last update 7/4/2001

  This page logs an attempt to fix a problem on my tek 475 scope. Both the vertical position potentiometers (pots) were so noisy to make my scope unusable. After failure in fixing the problem by turning back and forth the pots for the full rotation, I decided to open the scope to see if it is possible to squirt some kind of contact cleaner inside the pots.

Opening a 475 is an easy matter: just unscrew a couple of screws at the back panel and remove all the four stems from the back. Then gently slide the frame out of the box, paying a lot of attention not to touch any of those fancy components coming into sight during the manoeuvre.

Surprise: the to pots are armoured and no chance exists to squirt anything into them.

The only choice was to remove the pots out of the vertical amplifier board (vboard) to open them and then attempt a surgery. The operations looked immediately not so simple. The vboard is full of wires coming back and forth and I suddenly realized that is was also full of mechanical constraints. First of all I noticed a couple of resistors soldered one lead to vboard and the other to the vertical rotary switches. Here you can see one of them. Experience was also suggesting me they were mounted with many critical RF components electrically connected and physically very close to them.

So I connected to the Internet and looked for suggestions. No one seems have described maintenance procedures on our lovely oscilloscopes. But I’ve found and subscribed a mailing list in groups.yahoo.com titled “tekscopes”.

Here is what Fred de Vries replied to my message “tek 475 vertical position pots”

Hi Emanuele,

Those pots are probable Allen Bradly, and have a double section.

There are two type of AB pots, The plastic sealed ones, and the ones

which you can nicely open with four screws. To clean the pot, you

have to take it apart (first remove them carefully from the Vertical

amp board). Once you have opened them you need to cut the plastic

seal inside the black plastic section to get to the brown wiper

section.

If you have everything apart, remove all the old grease and clean all

sections with alcohol. Mind you that the whiper has a tiny carbon

pallet, which comes loose very easy.

Once this is done you need to re-apply new grease. I use for this

purpose lithium based grease from Dow Corning, Molycote DX. Other

types of grease I cannot recomend to use. This is probably the same

stuff that Bourns uses.

After this, everything has to be assembled again.

There is one small remark on this whole procedure, that is, if the

carbon conductive film is worn out, the result will probably

disapoint you.

I have personally used this procedure on a lot of AB pots. They are

the ones usuly having problems due to the aging of the grease. And

can be restored most of the time with a good result.

- Fred de Vries

Here is what Fred de Vries replied to my message “how to safely dismount the 475 vertical board”

Hi Emanuele,

You can safely unsolder the two resistors. They are connected to the

input FET. Just make sure that your soldering iron ground is on the

same potential as the scope, otherwise you could damage the input FET

(2N5911 discontinued by VISAY). The BW trig view screw (I think allen-

key 1/16") is best taken loos at the switch.

On most 475s there is also a strap from the main board to the

vertical board which can easy be overlooked. It is close to the HV

section. You have to unsolder it as well. The same with the Delay

line which is normally soldered to the vertical amp board.

Good luck.

Fred de Vries

For a complete list of messages go to groups.yahoo.com.

I want once again to thank you a lot all the participants to the discussion and who provided so many invaluable tips.

I then decided to proceed.

First of all I unsoldered the resistors. Here you can see the frame just before the operation; please note the wire (manually lime lighted in the pic) connecting the lead on the vboard to GND to protect the input FET . I also measured 18 VOLTS AC between my soldering gun and the scope chassis, so I also connected the gun to GND… crossed my fingers and the resistors were disconnected.

I’ve then spent two hours in dismounting the vboard and un removing the pots from it. What a nightmare! First of all the screws on the shafts are not 1/16”. They are smaller – but bigger of my 3/64”. Sgrunt. I had to adapt one

If any other people is about to start an adventure like this, please be sure to have a perfect and efficient unsoldering machine. I spent a lot of time trying to get rid of the tin that was keeping the pots on the board. My solder sucking tool was insufficient. I used a thin plaited wire to get the tin out of the PCB holes by capillarity.

The pots are now in my hands, but I don’t really know if my scope will work again…

By examining the pots, I easily figured out that, to dismount the frame, I  needed to unscrew the four screws keeping them assembled.

Doing so I have got the four components of the frame: one shaft support plate, two pots and a back plate.

Focusing on a single section pot, I easily figured out that to open it I have to get the black and white parts separated. Easy to say, much more difficult to do. The two part are tightly coupled  and even levering with a small precision screwdriver (1,2mm)  resulted in a failure.

CAUTION: this is the point where you can destroy your pots!!

Having spent so many time to get to this point I decided that failure was not an option. So I insisted. Levering with more force. And more. And more. And… I finally got it open! And these are all the pieces.

Everything has been cleaned with isoprophilic alcohol. I wan to spend two words on the wiper. Be careful. The metallic part of it (the real wiper) is not a single thin plate of steel as it seems to be: it is a set of steel wires of hair’s breadth. Do not clean then with Q-tips or you will easily destroy them of at least you will fill them up with plenty of cotton’s hair  you’ll never get rid of.

I made the clean operation on each section of each pot, remounted everything and … BINGO. It works perfectly. The pots are now fully and smoothly operational.

While remounting the scope I have aloso inverted the two trigger pots as the one for A trigger had the switch unoperative. As trigger B is used much less the trigger A I have the good switch on the most used trigger. The operation was trivial.

I don’t know if what I made on my pots is a definitive surgery. Until you don’t find any update here, is means that they are still working.

Today is 31/3/2001.

Ciao

Back to TEK475

Back to Emanuele  Girlando's Home page