Author Topic: Advice for cleaning Tektronix 475 pots and push button switches  (Read 314 times)

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Offline astrogator

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I have a Tektronix 475 oscilloscope which seems to be in good operational shape except that vertical controls and some of the switches are dirty. When I turn the controls the traces move erratically. The channel buttons also seem to need some cleaning and cause jumpy traces when pressed. Once I finally get the controls settled I can get two good traces when putting a sine wave into both channels.

I have tried sweeping the pots back and forth through their entire range several times but this has not helped much.

The pots look like they are sealed and I am not sure how I should go about cleaning them or the switches without doing damage.
I would greatly appreciate some advice on the proper technique for cleaning pots and switches on this scope from someone who has done it successfully.

Thank you in advance – I am new to this forum and looking forward to your reply.
 

Online tggzzz

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Re: Advice for cleaning Tektronix 475 pots and push button switches
« Reply #1 on: August 12, 2020, 06:11:04 am »
The search facility at the top right is useful, although arguably not as useful as google. It leads to https://www.eevblog.com/forum/repair/renovating-an-early-tektronix-475-oscilloscope-a-few-random-hints-and-tips/msg1438311/#msg1438311 That links to https://entertaininghacks.wordpress.com/2018/02/27/renovating-a-tektronix-475-timebase-switches-potentiometers-and-hf-response/

Be very careful with the various cam operated gold finger controls. They can be easily damaged. The correct technique is to slide smooth paper that has been dipped in IPA through the contacts, being careful not to damage the fingers.

The groups.io TekScopes archive will have many hints and tips. The techniques used for a 465 can be used for a 475.
« Last Edit: August 12, 2020, 06:16:10 am by tggzzz »
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Online med6753

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Re: Advice for cleaning Tektronix 475 pots and push button switches
« Reply #2 on: August 12, 2020, 09:54:19 am »
Excellent advice given above and as an owner of a 475A and two similar 465B's I can give a few hints that have worked for me.

For the pots. Even though they are supposedly sealed depending on type used the gap between the plastic and metal is enough to get cleaner in there without prying and risking damage. For this application I use Dexoit D100 and just spray it on top of the control and let it soak in. Twist the pot several times. Usually get decent if not perfect results.

For the switches basically same method. D100 on the exterior of the control.

The attenuator switch decks are a whole different animal. Yes, absolutely correct method is disassembly and cleaning with coarse paper and IPA. But disassembly not easy. I have gotten good results by direct spray of 100% IPA on the decks with them in place. But remove the attenuator blocks first. Then dry with a heat gun. Sometimes one or two positions still will have poor contact. Typically the 0.2V and 20mV range. In this case I will use a SMALL amount of dexoit D5 soaked in a cotton swab directly on the finger contact. Be careful and make sure no fibers left behind. If that doesn't work then unfortunately it will have to be disassembled and cleaned. And I have run into one instance where the deck was worn beyond repair and had to salvage one from a parts unit.     
« Last Edit: August 12, 2020, 09:57:17 am by med6753 »
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