How Not to Mod a Blues Junior...
or, How to Fix Simple Printed Circuit 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 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 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
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.
|Bend the ends of the leads so that they line up
with the tinned copper portions of the leads. Then solder down the ends,
as above, heat the lead, and flow the solder evenly over the lead, patch
wire, and trace. Push the component up from the other side of the board
so that it's tight against the board. Otherwise, if there's space to
push down on the component from the top it will tear the traces away
from the board. The base of the component and the patch have to form a
"clamp" that prevents vertical movement.
The Orange Drop caps have a crimp in the leads that holds the body of the component off the board, When there are crimped leads, it's not necessary to mash the component body down to the board; just press it so the crimp is tight against the top of the board.
|Remember the first picture? Here's 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 the recommended 27K. 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.|
|It turns out that the tack soldering job wasn't
so great, either. It broke while I was removing it. See above, under
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.
|Here's another Blues Junior that suffered
mightily under its former owner. He had attempted to do a head
You can see the holes behind the ribbon cables for the tubes--they're
badly drilled, complete with sharp edges. This is the "after" picture,
after I repaired the damage. In repeatedly removing and replacing the
circuit and tube boards, the owner had evidently broken some ribbon
cable solder joints. He then evidently wound up shortening the ribbon
cables, doing a bad job of soldering. He had cut some of the cables,
replaced the filament wires with solid hookup wire instead of stranded,
botched some solder joints, and generally made a mess of things. The
current owner, by the way, had bought it on eBay, after reading that the
mods were "professionally done." In fact, virtually everything that he
had touched was wrecked in one way or another.
As you can see, I had to strip out sections of the ribbon cables and replace them with stranded hookup wire.
Even the replacement input jack was done wrong, with solid, unshielded wire instead of shielded co-ax.
|While shortening or attempting to repair ribbon cables, our previous owner managed (I figure) to short out the screen to ground, and then short the plate to the screen on one of the output tubes. The result was this charred mess. The problem is that carbonized circuit board becomes conductive, so all of this had to be scraped away.|
|Here's what the damage looked like from the top. You can also see his inexplicable cutting (badly done at that) of the ribbon cables. Once I scraped away the carbon, there was nothing left of this area of the circuit board.|
|The short circuit drew so much current that it overheated the first power supply resistor--to the point where it bubbled the circuit board all the way through to the back. That's hot. I don't know why the fuse didn't blow.|
|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.
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