[sdiy] Jupiter 8 not booting

Phillip Gallo philgallo at gmail.com
Wed Feb 3 23:49:05 CET 2021


Looking at you schem excerpt ...

You can pull the rail end of R1 and connect it thru a switch to the pos
rail.
Doing this you can operate the power up reset at will.

Switch closed and everything quiescent  T1 collector should be ~ 80-100mV;
T2 Collector should be close to the positive rail.
Opening the switch you should see a short spike toward ground at T2
collector and a pos. ramp on T1 collector of about a volt or so.
Closing the switch you should see the same short spike but no ramp on T1
collector.

regards,
p


On Wed, Feb 3, 2021 at 1:21 AM Antti Pitkämäki <antti.s.pitkamaki at gmail.com>
wrote:

> 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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