[sdiy] Jupiter 8 not booting

Brian Willoughby brianw at audiobanshee.com
Tue Feb 2 06:39:18 CET 2021

I suspect that C7 or C8 is bad (or both). I had a bad capacitor in a TR-808 where touching it would make it work again, but a slight bump and it would stop working. This would explain your intermittent troubles.

If you had a digital/analog scope with recording abilities, you could look at what happens to the two Base and Collector voltages over time.

The circuit is a bit surprising. I've never looked at one of these startup Reset timing circuits before, so I never analyzed the time constants.

Ignoring all but the last stage, I would expect that TR2 could pull the Reset line low (active) rather quickly, because there's little resistance when the transistor is conducting, and that will discharge C9 rather quickly.

In contrast, when TR2 turns off, the 47 kΩ R4 and 10 uF (16 V) C9 would take 0.47 seconds to charge up to 3.16 V and 2.35 seconds to charge to basically the full 5 V. Of course, it depends upon the logic threshold for the Reset line, because it should trigger before the full 5 V anyway. Maybe the intention is for the system to keep the processor in Reset for a second or two while power stabilizes. This is the only part I find surprising, but maybe it's actually normal.

None of this explains 5 or 10 minutes, unless R4 is open-circuited. As far as I can tell, R4 brings the CPU out of Reset as soon as TR2 stops conducting.

Looking at the rest of the circuit, TR2 should conduct in under 1 millisecond after power-up. At some point, TR1 conducts and turns off TR2. I just can't quite figure out what happens to C7 that would control the timing of the Reset pulse. I assume that C7 starts with 0 V across it, then charges up to 4.31 V.

As I mentioned, the weirdest thing to me is the slow rise time of more than one second. Many modern processors would require a sharper rise time on the Reset line for proper operation.

I would expect that when not working, TR2 Base would be 0.59 V, not 0.14 V. That's because I assume R4 is good and TR2 is on. If that's not the case, then check R4.


On Feb 1, 2021, at 20:03, Adam Inglis wrote:
> Could I trouble some patient soul to cast their eyes over this circuit and explain what should happen?
> The JP8 CPU reset circuit is in the grey box on the right half of the image.
> <JP8 CPU reset.jpeg>
> My jupe has been taking its time booting these last few months, 5 or 10 minutes, and now sometimes is refusing to boot at all. (Occasionally it will boot as usual in a couple of seconds after going through the tune routine, so the fault appears to be intermittent). 
> Up on the bench, when failing to boot, I measure the reset pin 26 on the CPU as 1.4 v. When it boots successfully, that voltage takes a second to go from 1.4 up to 4.9 volts.
> So I’m suspecting something in the reset circuit.
> When working:
> Transistor 1 measures
> E  = 00 v
> C = 0.14 v
> B = 0.59 v
> Transistor 2 measures
> E = 00v
> C = 4.9 v
> B = 0.14 v
> When not working the only change is the collector of TR2 doesn’t come up above 1.4  v
> cheers
> Adam
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