Dave fixes a rather strange fault in his Tandy 102 portable computer.
Great video, Dave! I have both the Radioshack Model 100 and the Tandy 102 and the former has an intermittent fault where it plays possum for quite some time with no apparent reason. I couldn't yet find the energy to drill down this rabbit hole (all power, clocks are alright), but I will certainly do one of these days.
These are great machines - lots of fun, even for my young daughters that loved to type on the "minicomputers". Great keyboards also.
It was nice to see the fault on the scope. For those young players that don't have a scope and perhaps only have a DMM the fault would most likely have been easy to find with it. When I suspect a chip select issue with a circuit like this I will often compare the resistance to ground and VCC of all of the select lines. In this case they should all be pretty close, but the open one will be different and easy to spot.
Model 102 1987 $499 ($1,149 in today's dollars)
Model 100 1986 $599, 8K RAM upgrade $119.95
Model 100 1985 $799
I keep thinking it's a bad PCB wash and some etchant remained. I don't know of anything else that attacks copper like that.
That was a fun repair, it helps others to learn technique.
I keep thinking it's a bad PCB wash and some etchant remained. I don't know of anything else that attacks copper like that.
That would be my guess as well.
Dave, yellowed plastic is usually not caused by fire retardants.
I think the bromine myth may have been started by Nintendo. Apparently they had some controllers that yellowed quickly and had to come up with an excuse...
If you want to test the "bromine" hypothesis, maybe go outside and set fire to some pieces from old equipment and show us the results. FR plastic should self-extinguish in a few seconds - ideally as soon as you remove the flame. If it keeps burning, then it's just the regular stuff.
Proper fire retardant plastic is harder to mould, plus it's extremely expensive (like add a zero expensive!), so normally it's only used where regulations demand it.
For consumer products that are "built down to a price", manufacturers want the cheapest option possible. (I suspect this is why many early PCs were that consistent beige/white colour - that's the natural colour of ABS. Adding colourants would have made them more expensive to produce.)
ABS is particularly prone to yellowing. The polymer molecules are not completely stable and prone to oxidation. Their structure can be modified by absorbing a high energy photo, exposure to heat, or background radiation. (Our DNA suffers the same way.)
The resulting molecular changes cause the plastic to preferentially absorb more blue & UV light (which accelerates the process), leaving just the green & red to get reflected - which is why it looks increasingly yellow.
It happens to clear polyurethane as well. You may have noticed that the tubing used on industrial air lines also goes yellowy-brown over time. And the same thing occurs to the cellulose polymers in old paper, which is definitely not fire retardant.
Kosine, you are correct about the yellowing of ABS. It has to do with UV, oxygen, and residual olefins in the backbone of ABS. There are research publications on this topic.
Tandy used painted plastic on their U.S.-made computers, so they don't yellow. But the model 100/102 is really not a Tandy/Radio Shack design in any way -- it was designed and manufactured by Kyocera in Japan, and also sold with minor differences by NEC and Olivetti.
When keyboards were real keyboards.