show me 10-13Mohms or nothing."

Replace it!
Get a couple of 10M's in series and  to check your meters
« Last Edit: November 22, 2013, 11:03:54 am by tautech »
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Offline bob808

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Re: Tektronix 465 horizontal issue
« Reply #32 on: November 22, 2013, 11:09:10 am »
Luckily I have some 10Mohm resistors I just got today so I could build the Jim Williams pulse generator and it seems that neither dmm wants to go to 30mohm. But I insisted, I turned off the CFL lamp :D and I managed to get a read on one of the 22Mohm resistor. And it tested out good, but the other I forgot to mention that it isn't your classic resistor, it's a piece of pcb I guess with a trace on it, mounted perpendicular on the main PCB. The trace looks nice but the reading oscillates. I could get a good reading on the other as I saw that there were some diodes in series with the resistor so I just had to switch the test probes and I got the 22Mohms.
 

Online tautech

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Re: Tektronix 465 horizontal issue
« Reply #33 on: November 22, 2013, 11:23:45 am »
Thinking out loud.  :)
All the HV stuff is operating at milli-amps and any leakage will load the supplies up and create extra ripple. Dividers, caps, dirty boards etc. etc.
I suspect additional loading is what you are looking for.
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Offline bob808

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Re: Tektronix 465 horizontal issue
« Reply #34 on: November 22, 2013, 11:30:14 am »
At this point I may wait to be accepted in the TekScopes yahoo group and see if someone else had this problem before. Also I'm considering on building a good HV voltage divider so I can "see" what's happening in that part of the circuit.
 

Online tautech

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Re: Tektronix 465 horizontal issue
« Reply #35 on: November 22, 2013, 11:33:05 am »
 "that it isn't your classic resistor, it's a piece of pcb I guess with a trace on it, mounted perpendicular on the main PCB."

Could there have originally been 2 of these, one having been replaced with 22M?
« Last Edit: November 22, 2013, 11:39:06 am by tautech »
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Offline bob808

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Re: Tektronix 465 horizontal issue
« Reply #36 on: November 22, 2013, 11:48:24 am »
No, the pcb one is a multi tap resistor and pretty large about 6x2cm. I marked them both in the picture.
edit: wrong picture, fixed it now :)
 

Offline oldway

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Re: Tektronix 465 horizontal issue
« Reply #37 on: November 22, 2013, 06:40:22 pm »
Check ou replace C1421, C1422, C1423 and C1427.
 

Online PaulAm

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Re: Tektronix 465 horizontal issue
« Reply #38 on: November 23, 2013, 09:10:22 am »
That's a ceramic thick film resistor network.  Be careful around that.  I banged my head against a wall for a week before finding a cracked lead on one of those.  Also, they are practically impossible to find (and not cheap when you do)
 

Offline bob808

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Tektronix 465 horizontal issue
« Reply #39 on: November 23, 2013, 10:46:58 am »
Out of curiosity, can't you just replace them with HV resistors? You could make up the required values.
I've been admitted to the Tekscopes yahoo group and the guys there gave it a go but no luck so far. I'm back to where I was last night. Z amplifier seems ok. HV caps all check out.
I got some today but these are real small, round blue ones. 4k rated same capacitance, ceramic. Why the size difference? Better technology or?
 

Online PaulAm

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Re: Tektronix 465 horizontal issue
« Reply #40 on: November 23, 2013, 12:21:51 pm »
I've been tracking the discussion; looks like your puzzle has caught the interest of some good guys.  Keep at it; you'll learn quite a bit in the process.

If you had to, you could make up a new network, but you'd need to pay attention to power dissipation and particularly leakage.  You are dealing with a HV regulated supply.  Tek did some good engineering since we're talking about 30 year old scopes.

If the caps are really small, I'd worry about leakage and arc over in a HV situation.  If they're Chinese, I'd worry period.
 

Offline bob808

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Re: Tektronix 465 horizontal issue
« Reply #41 on: November 24, 2013, 01:26:06 am »
Here's a size reference of the old vs new capacitor. Same values. To be honest the original ones are 5kV and new ones are 4kV but that's very close. Both 0.0068uF.
Is there some other type of HV ceramic capacitor?
 

Offline bob808

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Re: Tektronix 465 horizontal issue
« Reply #42 on: November 24, 2013, 07:01:18 am »
I have cut the R1485 resistor so I can separate the Z-axis amp from the DC restorer. The problem remained the same, rounded top on the unblanking pulse on Q1478 collector and complete crap on the collector of Q1466. The pulse on the emitter of Q1466 looks nice. I've tested all the transistors in the Z-amp amplifier and they check out ok, no shorts. I've even replaced all of them with identical ones from all over the oscilloscope. Absolutely no change. I don't know what else to do at the moment.
The guys over Tekscopes group have been amazing in the level of commitment to my problem. They gave a lot of insightful suggestions and I managed to exclude the HV area from my problem (and that's good also as I'm not that exposed to HV anymore  O0 ) 
 

Offline markce

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Re: Tektronix 465 horizontal issue
« Reply #43 on: November 24, 2013, 10:38:16 am »
Hi Bob,
Just to encourage you, I've a similar problem with a Tek2236. It's smaller (trace starts a little late at fast sweep settings, the scope had other problems as well. Needed a power supply revision, so did that first. Thats fine now. Very interested in your findings on this one. Lately, I've tried to measure the horizontal trace issue, which is not easy to pinpoint.
Replaced a booster ceramic cap in the Z-amp output stage (have't looked at 465 schematics yet). Seems to change things a bit, but cap voltage of the one I used is too low (50V, need 200V), so I'm gooing to order a new one.
« Last Edit: November 24, 2013, 10:42:16 am by markce »
 

Offline markce

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Re: Tektronix 465 horizontal issue
« Reply #44 on: November 25, 2013, 09:05:55 am »
Had a quick look at the 465 schematics. The Z-amp is nearly the same in the 22xx series. Now I'm even more
interested in your findings from the TekScopes yahoo group.
 

Offline bob808

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Re: Tektronix 465 horizontal issue
« Reply #45 on: November 25, 2013, 09:51:46 am »
Hello.
At the moment I am in the same spot. I managed to replace some transistors only for a minimal improvement. I still don't have the solution.
I will make a full explanation when I fix it :)
In the meantime maybe you find the problem first  :-/O
 

Offline bob808

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Re: Tektronix 465 horizontal issue
« Reply #46 on: November 28, 2013, 10:39:51 am »
Finally, with the help of the pro guys over at TekScopes group I managed to solve the riddle.
It was a faulty transistor, more exactly Q358 on the vertical preamp board. I replaced it and it works good now. I'm still missing about half division on 0.05uS sweep speed but hey, I can live with that. It's pretty good now!
But there are a lot of things that could go wrong in that area. Do check the amplifier's transistors and capacitors. Voltages as well.
A giveaway for the bad Q358 was a higher voltage on it's base. It didn't fail completely so these things are harder to pinpoint.
I will post some photos along the way with some bad signals and test points, but now I will calibrate it one more time so I can put the case on.
 

Online tautech

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Re: Tektronix 465 horizontal issue
« Reply #47 on: November 28, 2013, 05:42:42 pm »
I have always been careful to have a vertical input setting that will not overdrive the vertical amps and display a vertical waveform that not will be larger than the screen. In my mind this only applies to analogue CRT scopes. Anybody in EEVBlog land like to add to this or shoot me down?  ;D Likely this was the cause of all your grief bob808
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Online PaulAm

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Re: Tektronix 465 horizontal issue
« Reply #48 on: November 29, 2013, 04:35:19 am »
Marginal parts can be a real bear to track down.  Congratulations on tracking down a tough problem.

There was some excellent analysis of the circuit; I've save the whole discussion for further study.
 


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