John Hudak wrote in
:

Hi:
I recently acquired a Tek 475 in what seems to be fairly good shape for
its age. It does have some minor problems that I'd like to take care of
and I am wondering if there is a website/discussion forum that could
address some of the following (or if somebody would want to respond
here, that would be great too!):

1. Noisey controls and switches - I plan on carefully cleaning with
contact cleaner (isopropel-based, no lubricant), lube shafts where
necessary.

NOT on the attenuator boards.Those contacts have extremely low wiping
force.
And the gold plating gets a hard film built up on them from plastics
outgassing,airborne contaminants,etc.To clean those switches,slip a narrow
strip of paper under the contact,then a drop of isopropyl on the paper,then
slide the strip back and forth.It's hard to do for the contacts on the back
sides of the atten PCBs. Eventually,those HF cam switch contacts lose their
spring tension and the tiny white plastic contact retainers crumble from
age.

It's possible on some of the pots to handdrill a tiny hole and use a
syringe to inject pot cleaner/lube and then rotate the pot shaft to
clean;the Bourns modular pots..black bodies with a brown resistance
element.You have to be careful to not let the plastic shavings drilled out
to fall back INTO the pot.

2. 2 MHZ wave forms are out of focus when the 100 MHZ and 200 MHZ bands
are selected, it is sharp in the 20 MHZ BW. - Suggestions?

You're picking up more noise with the wider BW. That's normal.
For 2MHZ,you can use the 20MHz BW limit and not lose much.
You can check the PCB screws for loose ones,there's several that ground
various points on the vertical preamp board.

3. Some floating of waveform when observing a trace for a period of time
- e.g. 2-3 mins.

Positional drift over time. I'd look to see if any thermal balance
clips/blocks are missing from stages in the vertical preamp.They were
placed on some differential pairs.
Where to get new ones,I don't know.Or you have some precision film
resistors going away;those would be specific to one channel.


4. Significant wave form bounce when switching from DC - gnd- back to
DC....the waveform reappears above or below the DC baseline with a bias
of approximately 10-20% of the voltage scale - but not on all scales.

I've seen where input BNCs become a tiny "battery",and generate their own
voltage,offsetting the signal,and making DC bal difficult to impossible.
It could also be just an internal DC bal calibration needed.IIRC,there's
one for between 5mv/10mv(or was it 2mv/5mv??)

5. Recal the scope.

I have a service manual. Any tips/pointers/suggestions as well as known
'weak' spots that I should investigate are welcome.
Thanks
John


On the HF trimmer caps,often a black silver oxide builds up on them,and
they freeze up,and when you adjust them,they become open due to the oxide
being an insulator. "if it aint broke,don't fix it" holds here.

Besides,if you don't have the TD pulser or the fast-rise low aberration
signal generator(PG506),I'd leave the HF cals alone,unless there's a severe
problem.

--
Jim Yanik
jyanik
at
kua.net