Fixing a Corroded Battery-Bay Module on HP-41CV Calculator

First let's get this next bit out of the way. I am not responsible for anyone's bad results and/or damage to their calculator or tools: insert standard CYA clause here. This post is "AS-IS".

This post shows how I was able to repair a HP-41CV calculator which had a badly corroded battery pad.

While the method described below worked well for my case, self-assessment of your skills will determine your success. This method requires at least intermediate soldering skills.

This post will show how to repair a single contact pad on the battery-bay pad. It will not show how to remove the assembly from the HP-41, as there are several good YouTube videos on how to do this. The focus will be just on the repair of the battery-bay pad once removed from the HP-41.

The photo below shows the damaged battery-bay and expansion port module as initially found in the HP-41. Note the blue-green corrosion on the right-most pad. The corrosion was extensive enough to have eaten through the copper at the point of contact for the battery pack terminal.

The following photos show one method to fix a corroded battery-bay and expansion port module pad on a HP-41CV calculator.

Tools and Materials Needed

Fine-pointed soldering iron
Flux solder paste and application brush
Brass shim stock (0.001")
Scissors for cutting shim stock
X-acto or scalpel knife
Bristle brush
Tweezers
Isopropyl Alcohol (IPA)
Super-glue

Method Procedure

Below is a photo of the whole battery-bay and expansion port module as initially remove from the HP-41CV.

The next step is to carefully remove as much of the corrosion as possible from around the pad being rebuilt. Use a moderately stiff bristle brush, together with IPA (IsoPropyl Alcohol), to gently scrub the module. When the area has been cleaned, let the module dry.

After cleaning, take a sharp scalpel or X-acto knife and gently scrape the conformal coating off the area above the pad area. Only scrap away enough to expose the underlying copper layer. Expose an area about 3mm in diameter.

The next step is to tin this newly exposed area. First apply a small amount of flux paste to the exposed copper area. Then, with a fine-tipped soldering iron primed with a bit of solder, tin the exposed copper surface. This should be done quickly to prevent damaging adjacent plastic areas. Unfortunately, the photo of this step was lost.

The new step will be the forming of an overlay made from 0.001" brass shim stock. Using scissors, cut out an overlay such that it closely conforms to the same shape as the corroded pad. Press the overlay onto the pad area so that it follows the contours of the underlying pad.

When a good overlay has been cut from the brass shim stock, the end of the overlay corresponding to the tinned area on the module should be tinned: both front and back sides.

Now the (somewhat) tricky part.

Press the overlay onto the pad area so that it covers the protruding area and the overlay tinned area is in contact with the scrapped-off and tinned area of the module. Hold the overlay in place with tweezers or similar tool. With a fine-tipped soldering iron, touch the overlay's tinned spot. The soldering iron heat should immediately melt both front/back solder on the overlay as well as melt the scrapped-off area solder. Continue to hold the overlay in place with tweezers and remove the soldering iron.

Once the solder has cooled, carefully insure the overlay still well formed over the pad area.

Finally, use superglue to attach the lower edge of the overlay to the under side of the pad area.

Below shows a photo of this whole step.