At first, I was hesitant at first to remove all of the modifications as I was unsure exactly what they were all intended for and due to the intrusive and destructive nature of how they were implemented, I wanted to be cautious that I could make calculated improvements to its performance and not make anything worse.
What where these used for? Perhaps it was for an external EWI (Electronic Wind Instrument) interface? Seems possible as the \’Source Mix\’ and Glide pots had been remapped to \’WIND\’ and \’ADSR\’ along with a toggle to disable the keyboard, a toggle for the mod wheel destinations and what seemed to be a 1/5 voice toggle.
Before starting to remove the modifications and get the synth back to its baseline configuration, I documented the layout of the modifications, including where the mod circuits were tied into the Prophet\’s PCBs, what components had been added to the PCBs and where components had been removed or altered. I took several pictures of the PCBs and also familiarized myself with the Prophet 5 schematics and board layouts. After several hours of background research, I was able to clearly identify where the mods had been jumpered into the PCBs, where traces were cut and parts removed or added. I labeled all of these locations with markers where possible so that in the event I needed revisit or reconnect a jumper to troubleshoot later I would not lose track of the changes I was making to get the Prophet back to its original configuration.
Modification Board Removal and Cleanup
One by one, I removed the mod boards and their respective jumpers to the Prophet PCBs. Once I got the mass of spaghetti wiring and veroboards out of the way, I needed to remove a few sets of resistor networks that were soldered directly to the board with buss wires. Once this mess was all cleared away, I was able to start the tedious task of repairing the cut traces. To do this, I used my microscope following the labeled locations I made previously to repair the traces and then verified the new jumpers by testing for continuity on the circuit. All in all there were about 20 cut traces across three of the four Prophet PCBs and this was quite time consuming, but also key to get done correctly to future-proof this part of the repair.
With the mods removed and the cut traces repaired, we were making good progress so far. The synth was now generally operable with most of its functions on the control panel working as expected. However, I was still encountering a few functional issues that needed a deeper dive. First, one of the voices was not working. Second, when trying to load the factory patches via \”load from tape\” (aka MP3 file on my iPhone), it was working intermittently and sometimes would not enter the load ready status. Lastly, the glide control was non functional.
After further review of the Prophet 5\’s schematic and a second, deeper visual inspection of the boards, I found two additional cut traces, (1) at pin 3 (clock in) on U330, a CD4013 logic flip-flop CMOS IC which was affecting the \”load from tape\” write to NVRAM function and (2) a CD4051 on PCB2 disconnecting the glide pot from the multiplexer. The cut traces were easy enough to fix, but after some additional thought, I decided to replace all of the CD4051 mux/demux CMOS ICs as these RCA versions of this era are prone to failure. I determined that while I was at it and had the synth pulled apart, I might as well socket and replace all 10 of these. So, with my Hakko FR-301 desoldering tool, I removed the RCA CMOS ICs and then got new replacement CD4051 ICs socketed. I have read other Prophet 5 repair posts where techs replaced all of the 4000 series CMOS ICs. I think this comes down to a matter of preference, effort and how much “future proofing” one wants to implement.
Once these issues were fixed, I moved on to replacing all of the tantalum capacitors, the polystyrene caps in the sample & hold and demux sections and the one electrolytic capacitors from the voice board. The question on what capacitors to replace on vintage synths and audio gear is debated and comes down to the specifics of the application of a particular capacitor, the type of capacitor (tantalum, aluminum electrolytic, polystyrene, PP/film, ceramic) as well as their age. If you are interested to dig deeper into this topic, below are a couple of links that are worth reading, though there are many others:
For the Prophet 5, we need to replace all of the tantalums that are connected to power rails. Though none on this synth have failed, they are known to fail with age and when they fail, they fail short which could cause damage to ICs and other components. I used high quality (Nichicon, Panasonic), low ESR electrolytics for the polarized tantalums on the power rails, and audio-grade electrolytics for the tantalums in the voice circuit.
The polystyrene capacitors in the S&H section are also known to lose their capacitance and fail with age. They are working ok at the moment, but they are quick to replace with the desoldering tool and the replacement film caps are inexpensive.
Lastly, I replaced the one axial electrolytic cap in the last stage of the audio output section. They tend to dry up with age, so it\’s simple enough to replace this as well.
Now that PCBs 1,2,3,4 and the power supply PCB 5 have been cleaned up and are working, its time to move on to more of the mechanical restoration work needed.