Welcome to Mindburner's DAC71-CSB-I & Prophet 5 Site - FAULT FINDING


 

 

Troubleshooting, Repair Information & Preventative Care


My approach to fault finding is not the most scientific. I generally replace old components with gusto as I feel it is worth doing and can help extend the life of the instrument. I am no expert:)
Handy tools include: A logic probe, dual trace scope and DVM. The prophet debug3 rom is also good for memory checks and repair although rev 3.2 onwards has a memory check built into the rom. Treat all information here as just my experience, it's not binding.

  Download the Rev 3.2 Technical Repair manual.This is a must for serious fault finding (Stanley Jungleib is the tech daddy:)
  Download the Operation Manual
  View a scan of the debug3 instruction sheet.

Below, some information on faults I have encountered.

Power Supply

In my opinion - check this is at least functional and voltages are within specification before going further..
Check the voltages on the PSU power rails with a meter. The voltage regulators can go out of spec. Ensure they are ok as this can cause lockups, especially if MIDI is installed. An extra diode can be added to one of the regulators for a bit more headroom. Google or try here for 'prophet 5 power supply mod'. Personally I would replace all the regs but again it may be overkill. The ones mounted to the back plane heat sink are a pain to test / replace. Mounting them on flexible leads can help. The legs can break off upon removal. Not a big deal though as if you’re removing them then you are probably going to replace them anyway. The tant capacitor on the 5v rail can short. It is recommended to replace this with a good quality electrolytic. Tuning and stability demand a solid power supply.

In 2015 I installed a 6V8CA TVS diode from the 5v regulator output. This was to stop a damaging voltage increase being dumped into the main circuit boards if the 5v regulator failed. In theory it should stop this.

TVS-dioderegulators mounted on flexible leads


Take care when operating near the power switch and transformer area as the heat shrinking can be dozed here and expose you to shock hazard! Been there done that!


Replace the large smoothing filter capacitors on the PSU board. In addition to smoothing the supply, these caps absorb AC surges. An older cap will not be as efficient as a new model and may let possible spikes through to damage IC’s etc. You can use slightly higher values e.g. 10000uf without a problem. You can also use units with snap in connectors, although you may have to modify the circuit board a bit to get them to fit.

Resistors

In the unlikely event that you need to replace resistors, check the tolerances as some are 1%. Resistors really need at least one leg lifted from the circuit board to check values as other components can effect the resistance measured. You can actually check a lot of them in circuit as many are duplicated for each of the voices. This ghetto way of checking can be used to spot a weird value that is different from the same resistor in an adjacent voice. Even if the values are wrong, the values should be uniform across the different voices for the most part. If you suspect a resistor, then lift it and check it properly with a meter.

Capacitors

Tantalum capacitors have a low WVDC. Definitely replace these as they only have a life span of about 12 years. I replaced mine with high quality 50v electrolytic types. Note that some of the tantalum are polarized. Some people prefer new Tantalums.


Decoupling capacitors

As logic IC’s switch, they produce ‘noise’ which should be smoothed by decoupling capacitors. These capacitors also prevent op amp IC’s from self oscillating. On the Prophet, these capacitors are the small yellow ones near most IC’s. They generally drop in effectiveness (their capacitance usually drops) and can leak frequencies into the power rails. There are lots of these 0.1uf capacitors but it’s worth replacing them  if you’re into long term survival of your synth. I used high stability ceramic ones from RS components (part no 1886237)

Sample and hold circuits

These are used in the Prophet to store the DAC values and the positions of many parameters affecting the instrument. The capacitors in these circuits can degrade causing the stored voltages to drop. This can cause problems with tuning and drift etc. Check the Sample & Hold capacitors (the silver ones, top left on the PCB4 voice board) also the associated op amp IC's. These can also stop voices functioning. Use high quality polypropylene replacements. Issues here though are more likely to be the associated opamps.

Individual Voice Issues

Voice, Envelope & filter IC's were made by SSM on the earlier revisions and Curtis on the later rev's.

A common fault is a voice not sounding.

VCO -This can be due to a dead CEM3340 oscillator chip. You can track down which voice is causing the problem using the synths controls. Drop down the volume of OSC A while listening to OSB B and visa versa. Lightly touching CEM3340’s will produce a slight distortion of the voice, showing you which voice you are on. Take care if trying this. Swapping CEM IC's can help detect the faulty one as the fault will move with the chip to a different voice. Bad chips also usually get hot. Be careful not to damage these IC's as they are very expensive to replace.

VCF - Filter controls not working on one voice can indicate a dead CEM3320 filter IC. Check the supporting resistors also. Again A quick and dirty way is to just compare resistors with a meter from different voices, but you may have to lift one leg of a suspect resistor to verify correct reading.

VCA - A voice not sounding can indicate a dead RCA CA3280E amplifier IC. You could try swapping these around to see if the problem moves with the chip. Again hot chips - not good.

S&H - A sample and hold IC can fail. Usually one of the 741 op amps, situated in the middle of the CPU board, or top left of the voice board.

 

serial number badgeprophet 5 - mega knobbage

74/4000 Logic IC's

My advice is to replace all the 74/4000 series logic IC's with modern versions which have a much smaller die (about .7 micron as opposed to 6 micron fabrication process used in the 1970's) and are vastly more reliable.
6um 4000 series chips can suffer from 'metal migration', a slow reaction where ions from the metallic gate of each MOSFET in the chip flow down to the substrate (electrical ground) of the chip over time, resulting in short circuit AKA dead chip.

Use turned pin sockets (I used gold plated ones) when replacing IC's as it's much easier to remove them in the future and they give trouble free use. It's worth spending the extra here. Check before ordering as some will have 14 pins and some 16. Mouser/RS/Digikey etc are good for these.

CPU/Clock

Another fault is the Zilog Z80 CPU not starting. This can be due to faulty or shorted decoupling capacitors on PCB3. Most of these were outside their tolerance on my P5. Also check chip U309-13 to ensure the pin goes high (+5v) on start. If not the CPU will be held in reset and the P5 will not start it's tuning routine. You can also try replacing the Z80 with a low power CMOS variant like a TMPZ84C00AP. The Z80 CPU itself is generally very reliable and fairly easy to get hold of if needed.

The clock circuit is usually very reliable but if a replacement oscillator crystal is needed, more or less any XO-12C 5Mhz 4 pin crystal will do. No need to buy a 'special' prophet one.

My Repairs

When I got the keyboard sometime before 2000 it was in a bit of a state. The tuning was all over the place and one of the voices was dead on both oscillators. This turned out to be a faulty op amp and a dodgy DAC71-CSB-I. The volume pot spindle was broken and the tune button inoperable.

In 2006 I finished a major overhaul of the instrument, including rebuilding the PSU. I also replaced all the keyboard bushings which was time consuming but improved the playability immensely.


In 2008 I had not used the prophet for a while and found that OscB2 was not sounding. I Managed to track the problem to a faulty resistor on the CA3280 amp chip circuit. This fault was a bit of a pain to find but very cheap to fix.

In 2009 I decided to take off all the wood panels and sand them down as they were covered in a nasty lacquer which had started to come off. The result was superb and really improved the look of the instrument.

Midi Install/GATE

My prophet now has a factory spec midi kit - which improves it's usefulness greatly. This one is slightly different as it plugs directly into the old USART socket rather than using a ribbon cable

 

Prophet 5 midi board.

A single note playing continually no matter what key is pressed can indicate a fault with one of the 4174 chips in the GATE IN buffer circuit. Near the battery. I can't remember which one exactly off hand. Can be usually blown because of incorrect voltage applied through the rear jacks.

General Issues

A problem can occur with the inter connectors between PCB 3 & 4. A molex connector with KK crimps. These can break and cause poor contact, causing lots of heartache. I have a few of these connectors spare so contact me if you need one, get in contact. Use a continuity tester to check the ribbon cables for shorts especially the 60 pin inter connector ribbon cable. Check all the circuit boards for damaged/lifted tracks and dry solder joints. Replace the internal battery at least every 5 years. Remember to back up all internal programs before though. Control potentiometers can be opened carefully and cleaned. Replace if necessary. Calibrate as per the service manual once in good operational condition.

In 2015 My Prophet turned on ok but there was no sound. After a head scratch I noticed this destroyed resistor! R319 a 10ohm was FUBAR. This was a strange issue as the battery checked out ok at 3v plus it really would have taken something more dramatic than a battery issue to cause this thermal damage. I checked the 7805 regulator and it was ok. Anyway I replaced the resistor with a quality 1/2 W version. This area is a bit of a mess and I shall tidy the battery connections when the cell is due for replacement, rather than lose my patches at the moment.