Prophet 5 with a missing power supply...

[gilwe]

So I started the refurbishing of the Prophet 5 I've been given,

which is missing the bottom cover and power supply.

 

After cleaning it I realise that the unit look quite good, not so very well kept but still not bad.

 

Missing/problematic parts:

 

- Broken Filter Env pot and missing black knob. Need and additional black knob to replace a knob which is missing the top metal plate.

 

- Bad keyboard bushings creating a severe "teething" probelm - ordered two sets of Pratt-Read bushings, one for my OBXa and a second for the P5. Keyboard look in quite good condition, once I replace the bushings of course

 

- One missing CEM3340

 

- Looks like a missing CA3280 as well...

 

- The bottom panel including the PS transformer, PCB and harness cables

 

I will start by restoring the keyboard, replacing all electrolytes and tantalums - quite sure some of the tantalums are blown.

 

Second phase will be building a power supply based on the Rev. 3.2 schematics and connecting to the main board, of alternatively trying to get these parts somewhere

 

Third phase will be a debug of the unit to full functionality.

 

Questions -

 

1. Does anyone have any of those parts for sale/trade. Most important is the PS unit, even a non-working PCB will do !

 

2. In case I don't find a PS, my intention is to build one on a prefboard based on the Rev. 3.2 schematics. Do you think this could work ?

 

3. Can just any 22v-24v AC transformer be used for the power supply, or do I need something special for the P5 to get working ok ? Any rare parts in the P.S ?

 

4. Are the jacks essensial for the Prophet circuit to work ?

 

Any help will be appreciated !

 

 

 

[80zclubkid]

Hi: Check ebay for a prophet 5 in very poor non working cond. to swap parts for under $300. Or call all the vintage synth repair shops in the world for these rare expensive parts. Or spend over $100 to ship to Wes aka FlatTop to refurbish. It might be cheaper overall to buy a new Prophet 08. But good luck, if you can get it fixed yourself. cheers:

[gilwe]

The thing that most interests me at this point is whether the unit will be able to function with only a powersupply attached to it, no jacks and harness....

Also, what type of transformer should I use and does this particular unit have the type of ROM for which you should disable two of the voltage rails.... ?

[80zclubkid]

Probabley not. You might need a repair manual with schematics /w part #s. If you know anyone that has a prophet 5, maybe they could get this pwr supply info, by looking inside their synth. or maybe not.

[80zclubkid]

Hi: Do whatever you must to restore it. Keep us informed on your progress & have a good fire extinguisher near by, just in case. cheers:

[ChromaLord]

On the other hand it probably sounds alot better without the power supply.

[OPEN OCEAN]

good luck man i will also have a look here...

[gilwe]

Here we go -

 

Powersupply is built, working almost fine. All +15v, -15v, +5v, 5.6 voltage rail show the correct voltage.

 

Problem:

 

I used a transformer of the voltage and amperage as appears on the schematics (36VCT, 2.8A). I chose a 3.2A, 2x18V transformer, although I wasn't 100% about it's being compatible, as the output unregulated voltages appear as 22v on the schematics.

 

Now I end up reading 36v on each of the +22v, -22v and +V rails.... Am I using the wrong type of transformer after all ?...

 

I heard suggestions about the 22 rails normally feature higher voltage than 22 and that this is quite normal. I recall a similar issue on my MKS70 (unregulated voltage higher than on diagram).

 

Any help will be greatly appreciated !

 

 

BTW, interesting point:

 

With those tiny heat sinks, the regulators almost don't dissipate any heat at all !

Is that because of the modern parts much more effeicient as compared to the 70s stuff ?

The P5 was notorious about its powersupply heating problems....

 

 

 

 

 

 

[flattop]

My assumption is that you have not placed the PS under the load of operating all the PCBs of the P5. You can not determine the heat generating capacity of the PS until then.

[mate_stubb]

 

My assumption is that you have not placed the PS under the load of operating all the PCBs of the P5. You can not determine the heat generating capacity of the PS until then.

 

 

Bingo. When those ancient 74LS series TTL chips start sucking down the milliamps, those little heat sinks will get right toasty.

[gilwe]

Guys,

 

what about the 36v instead of 22v ?.... My research shows that normal voltage on this rail while powersupply is not connected to circuit, should be about 28v...

 

flattop, on our last P5 discussion you suggested that I use 22v-24v "each-side" transformer.

Did you mean 2x12v center tap ?

 

Should I actually use a 2x12v to be able to get 24v ? is 36v too high any way ?

 

Also, as far as I understand, there's no need to connect the +22 and -22 rails at all, unless I set up the "Interface" ports. Does this make any sense ?

 

Thanks for the help !

[flattop]

Yes, the usual voltage is in the 28V range, +/- a couple. I cant, off the cuff, remember what they are used for. You need to take a look at the schematic to make sure. There are a few other regulators mounted on the PC boards. I can't remember where they get their source power from, though I think it is usually from the + or - 15V, if I remember correctly.

 

If you can't find any other use for it other than the digital/analog ports, then no worries. It is just that Sequential always had that type of voltage going to the digital board, even prior to the 3.2 update. You need to determine why.

[gilwe]

OK, thanks. I won't worry about what the unregulated rails are used for, as long as I have a power supply that provides correct voltages...

 

So what about the transformer type, should I use a 2x12v (or any other type) after all ? Or am I doing right and safe using the current one (2x18v), ending up having 36v on the unregulated rails ?

 

Thanks man.

[flattop]

You need a transformer that provides enough voltage for your regulators. Generally, going high is not as bad as going low. Generally you want a buffer, since you don't reliably know what is coming out of the wall. When I want reliable +/- 15V, I'd probably want to be in the 20-30V territory supplying those regulators.

 

You could always knock the voltage down if you are uncomfortable with it.

[gilwe]

I don't feel uncomfortable with the regulated voltages. They look fine.

What bothers me is whether it's ok to have 36v instead of the what-seems-to-be-normal 28v on the 22 rails, without causing any harm to the instrument

 

EDIT:

 

Meanwhile I replaced the 2200uf/25v capacitor with a 4700uf/40v (I assume it's ok to replace with a high capacitance value as it seems to be a filter cap). I did that because the cap voltage rating seemed too low (25v when over 30v are measured) and ended up having 33.5v at the 22 rails... Seems better isn't it ?

 

Bottom line - do you think I can stay and go on with this transformer and voltage output values, or do you recommend using something else there ?

 

Cheers!

 

[flattop]

You can always lift the pin 10 leg on the 74C02 and power it up with what you have and see what happens. I suspect that if it can handle the 28 volts (well whatever it is knocked down by the 75K resistor), it can handle 36 volts. Again, you could always knock it down yourself to the 28 volts.

[grimaila]

This portion of the circuit is a simple power detector. Note that the +22V is voltage divided by the ratio of (15K/(15K+75K))*22V=4.67V before being connected to U309 which will see it as a digital high. If the supply is 36V, this will increase to 6V. You are OK as long as the voltage connected to pin 14 of U309 is within the maximum supply voltage of U309. It appears that D303 will result in approximately 5V being applied to pin 14 of U309 in the absence of anything else driving Vm. Does Vm get driven by anything else or appear anywhere else in the schematic?

 

MikeG

EE

[gilwe]

Thanks guys, disconnecting pin 10 (and then I guess 13) and checking the voltages seems to be a good idea.

 

Meanwhile I got the tantalum caps and will start by replacing all tantalums/electrolytes.

 

There's somehing strange that I've noticed -

While all polarized caps appearing on the Service Manual's part list are tantalums, and I couldn't find even one standard electrolyte in the list,

I can see a few standard electrolytes on the main board PCB, some of them look like there were placed as replacements... Have the originals blown out in the past ? This could imply of a damage that might have happened to the board in the past, and that has been fixed.

 

Another strange thing that I found is a combination of 3 resistors soldered in series as one "package", and which are placed where the board says "P-047" (just near the battery). The "P-047" writing is covered by carboon, which indicated that something has fried at that spot in the past... The strange thing is that afaik, "P" indicates "Port" in that machine, so why would a combined resistor will be used at that place ?

 

Anyway, will update soon !

 

Thanks all !

[flattop]

There is a resistor that is used as a fuse in that area. If that was blown at some point in time, who knows what you are up against.

 

Yes, it is common for people to replace the tants with electrolytics. Usually because they are about a 1/10th the price.

[gilwe]

Thanks flattop,

 

do you mean a fuse-resistor ?...

 

Another interesting point btw, the battery (which is not the original one) is from 1986, what indicates that the synth has been like that for at least 10 years or so. It also has one CEM3340 missing, a missing CA3280 and I've found a CEM3340 that has been misplaced in it's socket...

 

I'm going to replace all tantalum tonight and connect it up !

 

Cheers !

[flattop]

It is a resistor whose purpose within context of the circuit is a fuse. Typically 1 to 10 ohms.

[ElectricPuppy]

re: higher than expected voltage on transformer: Are you reading peak, or RMS? I think transform windings are rated at their RMS voltage, not peak, which will be higher.

 

Higher-voltage caps is OK, but beware of going nuts and replacing lots of high-value caps with even higher ones. Too many will place a higher start-up load on the power supply.

[gilwe]

Thanks for the advice puppy!

 

flattop, I think I know what was happening there... as I said, the part list only shows tantalum polarized cap, no standard electrolytes.

 

While replacing all of them tonight, I found that the 10uf cap which is connected between the +15v and ground, and which is placed just near the resistor fuse, has been replaced. My guess is that there was a tantalum cap at that place which has shorted out and caused a short over the +15v rail

 

On the voice board, the two main filtering 10uf caps (one for the +15v rail and the other for -15v) have been replaced at some point, too.

 

My next step would be mounting the regulators to the big heat sink I got, and that I'm about to use finally (mounted to the rear panel), making the wiring between the transformer and the main board (including the audio out of course), and of course testing U309 voltages before letting the circuit absorb the 36v from the transformer

 

A small question -

 

Looking at the diagram, I found that most of the rear panel in/out jacks are "open" when no plug is inserted into them (i.e. at there's no short exist between any of the jack's legs).

 

However, I did find that the story is a bit different for the Sequencer Gate In and Amp CV In jacks.

 

 

 

My intention at this point is to only have an Audio Out jack, while leaving having all other in/out unimplemented (no jacks). Therefore I need to create

hard wiring between interface pins where needed

 

So if I understand correctly, for the Gate In port, I should hard-wire pin 5 (of J11) with the ground and leave pin 1 unconnected,

while for the AMP In I should hard-wire pins 14 and 19 ?...

 

 

A second question if I may - Should I expect the synth to play even though it is missing one CEM3340 and one CA3280 ICs ?

 

Thanks for the generous help !

[flattop]

Yes, that appears to be correct.

 

Yes, the instrument should make sound.

[gilwe]

Caps replaced, powersupply connected. No surprises really. Synth doesn't power up.

 

Checking volates I see something strange. All power rails are out of range:

 

-15v -> -9v

+15v -> +1v

+5.6v -> +2v

+V (+22) -> +3.5v

 

No hot ICs, smoke or fire. Just dead voltages.

 

I took the main board out of the synth and checked again, with only this board and the powersupply connected to it. Again, same volatges.

 

I started to take the removable ICs out, one at a time, to see if things get better (I thought there's a chance that one of them shorts). No change.

Connected the +22v and -22v power leads to the PCB, no change (I didn't use them at first).

 

Well, I guess there's a fried/bad IC taking all voltages down ??

Any other idea ? I've read that old coupling caps can cause the synth not to power up, but could these make voltages go bad ?