Fixing Simple Printed Circuit Board Mistakes
Every once in a while someone installing my mods gets in over his head, or something just goes wrong, and they ask me for help. I looked around on the Web for a page that shows how to fix common printed circuit board problems, so I could refer them to it. I was amazed that I couldn’t find such a page. So here are some examples of what not to do, and how to fix common problems when it all goes wrong.
I recently received this board for repair. The owner had attempted to install the green board reverb mod and the tone stack capacitors, and realized that he wasn’t hearing any bass or mids anymore. He had removed the tone stack caps again before he sent me the board. He had attached a weird piece of push-back wire to the cap that’s half-inserted into C20 and tack-soldered it to R40, and tack-soldered the re-routed R56 to the other side of R40. Neither tack-soldering joint held; one broke off in shipment and the other broke when I touched it. Neither would have survived the pounding that a combo amp gets from its speaker.
As you can see, three out of four of the pads for the tone stack capacitors are completely gone. This kind of damage is normally caused when you’re desoldering components. The leads are bent over onto the circuit board, and if you try to lift them while any of the solder is still adhered, the pad comes off. If you work carefully, suck all of the solder off, and make sure the lead is not adhered to any part of the pad, you won’t have this kind of problem.
Scorching on the board and widely-spread rosin indicates that the soldering iron was either too large or too hot or both.
This is nasty, but all is not lost.
I cleaned the board with solvent and scraped off the excess rosin. You can see how the lower left trace is partially torn off, too. The missing solder pad on the upper right breaks the continuity of the thicker trace.
The next step is to scrape around 3/8 inch of the green solder resist on the trace leading to each missing pad. Take light passes with the edge of a hobby knife. You want to expose the copper below, but not remove any of it.
The next step is to tin the exposed copper. Just heat the copper enough to flow solder onto it. Don’t overheat it, or you could lift the trace.
Put the new components into the holes. These Orange Drop tone capacitors have heavy leads. Here you can see that I bent the lead over sharply, so that it overlaps the tinned trace. I then heated it and applied a bit more solder, so that there was a long, smooth bond between the lead and the trace. You can hold the lead down tightly with a jeweler’s screwdriver. Solder won’t stick to a chromed screwdriver.
After it cools, you can trim the unsoldered portion of the lead. Make sure you don’t damage the trace when you clip the lead.
The upper trace that has to be connected in two directions to complete the broken circuit. This requires a different approach. I twisted a piece of wire around the capacitor lead. This is a cutoff from a thin resistor–perfect for the task. Never depend on a bridge of solder to close a gap; always use a patch wire.
Drop the loop over the lead and pull it tight.
There are other ways to repair printed circuit boards, such as metal-bearing ink pens that let you “draw” a new trace, or copper-loaded two-part epoxy that’s rugged and conducts well. They’re OK for repairing cracked traces, but neither is as strong as wire and solder for a long-lasting repair or for creating an attachment point for a component lead, as I’m doing here.
You can also use the loop technique to create a new solder pad. Just put a loop in the end of the wire, then connect the tail to the scraped trace on the board. Make sure that the component is firm against the other side of the board so that there’s no possibility of play through the hole. Motion can crack the solder joint or cause the trace to peel from the board.
Remember the first picture? Above is the right way to attach top-of-board mods: Crimp the components or wires so you have a strong mechanical connection before you solder.
Here’s the tone stack and reverb mod, all finished up. Solder has flowed nicely on the C20 connection (also crimped first), and on the R40 connections. The Orange Drop caps are glued together with hot melt glue to prevent vibration. Two more dots of glue hold the wire against vibration. Whenever you solder on the top side, always check the back side to make sure that you haven’t loosened the back-of-board connection.
We’re all set to button it back up.
Uh-oh, wait a minute! What the heck is this? Our modder decided to lower the too-hot bias by replacing the 22K resistor in R31 with a 27K instead of going with adjustable bias. Good, but tack-soldering the 27K resistor onto the cut-off leads of the old resistor is a bad idea, especially for bias. I pulled the board again to take a look at the back side–and do the job right.
Heating the stub caused it to desolder on the other side. This is a potential disaster waiting to happen. Losing the bias voltage will certainly take out the output tubes and maybe destroy the output transformer as well. Don’t mod onto stubs from the top of the board!!
It turns out that the tack soldering job wasn’t so great, either. It broke while I was removing it. See above, under “potential disaster.”
The solder pad and trace on the back of the board were in good shape, though. With a fresh resistor in there, the bias will be dependable–and cooler, reducing future heat damage from the output tubes and improving the tone.
Even arced-over, burned traces can be repaired. Damage like this can happen from a poor solder joint at the ribbon cable or from a screen grid failure in one of the EL84s. The carbon is conductive and it will arc again, even after you’ve repaired the solder joints.
The solution was to create new solder pads for the plate and screen wires. I cut the trace leading to the burnt area so there would be no possibility of further arcing or voltage leaks.
So the moral of the story is to do mods that are within your level of expertise. Learn how to solder and desolder on a broken radio. Make every fix “for the ages,” not something that looks marginal or is hanging on by a thread.
The ribbon cables are especially annoying to work on. If you break one connection, you have to shorten the whole cable. Then you have to get all of the leads into the holes at the same time. Like shortening one leg of a chair, things can go from bad to worse very quickly.
The basic mods to this board were done with a too-hot soldering iron, probably a cheapo plug-in with no thermostatic control. All of them showed board damage; I had to remove all of them (tone stack, presence, etc.) and start over again.