Ok, I bought a multimeter the other day - and I'll apologise in advance that I'm going to ask if someone could assist me in my completely oblivious experience in Electronics. So, I've gone through both speccies with this multimeter following the steps in THIS VIDEO
to look for faults. While I wrote the values down and tried to interpret them, either some of these are showing faults, or I have no idea what I'm looking at (probably both). I'm sure somebody here will know what's going on and be able to tell me if it looks like there's a fault??
Interestingly, the readings between the two speccies was pretty much the same (well, +/- a little difference but so close its almost the same). I set the resistance to 200K on the multimeter...Voltage Regulator
Both measured 22.1 (which at 200k, I thought was read as 22.1k ohm? Maybe I'm wrong?). According to the video, the reading should be greater then 100k, or an open circuit. Fault? Hmmm...Lower 16k 4116 chips
All chips I measured on the 12v, 0v and +/-5 v pins (as mentioned in the video). All chips in each machine had the same values - so I'm expecting that means that no single 4116 would be an issue. Both machines returned similiar results, though a small amount of difference between the two seemed to be noticeable. That said, as per images I've attached links to at the end of this post, the internals seem a little different so that could be the reason for the slight difference.Pin readings:
- 0v = 0.0 (both)
- +5v = 0.4 (rubber) and 0.8 (DK'Tronics).
- -5v = 22.2 (rubber) and 21.4 (DK'Tronics).
- 12v = 2.7 (both)
Both coils were read individually, and then across both to make sure the windings weren't damaged.
TR4 and TR5
- Coil 1 = 0
- Coil 2 = 0
- Both = > 34 (a high value apparently should be expected, or an open circuit)
From what I understand, TR4 is the one that is most important with regards to the circuits power. But I tested both using the diode test mode on the multimeter. The guide video indicated testing the base with both the positive and common when testing the collector and emitter pins. (I almost sound like I know what I'm talking about here). So I tested both of them since they were sitting close to each other anyway.
With TR4, Apparently the common as base should return an open circuit on the two side pins. A positive on the base should return a value between 0.6 - 0.9VTR4
TR5 - No idea if these values make sense (or if it was worth testing it), but...
- With common on center pin, both side pins read open circuit (both machines)
- With positive on center pin, both side pins read 812 (rubber) and 804 (DKTronics) - which I think at a 1mV resolution is 0.8V???
- Common on center pin, both sides read 885 and 878 (rubber). On the DKTronics, it read 883 on both
- Positive on center pin, one side was open circuit. On the rubber the other pin read 1841. DKTronics was 1886 (approx 1.8v???)
So, is there a fault anywhere here? The transistors kinda read OK, the inductor apparently reads OK.
The voltage regulator and lower 16k have that odd value which to be honest is likely me not really understanding how to read the value on the multimeter properly. Any help for this noob would be obviously greatly appreciated! Differences
I did note a few differences in the boards - that said, I'm guessing its an age difference in manufacturing that just means different components were used. The DKTronics board is my original one from 1983 - its got most the IC's soldered on board (ULA, Z80, Rom), where as the other is mostly socketted. The DKTronics one has a different RF block, and an extra wire/resistor all connected in. Its easier to see then explain... Images are on dropbox, and as it doesn't appear to let me embed the links into a post, the direct links are here...
That said, now I look at this macro piccy of the RF
on the older DKT one, I can see corrosion on the resistors, etc that I didn't spot looking at it normally (I don't have the best vision when it comes to getting close).
Thats probably likely the cause of the machine having a bad time (its got garbage on the screen - black, with vertical lines of flashing characters). The only reason I thought it was interesting is that there's the extra wire (compared to the rubber keyboard one) with plastic cover over it. Its not a problem as its how its always been since I got the machine as a kid, but more a curious thing.
Here's a picture with the tops taken off
the two RF blocks. Slightly different internally.
That said, they're almost the same (picture here
). Obviously the rubber keyboard version is more serviceable with those socketed mounts, and it looks a little more modern. That said, the poor older one has had a bad life living in boxes and damp spaces. If it can't be saved, I'd still like to get at least one working spectrum in the end...
Any tips, suggestions welcome.