Moog Polymoog Repair - Dead/Incorrect Sounding Keys - Synthchaser #162

hi this is syn Chaser from synth chaser.com I posted a picture of this polym mogue on social media last week and a couple people requested a poog video while I don't have the time to do a whole in-depth thing on the polym mo I thought I'd make a shorter video showing a very common polym Mo Problem and the two most likely causes of this problem first I'll update you as to where I am on this poly mogue I I bought this one broken as they usually are and it powered on and the LEDs and the push buttons were working but there was no audio output I checked the power supply and I found that the plus and minus 15 volt rails were shorted and sitting at around a half a volt each so I tracked down and replaced four shorted components uh there were three shorted tanum capacitors sprinkled throughout the synthesizer and there was a shorted diode in the power supply I also recapped the power supply as the Lilac Matsushita capacitors were were leaking so now now I've got all my power rails and I'm getting some audio output from the synthesizer problem we're going to be looking at in this video is dead or incorrect sounding notes uh for example check out mode 9ine as we work our way down the [Music] keyboard so clearly something isn't right here but what exactly is going on the poly mogue is one of those divide down synthesizers like most string synthesizers that most string synthesizers are just dividing down one master oscillator on the poly mogue there are two highfrequency oscillators located here and here each of them has a top octave generator chip that generates each of the 12 musical notes in the highest octave by having two of these the poly MO is able to offer both a saw too oscillator and a pulse wave oscillator that you can tune separately in order to get a far more diverse pallet of sounds than you could with a single divided down oscillator so the top octave is generated on these two cards here and then those notes are divided down on this board back here uh which has 24 divider chips to generate the frequencies for each key on the synthesizer bear in mind that the outputs of all of those chips are still square waves even for the notes that are later going to be saw to they haven't been shaped yet so those square waves are carried off by these white and Brown connectors uh down to three motherboards like this one here filled with these little cards which are called modulator cards so this is the center motherboard and there are two others that look just like it on the motherboard you can see these rows of film capacitors in the front and the back and there's a row of modulator cards one for each key on the keyboard on each modulator card there's a polycom chip that among other things includes a ladder filter so in a nutshell the the motherboards and the modulator card shape the waveforms the square waveforms that were generated by the divider chips and they they form the sounds that you hear out of the direct output of the synthesizer so backing up to what we were hearing before I I was in mode 9ine with both saw to and square wave oscillators on if I turn on only the square wave [Music] oscillator we can hear the majority of the keys are working and sound correct if I turn only the saw too waveform on we can hear that there's a problem with a lot of the saw tooth notes also if I hop over to the strings mode you'll notice that I as I work my way down the keyboard more and more keys are [Music] out so notice how C stops here and the C below it is also out so all indicators point to several of those frequency divider chips being out in fact when Reviving a dead polym Mo almost all the time I find a handful of these chips that have failed and we can confirm this with the oscilloscope so with all the the connectors and wires down here it's a little tight to probe the actual chips which are uh wedged in between the the uh the rows of uh connectors they have these uh wires over them but what we can do is we can jam the uh scope probe uh into the connector and we can view the outputs of the dividers from there so the square wave oscillator on the left half and the saw tooth is on the right half indicated with the brown wires the higher frequencies are toward the back and they get divided down additional stages each time you move towards the front so we can start with this um this square square wave oscillator and we can see that that uh pin one is okay pin 2 pin 3 4 five six so we'll move forward one chip or one connector we can see that that is a lower frequency as it's being divided down one another stage and then we can move to the uh to the last chip and actually you can see here this one is stuck in between logic levels so it's not uh this is stuck at ground ground ground ground stuck in a negative DC so so this divider chip here is bad this is um ic7 so we've already found one bad divider chip so here the uh the very first saw to connector I found a bad divider chip at ic5 so this is pin one and and that looks okay pin two there's nothing there pin three looks okay pin four looks bad pin five looks okay and pin six looks bad and uh the reason that we're seeing uh some some uh frequencies there is because the ones that we're looking at um pins 1 three and five that are that are working are coming from the top octave chip and the ones that aren't working pins 2 4 and six are coming from the divider chip so because this is the highest divider chip I'm not going to be able to verify that the two divider chips below it are working because they'll be missing their in in puts so I'm going to have to wait on checking that until I replace this this bad ship here at ic5 so here's another bad Sawtooth divider chip at ic14 see that one stuck [Music] High stuck high on uh most of the pins uh so there's one chip below this one so I'm not going to be able to check that one out now and here's another bad Sawtooth chip at ic23 so here I've pulled the divider board and I'm going to replace the chips and of course pulling this board is a pain in the ass like pretty much everything with the poly Mo because there are 20 27 connectors you have to disconnect in order to be able to remove this board so the chips are mm 5823 chips uh which are of course obsolete so I've made some dropin Replacements the SC 5823 and I'll be installing these along with some IC sockets for the chips that I replace these chips are used in other synthesizers as well including the mogue mg1 and The Liberation so if you own any of these synthesizers you might want to pick up some of these chips from my website and make yourself a survival kit so something to point out while I'm here so one of the uh three shorted tanum capacitors was was on this board and again the only thing I've done uh so far is is replace uh shorted components and recap the power supply uh eventually because these tanm capacitors are prone to short out I'd want to go through and um change out those the other tanm capacitors so this is one of the two top octave or high frequency uh oscillator boards this is the other one and you can see the tanum capacitor there while it didn't fail as a short clearly it's let out its magic smoke so uh dodgy tanum capacitors uh plague the poly mogue as well as uh a lot of other czers so I'm relatively lucky uh these four chips were the only of the the 24 divider chips that were bad so you can see on the one one that I replaced we get the uh the square wave and then all the way down to the uh to the lowest octave we're getting our we're getting our square waves there so uh now that I verified that all the divider chips are fixed I can uh put this board back in and reconnect the uh motherboard connectors so now with the divider chips replaced and and both saw and square waveforms there in mode 9 this is sounding a lot better and we get way more notes from our uh strings [Music] mode obviously there are still some issues I mentioned there are two common causes for the problem of dead or incorrect sounding Keys we just address the first the divider chips the second most common cause of this is the shitty connector that they used for the modulator cards something as innocent as moving your polym Mo across the room can cause one or both of the oscillators to go out for a given key or can make a key sound louder and brighter than the others as we just heard when I ran down the keyboard uh in this case the modulator card can be receded and the problem may go away to do this you need to pop the top lid of the synth off as it is here you need to unscrew uh some screws from these uh upper boards which fold up on hinged standoffs then you can pull a modulator card straight up out of its socket like this and then uh and then just recede it now bear in mind if your finger grazes a neighboring card you breathe too heavily uh you push too hard and the board flexes like this or or you just look at the thing funny uh you're probably going to knock other modulator cards out of whack in the process so if I recall correctly Moog had used a connector with non-standard pitch so unfortunately the polym mo can't easily be made more reliable by replacing these problematic connectors and that's a major reason why I won't take shipped in poly MOG repairs and when I sell a poly mogue I'll want it picked up in person while divider chips and the edge card connectors are the most common causes of the problem that we looked at that I've seen many root causes for these types of problems including defective modulator cards failed film capacitors on the motherboards and dirty or damaged key contacts as always troubleshooting first and then repairing is going to give you better results than a guess and check type approach anyway I've got much more work to do on this polym Mo but I hope you found this to be an interesting Peak inside the polym mo synthesizer I'm synth Chaser from synth chaser.com thanks for watching and have a great [Music] day Chaser chaser

Channel: Synthchaser

Views: 15,130

Keywords: synthesizer, repair, moog, micromoog, minimoog, model d, polymoog, memorymoog, arp, quadra, power supply, membrane, switches, matrix, odyssey, 4023, 4034, 4035, 4075, oscillator, 2600, tonus, grey meanie, blue marvin, ttsh, pro/dgx, pro soloist, filter, vcf, avatar, analog, keyboard, restoration, vintage, synth, synthchaser, omni, omni 2, oberheim, prophet, sequential circuits

Id: m2g8lSX_S-o

Length: 12min 51sec (771 seconds)

Published: Tue Feb 20 2024