Tektronix 475 Scope 'tune up' suggestions wanted

The pots are all the sealed type. You can clean them by either turning them back and forth FIFTY-SEVEN times, or if the pot has four little screws on the back, loosen the screws a few turns ansd then pry open the case so you can squirt in some cleaner.

That's your local Tv stations muscling into the circuitry. It should go away if you put all the covers back on and use a good coax cable between the signal generator and the scope.

Put the scope on a variac and turn the voltage up and down. There's likely a weak or poorly regulating power supply. The power supply capacitors are often marginal and need replacing.

John Hudak wrote in news:ejcsvk$7ht$ snipped-for-privacy@usenet02.sei.cmu.edu:

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.

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.

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.

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??)

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.

"Ancient_Hacker" wrote in news: snipped-for-privacy@h48g2000cwc.googlegroups.com:

I've handdrilled tiny holes and used a syringe or glued a needle to the cleaner tube.

One handy little trick is to wind a couple of turns of insulated wire around the knob and move the wire back and forth,rotating the shaft rapidly.

They can also LEAK electrolyte on the motherboard and eat thru the traces,or short out things.

Thank you for your reply and very valuable information...Sounds like you have lived with these things for quite a while! I don't have the TD pulser, am looking into getting an oscope cal box (e.g. Fluke) but, given your suggestions, I'll make sure all the other things, eg.voltages, clean contacts are taken care of and do the DC volt cal and go from there. BTW, In looking at the PS schematics, the 5000uf and 5500 uf filter caps are likely suspects for replacement (I haven't taken it apart to check, but just going on age). Are there are anything 'special' about them? e.g. Tek part only or can I order similar high quality caps from Digikey and be OK?

Also, the mesh filter on the fan is toast. Any suggestions on replacement parts? I have 10micron filter mesh that I use on my disk drives and was thinking of making a replacement from that...alternatives?

Again, tia, John

John Hudak wrote in news: snipped-for-privacy@sei.cmu.edu:

I worked for TEK for 21.5 yrs!

Any caps that fit will do,although you may have trouble finding them. Most people solder in radial-lead caps with values close to what the original was. Exact values are not necessary. Some cap positions used the electrolytic can's terminals to connect circuits on the PCB,so you might have to bridge together the vacant negative terminal holes.

That filter was just a piece of open celled foam,you ought to be able to find something similar.Too restrictive a filter and your airflow will suffer and the scope will overheat.

Jim Yanik schrieb:

Here in europe, cell foam filter for the kitchen exhaust or for stationary air condition system can be bought in the supermarkets. Or l look for shops with kitchen/household goods. I cut the size I need to fit in the rear of the 465/475/485 scopes, and this works well. Air flow is not suffering much.

hth, Andreas

The original parts are the ancient "FP" three-tab mount type, which I don't think are made anymore.

Also Tek did a partial no-no as they depend on the three tabs to carry high current from one side of the PCB to the other. So you somehow have to connect both sides of the big tab hole pads before wiring on the new capacitor.

A tight filter is going to be too restrictive. Look for some open-cell foam. Here you can get a lifetime supply, a couple square meters of the stuff, as replacement "humidifier belt" at any hardware store. Watch out, there is a carbon-filled variety that's used for air cleaners, you don't want carbon particles in your filter getting sucked into the scope!

A few vaues are:

If you can breathe through it it should be fine. I often get it wrapped around shipped items.