Cam switch pcb contact repair -- Suggestion needed


Rogerio O
 

Dear all,

Since I was unable to calibrate this unit and it was showing erratic behavior, I removed the main board for a closer inspection.
After removing the cam of the Step/Offset switch to clean the contacts with Deoxit I discovered that the pcb contact of one position was damaged and the PCB metal contact is almost gone.
The finger is intact.
I have posted a photo on the forum on the Tek 577 Main board repair album
This is a gold plated contact and I have no idea of how to repair it.
Any suggestions?
Thank you


Ananda
 

First, smooth out the edges where it is damaged. Peel off a small piece of copper from a new piece of pcb and try to solder it in place. It should be wide enough to cover the original copper. NEVER attempt to bend the contacts! The new copper should be thin enough so that the contacts will not touch it when it is not supposed to.
I think it is easy to peel off copper tracks from a perf board. Clean both sides thoroughly before attempting to solder it in place.


Harvey White
 

Would copper tape work well?  There used to be some "stick on the copper" types of PC board construction.  Would something like this help?

Harvey

On 5/19/2022 8:40 PM, Ananda via groups.io wrote:
First, smooth out the edges where it is damaged. Peel off a small piece of copper from a new piece of pcb and try to solder it in place. It should be wide enough to cover the original copper. NEVER attempt to bend the contacts! The new copper should be thin enough so that the contacts will not touch it when it is not supposed to.
I think it is easy to peel off copper tracks from a perf board. Clean both sides thoroughly before attempting to solder it in place.





Ananda
 

That is a great idea. I was thinking about where you can get some copper thin enough for the job. I would however remove the adhesive and solder it in place.
Since the contacts do not "move" but will only go up or down to make the connection, all you need is something really thin that will leave a gap as required.


 

I wonder how the contact points got damaged. There's basically not friction in these switches, so we must assume that either there was arcing, or that somebody manually caused the damage. I would suspect arcing, since the damage appears to be immediately under the contact points of the fingers.

PACE had a PCB repair kit called ThermoBond Cir-Kit that would probably work in this application. You can find instructional videos on-line. The ThermoBond Cir-Kits appear to still be for sale from PACE (at paceworldwide.com), and from resellers like TEquipment, but they're not particularly cheap (maybe not outrageous, given the going rate for a 577, though).

-- Jeff Dutky


Rogerio O
 

On Fri, May 20, 2022 at 12:52 AM, Jeff Dutky wrote:

Thank you all for the replies.
I checked ThermoBond but $500 is much more than I can afford.
After reading your suggestions I think I will try to delaminate a gold plated edge finger contact from scratch pcbs that I have collected for gold recovery, cut it to the size and solder it in place.
My guess is that I can't use copper since it will oxidize and develop a high contact resistance.

I was think of using a mini gas torch (from the back side of the board to cause the trace to delaminate
Any other ideas on how to remove them from the pcb?
Then, I will probably have to find away to clean the side that was attached to the pcb since it has a kind of glue.

One other question: Why not simply put a small "blob" of 60/40 solder over the "holes" and carefully sand it to flatten its top?
My feeling is that "holes" are so small the surface tension would be enough to keep the blobs in place.


Bert Haskins
 

On 5/20/2022 3:50 AM, Rogerio O wrote:
On Fri, May 20, 2022 at 12:52 AM, Jeff Dutky wrote:

Thank you all for the replies.
I checked ThermoBond but $500 is much more than I can afford.
After reading your suggestions I think I will try to delaminate a gold plated edge finger contact from scratch pcbs that I have collected for gold recovery, cut it to the size and solder it in place.
My guess is that I can't use copper since it will oxidize and develop a high contact resistance.

I was think of using a mini gas torch (from the back side of the board to cause the trace to delaminate
Any other ideas on how to remove them from the pcb?
Then, I will probably have to find away to clean the side that was attached to the pcb since it has a kind of glue.

One other question: Why not simply put a small "blob" of
silver solder?
60/40 solder over the "holes" and carefully sand it to flatten its top?
My feeling is that "holes" are so small the surface tension would be enough to keep the blobs in place.




 

On Thu, May 19, 2022 at 09:15 PM, Rogerio O wrote:


After removing the cam of the Step/Offset switch to clean the contacts with
Deoxit I discovered that the pcb contact of one position was damaged and the
PCB metal contact is almost gone.
Assuming you are talking about switch #33 this was a known problem.
An internal mod, dated Aug -93, was set up to address this problem.
I realize it's not much consolation once it has occurred but maybe this info
will be useful for other members with a 577:

//
I. PROGRAM OVERVIEW
A mod to prevent damage to Contact #33 in the STEP AMP SWITCH has been
implemented. A resistor (308-0365-00) is being added in series with the power
supply line to Switch #33. This will reduce the current spike. This update will
improve the reliability of the 577 series Curve Tracers.

II. PROGRAM IMPLEMENTATION
This mod will be implemented on all 577, D1 and D2 as they are brought in for
servicing including calibration. The mod can be installed at all designated
service locations. The Main Board part number will roll to -07, to show that the mod
has been added. This mod does not affect the calibration of the instrument.

III. TECHNICAL DESCRIPTION OF MODIFICATION
A 1.5 ohm resistor, p/n 308-0365-00, is installed in series with F391 on the Main
Board. The easiest way to do this is to lift up the end of F391 closest to the brown
wire. Place one end of the 1.5 ohm resistor into the space and solder the other
end to F391. Trim the 1.5 ohm resistor to the necessary length. This resistor will
be R391.

This mod reduces the current spike that comes from the power supply to Switch
#33 from 13 Amps to approximately 5 Amps. This will reduce the likelihood of
Switch #33, or the Main Board from being damaged by the current spike.

IV. PARTS LIST
Part Number Qty Description
308-0365-00 1 1.5 ohm resistor

No kit has been set up for this update. The resistor is a common part, so most
service centers should have this part in stock.
//

308-0365-00 is wire wound at 3W.

/Håkan


Rogerio O
 

Dear all,
I decide to use a gold plated finger contact from a PCB "donor" and solder it on the top of the arched contact.
The contacts were pulled off after baking the pcb in a oven to delaminate.
After polishing the contact, I applied solder to the copper side.
The contact was positioned in place and heated with an air heat gun to solder it in place
The finger plating was gone but since I have no replacement for it I have just polished it and soldered it in place.
I have posted photos of the repair in the " Tek 577 Main Board Repair" album for reference.
I checked continuity with a DMM and it seems to be working fine.
When the board is in place and tested I will come back and confirm that this repair works "in real life".
Roger


Ananda
 

Looks like you did a good job!