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Old 22nd May 2010, 9:24 pm   #1
Skywave
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Question Tektronix 475 scope: repair required.

Anyone interested in repairing a Tektronix 475 'scope - and earning a few £ in so doing?

Yes, I do realise that this is an unusual request to make here, but having spent countless hours trying to find the cause of a very elusive fault in the horizontal output amplifier (no X-deflection) & having eliminated ALL relevant components - I'm now completely & totally baffled.

I don't have a great deal of cash available to throw at this - so a professional repair company is quite out of the question - but I'm hoping someone here will take it on. Full Service Manual & notes, relevant to the fault, are available. Moreover, if you are local - and interested - you can even use my workshop here to do it: tea / coffee will be provided!

I'm happy to provide an outline of the fault here & now, but I will leave further, in-depth analysis to whomsoever - if anyone at all - takes this repair job on.

Thanks.

Al. / Skywave.
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Old 22nd May 2010, 9:57 pm   #2
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Red face Re: Tektronix 475 scope: repair required.

Of course, I should have added : "Replies by PM, please".

Al.
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Old 22nd May 2010, 10:00 pm   #3
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Default Re: Tektronix 475 scope: repair required.

Have you tried asking on the TekScopes yahoo group?
They might be able to give you a few pointers, so you can repair it your self.
Many ex-Tek employees seem to post there.
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Old 22nd May 2010, 10:29 pm   #4
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Default Re: Tektronix 475 scope: repair required.

Quote:
Originally Posted by switcher View Post
Have you tried asking on the TekScopes yahoo group?
Yes; to no avail.

Al.
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Old 22nd May 2010, 10:30 pm   #5
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Default Re: Tektronix 475 scope: repair required.

Oh dear!
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Old 23rd May 2010, 11:39 am   #6
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Default Re: Tektronix 475 scope: repair required.

Does the timebase work on any or some switch settings? My 485 had a really frustrating problem with the timebase circuits. Some settings worked, most interacted with other settings & it was a bit of a nightmare to see what was actually happening.

The 485 has 2 very small relays in the timebase switching. One of these had stuck. Sounded as if it was changing but didn't. I eventually swapped them over & got 1 timebase working. Then fixed the other one. Not had the problem since......

My copy of the 475 manual is very poor but the 475 looks to have a fairly similar circuit & at least 1 relay hidden away inside. I am too far away to be of much help in south Devon unless you ever come down here?

Phil
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Old 23rd May 2010, 2:13 pm   #7
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Default Re: Tektronix 475 scope: repair required.

Hi Al,
I'm sure you'll kick yourself if you let someone else have it and it turns out to be a simple fault that you have overlooked.
Perhaps you have put too much time into it in one hit and it might be better to put it one side and come back to it with renewed vigour?

Regards

Rob
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Old 23rd May 2010, 2:20 pm   #8
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Default Re: Tektronix 475 scope: repair required.

If you do find anyone I'd appreciate knowing and am not all that far. I have a 475A (200MHz version) which although working I would love to have serviced to bring up to scratch but have also been frightened by 'standard prices' of specialist companies.

Dave
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Old 23rd May 2010, 3:24 pm   #9
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Default Re: Tektronix 475 scope: repair required.

Quote:
Originally Posted by Oldtestgear View Post

My copy of the 475 manual is very poor but the 475 looks to have a fairly similar circuit & at least 1 relay hidden away inside. I am too far away to be of much help in south Devon unless you ever come down here?

Phil

Hi Phil (or anyone else),

I've just put up a .PDF of the full manual on my Web space, I'll leave it there for a few days (it's biggish > 13Meg) :-

http://evingar.www.idnet.com/Tek_475...61U_038241.pdf
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Old 23rd May 2010, 4:21 pm   #10
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Arrow Re: Tektronix 475 scope: repair required.

Quote:
Originally Posted by Guitarist28 View Post
Hi Al,
I'm sure you'll kick yourself if you let someone else have it and it turns out to be a simple fault that you have overlooked.
Perhaps you have put too much time into it in one hit and it might be better to put it one side and come back to it with renewed vigour?
Regards
Rob
Those are constructive & worthwhile insights, Rob, for which thank you.

Not wishing to sound immodest, I have had my fair share of fault-finding on analogue electronics, including repairs to Tek. 475 'scopes over the years. However, this one really has got me beaten! So much so, that it's now reached the point where I will gladly pay someone to prove to me what the fault cause is - and the only way to do this is for that person to have it on his (or my) bench, get stuck in - & clear the fault. I doubt very much if it will be a 'quick job'. (But I could be wrong! )

Yes, it may well be a simple fault that I've overlooked - but I can only work with the tools that I have available - in this case, the manual, my well-equipped workshop, my experience and my ability to analyse the circuitry. If, post-repair, I am told "It was simply because of blah - blah", then not only do I end up with a repaired 'scope, but then I will also have learnt some very valuable knowledge - which may well also include 'technique'. This is worth paying for. We all have room to improve our diagnostic skills.

As regards 'walking away and coming back afresh', I know exactly what you mean! I am not going to go into the extremley long tale here of everything & all the diagnostic techniques I've tried - with all of them to no avail. It's one of those faults that just seems to be not possible to exist

Al.
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Old 23rd May 2010, 4:33 pm   #11
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Default Re: Tektronix 475 scope: repair required.

Hi Al,
Yes, I do know that feeling. Its very annoying when a problem exists that can't be resolved and yet you have tried every possible method to cure it. Perhaps you are right that it is time to walk away from it.

On the question of paying someone to cure/fix the problem I'm sure that we have all paid out money for equipment to be fixed only to work properly for 5 minutes and the original problem to reappear. I can recall a few VCRs I have owned in the past to be short term 'fixed' for a tidy sum to repair shops.

Regards

Rob
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Old 23rd May 2010, 4:34 pm   #12
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Arrow Re: Tektronix 475 scope: repair required.

Quote:
Originally Posted by Oldtestgear View Post
Does the timebase work on any or some switch settings?
Phil
O.K - despite my earlier comments, here's a very brief synopsis of the fault conditions.

It doesn't work on any T/B settings. The fault appears to be that of incorrect d.c. biassing in the d.c. coupled horizontal amplifiers. All associated semi-conductors have been replaced with known good Tek. ones. Capacitors and resistors have been checked ( = O.K.); d.c. supplies are present and correct; the CRT is not at fault; there are no defective switches / connectors / pots. The drive waveform to the horiz. O/P stages is present and correct.
That simply leaves the raw pcb substrate as the last, unproven component. And I do not believe that this is the culprit!

Al.
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Old 23rd May 2010, 4:42 pm   #13
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Arrow Re: Tektronix 475 scope: repair required.

Quote:
Originally Posted by Guitarist28 View Post
Hi Al,
On the question of paying someone to cure/fix the problem I'm sure that we have all paid out money for equipment to be fixed only to work properly for 5 minutes and the original problem to reappear. I can recall a few VCRs I have owned in the past to be short term 'fixed' for a tidy sum to repair shops.
Regards
Rob
I know, I know! However, the significant issue here is that I will have confidence in the skills & attitude of anyone who takes this job on via this Forum. Such a person will not be entirely unknown to me & it is likely that his 'expertise' will have been well-established by virtue of Posts & Threads that he has previously made to this very Forum.

Al.
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Old 23rd May 2010, 7:01 pm   #14
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Default Re: Tektronix 475 scope: repair required.

Hi Al, you have a sweep into the horizontal amplifier? What's on the plates?
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Old 23rd May 2010, 7:05 pm   #15
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Default Re: Tektronix 475 scope: repair required.

Whilst I doubt there's much I can mend that you can't, I'm more than willing to offer a fresh pair of eyes.
Feel free to PM, Rob.
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Old 23rd May 2010, 7:07 pm   #16
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Default Re: Tektronix 475 scope: repair required.

There must be someone on here with a working 475 to compare notes with, surely.
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Old 23rd May 2010, 7:14 pm   #17
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Default Re: Tektronix 475 scope: repair required.

http://www2.tek.com/cmswpt/madetails...ci=15845&lc=EN

Tektronix has a free copy of the 475 manual. The photos & illustrations are really good quality. Well worth grabbing. Link courtesy of the Tekscopes group.

Phil
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Old 23rd May 2010, 11:04 pm   #18
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Arrow Re: Tektronix 475 scope: repair required.

Quote:
Originally Posted by switcher View Post
There must be someone on here with a working 475 to compare notes with, surely.
Indeed: I had another, fully-functional 475 on the bench at the same time and duplicated various test conditions & measurements on each. Nevertheless, I was unable to pin down the cause to any particular component(s).

Al.
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Old 23rd May 2010, 11:08 pm   #19
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Default Re: Tektronix 475 scope: repair required.

Quote:
Originally Posted by PJL View Post
Hi Al, you have a sweep into the horizontal amplifier? What's on the plates?
Nothing; the d.c. biassing conditions in the d.c.-coupled, multi-feedback, differential pre-amp. & O/P amplifiers are incorrect; the O/P stage is being turned hard on.

Al.
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Old 24th May 2010, 10:06 am   #20
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Default Re: Tektronix 475 scope: repair required.

Hi Al,

How accessible is it?

Might be easiest using another scope and look for the sweep on base Q1124 then collectors Q1124, Q1134, Q1152, Q1162.
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