Re: Suggestions for rehabbing a 466 w/ DM43

Fabio Trevisan
 

David,
Yep, essentially that is the idea but the solder doesn't actually need to
touch the board (although it doesn't hurt either).
Since molten solder has a high surface tension, you fill the crucible with
enough solder so that the molten solder surface is above the crucible edges
so, when you get the board closer and closer, the solder touches first the
tip of the components and it's surrounding solder... In that moment, the
solder around the leads immeditaly melts and is "pulled" by the Crucible's
solder (by capilarity or surface tension, I'm not sure exactly what's the
name of mechanism by which it happens).
You don't actually need to take the board closer than that because at this
point, the solder around all pins is already molten and the component is
free to be pulled from the other side.
In practice you apply some pulling force to the component before and as you
get the board closer and closer to the crucible.
At some point the component gets free and comes out and you can lift the
board away from the crucible.
If you apply some flux to the board before is even better as it helps the
molten solder to "wet" the pins/pads.
When you lift the board, it leaves the pads with an even and thin solder
coating.
Large holes are drained out and smaller holes remain filled with solder as
the hole's capillarity "hold" the solder inside them.
As with everything else, some practice is required and you can train
yourself on scrapped PCBs of any unrepairable device or equipment, such as
of computers, laptops and modern scopes :-).
Kidding... One cheap stuff that is often discarded such as desktop PC power
supply boards usually have the kind of bigger components (like big
transistors hooked together with their solderable heat sinks) that allow
you to get the "touch" quickly.

Brgrds,

Fabio

On May 3, 2017 10:01 PM, "David Berlind david@... [TekScopes]" <
TekScopes@...> wrote:



So, the PCB essentially had to make contact with the melted solder in the
crucible?

On May 3, 2017 8:07:40 PM "Fabio Trevisan fabio.tr3visan@...
[TekScopes]" <TekScopes@...> wrote:

Hi David,
I didn't but I wish I did because now I'm having to work around quite a
few
lifted pads and stripped-off metalized vias.
I do have, however, experience of a former job at a computer manufacturer
in Brazil where we had small crucibles, of about 3 cms diameter, to
unsolder hard stuff just like this... Multi pin connectors where the pads
were big (and retains a lot of solder and drains a lot of heat) all that
were a pain to remove by any other means, and were so easy to remove using
the crucible.
It was just a matter of carefully turning the PCB solder side down over
the
crucible so that it would melt all the pins simultaneously, wait for no
more than 2 seconds and pull the connector away.
They would come out so quickly that we could hold the connectors bare
handedly.
Of course that it has its down side...
It takes you to remove the board (which is not easy on the 4xx series
scopes), and sometimes takes additional measures such as removing
components around the area where the crucible will have to get close to
the
PCB, either not to damage the components or to clear the area so that you
can actually put the board in contact with the molten solder.
Back then, at a factory, we did that simply because it was faster and
cleaner, and usually there wasn't the down-side of having to disassemble
the equipment, because it was already disassembled.
At a repair shop, dealing with equipment that's still current, I think it
wouldn't be practical for the day to day use, due to the down-sides and
due
to the availability of parts to replace, should they get damaged in the
removal process..
Back to the restoration business (where we are) when a PCB that is, at
least, hard to get, and when you also don't want to destroy the old caps,
because you want to use their packaging as mechanical support for the new
ones, as I had to, I think that the additional preparation work is worth
it.
I wish I had a small crucible at hand when I started removing the caps.
But I fooled myself I would do it easily and I must confess I regret for
having insisted on doing it the hard way.
Later I went on looking for crucibles and found small ones for as cheap as
Brazilian 110,00 which is roughly 30,00 dollars.
I will look after one to have it around for the next occasions.

Brgrds,

Fabio


On May 3, 2017 7:06 PM, "David Berlind david@... [TekScopes]" <
TekScopes@...> wrote:



Hi Fabio,

I suspect that I will one day end up having to recap my 466... I was
curious about this statement:

*"get yourself asmall soldering crucible... because it takes too long to
unsolder thecapacitors using regular solder wick and solder vacuum-pump
and
the PCBsuffers. It's almost impossible not to end-up lifting some pads
and/ortracks or ripping-off some of the metalized thru holes (vias)."*

Can you explain how specifically you ended up using the crucible?

Thanks.

On Wed, May 3, 2017 at 5:43 PM, Fabio Trevisan fabio.tr3visan@...
[TekScopes] <TekScopes@...> wrote:



Hello Ryan,

I have a 464 (they're quite similar to the 466, exception to the H.V.
section that is simpler than that of the 466s) which I have went
through
all sort of minor problems since I bought it about 9 months ago.
Last "event" was that if finally blew one of its large P.S.
electrolytic
capacitors, the 1200uF x 120V.
On this last event (quite recent), I posted a question on this forum,
for
which I had quite some good advice from the folks.

Search for the thread:
Tek 464 - Big Caps recommnedation - Pot grease recommnedation

The original caps "form factor" is not in use presently and most you
will
find on the market are plain "Radial" caps, (with plain leads) or
"Snap-In"
caps that are usually short and fat (and some of them won't fit on the
1"
available space).
Neither fits-in mechanically / physically as the originals and both
require
some adaptation.
In recapping yours, you will have to make up your mind on either
following
the path of that guy you mentioned who recapped his 465 with Snap-In
capacitors (and connected them with wires and held them with plastic
brackets)... Or...
Buy pairs of smaller valued capacitors and mount them "In" the cans of
the
original caps (after opening them up and disposing their original
innards).

I followed the latter path and I`m just about to finish doing it...As
soon
as I can I will post to the pictures area of Tekscopes.
My solution was as follows:
500uF x 50V was replaced by 820 x 100V (it fits inside the can).
250uF x 150V was replaced by 330uF x 250V (if fits inside the can)
3 x 5500uF x 30V were replaced by 3 pairs of 3300uF x 63V (and each
pair
fits well inside the can)
1200uF x 100V were replaced by a rather long pair of 680uF x 160V (I
wish I
could have found shorter ones) They were 2 inches tall (each) and the
association didn't fit inside the can and I had to open the top of the
can
to mount them inside.

You will notice the C and V values are larger in all of them than the
originals, and this is not by chance. It's rather an advice from the
folks
of this forum to make up for the overall smaller ripple current ratings
of
the modern electrolytics.
Another advice is to try to have them all of 105C grade (the original
ones
were all 85C).

Last but not the least, on desoldering the old ones, if you plan to
follow
the 2nd path (and re-use the old capacitors base and cans), get
yourself
a
small soldering crucible... because it takes too long to unsolder the
capacitors using regular solder wick and solder vacuum-pump and the PCB
suffers. It's almost impossible not to end-up lifting some pads and/or
tracks or ripping-off some of the metalized thru holes (vias).

If you don't plan to use the older capacitor bases and cans, its better
to
just cut the old ones with a dremel cutting disc and pulling the
terminals
one by one.

Rgrds,

Fabio

2017-05-03 17:17 GMT-03:00 Ryan Stasel rstasel@... [TekScopes]
<
TekScopes@...>:




Hi All,

I picked up a Tek 466 w/ DM43 locally for $30 this week, and after
replacing the main fuse, it powered up, and after fiddling with
things
for
a while, it’s mostly “working”. Checking all the voltages, things
seem
good
and within tolerance. But it’s obvious all the caps are original to
the
unit… which, I have no good date on since I can’t find a serial
anywhere
(there is a hand written label on the tube shielding that says
92615).

Anyway, it’s pretty clear all the switches and pots need cleaning (do
most
suggest just using Deoxit spray, and maybe Fader Lube for the pots?),
and
the main caps need replacing. I’m also seeing a couple axial
electrolytic
caps on the “main” board (looking at the screen, the board along the
right
hand side) need replacing (they’re showing corrosion on the leads).
But
I’m
also curious if I should pull any of the socketed transistors or ICs
and
spray the sockets with cleaner and reseat, etc. The unit still acts a
bit
weird from time to time (screen blooms like it’s doing some storage
mode,
not showing both traces, not properly grounding the inputs when gnd
is
selected, etc).

I’m happy to link to pictures, etc… everything looks good, but
obviously
hasn’t been touched much since the unit was built. I’m also really
interested in what caps I should use for recapping the Power supply.
They’re a very odd size (tall and skinny), and looking online, I see
someone recapped a 465, but the new caps didn’t really match in size
at
all
so jumpers were needed. I’m pretty sure these size caps aren’t really
made
anymore, so I’m all for suggestions.

If anyone’s interested, it looks like my unit was tested by a
Kreurauko
(or something like that)… and the DM43 has “Donna” written on the
board
in
“Sharpie”. =)

Thanks very much!

-Ryan Stasel

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