Power Supply Pics: After Rebuild

After ordering and receiving all of my shiny new components, I began replacing the old capacitors with the new ones. When I got to the big caps, I discovered something inconvenient. The leads of the new caps are thicker than the leads of the old ones. The old caps leads are 0.8mm thick and the new ones are 1.0mm thick. Typically, such a small difference would not be a problem because most PCBs holes are oversized enough to accommodate a small difference like this. However, the designer of this PCB had a very bad habit of making the holes a very tight fit for all of the parts on the board. This problem also became obvious when I was installing the new headers. The holes were so tight that I could barely get the new headers to go into the board. After an extensive search for some new caps with thinner leads, I found that every model of radial aluminum electrolytic capacitor that I found that was 20mm in diameter or over had 1mm thick leads. I could not find any with 0.8mm leads that were larger than 18mm in diameter and all of the parts with sufficient capacity and voltage ratings were 20mm or larger. It appears that we are stuck with 1mm leads.

On some PCB's, the problem of tight holes could be solved simply by drilling them out slightly larger. For single sided PCBs, this doesn't cause additional issues. In this case, The PCB has a ground plane on the top side that many of the caps are connected to. Drilling out the holes removes the thin layer of copper (the via) that passes through the holes. For the holes that must connect to the ground plane on the top of the PCB, removing the via will disconnect the component pin from ground. When the pin is soldered to the pad on the bottom side, there's still no direct connection between the pin and the pad on the top side. That's what the via was for, to connect the pad on the bottom side to the pad on the top side. All of the large caps have one pin that is connected to the ground plane on the top side, so if the holes are drilled out to make the caps with the thicker leads fit, then the ground connections are destroyed and so must be somehow restored. Many people would add a bunch of heavy wires to the bottom of the PCB after installing the new caps to connect the appropriate pins to a ground point somewhere. There's a much better and cleaner looking method that I will show you below.

Click the images below to see a larger hi-res image.

1/ Remove the large capacitors from the board so that the board looks like the following picture. Note that the new C30 component was a smaller size and it had 0.8mm leads so it was a direct fit and didn't require the modification. Drill both holes for each cap out to accommodate new 1mm diameter leads.
Use this size: D 0.043", bit #57, 3/64".

 

2/ Drill 7 new holes (D 0.043", bit #57, 3/64") in the locations shown. Also scrape some of the green soldermask away from the hole to create a solderable area (pad) around the hole. These holes will be very close to the grounded holes and must not pass through any other traces on the bottom of the board. The locations shown meet this requirement.

 

3/ Obtain some solid tinned wire (leads previously clipped from components are perfect). Cut pieces about 1/2" long. Bend one end of each segment 90° to create a small 2mm (0.08") hook. Drop one segment into each new hole as shown by the arrows in the pictures (7 places total).

 

4/ Solder the hooks to the new pads on the top side to secure the new solid wires.

 

5/ Now turn the board over. The new solid wires should be sticking out as shown.

 

6/ Insert the new capacitors and solder the non-grounded pin on each one as shown.

 

7/ Bend over the solid wires so that they overlap the ground pads as shown.

 

8/ Using small flush cutters, cut the excess length of the solid wires so that the wires just span the pads as shown.

 

9/ Solder all of the grounded pads and then clip the excess cap leads as shown. This completes the installation of the large caps and the grounding modification.

 

The next 3 pictures show the original headers and the factory original connections that were added to them on the bottom of the PCB. After you install new headers, you must restore these connections. The best way to remove the old headers is to first clip their pins on the top side right at the underside of the plastic body. With all of the pins cut, you can discard the body. Then heat each pin individually with an iron and slide the pin out. After all of the pins are removed, go back and desolder the holes.

 

Before installing new transistors and regulators, I wanted to improve the way they are mounted. After removing them from the heatsink, I drilled new holes in better locations and tapped the holes with a 4-40 thread. I then cleaned the heatsink and remounted the components with new 4-40 x 3/8" machine screws and new mica insulators and heatsink grease. Don't forget to also install the proper nylon insulator on the screw so that the screw doesn't short the component tab to the heatsink. Also, put a thin layer of grease on both sides of the mica insulators, not just one side.

Even if you don't want to tap the new holes, you should still drill new holes and relocate the components. The original holes were designed for a much larger component type. The excessively large holes greatly reduce the contact area of the heatsink to the component and this will degrade heat transfer and the component will operate at a higher temperature. Also, due to the large hole size, it is very easy for the mounting screw to cause deformation of the component soft metal tab. This will further diminish the contact area and cause the component to operate even hotter still. The worst case scenario is that if there is enough deformation of the component metal tab, the component body can fracture leading to catastrophic failure of the component.

 

Here are some pictures that show where all of the new parts were installed.

 

A word about the diodes. I replaced all of the diodes on this board, but these are not the ones listed in the parts list shown on my site. The components that I used are functionally equivalent or superior to the parts that I recommend that you use. I only used the components that I did because I happen have a bucket full of them.

Now, since power rectifiers (diodes) tend to get pretty warm during operation, I like to raise them above the surface of the board. The additional space between the component body and the board protects the board from the heat produced by the component. Also, the height of the component requires it to have longer leads and the longer leads act as heatsinks that help direct more heat away from the component.

Also notice the "S" bends that I made in the component leads. This prevents the lead from punching out the pad on the bottom of the PCB should the component ever be physically struck. The lead may bend, but it won't push through the hole and damage the copper pad and trace on the bottom of the PCB.

 

Here are all of the old components. More junk for the landfill. They did their job for over 27 years and were still working before they were removed. All in all, I'd say that's a pretty good run.

 

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