[sdiy] Prophet 5 rev2 conundrum

brianw brianw at audiobanshee.com
Wed May 3 09:24:42 CEST 2023

This is why a 'scope with recording abilities would be helpful. I suspect that the tuning operation is starting, but a fault is found so quickly that the tuning subroutine quits immediately. With a 'scope, you'd see the signals on the multiplexer chips and the CV outputs change when the tuning starts, and there'd probably be some clue as to why it stops faster than a human can observe.

Nathan's suggestion to look at the CV output during tuning seems like a good next step. I realize that tuning "doesn't start" - but I suspect it's actually trying to start, running a million instructions per second, and just stops on the first problem. i.e. Although the algorithm will hang on a bad oscillator, it might not hang if some other problem is detected.

The fact that everything other than the tuning function is working tells me that the CPU and firmware are all running correctly - unless you have a bad EPROM that only has bad data in the tuning subroutine.

If anyone has disassembled the P5R2 firmware, that might help. I've done this for the Matrix-12, but it's a tedious task. Understanding what tuning does might help discover what's wrong with this voice board.

I was wondering if the tuning button could be shorting out something, but since the power-up tuning isn't taking the usual 5 to 7 seconds, that would mean it isn't the button.


On May 2, 2023, at 11:45 PM, g m montalbano wrote:
> These are all good points.  However ---
> All oscillators are sounding; all voices are playing; the two 4051 mux ICs are receiving their correct timing signals; all signal paths on the voice board are functioning.  The tune procedure is just not starting.
> On the rev2, if one or more of the oscillators is bad, the tune procedure will hang up; but in this case, the procedure is not starting at all.
> The rev2 CPU board will boot without the voice board connected to it; it comes up immediately, without tune -- this is the effect I'm seeing now, even though the voice board IS connected & otherwise functioning correctly.
> On Tue, May 2, 2023 at 11:22 PM NT wrote:
>> On the first paragraph of page 2-20 in the service manual it says if any oscillator is missing from the tuning mux it will abort. Greg - you haven't mentioned it but do you get signal from any/all voices? Or presumably you the SSMs are tested and you're still hitting this issue?
>> If the SSMs, mux, comparator, and path to connectors are all good, i'd start looking earlier than the VCOs. paragraph 1 of 2-20 says that auto tune engages all S&Hs that control tuning, so I would check each VCO to make sure they're getting set to the required 300hz at tune start. Even if they're working OK outside of the auto tune procedure, something could be causing one to go way out of range during tune, so all 4051s and opamps before the VCOs should be looked at. 
>> Service manual for everyone's reference: http://www.synfo.nl/servicemanuals/Sequential/SEQUENTIAL_PROPHET-5-REV2_SERVICE_MANUAL.pdf
>> Nathan
>> On Wed, May 3, 2023 at 1:50 AM Adam wrote:
>>> Is the CPU jumping to the end of the process prematurely? What determines the end of the process? Does the CPU receive a signal form the voice board?
>>> A
>>> On 3 May 2023, at 1:50 pm, g m montalbano wrote:
>>>> I've been looking for that -- the problem here is that the tune process won't even start.  Usually, on powerup or pressing Tune the front panel goes dark, the CPU section enables the 8253 programmable interval timer and the individual oscillator inputs to the two multiplexer chips are read through the LM311 comparator back to the CPU board.  But NONE of that is happening. Pressing Tune causes a split-second blink of the front panel, and nothing else.
>>>> ~G
>>>> On Tue, May 2, 2023 at 7:49 PM brian wrote:
>>>>> The Prophet 5 Rev 2 uses a multiplexer chip to connect only one oscillator at a time to the comparator input. This should effectively remove all other oscillators from the tuning. However, it's theoretically possible that there could be some bleed-through from other oscillators if the multiplexer doesn't have perfect isolation. i.e. crosstalk.
>>>>> Brian
>>>>> > On Tue, 2 May 2023 16:20:38 -0700 g m montalbano wrote:
>>>>> >> The rev2 P5 is problematic in that each oscillator & filter has a number of
>>>>> >> opamp, CMOS switch and resistor inputs that can cause wild differences
>>>>> >> between voices ( that fabled "analog warmth" that some people think makes
>>>>> >> the rev2 superior to the rev3, but for service purposes is a pain in the
>>>>> >> balls).
>>>>> >> Will attempt to swap out all oscillator chips and monitor all CMOS and
>>>>> >> opamps to see if anything is happening there.
>>>>> >> 
>>>>> >> ~G
>>>>> >> 
>>>>> >>> On 2023-05-02 2:11 p.m., g m montalbano wrote:
>>>>> >>>> I've been working on these for twenty years, and thought I'd seen
>>>>> >>>> pretty much everything that could go wrong with them.
>>>>> >>>> Now, I have one that works perfectly except for the fact that it will
>>>>> >>>> not go through the tune procedure, either at powerup or when the Tune
>>>>> >>>> button is pressed.
>>>>> >>>> 
>>>>> >>>> The confusing part is that the fault has been narrowed down to the
>>>>> >>>> voice board: if I swap in a working voice board, the unit functions
>>>>> >>>> correctly; if I install this voice board in another unit, the problem
>>>>> >>>> follows.
>>>>> >>>> 
>>>>> >>>> As far as I can see, the ONLY parts on the voice board that relate to
>>>>> >>>> tuning are the two 4051 multiplexers (U423 and U428) and the LM311
>>>>> >>>> comparator (and its associated resistors & caps). All of these
>>>>> >>>> components have been replaced; traces and resistors have been checked
>>>>> >>>> &  measured. All traces to the 60 pin connector between the CPU and
>>>>> >>>> Voice boards are good, as is the 60 pin cable.
>>>>> >>>> EVERYTHING is there, and everything works correctly except for Tune.
>>>>> >>>> 
>>>>> >>>> I've been hitting my head against this for a couple of weeks, and
>>>>> >>>> would appreciate any suggestions as to where to look.
>>>>> >>>> 
>>>>> >>>> Thanks.
>>>>> >>>> ~GMM

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