Author Topic: Tektronix 475 repair assistance  (Read 12231 times)

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Offline Specmaster

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Re: Tektronix 475 repair assistance
« Reply #75 on: October 22, 2017, 10:08:11 am »
Thanks for the link, I'll keep an eye on that. At the moment there's nothing near me scheduled but that can change of course. I see you're being spoilt with at least 2 a year so I guess it must the biggest one in the year?
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Offline bd139

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Re: Tektronix 475 repair assistance
« Reply #76 on: October 22, 2017, 10:20:56 am »
No idea yet. Sounds good though. My local amateur radio club are running it which should be interesting.
 

Offline Specmaster

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Re: Tektronix 475 repair assistance
« Reply #77 on: October 22, 2017, 10:32:58 am »
Shame that its a real thrash to get to it and also the timing, is when SWMBO will be at home, been a weekday it would not have been so bad, I get any booty in without without the tongue wagging and disapproving looks  :-DD
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Offline tggzzz

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Re: Tektronix 475 repair assistance
« Reply #78 on: October 22, 2017, 10:52:27 am »
Thanks for the link, I'll keep an eye on that. At the moment there's nothing near me scheduled but that can change of course. I see you're being spoilt with at least 2 a year so I guess it must the biggest one in the year?

There are many many more. Near me there are some in Frome, Chippenham, Newport, Newbury. There's a good one in a field in Luton/Dunstable in springtime. There's always the national hamfest, but I haven't trogged up there.
There are lies, damned lies, statistics - and ADC/DAC specs.
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Offline bd139

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Re: Tektronix 475 repair assistance
« Reply #79 on: October 22, 2017, 10:59:31 am »
Dunstable and Newbury are doable for me as well.
 

Offline tggzzz

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Re: Tektronix 475 repair assistance
« Reply #80 on: October 22, 2017, 11:09:43 am »
Dunstable and Newbury are doable for me as well.

Dunstable is tolerable (for me) since I can fit in a visit to TNMoC in the afternoon.
There are lies, damned lies, statistics - and ADC/DAC specs.
Glider pilot's aphorism: "there is no substitute for span". Retort: "There is a substitute: skill+imagination. But you can buy span".
Having fun doing more, with less
 

Offline bd139

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Re: Tektronix 475 repair assistance
« Reply #81 on: October 22, 2017, 12:07:43 pm »
That’s a reasonable day out. Will do that as well I think :)

Back to the 475, I’ve traced the problem with the trace fading out at high sweep speeds is proportional to the A gate output. I found this by connecting the A gate output to channel two. If you move the trace as far left as it will go you can actually see this on slow sweep speeds but obviously the sweep is much slower so it has a less pronounced effect.

Ergo timebase debugging time this evening. Plan is to check the input and output of the blanking amplifier on the timebase and then slowly bisect the system until I find a hard edge to slow edge transition.

It’s my youngest’s birthday today so no time for anything during the day.
 

Offline bd139

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Re: Tektronix 475 repair assistance
« Reply #82 on: October 22, 2017, 09:01:14 pm »
Ok this one is a pain in the arse. The unblanking is taking too long to happen.

I've got two scope traces below.

TP588 (yellow) label is the output of the trigger generator.
BLK (light blue) is the z-axis output. As this increases it increases the intensity of the trace.
SWP (dark blue) is the horizontal sweep. As this decreases, the trace moves from left to right.
Q574 (pink) is the collector of the input transistor of the blanking circuit triggered by TP588.

The output at TP588 and Q574 are absolutely spot on and nice and sharp. BLK is however soggy and looks like it has an RC-time-constant function on it.

As you can see with a relatively slow trace, when the trace starts, there is enough time for the unblank to happen before the sweep gets very far.



On a relatively fast trace, it hasn't even finished unblanking before the trace is done



Aaaand I'm done for the day  |O
 

Offline bd139

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Re: Tektronix 475 repair assistance
« Reply #83 on: October 22, 2017, 11:44:57 pm »
Spent some time on this and it looks like there were some problems with decoupling capacitors again. Replaced a bunch of tants and it's looking better already. Sweep doesn't completely disappear now. Looks like the gain of the transistors were being shot by the tants gone high ESR. Grrrrrrr.

I've run out of tants now so just put down another £10 worth. Decided to stock up this time so bought 60 of the damn things in 1uF, 4.7uF and 10uF, 20 each.
 

Offline Specmaster

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Re: Tektronix 475 repair assistance
« Reply #84 on: October 23, 2017, 12:33:51 am »
 :wtf: Now your making me thing that I ought to pull mine and check them individually instead of doing a short circuit test, it might cure my flyback issues / blanking issues.

These tants are not very reliable then, your's have only been installed for 7 days at the most  :-- 
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Online tautech

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Re: Tektronix 475 repair assistance
« Reply #85 on: October 23, 2017, 12:53:26 am »
:wtf: Now your making me thing that I ought to pull mine and check them individually instead of doing a short circuit test, it might cure my flyback issues / blanking issues.

These tants are not very reliable then, your's have only been installed for 7 days at the most  :--
I very much doubt any replacements failed, BD139 knows what he's about  :) , they'll only be the ones he was too lazy to replace earlier without having to dismantle the scope.
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Offline bd139

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Re: Tektronix 475 repair assistance
« Reply #86 on: October 23, 2017, 07:49:26 am »
Exactly that. Especially the lazy bit  :-DD

I did the interface board but not the trigger board. There are about 70 of the damn things in this and I’ve replaced 35 of them so far as it’s hard work and I ran out of tants :) . I’m trying to do as little work on it as possible really as each time you dismantle it you risk causing another problem to contend with.
 

Offline tggzzz

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Re: Tektronix 475 repair assistance
« Reply #87 on: October 23, 2017, 08:36:20 am »
I’m trying to do as little work on it as possible really as each time you dismantle it you risk causing another problem to contend with.

I refuse to admit that I have sometimes replaced tants without removing the board. All I do is note that Tek didn't bend the leads to hold them in, so it can be possible to heat the leads next to the capacitor, use a solder pump/wick, then pull the lead out. (The possibility of solder drops elsewhere in the case? Puts hands over ears. Lah-la-lah-la-lah; did you say something? :) )
« Last Edit: October 23, 2017, 08:43:54 am by tggzzz »
There are lies, damned lies, statistics - and ADC/DAC specs.
Glider pilot's aphorism: "there is no substitute for span". Retort: "There is a substitute: skill+imagination. But you can buy span".
Having fun doing more, with less
 

Offline bd139

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Re: Tektronix 475 repair assistance
« Reply #88 on: October 23, 2017, 08:44:12 am »
I haven't removed any boards at all yet in this. All the replacements are done from the top. If you do it with wick, the probability of solder splatter is minimal. I'm using multicore solder so it's no-clean flux as well. I'll clean any high impedance traces with IPA but that's about it. The holes are all plated so it makes it nice and easy :)

When I refer to dismantling it, it usually means taking the case off and rescrewing the feet onto the base so you can stand it up while working on it.

Yes I'm that lazy :D
 

Offline Specmaster

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Re: Tektronix 475 repair assistance
« Reply #89 on: October 23, 2017, 08:46:59 am »
I can't stand my 1740A up on its feet, it doesn't have any  :-DD
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Offline bd139

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Re: Tektronix 475 repair assistance
« Reply #90 on: October 23, 2017, 08:49:24 am »
Ah yes. Was rather lucky with this one. At least on the 465/475 scopes you can stand them up on the BNC box that sticks out of the rear if your feet have disintegrated.

Came across this the other day: https://www.thingiverse.com/thing:1410827/
 

Offline tggzzz

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Re: Tektronix 475 repair assistance
« Reply #91 on: October 23, 2017, 08:49:42 am »
When I refer to dismantling it, it usually means taking the case off and rescrewing the feet onto the base so you can stand it up while working on it.

Yes I'm that lazy :D

You're not a very experienced Wally (as in Dilbert); be more assiduously lazy :) I've never rescrewed the feet, since Tek had the forethought to make the dismantled case proud enough that components weren't damaged when it was laid on any side.
There are lies, damned lies, statistics - and ADC/DAC specs.
Glider pilot's aphorism: "there is no substitute for span". Retort: "There is a substitute: skill+imagination. But you can buy span".
Having fun doing more, with less
 

Offline tggzzz

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Re: Tektronix 475 repair assistance
« Reply #92 on: October 23, 2017, 08:51:37 am »
I can't stand my 1740A up on its feet, it doesn't have any  :-DD

Rubber doorstops, hacked with a stanley knife, are adequate.
There are lies, damned lies, statistics - and ADC/DAC specs.
Glider pilot's aphorism: "there is no substitute for span". Retort: "There is a substitute: skill+imagination. But you can buy span".
Having fun doing more, with less
 

Offline tggzzz

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Re: Tektronix 475 repair assistance
« Reply #93 on: October 23, 2017, 08:53:09 am »
Came across this the other day: https://www.thingiverse.com/thing:1410827/

There's a chap on fleabay who manufactures new feet on his lathe. He claims to have gone to considerable trouble to get the right material, which is more important than the shape.
There are lies, damned lies, statistics - and ADC/DAC specs.
Glider pilot's aphorism: "there is no substitute for span". Retort: "There is a substitute: skill+imagination. But you can buy span".
Having fun doing more, with less
 

Online tautech

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Offline bd139

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Re: Tektronix 475 repair assistance
« Reply #95 on: October 23, 2017, 09:07:25 am »
When I refer to dismantling it, it usually means taking the case off and rescrewing the feet onto the base so you can stand it up while working on it.

Yes I'm that lazy :D

You're not a very experienced Wally (as in Dilbert); be more assiduously lazy :) I've never rescrewed the feet, since Tek had the forethought to make the dismantled case proud enough that components weren't damaged when it was laid on any side.

I only rescrewed the feet because I needed access to the Z axis input and gate outputs while I was probing it from three sides.   They certainly made these nice to debug.
 

Offline Specmaster

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Re: Tektronix 475 repair assistance
« Reply #96 on: October 23, 2017, 09:21:11 am »
I can't stand my 1740A up on its feet, it doesn't have any  :-DD

Rubber doorstops, hacked with a stanley knife, are adequate.
I had already looked at doing that but discounted the idea as the rubber stops are only 20mm high and I need at least 30mm, ideally 65 to allow the power cord to left and so it can be powered in the upright position if required.
Who let Murphy in?

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Offline tggzzz

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Re: Tektronix 475 repair assistance
« Reply #97 on: October 23, 2017, 09:36:54 am »
I can't stand my 1740A up on its feet, it doesn't have any  :-DD

Rubber doorstops, hacked with a stanley knife, are adequate.
I had already looked at doing that but discounted the idea as the rubber stops are only 20mm high and I need at least 30mm, ideally 65 to allow the power cord to left and so it can be powered in the upright position if required.

How irritating: I'm wrong, you're right. I forgot that I did that and merely rested it on the CRT's cover, at an angle.

Since then I've replaced them with 30mm bars of (IIRC) nylon. Drilling the holes for the screws was a right bast**d since that plastic has a low melting point; and I don't recommend it. A better choice of plastic would help.
There are lies, damned lies, statistics - and ADC/DAC specs.
Glider pilot's aphorism: "there is no substitute for span". Retort: "There is a substitute: skill+imagination. But you can buy span".
Having fun doing more, with less
 

Offline Specmaster

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Re: Tektronix 475 repair assistance
« Reply #98 on: October 23, 2017, 09:50:48 am »
I can't stand my 1740A up on its feet, it doesn't have any  :-DD

Rubber doorstops, hacked with a stanley knife, are adequate.
I had already looked at doing that but discounted the idea as the rubber stops are only 20mm high and I need at least 30mm, ideally 65 to allow the power cord to left and so it can be powered in the upright position if required.

How irritating: I'm wrong, you're right. I forgot that I did that and merely rested it on the CRT's cover, at an angle.

Since then I've replaced them with 30mm bars of (IIRC) nylon. Drilling the holes for the screws was a right bast**d since that plastic has a low melting point; and I don't recommend it. A better choice of plastic would help.

I did find some metal ones on Ebay that I thought about but discounted them as well because the price was something like £40 for 4, almost as much as the scope itself and at the end of the day they did nothing to towards curing the problems it had electronically so was considered to be a wast of money at this stage. If the scope comes out the other side of the restoration OK and is considered to be a keeper, that then becomes an option, but for now I just want a cheap solution.
Who let Murphy in?

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Offline bd139

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Re: Tektronix 475 repair assistance
« Reply #99 on: October 23, 2017, 11:43:36 am »
I saw one a 1740A on ebay a few weeks back where someone had made wooden feet for it!
 


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