Electronics > Repair

Tektronix 475 vertical position knob problem

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David Hess:

--- Quote from: iDevice on September 07, 2014, 10:01:18 pm ---
--- Quote from: David Hess on September 06, 2014, 04:49:15 pm ---I recognize the part.  It is a 470k 1/8 watt carbon composition resistor which was selected because they have good overload capability and tend to fail open.  1/8 watt carbon composition resistors are practically extinct and I do not believe anybody makes them anymore.

I do not have any good suggestions for an ideal replacement.  A 1/4 watt carbon composition resistor will certainly work but so will any 1/8 watt film resistor.  The best option might be a thick film 1/8 watt or lower chip resistor with a high voltage rating like 500 volts.
--- End quote ---

Carbon composite resistors are also sometimes chosen for their low inductance which could be the reason here in a high frequency circuit so a film resistor could change it's characteristics.
That's why these resistors are still widely available at the usual suppliers like Mouser etc.
--- End quote ---

That is often the case but here, the resistor is bypassed by a capacitor and its self inductance has a negligible effect because it is part of a high impedance circuit.  It was chosen for its overload tolerance and propensity to fail open.

My 475 may have a related problem. Ch 1 INTERMITTENTLY jumpy (vert) and fuzzy (with signal). Moving the input jack around seems to affect it, clears it up for a while. I suspect same areas shown, since a lot of those components might be subject to vibration. I'll have to take a look sometime soon.

Thanks for the great photos, and info!!!


With the jumpy channel 1 - mine too has these sort of issues. If you open the cans where the attenuators live you can see a bunch of gold switch contacts in there. Turn the volts/div dial until a switch opens, stuff a piece of isopropyl alcohol soaked paper between the switch and the contact, gently(!) move it around to clean the contact and pull it out.

Other issues I had were the AC coupling capacitor is not soldered but socketed. Tapping it changed the waveform. Pulling it out and pushing it back in seemed to make lots of this go away.

Finally, there are a few of these gold contacts underneath the grey volts/div selector barrels. I think the ones underneath set the gain for the cascode amps so if these switch contacts are dirty it makes a *big* effect on the signal. Same procedure can be applied without removing the unit to clean them. You can get to them from the side between that lump of metal and the underside of the attenuator can.



Thanks for that info. When I get the chance to go in it, I'll be looking at all that. I think IPA is much safer alternative to other cleaners for this work. You want something that evaporates completey, does not leave any film, and doesn't melt plastic!!!

Lots of places for intermittents!!!


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