DIYbanter - View Single Post - Tektronix 485: How to get to vertical attenuator?

wrote in
ups.com:

Andreas Tekman wrote:
Looks like the pot needs to be replaced or at least to be re-lubed.
Since t will be difficult to get a irect replacement, try this: get teh
pot out, drill a very small hole in the pot housing and apply 1 -3
drops of contact cleaner/lube "Kontakt60" into the housing. Turn the
the pots shaft for a couple of times. Repaeat in necessary. Let dry at
least for half an hour .

I used this procedere a dozen times with success, the 485s I repaired
still work smoothly.

Thanks! I can't find Kontakt60, but I can probably find something else.

The only remaining thing is the 1M/50R bulb. I wonder if it could be
replaced by a LED (with the appropriate resistor).

That is what I would do.The incandescent bulb draws IIRC,at least 50ma,so
the IC will easily switch a LED at 5-10ma. You even have enough V to run a
bright-white LED,but I would use a 3mm LED instead of the usual 5mm.
I installed LEDs in other TEK test gear on my bench because I tired of
dealing with hard to access lamps.(and also like to mod things!) 8-)

The schematic shows
this bulb connected to +5V on one side and to an IC (through a 10R
resistor) on the other. I'd guess the IC pin goes from +5V to GND when
it wants the bulb to light up, so I could put an LED plus a 270R
resistor in place of the bulb. Has anybody done this? If not I will
probably remove the A channel and check what's supposed to happen.

Best,

- Cesar

--
Jim Yanik
jyanik
at
kua.net