Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems.

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Old August 22nd 18, 05:33 PM posted to sci.electronics.repair
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Default Removing Large Electrolytics

On Tuesday, August 21, 2018 at 7:46:24 PM UTC-4, Cursitor Doom wrote:
I have just spent about 40m trying to unsolder an electro on the board of
the Tek 466 I'm currently working on. It showed a ESR of 45 ohms so I
needed to hook it out of circuit just to be doubly sure it was really
that bad.
The electros in question are all fairly large ones in the (linear) PSU
section. For some reason they've not used separate boards for the various
sub-circuits, so the PSU is just one area of a v.large board which also
has components for all sorts of other functions. Anyway, The cap in
question has 5 connections to the board: 3 equi-spaced ground tabs around
the outside that come straight from the case of the cap and the 2 + and -
wires close to the centre. Given 5 in total through-hole connections,
it's proving very difficult to remove the cap using suction pump and
solder braid cos I cannot wiggle it at all. The best solution IMO would
be to heat all 5 tabs at the same time whilst pulling the cap's case from
the other side until it breaks free. Is there any tool that enables you
to do this? I fear the amount of ****ing around I'm having to do
otherwise will lead to delamination of the PCB traces and an ugly mess.
I cannot even slip a junior hacksaw blade between the bottom of the cap
and the PCB and cut it free cos it's obscured by other components.



You need to preheat the board. BTW, rocking a component while desoldering is a recipe for disaster if the board in question is a multi-layer. An internal foil can be separated from the plated-through hole if physical assertion is used.

If you can't remove the board to put it in a pre-heater, aim a hair dryer at it for about 10-15 minutes (a real heat gun can blister it). When the board is good and hot, it will have far less propensity to sink off heat from your soldering iron tip.

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Old August 22nd 18, 10:36 PM posted to sci.electronics.repair
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Default Removing Large Electrolytics

Once you remove the solder "ADD MORE". Yep, add more. Suck that out.

It has the tendency to heat other areas.

You can also add a solder removal alloy form Chip-Quik.

You can break off the tabs sometimes. You can cut the can sometimes. I'd use these methods at last resort.

Solder with a low temperature allow after removing most of the solder and suck out is my best suggestion.
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Old August 23rd 18, 12:08 AM posted to sci.electronics.repair
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Default Removing Large Electrolytics

On 08/22/2018 06:12 AM, Cursitor Doom wrote:
On Tue, 21 Aug 2018 21:28:10 -0400, Phil Hobbs wrote:

+1 for can-opening it to save wear and tear on the board. If you
measure 45 ohms ESR in-circuit, it won't be better by itself.


Well, it's hardly surprising for an electro of this age. Clearly the
original component from a scope with a s/n indicating it was manufactured
44 years ago! What was more surprising was that only 2 out of 6 electros
in the PSU section showed abnormal readings (the remaining faulty one has
gone leaky).


I have a Keithley 410 Micro-Microammeter from about 1960 that has all
its original electros and still works fine. Of course it uses an
electrometer tube, so it takes a good couple of hours' warm-up to settle
down on the 100-fA FS range, but it eventually does.

It has way more soul than my 610C(*), but really isn't nearly as good a
meter. (Don't anybody tell it, though--it has an honoured place on my
bench shelf.)

Cheers

Phil Hobbs

(*) "Now Nortons and Indians and Greeveses won't do...They don't have a
soul like a Vincent '52." -- Richard Thompson

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

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Old August 23rd 18, 10:45 PM posted to sci.electronics.repair
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Default Removing Large Electrolytics

Cursitor Doom prodded the keyboard with:

On Tue, 21 Aug 2018 21:54:12 -0700, stratus46 wrote:

I use a Metcal which is a 40 Watt iron very well controlled but
when removing caps from computer motherboards where there are no
thermal reliefs to the ground plane, NO soldering iron can unsolder
it alone.


You're not kidding. My largest iron is 40W/80W switchable. 40W won't
touch it and I'm not risking 80W! The biggest problem seems to be
the cap case - a long 1" dia aluminium tube with 3 tags at the
bottom going straight to PCB grounds. It's acting like a very
effective heat sink. Going to have to trash the caps from above.

Have you tried making a soldering bit with a piece of split copper
tube and a cooks torch to heat it. Apply to all the pins up at once.

--
Best Regards:
Baron.
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Old August 24th 18, 02:24 AM posted to sci.electronics.repair
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Default Removing Large Electrolytics

Some styles of electrolytics may be particularly difficult to remove gracefully. The ground tabs of the FP style were often twisted for mechanical stability before soldering. And sometimes, the pins were folded over before soldering.



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Old August 24th 18, 09:27 AM posted to sci.electronics.repair
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Default Removing Large Electrolytics

On Friday, 24 August 2018 02:24:43 UTC+1, wrote:

Some styles of electrolytics may be particularly difficult to remove gracefully. The ground tabs of the FP style were often twisted for mechanical stability before soldering. And sometimes, the pins were folded over before soldering.


If enough solder is removed, twisted tabs can then be twisted back. A bit of remaining solder can be ignored, it lacks enough strength to get in the way. Folded over legs can be pushed up with a soldering iron before attempting removal.


NT
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Old August 25th 18, 12:33 AM posted to sci.electronics.repair
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Default Removing Large Electrolytics

On Fri, 24 Aug 2018 13:18:02 -0400, Phil Hobbs wrote:

This might be a good use for Chip Quik--it's a bismuth-based solder that
drops the liquidus point of a joint low enough that gentle use of a heat
gun will melt it.


I had to use one of these so-called multi-tools to slice through the
pins. There was about 2mm of clearance between the bottom of the caps and
the PCB. This is one of those cutters that vibrates from side to side at
high speed and the blade had *just* enough reach to cut through the most
inaccessible pins. Double checking after removal confirmed they had all
definitely failed (3 out of 6 altogether).





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Old August 25th 18, 12:42 AM posted to sci.electronics.repair
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Default Replacing Large Electrolytics

I had 3 faulty electrolytics; all 30V/5500uF. In my spares bin I found 6
NOS (new old stock) electros of values 35V/3300uF which when stacked 2
high and wired in parallel occupy - fortunately - the same footprint as
the ones that failed.
These spares, though unused, are probably 40 years old. Will they need re-
forming gradually over 24hrs before installing?




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Old August 25th 18, 03:06 AM posted to sci.electronics.repair
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Default Replacing Large Electrolytics

On Saturday, 25 August 2018 00:42:23 UTC+1, Cursitor Doom wrote:

I had 3 faulty electrolytics; all 30V/5500uF. In my spares bin I found 6
NOS (new old stock) electros of values 35V/3300uF which when stacked 2
high and wired in parallel occupy - fortunately - the same footprint as
the ones that failed.
These spares, though unused, are probably 40 years old. Will they need re-
forming gradually over 24hrs before installing?


I'd get newer ones if possible. At that age they'll be dead or near to it. In apps where high ESR is acceptable they last better, but still 40 is 80 human years for lytics.


NT


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