Why did 1970s kits come with color blue/red circuit diagrams?

[bzotto]

Early micro kits came with (typically) beefy instruction binders, full schematics, and board layout diagrams. Sometimes, red/blue PCB diagrams would be included with the instructions. Attached is one such example apparently from OSI; SWTPC did this too I think in some cases. I know that such diagrams were produced and used in house in as a step in to-scale photolithography of double sided PCBs. But it seems to me that high quality color copies cost money, and the added value of having full red/blues doesn't seem worth much to an end-user who is only assembling a board. Unless the customer was expected to use a high-quality image like this for some sort of photo process of their own? Can anyone--especially someone who was there at the time!-- shed any light on this? Thank you!

 

[Al Kossow]

Two-sided printed circuit boards were laid out by hand with red and blue tape
The sides were then color separated to produce top and bottom.
The pads are black so they show up on both sides.

 

[bzotto]

Two-sided printed circuit boards were laid out by hand with red and blue tape
The sides were then color separated to produce top and bottom.
The pads are black so they show up on both sides.

Thanks Al. What would the value be to an end-customer of having one of these sheets provided in the kit documentation? I get that it's a nice bit of clarity, but was it just "hey, we already have this red/blue sheet from when we made the boards so let's just include a copy so people can reference it in addition to schematics and layout diagrams" ? Or was there some other specific purpose a customer might put it to?

 

[Eudimorphodon]

Unless the customer was expected to use a high-quality image like this for some sort of photo process of their own?

Or, just tossing this out there, attempting to photocopy this would result in a poor quality mess that would make it harder for someone to rip off and clone your PCB layout on the cheap from the manual…

 

[DDS]

This was common practice for Heathkit products. Using color allowed a new kit owner to see part placement, top side traces, and bottom side traces all on one sheet. But, yes, it seriously mucks up any attempt to share via black and white copies. I have a copy of the drawing for the power supply board for an H11. The ground planes render as a black blob that covers most of the sheet of paper.

So you copy that sheet in color.

 

[Eudimorphodon]

So you copy that sheet in color.

Of course color photocopies were a little... rare, in the 1970's.

 

[DDS]

I'm thinking this is one like the mystery behind the reduced sized heads on the #8-32 machine screws that secure the case on 5150s and 5160s. Someone obviously made the decision but if they wrote their reasoning down we haven't found it and they're probably no longer among the living and can't explain.

OTOH, it's entertaining to speculate and likely doesn't hurt anything or anybody.

I say go for it!

;-)

 

[bzotto]

This was common practice for Heathkit products. Using color allowed a new kit owner to see part placement, top side traces, and bottom side traces all on one sheet

It sounds like this may just be the simplest explanation-- that it was just considered to be a useful enough aid in constructing the kit, so it was worth the cost for Heathkit (OSI, etc) to reproduce it and put it into the kit.

On the flip side, it seems to me that it might make it easier to rip off the design-- you buy one original kit, you take the red/blues to your local PCB fab, and they can do perfect reproductions of it directly. But frankly, it has never seemed to me that the intellectual property angle was ever especially tightly held in this era-- every one of your customers had to be intimately familiar with your design just to build it and get it to work. (See also: Altair --> IMSAI, etc)

 

[Chuck(G)]

Somewhere, I think I've still got some of these red-blue copies--but I can't for the life of me recall what they were for.
I'm thinking that the originals were done on blueline gear, but can't recall if it did red. Basically, a diazo process.
Mostly what I recall from blueline duplication was the smell of ammonia...