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Aaron Wright
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Casiotone CT 6500 - Turns On , No Sound
Aaron Wright replied to HAILHAIL's topic in Classic Casio Discussion
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. -
Casiotone CT 6500 - Turns On , No Sound
Aaron Wright replied to HAILHAIL's topic in Classic Casio Discussion
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. -
Casiotone CT 6500 - Turns On , No Sound
Aaron Wright replied to HAILHAIL's topic in Classic Casio Discussion
@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. -
Casiotone CT 6500 - Turns On , No Sound
Aaron Wright replied to HAILHAIL's topic in Classic Casio Discussion
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. -
Casiotone CT 6500 - Turns On , No Sound
Aaron Wright replied to HAILHAIL's topic in Classic Casio Discussion
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. -
Casiotone CT 6500 - Turns On , No Sound
Aaron Wright replied to HAILHAIL's topic in Classic Casio Discussion
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! -
Casiotone CT 6500 - Turns On , No Sound
Aaron Wright replied to HAILHAIL's topic in Classic Casio Discussion
The big power supply capacitors, the 4051, and the 4053 came in today, but as of installing and testing them, no progress has been made. The power supply still produces a similar (perhaps now even sharper) waveform, here: And there is still no sound coming from the output of the TL082 directly above the 4053, although I actually do see some scratchy remnants of the waveform when I play keys. However, it's inaudible over the buzz. There's one strange thing I noticed when testing the input of the TL082 (pin 5 of the 4053), which is that if I strike the same key several times in succession, the key will only register for about a one second period, then nothing will register for another second, and so on. Strangely, this doesn't apply to holding a single key down (the key just continues to play, but only if I strike it during one of its "capturing intervals"). This only happens at pin 5 (and pin 12, on the opposite side) of the 4053, and this happened before I replaced it too. Those two pins are only connected to the TL082 above it, so I'm not sure what's causing the regular drop in keys, or this behavior in general. When listening on the other pins, I can't find anything that correlates with this drop in new note sounds. Furthermore, if I'm holding down a key, I still can only press additional keys during the capturing interval, otherwise they will not play through the 4053's output, even though it is audible on its input. All these "dropped keys" somehow only apply to the output of the 4053, since they are still playing on the input side. I hope I explained that well enough, since it's pretty confusing. I also noticed that there's a little 'blip' in the voltage across the main board whenever I strike a key, even if no sound is produced. And another thing! When I play two keys at the same time and listen to the sound on the 4053's input, sometimes the keys will play harmoniously like they should, other times the combination sounds noisy and almost overdriven, and this seems to correspond to the drops in the output, so I guess that's the only thing that correlates with the drops that I've found so far. Seems like a timing issue somewhere. I'm wondering if maybe the choppy voltage is periodically syncing up with something logic-related on a clock pulse and causing something to go wrong. Could the rectifiers be at fault? I decided to test the two rectifiers' waveforms. Here are my results: 5V Rectifier ( + ) 24V Rectifier ( + ) 24V Rectifier ( - ) 24V Rectifier (between + and - ) I'm not sure what to make of this, whether this is normal or abnormal, but it doesn't seem quite right to me. I should note, the 60Hz AC coming into the rectifiers is a lovely perfect square wave. In the next week or so, the 13600 and the replacement TL082s will get here, and hopefully I'll be able to get some sound out of the speakers from the drums, at least. For now, the vexing continues. As usual, any help or insight is appreciated. If anyone with a similar keyboard wants to prod their rectifiers (or at least come up with a less funny way to say that), that would at least provide a clear basis of comparison. For now, I'm returning to the service manual to try out its waveform tests. Edit: I also may very well reflow all the solder joints on the power board just to be super extra sure none of them are faulty. -
Casiotone CT 6500 - Turns On , No Sound
Aaron Wright replied to HAILHAIL's topic in Classic Casio Discussion
I decided to go ahead and order some replacements for the three big caps on the power board, the three TL082s, the 4051, the 4053, and the NJM13600 just to be safe. Hopefully some progress will be made next week after installing those. I'll probably finally reassemble this thing and get it off of my dining room table tomorrow, so if there's anything anyone would like me to try before I put it away to wait for the new parts, I'm all ears. As a side note, I'm kind of curious to know what anyone else's waveforms look like on the 5V and 15V rails (or on the output with no keys being pressed) on a similar keyboard, since after doing some more reading up on smoothing capacitors, I realize I'd expect at least a similar shape to what I was getting (but on a lesser scale) during normal operation due to how smoothing capacitors work on fully rectified AC. It's possible that the hum isn't even an issue for me anymore, since it's barely audible through either the speakers or the headphones now. Regardless, new capacitors are on their way. -
Casiotone CT 6500 - Turns On , No Sound
Aaron Wright replied to HAILHAIL's topic in Classic Casio Discussion
As a newbie, I am apparently limited to two posts a day, so apologies that my updates will have to be portioned appropriately. Anyway! I resoldered the TL082CPs in a different order, and it turns out I was wrong about there no longer being sound output-- it's the same as always, though the hum actually seems quieter than it was in my initial testing. I looked closely at the power board and didn't find anything loose, though some legs of one of the transistors were apparently cracked around the solder pads, so I reflowed that solder and several others just to be safe. I'm actually considering reflowing every single soldered connection on the power board simply because there aren't that many of them and it might help, but I'll only do that if I run out of ideas. I also looked at the amp board for similar issues but found none. I found a USB sound card I had laying around and used the line in per @IanB's suggestion, which has been interesting and enlightening. The "buzz" everywhere is exactly 120hz, which suggests yet again that it's something in the power board, but I'll have to look closer into that. Here's the waveform, if you want to see: It's about the same at every point with positive voltage that I test, going back to the power board, but I'm reluctant to poke my "line in" input around the power board because I don't want to fry my sound card if I poke the wrong part, so I'm not sure precisely where it starts. All sound coming from the board is mixed with this waveform, it's never just the sound on its own (save, perhaps, for pin 1 of the BA9221 DAC, though I think that's mostly due to the much higher voltage). Secondly, I found that drums sounds are being produced correctly and are making it to the amp board. The same cannot be said of the keyboard sounds, which don't make it out of the main board. The top left op-amp, TL082-2, seems to be where the sound stops, and this is one I swapped out from TL082-3 down lower. I think that suggests that that might be a problem piece. I can get sound from the non-inverting input, but none of the other pins. I measured some voltages: Pin 3 (non-inverting input) with no sound: 5.70-5.72V Pin 3 with sound: same as above plus 2mV Pin 2 (inverting input): constant 4.22 volts Pin 1 (output): constant 13.64V regardless of sound The other side of the chip has around 200mV on all the input and output pins, which does vary with keys pressed, but I can't hear any sound out of them (probably due to the... low voltage). To reiterate, there is no sound from the keyboard past that chip. It could be that TL082 is at fault, or possibly the 4053 below it, or maybe even the 4051, if they're supposed to contribute anything more than an audio output signal to the TL082. Finally, the sound output on the amplifier board seems to stop at the NJM13600. Both input pins on both sides (total of 4 pins) have drum sounds on them (but no keyboard sounds) and neither of the outputs produce anything other than the trademark buzz. There is no sound from any of the other pins either. So, to summarize, I think there are three problems: 1 - 120hz buzz from the power board 2 - no keyboard sound past the second op amp on the main board 3 - no sound at all out other than buzzing from the amplifier board, stopping at the NJM13600 amplifier. Tomorrow I'll continue to read the service manual and see if I can figure anything else out or narrow down the issues any further, concentrating on the hum and the power board first and the amplifier circuit second. Thanks for you guys' help as always. I might update this post with new information if anything else comes up, but we'll see. -
Casiotone CT 6500 - Turns On , No Sound
Aaron Wright replied to HAILHAIL's topic in Classic Casio Discussion
Evidently I've had the concept of impedence wrong in my head for a while, so thanks for the tip. I somehow got it in my head that the impedance would have just resulted in a quieter speaker and a lower likelihood of blowing my eardrums out when listening for a tone. I've learned my lesson. The op amps I was swapping are the cheaper TL082s, which was why I figured it would be alright to swap them. Mostly, I got carried away and desoldered them with the intent to test them individually, but thought it would be easier to see if swapping them changed the point of failure and probably wouldn't hurt anything. Worry not though, I was being extra careful with everything irreplaceable. The "choppy" sound was what I assumed was the sound going into the expander circuit, since I did read up on that in the CZ-1 service manual (thanks for the CT-5000 manual, by the way) and it was coming out of the op amp directly after the DAC and before the 4051s. It sounded like I imagine the unexpanded waveform printed in the service manual would. That midi info is excellent and is exactly what I was interested in doing. Pretty incredible that that much control is available through midi alone, as someone whose only previous experience has been with some comparatively dumb keyboards. I'll definitely be using your software! Finally, I did read about the separate drum synthesis, but wrongly assumed it was still on the same board. There's no sound from the drums either, so I guess I'll have to poke around (less literally this time) the spooky power board. That transformer is MASSIVE! I'll update as I find out more and hopefully don't get zapped by any capacitors. -
Casiotone CT 6500 - Turns On , No Sound
Aaron Wright replied to HAILHAIL's topic in Classic Casio Discussion
Hi, I'm the new owner of this CT-6500, and I spent the last few days disassembling, soldering, and experimenting to see if I can get it working. I'm not sure if it's against forum etiquette to continue the thread like this instead of making a new one, but I figure it makes sense to keep the current discussion in place and add on to it. I'll move it to a new thread if anyone raises any objections. Anyway, I fixed the broken board and confirmed with a multimeter that the sliders are now doing what they should. Unfortunately, the speakers/headphones/line out still only produce a hum after starting, and generate no sound whatsoever regardless of volume. I even experimented with the aforementioned relay, finding that the contacts were stuck in the wrong position, hence the pop on startup, and I even tried bypassing it to see if that made any difference, but it doesn't seem to be the source of the hum or the lack of sound. I found the service manual for the CZ-1, since I couldn't find one for the 6500, and thankfully the relevant components are almost the same, so I tried following the sound generation backwards from the output to find a place where I could actually hear some keyboard sounds, and there's a loss somewhere after the BA9221 DAC and the several TL082CP op amps at the top left of the main board (M5157-MA1M). I can hear some choppy and not-yet-completely-analog sounds from the output of the BA9221, which get quieter after the first op amp (which seems... wrong) and eventually gets lost entirely as I move further down. Part of the problem could be that I'm testing it with an 8 ohm impedence scrap speaker with the negative terminal hooked up to the chassis ground, which might not be the proper way to be testing the op amps further down the line, but I haven't found any guidance on how to do it better. I also tried fiddling around with the DAC offset voltage, which didn't do anything and I'm not even sure I was doing it right since the components on the CT-6500 are labeled differently from the ones on the CZ-1. There is an adjustable dial right by an 8A9221 and TL082, but it's labeled VR2 instead of VR3, the voltage between pin 13 of the DAC and pin 1 of the op-amp is something closer to 5V than the service manual's specified 3 +/- 3mV, and adjusting VR2 has no apparent effect on the voltage or the sound of the keyboard. Anyway, I poked around in the wrong place and now don't get any speaker output because I think I fried an op amp, so I'm going to swap them around on the board tomorrow and see if anything changes. I am also suspicious of the two chips labeled "5K4 T / HD14051BP", the functions of which I am uncertain. Just spitballing here, but the hum from the speakers/headphones is exactly the same as the sound produced by putting the negative terminal of a speaker to chassis ground and just poking around until I find a positive lead somewhere on the board, so that suggests it must either be a short somewhere on the main board, a problem with the power board (maybe the transformer?), or this is normal noise that's ordinarily supposed to be removed by all the sound processing circuitry. I suspect the transformer, just because I own an electric piano with a buzzy/static-y speaker that sounds similar, which I was told could be an issue with a transformer. Also, the shattered volume control board is on the same side as the power board, so even though there's no apparent damage on anything else, it's not outside the realm of possibility. Sorry for the long read, but hopefully someone might have a better clue than I do from all that information, since this is my first time diving into a Casio or doing any significant repair work on audio circuitry, though I do have some basic familiarity with audio circuitry from digging into a different keyboard for a personal project a while back. Is there a particular direction I should be focusing on here (the power supply vs the DAC on the main board)? Really hoping to get this thing working as more than a midi keyboard and to hear its amazing sounds, and I'd even like someday to try hooking an arduino up to this thing to get more individual control out of the phase distortion synthesis than the presets it offers, if possible. But I digress. One thing at a time.