We are spoiled these days because you can shop online and get all manner of inexpensive electronic goodies shipped to your door. This is due to the fantastic electronic fabrication workflow that has grown into a global powerhouse, facilitated by complex yet inexpensive integrated circuits! But it took a few intermediate steps to get here, and one of those is known as a couplate.
When I was a kid, the big deal was to find an old radio in the trash. You could spend a few hours stripping all sorts of parts from the thing and add it to your collection for a future project. Of course, old radios from the 1970s and earlier had a lot of the usual parts we use today, even though many of them were bigger — no surface mount parts yet. Since older radios were the usual find in a dumpster, tubes were common but you could find some transistor radios.
Once in a while something older. There would be a little box with some wires poking hiding in an old radio from the 1940s or 1950s (too early for ICs). In a way, though, these were predecessors to the Integrated Circuit and they went by a few names, depending on who sold them. PEC (Printed Electronic Circuit), a couplate, or a BulPlate, are all names for hardware that was a stepping stone between discrete circuitry and ICs.
Collections of Passive Components
PECs were most common in tube sets and they didn’t have any active circuitry. They sere often set up to handle audio filtering or some other common task using just resistors and capacitors. Companies known for these devices Areovox (PEC), Centralab (couplate), or Sprague (BulPlates). The example shown here, the PC-33 from Centralab, sold for fifty cents and had three terminals
The PC-33 is not terribly impressive, but a PC-151 (see right) had 7 pins and the schematic is quite impressive. The couplate itself only had the bold components, not the tubes and other components showed in gray. That little jewel cost about $1.15. Doesn’t sound like much, but in 1950 terms that was like $12 today. In fact, we found one on eBay for $11.90 so maybe there’s something to that.
Another place you would find these were in TVs where a vertical integrator PEC could help with sweeping the CRT. It was basically three resistors and three capacitors set up to help generate a vertical deflection ramp.
There’s never been an electronic device like Centralab’s Ampec. It is one compact unit permanently bonded to a master plate with all components of an audio amplifier — tube sockets, capacitors, resistors, wiring. It’s a full three tube three stage speech amplifier.
Centralab Ampecs are widely used in hearing aids, for the most trouble-free performance ever attained. Ampec has other interesting applications, as mike pre-amplifier, etc.
The module — without tubes — was an inch and a quarter wide and nearly as long. An Allied Radio catalog described it as “no larger than a book of paper matches” and sold it for $15.29, including the tubes.
The American Radio History site has so many old magazines and catalogs and, as usual, it didn’t let us down when looking for more information about these old components. The December 1949 issue of Radio and Television News (PDF) has the start of a two-part article entitled “Printed Circuits.” The cover with the smiling housewife, beaming at her brand new intercom while her five inch TV set perches next to the sink is priceless, too.
According to the article, Centralab began mass producing printed circuit boards in 1945 for a mortar shell proximity fuse. But these were not printed circuits in the sense that we think of them today. A ceramic substrate was the base for silver and graphite paint applied via silk screen. The silver makes wires and the graphite makes resistors. Seems like conductive ink circuits isn’t at all new concept!
The ceramic was fired in an oven and capacitors were attached. In some cases, conductive paint on both sides of the ceramic would form capacitors, too. Of course, small flat inductors were also possible. Supposedly, inductors could be covered with an insulator and painted with ferrite paint to increase inductance, but that doesn’t sound like it would get you very far. In more modern times, this same basic technique is how you make hybrid thick film circuits, not uncommon in high-reliability applications.
The author notes that you can use other methods such as rubber stamping or lithography to ink the printed circuit board. There was even talk of using decalcomania which is exactly what it sounds like, but I still had to look it up. There were many other methods that didn’t catch on over the long term. For example, “dusting” had metallic dust spread on a substrate and sintered in place much like some metal 3D printing processes. We were especially amused by the cylindrical boards that built circuits around a glass tube.
Build Up and Teardown
The second part of the article (PDF) covers creating your own printed circuits using conductive and resistive paint from DuPont. Since DuPont wasn’t going to sell you a few ounces of these paints, an enterprising Michigan company was selling smaller quantities and kits. You had to find your own substrate and we find it amusing that they suggested using an asbestos board.
A Russian site has a great teardown of an old tape machine that used modules like these. The tape player may look bulky by today’s standards with its large battery and tubes. However, for its day it was quite svelte and cheaper to manufacture thanks to the couplate technology.
It is hard to remember a time when consumer electronics were wired by hand with real wires. We’ve all seen computer backplanes that looked like plates of spaghetti. Printed circuit boards would change the face of electronics forever. Integrated circuit modules, even more. A look at the very early birth of these technologies is sobering when you realize all this was less than 100 years ago.
These methods didn’t last long. By 1969, boards were a bit more like we think of them, although there was still a ways to go. You might enjoy the Tektronix video talking about those kinds of boards, below. And it’s also worth a mention that one of most mesmerizing component assembly periods is tinkertoy and cordwood construction.