Aaron Wright

Upon further inspection and getting seriously sidetracked, I found that the CZ-5000's power supply schematic (pictured) doesn't actually match the CT-6500: The bottom-leftmost 100 ohm 1W resistor doesn't exist on the CT-6500, and the 100 ohm resistor to the right of it is actually 150 ohms. From what I can tell, the 4.7 ohm resistor to the right of those transistors is where the 100 ohm 1W resistor should be, which doesn't seem right. The transistors also don't match up: T3 should be a 2SA933 PNP transistor, and on my board it's labeled T10. I checked the voltages between the legs of that transistor with and without load, and it didn't approach any limits spelled out in its datasheet. It matches more closely with the CZ-1's PSU, but there are still differences: I got sidetracked trying to understand the parenthesized voltages labeled on this diagram, and also trying to find a higher quality scan of the CZ-1 service manual, because some pages (like this one) are practically illegible. Here's what I have going on, which I know isn't very helpful without the traces underneath, but bear with me: The 100 ohm transistor at the top is not relevant to the -15V circuit. The voltage going to where the 1W 100 ohm transistor would have been is about 25V and comes straight from the rectifier in the top left, which drops to around 24.3V when the stereo chorus board is connected. In that poorly labeled photo, the two zener diodes are directly above and to the right of the 150 ohm resistor. The bottom one is the -18V zener, the one above is -15V. The capacitors on the right with the green marks on top are ones I replaced the other day. The two big ones on the left were replaced 2 years ago. The two in the middle have not been replaced yet. You can see above the middle two capacitors that there's another RD18JSB -18V zener diode, which doesn't show up anywhere on the CZ-5000's schematic or the CZ-1's schematic. I'm probably going to take it upon myself to make a CT-6500 PSU schematic (at least for the relevant -15V rail) in order to better understand what I'm dealing with. I didn't get much time to look at this today, but I'll do some more in-depth testing tomorrow.

I also want to add, the biggest issue with this keyboard right off the bat was that the mixer/volume control PCB was smashed across the master volume slider. I wonder if that could have caused a short which overwhelmed the PSU (maybe the zener diode?), or overwhelmed something on the stereo chorus board. The PSU has two zener diodes Side note: there is a mismatch between the CZ-5000 and the CT-6500 in the power supply schematics: One of the zener diodes does labeled RD16E is actually an RD15E on my board, and that's the one I suspect is an issue. However, I measured the resistance across them both ways and found it to be consistent between the functional RD18E (which did measure at a constant -18ish volts under load) and the seemingly less functional RD15E (raising to -2V as discussed). That is, it had higher resistance one way than the other. This test may have been too superficial, as the positive ends of these diodes are connected and I didn't measure their resistance under a load. I still have much to learn about electronics and I'm not even sure how a zener diode under load could have its voltage drop like that. In theory, if it fails it should fail short or fail open, and if excess current is drawn beyond its rating then it should dissipate that as heat to maintain its rated voltage, so this alleged failure mode is kind of bizarre to me! Also, ,I went ahead and replaced the two 470uf 16V capacitors stabilizing the +15V and -15V rails on the PSU with some much newer ones, which did not change anything at all. There is also one 200uF capacitor and one 220uF capacitor on the relevant lines that I haven't replaced yet, but I kind of doubt those are the problem. I will continue to poke around the diodes and the stereo chorus board and further investigate my "maybe the smashed volume sliders are what ruined everything" theory in the coming days. My replacement PNP transistor for the amp board should arrive on Monday, which will allow me to resume testing the amp board in a more representative state.

I did exactly all of these steps yesterday, but I was anticipating doing more work today on a write-up before I got sidetracked. The main board draws 30mA on the -15V line (raising the line from -15V to -12.8V, which still works), and the stereo chorus board draws an additional 25mA on -15V, which somehow brings it up to -2V. I checked every chip, none seem warm. I'm partly suspecting a bad zener diode on the power supply.

I was doing some research into the DAC offset voltage. For some reason, no matter what I did, I couldn't get mine to change from ~1.3V, when it's supposed to be ~3mV. I noticed that my replacement TL082 chips actually have the part number "TL082CD", whereas the original chips have the part number "TL082CP" I tried swapping an old chip in place of the new one by the DAC (yay for IC sockets), and lo and behold, it brought the DAC offset voltage down to 3mV. However, this had the downside of lowering the volume to near-unlistenable levels. What's more, I could still hear the fuzzing that happens when multiple keys are handled by the same 933 sound generation chip, so it didn't even solve my main issue. I tested with several TL082CD and TL082CP chips and found the results to be consistent (TL082CP is quieter but brings the DAC offset voltage to 3mV). I decided to order a new BA9221 DAC chip, since that seemed to be the issue. I decided to test the troublesome TL082-3's outputs per the service manual, this time seeing if having the amp board completely disconnected made any difference (it didn't): No dice. Lots of noise on the DAC output, and a strangely quantized waveform on the expander circuit (I suppose because it's trying to expand the noise...) Then, to narrow things down even more, I decided to disconnect the stereo chorus board, and viola! I'm seeing normal waveforms for the first time! Now I have to cancel my BA9221 order. (I also tried swapping the newer TL082CD chip in there and it produces the same results) This made me curious: what combination of things would cause the DAC waveform to collapse? I found the following connections would do it: main board to stereo chorus board power supply board to amp/midi board (amplifier section) But why? Well, it turns out one of those reasons was that I broke the leg off a PNP transistor on the amp board while trying to troubleshoot the volume control circuit, and my temporary fix of pulling the 13600 op amp DC bias to ground to get sound out of the speakers was apparently drawing too much current for the power supply to handle. Disconnecting my "fix" now allows the amp board to be plugged in without affecting the DAC waveform (I have ordered an actual replacement transistor, don't worry). I started poking around at the PSU voltages, measuring them before and after plugging in various connectors. I found trouble only on the -15V (-Vcc) line. Here are my findings: Nothing connected to power supply: -16V (no waveform, obviously) Only main board connected: -15V (good) Only amp board connected: -15.3V (good) Main board and amp board connected: -10.5V (good) Main board and stereo chorus board connected: -2V (bad) Main board, stereo chorus board, and amp board connected: -2V (still bad) Connecting or disconnecting other wires to the amp board or stereo chorus board made no difference to voltage. So with that in mind, I have to suspect either that the power supply has some issue with the -15V line (zener diode or capacitor issues maybe?), and/or there's issues with the stereo chorus board, or with the main board somewhere near the connector to the stereo chorus board. The good news is, with those nice waveforms, I was able to tap in to pin 1 of TL082-3 with some tiny speakers to hear what this keyboard is supposed to sound like, for the very first time. Some other things I learned: The overdriven sound when pressing multiple keys is still there on the DAC output, but is completely gone after the expander circuit. While listening to the clean sound, I was able to hear the hissing/fuzzing of an uncalibrated DAC offset voltage, and so I adjusted that until it was completely gone. I'm pretty sure that's what the fuzzing you were talking about was @pianokeyjoe, which is much less serious than what I was experiencing. The schematics for the CZ-5000 show the two outputs of the topmost op-amp on the main board (TL082-2) as each passing through a 6.8k resistor before being tied directly together and sent to the stereo chorus board on the red wire. In reality, the two outputs (which somehow have been separated into sounds from each of the two LSI 933 sound synthesis chips) pass through 3.3k resistors and then continue separately on one red and one white wire to the stereo chorus board. The schematics for the CZ-1 show the TL082's two outputs continuing independently to the stereo chorus board, but on completely different wires (side note: most of the CZ-1 service manuals I've found are almost illegible): I think the CT-6500's stereo chorus board and its interface probably don't match either the CZ-5000 or the CZ-1's, and therefore are undocumented. That sucks! I've only found a few things on the stereo chorus board that even tap into the -15V line, namely its several BA4558 8-pin op-amps (very similar to the TL082, I wonder why they picked a different part?), but I'll look for more next time. What's strange is that the stereo chorus board did work despite its negative effect on the DAC on the main board. This suggests that there's either some kind of passive component failure involving the stereo chorus board or that it really is a case of the power supply choking under load on the -15V line. That's just my guess, though. Maybe some active components could have a failure mode where they just draw too much current but don't blow themselves up, but that seems unlikely since I've tried feeling every IC after powering off to see if any were unusually warm (none were). That's all for today. I'm feeling very close to solving this thing.

No, the DAC is on the main board, getting fed directly from the music synthesis (933) chips. I wonder if it's possibly power supply related, like maybe your amp board was doing something to the power supply that was affecting the sound synthesis. Just an idea. It could also be that yours was overdriven by some offset voltage shenanigans on your amp board and it just happened to have a similar effect. I think the deciding factor as to whether you had the same problem as me would be if it was possible at all to play two keys at once and have them sound "normal" (which I can do if I start pressing the second key during a different one-second interval from the one the first key started on), or if it would always be fuzzed no matter when the keys were pressed. If the latter, we may be having different issues. Or maybe I'm having multiple, overlapping issues. I feel like I'm getting close, at least.

I apologize for the triple post, since I keep not expecting to find something new before I have work in 2 hours, but I might have narrowed the problem down even further. I can't read any waveforms from the DAC output at all, measuring at op amp TL082-3 pin 7. Here's a comparison of the test procedure in the manual compared to my actual readings with those same settings, as I hold down a C4 flute: I even replaced the TL083 with a fresh one (and socketed it just in case), which didn't change anything, so I suspect the BA9221 is misbehaving. Thoughts?

A new lead, perhaps? On the topic of the weird 0.5hz alternating of TL082-2's outputs depending on when a key is pressed, I've found that having sounds from two keys outputting on both channels actually sounds good, but when they're on the same channel, it sounds bad and overdriven/fuzzy. Here I'm measuring pins 1 and 7 (the output pins) on TH082-2 on the main board, right before the output to the stereo chorus board. Figure 1: the two keys were pressed on different cycles and are thus on different outputs, and this sounds correct coming out the speakers Figure 2: the two keys are pressed during the same cycle and are combined on one output, which sounds bad and overdriven/fuzzy. In theory, the sound produced by holding two keys on the keyboard shouldn't alternate between sounding good and sounding bad depending on when the keys are pressed in some arbitrary 0.5hz cycle, so I think I must be getting close to the real problem. I will poke around the sample/hold circuit and the expander circuit before it some more and see what I can find.

Today's progress: I replaced the CR2032 battery with a modern holder and a new CR2032. This didn't change anything, but at least it's up to date now. I checked every single resistor on the amp board, and every single one is within spec (I had to remove a few which weren't measuring properly in-circuit, but which read correctly when disconnected) I started checking the test points in the CZ-5000's manual with my oscilloscope, and did not find what I expected to find. EDIT: turns out the CZ-5000's service manual uses a slightly different part number for the 933 chips and has a different pinout for them. The CZ-1's manual is much closer to the CT-6500's and has the correct part number (UPD933AC), and it was because of this that my tests came out wrong. Unluckily for me, the SH signal on pin 23 of the CZ-5000's UPD933 looks very similar to the data from pin 23 of the CT-6500/CZ-1's UPD933AC, so I got misled. The correct pin for the SH signal on the 933AC is pin 13. Here's the test procedure I tried, followed by my results on pin 23 (per CZ-5000's service manual's instructions), followed by the correct results on pin 13 (the equivalent SH pin on the 933AC used in the CZ-1 and CT-6500). Just putting this here in case some poor soul runs into the same problem as me in the future. I will do more testing, but I'm not sure what to suspect since every resistor on the amp board seems to be within spec, the amp board doesn't seem to have any dedicated filtering sections as documented in the service manual, and the DC bias of the 13600 op amp seems like it can just be tied to ground to bypass the volume control, and I'm not sure what other DC bias exists on the amp board. I kind of suspect the expander circuit on the main board or the stereo chorus board could be places worth investigating. Surely the fuzzing sound needs to be subdued before effects can be properly applied to it, right? Doesn't the contracted waveform look overdriven?

Since someone else posted in this dead thread a year later, it seems like it's fair game for me to do so as well. After a tumultuous two years, I finally made enough room on a table to have a crack at the ol' keyboard again, and I'm now armed with two years of experience, some actual solder flux, a fancy oscilloscope, and a little bit of free time. I managed to at least partly solve the mystery of the amp board outputting nothing -- when I connected my scope's negative clip to pin 1 on the 13600 op amp (AMP BIAS INPUT A), I suddenly got sound out of a speaker for the first time ever. After some experimenting, I found that grounding the amp bias input would get both speakers working. However, the "fuzzy", "overdriven" polyphonic sound still remains, and I'm back to scratching my head. I've tried adjusting every trim pot on every board, and haven't heard any difference in sound output even once - not even when power cycling between adjustments. I am at my wit's end today, but next time I'm going to go through all the oscilloscope test points in the CZ5000's service manual to see if the CPU board is even working correctly, because I suspect there's some sort of subtle timing issue that causes this fuzzing sound only when multiple keys are pressed simultaneously. Hopefully I'll be able to get to the bottom of this before the time I've spent far outweighs the cost of getting a working CT-6500 on eBay.

I didn't mean to imply that the 933s, CPUs, or the sample and hold circuit were running at 1Hz, only that by listening to the two separate waveforms output by the final TL082, they do indeed alternate, in a sense, once per second. I'll try to explain the phenomenon in as clear of detail as I can so that there's no further confusion. If I listen to one output of the TL082 and press some keys, sound will only be produced by those keys and come out of that output if I press those keys during a one-second interval. It switches every second between capturing keys and not capturing keys. If I hold a key which it has captured, the sound will continue to come from whatever output the key started on, even during periods when it is not capturing new keys. If I listen to the other output, then any keys I pressed that didn't play from the first output will play there instead. Apparently they're combined in the stereo chorus board in the end, so there's no loss of sound at all once that happens. It's not that each 933 seems to be producing sound on alternating seconds, only that they seem to be capturing keys on alternating seconds. I did understand what the manual was describing, I was just citing it as the only clue I got about multiple waveforms being output in the end, and advising you not to look too closely for anything regarding a 1Hz trading behavior between the 933s, since that isn't mentioned anywhere in the manual and looking for it would be fruitless. I was going off on a bit of a tangent with the DAC offset voltage, but it's good to know that that's not responsible for the overdriven sound of pressing multiple keys. I was just trying to find a potentiometer that actually affected the sounds per @pianokeyjoe 's suggestion, but the DAC was the only trim potentiometer adjustment documented in the manuals. Then, I found the voltages were not within spec and got carried away. There are only 6 potentiometers on these boards (not including the "tune" dial on the back): one that adjusts DAC offset voltage, two that control the Low-Pass-Filters on the stereo chorus board, two that affect drum sounds, and one on the amplifier board that I have no clue about. The issue does seem to be present right at the output of the DAC, though. I don't have any leads anymore as to the source of the overdriven sounds of pressing multiple keys. Here's a recording of what I'm hearing, in case that helps. I'm pressing two keys at the same time, over and over, and it only sounds the way it's supposed to three out of 8 times. Very odd. Edit: it doesn't seem to be affected by where I'm listening in on the sound, whether it's from the DAC output or the input to the amp board after the stereo chorus effects. Edit 2: that is correct @IanB . The only thing that's technically 1Hz is the sync signal between the CPUs. I have to mention this in an edit because I've reached my 24hr post limit yet again.

It doesn't seem to indicate any specific timing in the service manual. I only found out from these pages that there were two alternate waveforms at all: Anyway, next step is to determine why my DAC offset voltage is misbehaving. It's strange that the manual doesn't go into any detail about the role of the offset voltage in the timing of simultaneous keys.

@pianokeyjoe the chorus feature definitely does work, so I don't think anything is amiss there. I am aware that the sound out of the main board is mono and is only given a fake stereo effect by the chorus board, I just didn't realize that the op amps were carrying separate audio from the main and sub CPUs all the way to the stereo chorus board. The "dropping keys" turned out not to be an issue as I later wrote in my last post, since I was just listening to the output of one CPU at a time, not realizing they were processing keys and sounds on an alternating 1Hz cycle. So, from the output, it sounded like keys were only being detected on every other second, but in reality, the undetected keys' sound was coming out of the other output of the board, which I didn't even realize was a possibility until I got to the part of the CZ-1 manual documenting that part of the audio generation. I've been fiddling with the DAC offset voltage as per the service manual (turning the VR2 potentiometer by the BA9221 and TL082-3), but can't get it to affect anything. It's odd- it does not affect the voltage between pins 13 or 19 of the 9221 and pin 7 of the TL082 unless I disconnect the stereo chorus board. Even then, though, there is still a "fuzzy" sound at the outputs of the main board when more than one key is pressed (around two-thirds of the time, at least), despite the offset voltage being set appropriately. Messing with that pot while playing keys produces no changes either. The fact that the stereo chorus board being connected affects the quality of the sound output of the main board and the DAC offset voltage leads me to think there could be a problem on that board after all, but that wouldn't quite explain why the DAC trim offset voltage seems to affect literally nothing at all. @IanB that's excellent. I have a smaller yamaha portasound pss-450 which I modified to work as a midi keyboard and was thinking of doing something just like that to avoid having to find a place for this table-consuming monster. Also, as seems to be the trend with my posts, just before sending this, I checked something else and realized: I wasn't actually plugged into one of the line out jacks, I was plugged into the volume control pedal jack. I've had the part of the unit with the port labels face-down and got them backwards. Evidently, line out is very quiet and has a lot of white noise. But then again, the amplifier board's power cable still isn't plugged in, so it could be that some amplification is ordinarily supposed to take place there.

I tested the continuity, and every single chip in the keyboard is properly grounded, though the case ground seems to be a "lifted ground" as every ground point I tested has a resistance of exactly 101.7 ohms. I also forgot to mention that both MIDI input and output work, with the caveats mentioned of strange feedback to the board voltage, but otherwise perfect functionality. I tried replacing the TL082 where the output was stopping, but it did nothing. I did manage to lift off some pads when desoldering though, that was (not) fun. Very interestingly, I found that if I disconnect the large black and yellow ribbon cable between the main board and the stereo chorus board (edit: not actually the case!), the TL082 actually does have sound coming out of it, but for some reason having that cable plugged in kills the sound completely. The dropped keys issue is still present, of course. If I disconnected the cable in the top left that carries the audio output from the TL082 to the stereo chorus board, the sound quality from the TL082 improves dramatically. Something must be going wrong on the stereo chorus board, but I'm not sure what. Here's what's really exciting: I discovered that the outputs from the master and sub CPUs are alternating on each 1Hz cycle. The two sides of the TL082 carry sound from the two CPUs, and I just didn't notice until I got really deep into the CZ-1 service manual. I think there might not actually be a problem! I think the two different CPUs are probably supposed to be combined in the stereo chorus board, which seems to be the point of failure. It figures that that's the one board I haven't unscrewed, examined (apart from testing the ground of all the chips and looking for loose parts), or otherwise messed with, so that figures. Also of note, when the plug carrying audio to the stereo board is disconnected, the audio going in is both clear and free of buzzing, so I think the buzzing voltage from the CPU and the 1Hz ticking are no longer problems. Right after writing that and concluding that the stereo chorus board might be the point of failure, I did some testing in different configurations with cables connected or disconnected, and found that the only thing that actually triggers the loss of sound is connecting the power cable for the amplifier board. This leads me to conclude that either the power supply or the amplifier board is the point of failure. I suspect something in the amp board, maybe the STK4332 itself? The great news is, I'm actually getting sound out of it with the amp power disconnected! Line out works like a charm. Kind of ironic how simple it was to get sound out of this thing in the end. There is still an audible buzzing over the line out, though, so perhaps there is something minor going on in the stereo chorus board after all, or it could just be more amplifier antics. Now, I have some qualms remaining about the keyboard that I might need someone with a working similar keyboard to help me out with. Most of the time, if I press more than one key at the same time, it sounds rather discordant and overdriven, even though it doesn't actually appear to be clipping anywhere in the waveform. I feel like this isn't normal behavior, and I should be able to play some dang chords. Secondly, if I'm holding any keys down, any drums playing will suddenly seem to have a fill-in in the middle of its cycle and will restart, which might be intentional behavior, but I don't have access to those pages of the owner's manual to find out. If I set it to "synchro" and trigger it with a key press, I can continue holding down that key and the drums will continue their cycle as usual, it's only when I hold down another key after that point that they will start to do their weird thing and reset. Similarly, if I hold down some keys and then press start, the drums will play uninterrupted. If either of those are indeed abnormal behavior, they sound like a pain to diagnose, but I pretty much got this thing entirely for the sound, so I may as well do my best to improve what I can. I'm pretty ecstatic that this thing isn't a lost cause for sound output, and the real issue seems to be just around the corner. Hopefully soon everything will be in full working order.

I fixed the volume board by soldering some wires in to connect every solder joint that had cracked traces between them. I confirmed my work with continuity tests and by making sure the resistance values were changing between the inputs/outputs in accordance with moving the potentiometers around. You might be right about those boards, but at least everything in them is replaceable! I can assure you, without naming a price, that the deal I got makes it very hard to complain, even if getting sound out of it might end up being a lost cause. The other day I accidentally knocked one of the soldered wires (from the master volume input to the corresponding point on the potentiometer) off by rubbing it against part of the other half of the case, but unless I'm mistaken, that doesn't seem to be an issue when the sound isn't even making it through the little op amp on the amplifier board yet, and I didn't feel like setting up my soldering equipment again so I'm leaving it alone for now. It seems that the master volume slider adjusts the volume *after* it is put through the final amplifier (or possibly just before that) If I truly can't get anything fixed, I'll probably look into a more modern power supply solution and lifting audio directly from the main board and amplifying it myself, though I'd lose the stereo chorus and other effects from the secondary board in that case. That's the worst-case scenario though. Edit: I just reached my post limit for the day, so I'll have to report ground test results tomorrow @IanB, and I'll finally take the time to test midi input. I got sidetracked yesterday by the buzzing on the voltage when midi out was plugged in.

I tried hooking it up with MIDI, and it works perfectly. No dropped keys or anything. I think you were on to something, however. If I have a device plugged into the midi out port, and I play a drum sequence, the voltage suddenly gets distorted across the board, producing a loud buzz. I mentioned in an earlier post how there would be a blip in the voltage whenever a key was played, and I think it may be connected to midi output. If I disconnect the amplifier board's power entirely, the distortion no longer happens, and the blips in voltage no longer occur when I press a key. Strangely, the buzzing only happens when I have a midi cable plugged into the output. the cable doesn't even have to be plugged into anything at the other end. Also, the voltage has always been reading correctly on my multimeter, so no problems there. I had the idea that maybe the board was resetting itself as well, but thought that couldn't be true since held keys continue to play. HOWEVER, I tried holding a key down while playing drums and listening to the drums' output, and when holding down any keys, the drum beats would start over abruptly when halfway through their cycle, not corresponding with the timing of the cyclical detection of keys in conjunction with the 4053 as mentioned before, but apparently the phenomenon are connected. Very strange. I also replaced the 13600 today, but all behavior remained the same. Drum sounds are making it to the 13600, but not out of it. I wonder if perhaps this is related to the volume control circuit on the amp board, but I have no ideas for testing this yet. Also, a fun thing I found out: if you disconnect the power cable from the amplifier board, the ground of the board is somehow connected to the drum sounds, so I can hold the positive end of my "oscilloscope" probe and touch the heatsink for the big amp and I'll start hearing the drums on my computer. Anyway, a recap of what's known: POWER BOARD (M5157-PS1 & M5157-PS2): 120hz buzz from rectified voltage, audible when probing the board and listening to the waveform, and making waveform analysis difficult (might not be an actual problem since all the voltages are still correct on a multimeter in the end). MAIN BOARD (M5157-MAIM): intermittent sound past the 4053 (which has been replaced and the issue persists), in which new key sounds only register within one-second windows which are spaced one second apart (approximately). held keys will continue to play their sounds as long as I start holding them during a so-called capturing interval. if any keys are held, drum beats will reset halfway through their cycle, independent of the capturing interval. the input to the 4053 has all the missing sounds. no sound at all out the TL082 op amp directly above the 4053. AMP BOARD (M5157-ASIM): no sound out from the 13600 op amp, even after replacing it. when power is supplied to the amp board, any keys pressed will cause a blip in overall board voltage. when any midi cable is plugged in to MIDI OUT and a drum sequence is played, the constant midi messages cause repeated blips in the board voltage, adding a loud buzz to the sound of all points with positive voltage on any of the boards. Yes, a real oscilloscope would sure come in handy right about now. As would a service manual for the CT-6500 in particular, since there are some differences in the main board and the amp board between the CZ-5000 service manual and the CT-6500 (such as a completely different headphone amp in the CZ-5000 that is not present in the CT-6500, and the layout of the entire main board being different), but I might be a few decades too late to find one of those. For now, just waiting for the new TL082s to arrive, though I think the best those could do is finally let some sound out of the main board. If anyone wants any high quality photos of parts of the board, or video or audio recordings of the sounds it makes in various places, I'd be happy to provide them. ------------ As I was about to sign off on this post, I decided to do some poking around on the outputs of the D933 chip in the bottom left, and found some blips that matched up with the keys being detected that make it through the 4053. I've traced them back through several chips, and I think I'm going to have to draw a diagram to keep track of them all. I have not yet been able to determine the source of the blips, but I have some theories. While I was tracing those, I also discovered that several lines on the right side of the board make a different sort of buzz from the normal DC voltage on the board, and they have a blip that sounds like a loss of voltage once every second or so. To differentiate, I'll call the first sorts of blips (which include key presses, but only the ones in the capture interval) as "key blips" (a very technical term) and the other kind as "power blips" (for lack of a more descriptive term). The key blips seem to originate in the 933 chips in the bottom left, specifically, pin 3 on both of them produces the same blips for each key pressed, cutting off for a second, then resuming detection. Without any real documentation on the UPD933AC, only the low-resolution labels on the service manual scans, it's hard to say what that's doing. I had to reference the CZ-1 manual for these, since that's the only one with the same chips. Pin 1 has all the blips for all keys pressed at all times, which is what I would expect from pin 3. The fact that pin 3 on both 933s aren't connected suggests to me that neither of them are broken, that there's something with the input that's affecting the output, but I'm not sure what it is. Perhaps more importantly, though, the reset pins on the 933s seem to be being triggered once a second, or at least the signal is brought low on those pins once a second. If that were true, that might explain the issues. That reset pin is connected to pin 44 of the leftmost uPD7811G, which is the RD (read strobe) pin, which the datasheet indicates goes low "to gate data from the external devices onto the data bus." It's also connected to pin 12 of a 74LS08 to the right, a quadruple two-input AND gate, though that's an input, so it seems that the uPD7811G is the source. However, it seems like this is expected behavior, otherwise why would the read strobe pin be connected to the 933s' reset pins? I was probably searching in the wrong direction. I have not been able to trace the power blips to anything concrete, the whole right side of the board seems to be plagued with them. I wonder if perhaps one of the crystal oscillators could have been damaged in whatever impact broke the volume panel board. Anyway, now I've stayed up until almost 6 AM trying to find a definitive source for either the 1Hz blips or the dropped inputs, to no avail. Sorry for the info dump, it looks like I might have to become an expert on these components if I want to get this thing working right. Tomorrow I'll try to spend some valuable not-middle-of-the-night hours doing more investigative work. Until next time!