Prior to building a cold start relay I examined the two boards I have.
Both are out of '73's and are identical in general appearance. By that I mean they appear to be the same circuit board schematic and all passive components are present. Here is the front and back of one of them:
However, the values of some resistors and transistors vary between the two boards. Could be several reasons for this - the system is much more forgiving than I realize and so it doesn't matter what the builder grabbed out of the pile, or it was a planned evolution when they realized it wasn't working as designed.
I've seen timer boxes that folks have posted that are slightly different. These circuits also have different passives; in this case they even may be different numbers of diodes. Again, could be a natural evolution of the board, or perhaps they had multi-climate versions. Who knows. As an example, here is one from this thread (photos courtesy of the original thread posters). See how this one does not have a "D2" (diode 2) and also has two bridge resistors soldered in the back (normally attributed to a design error on the board). Also a "cooling fin" is wrapped around one of the transistors.
Components consist of capacitors, resistors, transistors, diodes + a 12v relay.
- Capacitors can be used as timers or as "batteries". Once filled they will drain at a certain rate based on the size of an attached resistor. Once voltage drops below a specified amount, the transistor will flip a switch. My gut is that each of the capacitors (there are several types on the board, but I'm looking t the blue axial ones) have a resistor and possibly a diode tied to it.
- The system is triplicate more often than not. Perhaps this aligns with the three phases of timing circuit?- Something to explore.
- Replacing a burned passive component may kickstart a failed OEM board, but based on age and the changing characteristics of the remaining passive components I'd be curious to see if the board will exhibit OEM behavior. Something else to explore.
- The 12v relay (the big rectangle with a circular top) is no way available today in identical form-factor. Plenty of relays available to take its place but need to understand specs.
- These are two spade connector styles, one mounted near the edge, one behind it (Longer because it must climb over the near side before mounting to the board).
- They are in triplicate. 2 styles x 3 sets = 6 total.
- They appear to be aluminum stock.
Pretty sure that I will have to hand craft these individually, or build a daughter board to permit more standard male PCB terminals. I'm going to try handcrafting first but if they don't turn out nearly identical then I won't be happy and will have to turn to plan B.
- (Somewhere in a German factory there is a bag of these pieces.)
- @Ray - these are the challenging pieces.
Printed Circuit Board
The boards I have examined have inconsistent soldering quality (not a surprise) and/or peeling solder traces. Heck its been 40+ years. Most often have burnt components too (or at least look burnt). Also the original one layer board material is significantly different than today's boards. Compare the new boards below with the original style.
Building a PCB out of modern materials is important b/c I think they will last much longer and I don't want future owners of my two cars to worry about it. The good news is that v1.0 of the board has been built. While it looks great it remains to be tested. I am sure that some modifications will be necessary in order to support new passive components with new dimensions (for example the 12v relay is totally different).
Also --> While I understand the general design of the system and its purpose, the nuances of the component relationships + behavior are a bit unclear and I am excited to finish the test harness to better understand how all of it works together.
A first go, v1 I'll call it!
Next up --> blog post walk through of the test harness and then probably another blog as a walk through of the circuit behavior so that I know what I am looking at when I test it. Fun hobby!