Re: u800 again


Tom Jobe <tomjobe@...>
 

Thank you Chuck,
For your explanation of how you go about replacing U800!
Like many others, I have a 24x5 with what is believed to be a dead U800.
I bought a replacement U800 chip that came with no guarantees, so I
would like to put it in a socket of some type for easy removal.
Your solution provides both the socket and a heat sink, so it's the best
solution I've ever seen.
My question is.... Is it possible to do your style of fix without
removing the mainboard?
In looking at how the 24x5 chassis is constructed, is there is a chance
this could be done from the side of the chassis after removing some of
the pieces, leaving the mainboard in place?
Could you comment on such a plan?
If the mainboard must come out, could you comment on that procedure?
Thank you!
tom jobe...

On 3/23/2017 8:43 AM, Chuck Harris cfharris@erols.com [TekScopes] wrote:

When I replace U800, I go a different route. I remove the
old part, and install two machine tool contact SIP socket
strips. I then make a little gull shaped heat sink out of
a 1/2 inch wide 14ga strip of aluminum. The heat sink strip
is punched so that it fits over the two threaded studs, and
fits snuggly between the SIP strips. Then U800 plugs into
the socket, and clamps down on the heat sink strip using
the nuts.

There is no stress on the pins, as they are socketed, and
the package has uniform pressure on it, because of the
heat sink making a flat spacer.

And yes, I still don't think that a heat sink is necessary,
but it is a better way of mounting the U800 package, and
customers that want their U800 replaced are usually already
shell shocked at the price, believe the stories they hear
on the net, and don't want to repeat the experience.

On another note, I have seen some of the later 2465B scopes,
that have MAXTEK U800's that have a different pin profile,
and no star washers under the chip body. Further evidence
that Tektronix knew that the star washer kludge was a shoddy
way of mounting that package/part.

All the effort they went to to save a dime for a simple
aluminum shim that was needed under the package....

-Chuck Harris

'G. K.' glkinst@yahoo.com [TekScopes] wrote:
Recently, I had a similar problem on my 2465B. After much
"head-scratching" and
comparing waveforms, I discovered that pin 13 on U800 (HSA signal
pin, test point
59) was loading the signal from U650. The HSA signal was only about
0-2V, not 0-5V
per the manual. That pin measured about 800 ohms to ground with the
instrument
off. After replacing the U800, the HSA signal was back to 0-5V and
the readout
works fine, traces are perfect. The scope is back to normal
operation condition.

I have not observed this failure before in the U800, but it seems
related to
stress/thermal cycling of the chip. After removal of the bad U800,
the resistance
of pin-13 to ground pin is back to megohms. Mechanical stress is a
serious problem
with his chip. So use caution when replacing it; don't over tighten
the nuts and
crush the star washers. Mount the chip first using light torque on
the nuts and
then solder the pins. Also, I always remove the entire mother board
to replace the
U800. - George

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