Re: 475 questions


ciclista41@...
 

Graham,

Well, my scope is peppered with the kind in the photo. Maybe they re-spec'ed the caps later in production, or maybe the US Army bought enough in their contract with Tek that they were able to spec what they wanted. Mine is from November of 1982, if the date code on the AC power cord box inside the case just above the transformer is original.

I went ahead and pulled the two tantalums out of the circuit to measure them. I broke a lead off the first one, pulling too hard. Because I only have access from the top of this A7 board unless I remove it, which I don't want to do, my solder sucker wasn't able to slide over the clipped end of the leads, and I had to just use a sharp tip on my iron. Did much better on the second one, which came out easily by that second method. I then used the sucker to remove the remaining solder from the holes in the board for when I replace the broken one.

Yes, you were right to question my belief that they were bad. Out of circuit, they both tested well within the green on my ESR tester. So much for being able to reliably test capacitors in circuit with that thing. They weren't even close to being in spec when tested with it in circuit. I was thinking that these dipped tantalums were generally bad in boards this old based on numerous threads in this forum and others saying how often these were bad. Some said they routinely replace them all when they see them in a scope of this era. I have tested dozens on this board and only found about a handful that this tester considered bad. So, considering that these two were among the worst that I tested in circuit, I decided to pull them, as they'd be less easily accessible later when the A8 board is back in place.

So this makes me question whether the tantalums in this board are among the infamous ones. I hope not, obviously. Your statement that my photo showed what looked like a very '80's tantalum made me think maybe the infamous ones are some earlier 60's or 70's version. How can I know, other than pulling them all? At this point, I don't see the point of pulling any of them unless they show up as faulty later, when there is a trace on the scope and I can run it through its paces and make sure it's working as it should.

Speaking of working as it should, Tektronics calls for several specialized calibration tools in the manual. If the scope is running well, with no serious issues other than calibration, can it be calibrated without resorting to the purchase of such tools? I briefly noted some shops wanting a couple of hundred dollars to calibrate a scope. Definitely don't want to do that! If I thought I had to have someone else calibrate it, I probably wouldn't have taken on the project and just invested in a new digital scope.

Bruce

On Tue, May 26, 2020 at 09:09 PM, VK1GVC wrote:


Bruce, the link to the pics on TekScopes worked for me and that
capacitor looks very 1980's tantalum as Michael confirmed in a later 475
manual and the Tek Common Parts manual.  Ceramics of that era were
commonly flat circular disks, very different from what you have.

Roger re testing in place - very problematic.  Best to remove them to
eliminate any ambiguity and as you have reported success in desoldering
then that's the best option while you have access.  BTW I'm curious to
know why you *believe* that they are faulty - mere suspicion based on
type and age, or something else?

You quoted your post of 22 May to Harvey a few minutes ago in which you
sought advice about what can and cannot be substituted when replacing
1982 components in 2020.  Has that qn been answered to your satisfaction
or is it still a live issue?  The short answer is: it depends.  The long
answer really has to address a specific component in a specific
application.  But the laws of physics haven't changed a lot in the last
40 years so there is almost certainly something out there which can be
bought/found/made/adapted or cajoled to do the job.  If you need a 1amp
400V rectifier diode then a IN4004 of the 70's or 80's is just the same
as one from the factory today.  If you need a very specific high-spec
module made for or by Tek for a very challenging application 40 years
ago and now out of production ... oh dear, you've got a problem.
Fortunately we now have TekScopes, some wikis, eBay and of course the WWW.

Graham

On 27/05/2020 1:24 pm, ciclista41 via groups.io wrote:
Hi Graham,

I posted a photo of the C1091 tantalum, along with the plaque stuck to the
back of my scope here:

https://groups.io/g/TekScopes/album?id=247625&p=Created,,,20,2,0,0

Bruce



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