Casiotone CT 6500 - Turns On , No Sound

[picklebush]

On 12/8/2021 at 10:54 PM, pianokeyjoe said:

While a dead battery will not necessarily cause the no sound on THIS model keyboard(it will for the CZ101 internal sounds), it can leak and cause damage that way. I suspect a crack on a board, or the amp board having faulty output chip or the ironic loose wires scenario. Be slow, be methodical and be patient when you check everything. Its worth it. I did not see anything obvious on the component si

I really appreciate this forum. I'm new to learning keyboard not but not music. I have several guitars. I too picked up an old CT-6500 and when inspecting it, I should it and heard something pop around.  I came home and discovered that the drum kit doesn't work. So, me, being impatient on my super dirt cheap purchase, decided to take it apart. I found a lonely 1 amp /250 V fuse  bouncing around. I don't see a location for it. Is this the fuse that might control the drum kit and such? Also you say there is a coin type battery, but I only found one, but it's not easy to remove. Is this it in the pic?

pics attached.

 

Again, I appreciate anything anyone can add to this old topic.

 

https://imgur.com/gallery/jwiDql1

[pianokeyjoe]

I have to say, the coin battery is soldered on the board, and no, it is not that easy to pop off. You will need to desolder the battery off the board and then install and solder a coin battery holder instead that you can then swap dead coins out far easier like on computers and modern synths. If you do not know how to do this work, get some one that does. Never try your self or you will ruin the synth needlessly. The fuse came off the power supply board only. Check the Power supply board and you WILL find the missing fuse location. The other place that uses a fuse may be the Audio amp board which is on the opposite end from the power supply and is installed to the TOP section of the keyboard next to the right speaker and has a long aluminum plate that is the POWER AMP heatsink. Which could explain the no beats volume. But that could be a dead or dirty slider too since the slider is more exposed to the elements. A fuse bouncing around inside and yet the keyboard powers up, is odd so check that PSU board and POWER AMP board for that mystery. The no sound for beats, check your sliders. For the battery, you could snip the leads right AT THE BATTERY, and then you can solder a coin battery holder with wire leads pre wired, to the appropriate battery leads that you leave on the board(you cut only the battery away and leave the lead stumps in), making sure the (-) and (+) leads correspond to the new coin battery compartment lead wires(Black for Negative or (-), and red for Positive or (+)).

[picklebush]

Ok that’s what I needed to know, the other location. It’s not the power or the board next to it. I count 4 good fuses there. I will check next under and to the right speaker. I am confident enough to solder a new coin holder in place! Thank you!

 

..regarding the no beats/slider, none of the other sliders have any issues as far as weird sounds when I move them. I’m hoping this missing fuse will fix the entire beat section. None of the lights come on at all in the beats section…

 

thanks again for your help!

Edited January 30, 2023 by picklebush
I can only make two post a day, so I just edited this one add some

[picklebush]

On 1/29/2023 at 6:36 PM, pianokeyjoe said:

The other place that uses a fuse may be the Audio amp board which is on the opposite end from the power supply and is installed to the TOP section of the keyboard next to the right speaker and has a long aluminum plate that is the POWER AMP heatsink.

I can’t find it. I’m there… got heat sink out… no fuse holder anywhere.ppft. I’m at a loss. See pic https://imgur.com/a/lQkMIfG

[pianokeyjoe]

I will be back home in a month. I will have to take my unit apart and follow along to see what is what. Since you have power to the unit, the extra fuse may be some stray from what ever person tried to mess with it before you. As for the rhythms section, seems it may be a bad slider

[CYBERYOGI =CO=Windler]

On 1/11/2022 at 1:02 PM, IanB said:

At one time I was considering trying to write a new ROM to give more flexibility (e.g. 16 single line voices, or 8 dual line Monosynths running different patches, etc), but documentation on the 7811 microcontrollers is sparse and incomplete and it looked very much like a rabbit hole I'd regret falling down so I abandoned that project.

 

The D7811 CPU nowadays has been documented by the MAME team to emulate Casio RZ-1 etc.

https://www.cpcwiki.eu/index.php/UPD7810/uPD7811

http://zine.r-massive.com/casio-rz-1-firmware-hacking/

 

However the D7811 is a D7810 with 4K internal ROM, so (depending on its wiring) it may be impossible to make it execute different code from an external eprom if it reuses the same pins for something else.

Edited February 8, 2023 by CYBERYOGI =CO=Windler

[Aaron Wright]

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.

[pianokeyjoe]

Hello @Aaron Wright! I check the forum classic Casio section several times a day for years now so you are not writing in a dead thread by a long shot! So your situation is the same as mine for ONE of my 3 CT6500 units. It has a bad AMP board and that is very easy to tell IF you had a second unit that does work but say, has broken external parts and you would just transplant the good amp board from the physically damaged(in shipping in my case) CT6500 into the CT6500 that has everything looking good but needs a good amp board.. The issue in your case is that very thing.. bad amp board. Sadly if you do not have more than one unit, you will need to fix the amp board you have. Best thing to start with is finding the Filter and DC bias section of the AMP board. Leave the big Synth boards alone except for the CR2032 battery inside, you WILL need to get rid of that and put in a coin battery holder in it's place with a new battery! The Dead Battery can and will cause issues if left in the synth/home keyboard CT6500. That fuzzing output is the filtering section and DC bias being off due to some short or in my case, an open resistor somewhere. Again that sound is what you get from the synth chip in raw form. You need the biasing and filtering to tone it down to analog warm sounding levels. Even then, you DO hear the digital artifacts when you press certain presets without pressing any keys yet, and when you release keys after hearing a tone(Normally the same fuzzing but very very quiet and you have to put your ear to the speakers to hear it). Yamaha FM synths and keyboards have this same fuzzing artifact in their preset tones after key release or when you press the touch responsive keys very soft you hear it too. I hope you can get your unit fixed cause it is a very nice preset based CZ style keyboard, not unlike the CZ230S and maybe the CT5500?? Oh the battery in the CT6500 is for storing a custom synth sound you can make via midi and if I remember correctly, the internal recorder? I could be wrong on the recorder though.

[Aaron Wright]

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?

Edited September 29 by Aaron Wright
removed unnecessary info from before I caught my mistake

[Aaron Wright]

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.

Edited September 29 by Aaron Wright

[Aaron Wright]

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?

[pianokeyjoe]

Sorry @Aaron WrightI had to google the part number for the DAC lol. Ok, so after you showed the expander circuit which I find baffling why a dac and then an expander to get the analog sound out of the digital waveform in the first place but eh.. Casio and Yamaha did things different in the 80s to get that magic signature sound from their keyboards. Ok, So the first thing I thought was the DAC is bad because duh! You getting digital sound where analog is supposed to be AND the digital sound artifacts of pressing more than one key, which is fuzzing, but louder than normal sound volume. So check the DAC chips. You should have 2 at the very least, on the main or melody sound section, one on the rhythm section and normally on most classic Casio accompaniment keyboards you would have the third for chords/bass/arpeggio? I could be wrong. I do not have my board accessible still and it is 2024 going on 2025.. I am slacking, I know, Sorry. I can 100% tell you when I changed my amp board out, the problem went away for ME. So at least in my case, the issue is ON the amp board..

[pianokeyjoe]

Is the DAC chips on the amp board?

[Aaron Wright]

7 hours ago, pianokeyjoe said:

Is the DAC chips on the amp board?

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.

[Aaron Wright]

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.

[CYBERYOGI =CO=Windler]

Check the power consumption of the stereo chorus by inserting a mA meter into its -15V line. If it pulls it up to -2V then either a semiconductor went bad (do any of the chips run hot?) or the PSU outputs way too low current on its -15V line.

 

The sample & hold circuit in 1980th Casios functions basically like the distributor of an old car engine. Like the distributor assigns the ignition voltage to different spark plugs depending on the rotation angle, the sample & hold demultiplexes the signal behind the DAC among multiple audio lines by timing signals to produce multiple analogue sound channels (e.g. stereo or to permit a separate chord volume potentiometer) from a single DAC. Bontempi did similar things in stereo keyboards, while cheaper mono variants just used a resistor and capacitor to sum the output without demux..

[Aaron Wright]

2 minutes ago, CYBERYOGI =CO=Windler said:

Check the power consumption of the stereo chorus by inserting a mA meter into its -15V line. If it pulls it up to -2V then either a semiconductor went bad (do any of the chips run hot?) or the PSU outputs way too low current on its -15V line.

 

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.

Edited Friday at 04:19 AM by Aaron Wright

[Aaron Wright]

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.

[CYBERYOGI =CO=Windler]

A zener diode normally is no high resistance part. A small one can handle about 100mA, and if overloaded (pulled to far above its breakthrough voltage) it would run boiling hot. Which voltage is on the lower left 100 Ohm 1W resistor (coming from mains trafo?). Does that voltage collapse too when connecting the stereo chorus circuit?

[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:

 

4 hours ago, CYBERYOGI =CO=Windler said:

A zener diode normally is no high resistance part. A small one can handle about 100mA, and if overloaded (pulled to far above its breakthrough voltage) it would run boiling hot. Which voltage is on the lower left 100 Ohm 1W resistor (coming from mains trafo?). Does that voltage collapse too when connecting the stereo chorus circuit?

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.

Edited Saturday at 09:08 AM by Aaron Wright
placed quote in a more appropriate context

[IanB]

Hello everyone I'm just having a read of this thread (haven't paid attention as my day job is intense hours wise at the moment). Nothing to add but all most interesting!

[pianokeyjoe]

Remember when doing your troubleshooting that the CT6500 has built in speaker system and the amplifier and PSU sections will be different due to this fact. The synth/accomp section is also different from the CZ line due to the added rhythm/accomp section. The presets are but 40 in all and there is a hidden rewritable internal sound preset that is only accessible via midi as @IanB has pointed out. This keyboard is very different beast from the CZ line. You will need to do some process of elimination troubleshooting as you have been doing, for sure. I do not have my units handy still so I can not add more info to help but this: The PSU may need to have caps redone! I smelled electronics when I last powered my 2 good CT6500 units!

[IanB]

Small "further to" what @pianokeyjoe just said, the CT can operate as 4 synths in one (same number of voices, divided into 2 poly and 2 mono synths) and you can write to the edit buffer on all of them so repatch them all using their separate MIDI channels. The rhythm section also operates on a separate MIDI channel. So the CZ schematics offer help, but there are various differences as we already have seen.

Edited 17 hours ago by IanB