[sdiy] Jupiter 8 not booting

Antti Pitkämäki antti.s.pitkamaki at gmail.com
Wed Feb 3 09:18:20 CET 2021


I love reading these troubleshooting cases. They're like good detective
stories.

Antti

ke 3. helmik. 2021 klo 9.38 Adam Inglis (synthDIY) <synthdiy at adambaby.com>
kirjoitti:

> Thanks all for the replies. I always find it educational seeing how people
> think about troubleshooting these things.
>
> Yes I thought TR2 would have to be a likely culprit - but why did it only
> fail at start up and never during operation once started?
> The only other thing connected that I could trace to the reset line was a
> gate that selected a RAM chip - must be a start up requirement.
>
> Before I started cutting traces etc I pulled the board out and measured
> the ESR of the three electrolytic caps in that little circuit - and lo and
> behold C7 read high, and C8 was off scale (i.e. more than 99 ohms on my
> little tester). Removing C8 you could see the corrosion/damage. I replaced
> all three and the problem does indeed appear to have been fixed!
>
> Many thanks once again - and Brian, thanks for the circuit analysis. I had
> muddled out a rough idea something along those lines. Now that I think of
> it, my DX7 has a similar circuit and has given me similar trouble in the
> past.
>
> cheers
> Adam
>
> > On 2 Feb 2021, at 3:39 pm, Brian Willoughby <brianw at audiobanshee.com>
> wrote:
> >
> > 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.
> >
> > Brian
> >
> >
> > 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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> >
>
>
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