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Prophet 600

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Re: Prophet 600

Postby nickle15 » Thu Mar 25, 2021 6:20 pm

jjlonbass wrote:A short between pins 22 (D3) and 23 (I) of the 4514 could cause the bottom segment of the displays and "ARP ASSIGN" to be stuck off. Similarly a short between pins 10 (S2) and 11 (S0) or between pins 11 (S0) and 12 (Vss) would affect keypad buttons 0 to 7.
Other combinations are possible!

John

Thanks - this is very helpful!
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Re: Prophet 600

Postby nickle15 » Thu Mar 25, 2021 6:20 pm

Hugh Robjohns wrote:Its probably self-evident that these new faults will undoubtedly be the result of your difficulties with lifted tracks around the replaced chip, so lots of careful scrutiny around those tracks and the new links added to replace them will be required.

Glad to hear about the static precautions.

That sounds like a good place to start this evening. Thanks!
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Re: Prophet 600

Postby Folderol » Thu Mar 25, 2021 8:24 pm

A bit of advice with old, thin and delicate broken trackwork.
Rather than trying to rejoin it directly, find the two joints/component legs either side of the break, and run a very thin flexible wire between those, following the line of the track. That way you are using two secure anchors, and making minimal change to the electrical characteristics.

If you use rigid wire, you run the risk of more damage with board flexing.
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Re: Prophet 600

Postby nickle15 » Thu Mar 25, 2021 8:37 pm

Folderol wrote:A bit of advice with old, thin and delicate broken trackwork.
Rather than trying to rejoin it directly, find the two joints/component legs either side of the break, and run a very thin flexible wire between those, following the line of the track. That way you are using two secure anchors, and making minimal change to the electrical characteristics.

If you use rigid wire, you run the risk of more damage with board flexing.

Good tip - I appreciate it! On the three broken lines we soldered directly to the pin of the new chip on one end and then onto the trace itself on the other side of the break. We would need a lot of wire to run all the way from the new chip to the next joint but that might be worth doing. If nothing else I would feel more confident that we're not bridging anything.
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Re: Prophet 600

Postby Folderol » Thu Mar 25, 2021 9:31 pm

Hmmm. Are you soldering chips directly into the board? Unless there are space constraints I always put them in chipholders, so there's no risk of damaging the chips in the soldering process, as you don't insert them until all the work has been done.
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Re: Prophet 600

Postby nickle15 » Thu Mar 25, 2021 10:32 pm

Folderol wrote:Hmmm. Are you soldering chips directly into the board? Unless there are space constraints I always put them in chipholders, so there's no risk of damaging the chips in the soldering process, as you don't insert them until all the work has been done.

We soldered that one in, yes. We just put it back the way the original had been installed. A chip holder sounds like it would have been a good idea though. I didn’t realize you could add one wherever you wanted to - is that an option?
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Re: Prophet 600

Postby Folderol » Thu Mar 25, 2021 11:04 pm

nickle15 wrote:
Folderol wrote:Hmmm. Are you soldering chips directly into the board? Unless there are space constraints I always put them in chipholders, so there's no risk of damaging the chips in the soldering process, as you don't insert them until all the work has been done.

We soldered that one in, yes. We just put it back the way the original had been installed. A chip holder sounds like it would have been a good idea though. I didn’t realize you could add one wherever you wanted to - is that an option?
Yes, nearly always. The makers didn't do it partly to save a few pence per chip and also because it's quicker on automated systems. Of course, this isn't possible on modern surface-mounted components :(
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Re: Prophet 600

Postby nickle15 » Fri Mar 26, 2021 1:39 am

Ok we found pin 22 is shorted to ground. It’s adjacent to 23 which is supposed to be grounded. No physical evidence that they are shorted together. Pin 22 is the d3 output. We are replacing the chip again. As part of that repair we are also installing the chip socket and will use rework wire to make our jumpers run from pin to junction.

It’s possible I mishandled the chip to cause the problem. It’s also new old stock. Either way, lessons have been learned and we’ll try again!
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Re: Prophet 600

Postby nickle15 » Fri Mar 26, 2021 2:16 am

jjlonbass wrote:A short between pins 22 (D3) and 23 (I) of the 4514 could cause the bottom segment of the displays and "ARP ASSIGN" to be stuck off. Similarly a short between pins 10 (S2) and 11 (S0) or between pins 11 (S0) and 12 (Vss) would affect keypad buttons 0 to 7.
Other combinations are possible!

John

You’re a genius. Check my newest post!
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Re: Prophet 600

Postby jjlonbass » Fri Mar 26, 2021 10:07 am

Thanks nickle15; no genius involved, it's just that I've been working with electronics for a long time and have seen many of these sorts of problems.

Folderol's advice is good with regard to fitting an IC socket here, although you should fit a good quality one to avoid puzzling problems in future. Go for a "turned pin" type if you can.

If you're replacing faulty ICs, in order to avoid PCB track damage when removing, it's a good idea to cut all of the IC pins close to the body of the chip, remove the body then desolder all of the pins. If you're lucky, the pins will just drop out of their own accord with the merest touch of the soldering iron. It sounds a bit brutal, but the old chip is useless anyway.

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Re: Prophet 600

Postby jjlonbass » Fri Mar 26, 2021 10:52 am

Just a thought, the service manual shows the artwork for each of the PCBs and it can be seen that board 1 is double sided i.e. it has PCB tracks on both sides.

Double sided PCBs usually have through plated holes that join the top PCB pads to the bottom ones. However, if this board doesn't have through plated holes for all of the 4514 pins you won't be able to use an IC socket and also you'll need to solder the IC's pins to both top and bottom side pads.

You can check for this by removing the old chip and measuring the resistance between the top and bottom pad of pin 2 for U104 which only shows a blue PCB trace on the artwork, then repeat for the top and bottom pad of pin 23 which only shows a grey PCB trace. If you see roughly 0 ohms in each case then the board is through plated and you can use your IC socket.

John
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Re: Prophet 600

Postby nickle15 » Fri Mar 26, 2021 1:24 pm

jjlonbass wrote:Just a thought, the service manual shows the artwork for each of the PCBs and it can be seen that board 1 is double sided i.e. it has PCB tracks on both sides.

Double sided PCBs usually have through plated holes that join the top PCB pads to the bottom ones. However, if this board doesn't have through plated holes for all of the 4514 pins you won't be able to use an IC socket and also you'll need to solder the IC's pins to both top and bottom side pads.

You can check for this by removing the old chip and measuring the resistance between the top and bottom pad of pin 2 for U104 which only shows a blue PCB trace on the artwork, then repeat for the top and bottom pad of pin 23 which only shows a grey PCB trace. If you see roughly 0 ohms in each case then the board is through plated and you can use your IC socket.

John

Thanks again - we'll double check that when we get back in there.

We actually did cut the pins of the old chip in order to remove it and I'm not sure how the traces came loose. Unfortunately the pins didn't fall out of their own accord but we stood the board up and worked from both sides to remove the pins. It worked well!
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Re: Prophet 600

Postby Folderol » Fri Mar 26, 2021 1:59 pm

I use a pair of tweezers with the tips lined with thin cardboard to pull the cut pins out from one side while heating the other side. The cardboard doesn't chill the metal, and actually lasts quite a long time before you need to replace it.

Personally I don't like the idea of soldering both sides for chips where there is broken through plating. It puts a lot of extra heat stress on the chip. With a fine tip iron it is usually possible to get in to the pins on sockets, and it doesn't matter if you melt just the edge of the plastic. Where this really is not possible, my usual approach is the thread a single strand of fine wire through the hole and tack it down on both sides with just a tiny bit of solder. This provides a path to wick the solder through when you fix the holder in place.
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Re: Prophet 600

Postby nickle15 » Wed Apr 14, 2021 3:19 am

So, progress. We used a chip holder and got a new 4514 chip installed tonight. It took quite a while to get it all done but when we finished we fired it up and it made sound! The display was back to normal as well. However, now we don't have any sound coming from keys C2-G2. The original keys that didn't sound are working now. So even though it's not perfect it's certainly progress. We're going back in tomorrow night to check for any other damaged traces first and then go from there.

Again - thanks to all for the invaluable tips and advice through this process. It's not over yet but it was an encouraging evening to say the least!
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Re: Prophet 600

Postby Martin Walker » Wed Apr 14, 2021 2:34 pm

This is most encouraging news - you are moving forward at last! :thumbup:


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