Re: 475 questions


n4buq
 

Hi Bruce,

Okay - I was thinking the 475's caps were the same as the 465 but perhaps not.

It's a custom board I designed and had manufactured. BTW, if you notice, the board shown has a trace that had to be removed for that version of the board (I made a mistake...). I had another batch made where that wasn't necessary.

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "ciclista41 via groups.io" <ciclista41=yahoo.com@groups.io>
To: TekScopes@groups.io
Sent: Thursday, May 28, 2020 2:22:48 AM
Subject: Re: [TekScopes] 475 questions

Hi Barry,

That certainly looks like a neat way to handle it! I've already order the
disc-shaped ones for this board. As far as I know, there isn't a similar
set-up available for the 475, but I like the single board with stand-offs.
My 475 has six large caps there, rather than five, although I read
somewhere that two of these are in parallel, so can be replaced with a
single cap. Was that solution a custom job, or purchased ready made?

Bruce

On Wed, May 27, 2020 at 08:03 AM, n4buq wrote:


I have hesitated to mention this as a possible approach to the filter caps
in
the 475 because:

1. I don't know for sure whether the layouts between the 465 and 475 are
identical
2. The design is, well, a bit kludgy (if I do say so myself)

However, I thought I'd pass it along.

Pictured in this album is an implementation of a separate circuit board
that
provides that ability to use common snap-in caps as replacements. My
intent
for this was to make it possible to replace those caps again (should it
every
become necessary) without further desoldering/soldering on the main board.

https://groups.io/g/TekScopes/album?id=64929

There are small disc adapters that do somewhat the same thing (I have
some);
however, if a future cap replacement becomes necessary, I'm not sure it is
possible to do that without removing the disc which, of course, involves
solder work on the main board.

Maybe one day I'll afford a snazzy desoldering tool which would probably
(certainly?) make that part much easier for me but, for those large can
connections, it's not fun with a 25-year-old Radio Shack solder sucker.
The
can, itself, is a rather large heat sink/spreader which further complicates
that process.

Anyway, FYI.

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "VK1GVC" <vk1gvc@iinet.net.au>
To: TekScopes@groups.io
Sent: Wednesday, May 27, 2020 9:40:45 AM
Subject: Re: [TekScopes] 475 questions

Bruce, I treat tantalums like I treat people - assume they have good
intentions unless I learn otherwise.  If you remove lots of them without
reasonable suspicion then you'll probably do more damage than good as
Eric has warned.  As tantalums usually fail to a short circuit, a
failed/shorted tantalum will pull down the voltage at that point to
close to zero and this will/should be straighforward to find - later.
At this stage, if I haven't missed any key points, you've found 2
sick/dead capacitors in the +50V supply which MAY be the cause of all
the wrong voltages you've measured in the low voltage power supply
section.  So replace those caps first, then adjust all of the LV power
supply to meet calibrations specs and see what happens.  You might have
a working scope if luck is very much with you, or you might be one step
on a long journey towards that goal.

And when you replace the tantalum which lost a leg, make sure you
install the new one with the right polarity.  Tantalums don't like
reverse voltage and they can get hot, emit smoke and explode.  I've seen
it happen when a workmate powered up some newly assembled eqpt for the
first time and heard a hissing sound so he bent down for a better look.
A cylindrical metal tube type tantalum then exploded and flew off the
PCB, burning itself onto the side of his neck. Very painful ... and the
language was terrible.  And very lucky he didn't get hit in an eye.

BTW here's a different type of tantalum which I don't think has been
mentioned yet - see the colourful little blob in the lower left of the
picture at this link:
https://groups.io/g/TekScopes/photo/12901/22?p=Name,,475,20,1,0,0

It has a black top, brown middle and green bottom ... the colours are a
derivative of the resistor colour code and I think there's a spot on one
side too.  Much better dressed than a plain boring tantalum!

Graham

On 27/05/2020 3:28 pm, ciclista41 via groups.io wrote:
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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