Rookierafa Posted March 22 Share Posted March 22 Hi! I am trying to fix a Casiotone Ct401 for a friend. He complained that the keyboard started smoking and stop producing sound, it was disconnected and stored until now. Since we are in an area where audio tech´s are not available, and send this to a distant technician is economically unviable, because of the low commercial value of the keyboard, he asked me if I could try to fix it. And because I have some spare time a accepted looking at this as an opportunity to learn something new and expand my little electronic knowledge. I looked online for people with similar problems with this Casiotone models and found that usually it is a paper cap supressor in the power supply module that usually cause this issue and found that most of them solved the problem just changing it. I examine the module and that it was the burned cap ! https://imgur.com/4vpcaD1 https://imgur.com/S1WxpVt I started rubbing my hands with joy, it was going to be an easy solution. So I thought that although the paper caps are unreliable, my friend would be ok maintaining the originality of the object. https://imgur.com/IoYWmj8 https://imgur.com/MS6SMZ5 I assembled everything to test it and, nothing happened, still no sound. Then I tried to troubleshoot basic stuff like: Speaker- ok Nothing connected or stuck to the headphone line, cleaned, re-solder pins.- (started to ear very low almost unperceptive sound of the accompaniment rhythm and melody sounds, but still no proper sound) Volume Pot resistance (continuity of the cables also)- ok Visual inspection -no cracks anywhere in solders or boards Key Bed cleaned and checked - ok Key Bed ribbon cable continuity - ok One dirty 22u100V Cap from the power supply removed and checked - Ok Cables from power supply to mainboard continuity- Ok So after this I got the Service Manual, and measured the points, and found out something that I am fighting to understand because of my rookieness. I found that the +VCC voltages reads ok in the main board it gets +15 and in the power supply side reads +25 , the 5+ V in the main board reads ok, the ground pins reads 0 everywhere, but every single place where it is supposed to read -15V in the ,ain board reads 0 nd in the -VCC in Power supple board, reads 0 too. https://drive.google.com/file/d/1oND9ugvzNN9W4g83hsgiObp5lFSWdhh4/view?usp=drive_link https://imgur.com/SdWYmyH https://imgur.com/EeMtHD7 After this I started to suspect of the transistor 79M15, downloaded the datasheet and found out that this transistor output is -15V. Again I thought, Voilá! https://imgur.com/HORbGy2 But No no! I ordered a spare one and changed it, and measured, and finally I have -25 in -VCC in the power supply board but nothing in -15 point in the main board. This piece o s$%# still doesn´t sing, just started to make a high pitch noise, and with headphones I can ear a little bit better the accompaniment rhythm but nothing close to the supposed sound. https://imgur.com/rjj0jEX And that´s it I am stuck! Can anybody, please, help me to find out what is going wrong with this? I still thought maybe it can be the STK020 Power amplifier is bad, but I don´t want to order more stuff without being certain of what it is happening with this. Thanks in advance 1 1 Quote Link to comment Share on other sites More sharing options...
Chas Posted March 22 Share Posted March 22 (edited) Those RIFA film capacitors are a well known failure issue in many old electronic devices, and are often referred to as "RIFA Bombs" by those in electronics. My own CT403 also suffered the (quite spectacular!) exploding RIFA power supply capacitor - I never knew that such a small component could produce so much smoke! Interestingly, even while billowing smoke my 403 was still functioning and producing sound until I hurriedly hit the power switch. Luckily for me, a replacement (and better quality) capacitor soon had my 403 up and running fine again, and has worked without issue ever since. Unfortunately, I can't offer much more than what you've already found out with the 401 you're working on. However, I did see recently someone with a similar issue with an MT65 (I think). It too was not producing sound correctly, with just some very faint evidence of the rhythm/ accomp. section coming through among a bunch of noise. IIRC, it turned out to be the DAC (or Sample & Hold, I forget which) downstream of the LSI, and once that was replaced, it was all working again. I can't for the life of me find that exact post though... BTW, the 401 might be more valuable than you think as it is one of the models that has the now famous Michael Jackson "Frog" preset Tone! Edited March 23 by Chas 1 Quote Link to comment Share on other sites More sharing options...
Rookierafa Posted March 23 Author Share Posted March 23 Thank you for the info. Still trying to find the faulty part. I re-solder the transistor and the high pitch noise vanished. In any case I am still just getting very faint rhythm and melody noise. 1 Quote Link to comment Share on other sites More sharing options...
Rookierafa Posted March 24 Author Share Posted March 24 Hi all. I checked the main board and found that near the Power Amp there is +15V and -15V, but when I measured the op amps I found out that I have +15 V in both sides, pin4 and pin 8. I think the circuit need´s one to be positive and another one to be negative. Anybody more experienced can tell me how this is achieved or if this gives a better clue of what maybe is causing the synth to go mute? 1 Quote Link to comment Share on other sites More sharing options...
IanB Posted March 24 Share Posted March 24 Just to clarify, what's the voltage on pin 19/brown coming out of the Power supply? Quote Link to comment Share on other sites More sharing options...
Rookierafa Posted March 24 Author Share Posted March 24 (edited) In the pin 19/brown and in the pin 20/yellow the reading is 0V. Edited March 24 by Rookierafa 1 Quote Link to comment Share on other sites More sharing options...
IanB Posted March 25 Share Posted March 25 Ok, so it looks like you have no output from the 7915 you replaced (it's a voltage regulator not a transistor btw). Check what voltage you have on its 3 pins. The input is the centre one (2) the output is the right one (3). Is it bolted to the case or a heatsink? Its metal tab is connected to its input so if the heatsink is not isolated or it's bolted to common metalwork it should have an insulating washer. If it has no input, check the fuse before it and the bridge rectifier before that (feel free to post a photo of it). You probably have +15V on both pins of the op amps etc because there's not voltage supply on the negative supply pins at all so the +15 is just backfeeding through them. Also note that the 7915 has internal short circuit protection so it may be fine, but is shutting itself off due to a short circuit on its output, so it may not be the culprit but we will start there. If you can disconnnect the PSU outputs from the other board(s) to check the voltages that would be useful to do for that reason. If there's -15V on 19/brown when it's not connected to anything else, the problem is further down the circuit... Quote Link to comment Share on other sites More sharing options...
Rookierafa Posted March 25 Author Share Posted March 25 (edited) Hi! Thank you in advance for helping. With all the cables for the main board disconnected. Measuring 7915: center pin = 0.08V (approx) right pin = 0.00V left pin = 0.00V Measuring the fuses: - VCC gives -27.8V +VCC gives +27.4V +VA/ +VDD +15.6V -VA -15.9V The 7915 is bolted directly on the PCB, no metal touching it. Measuring point 19/brown reads 0.00V. https://imgur.com/gallery/zsvfbwv https://imgur.com/gallery/H8g4ddD Edited March 25 by Rookierafa 1 Quote Link to comment Share on other sites More sharing options...
IanB Posted March 25 Share Posted March 25 Hmm. The output of the fuse, the capacitor and the input of the 7915 are all directly connected on the circuit diagram so should all be at the same voltage, it's the -VCC fuse. You said you have measured the voltage at the fuse but didn't mention checking the fuse itself. Is this voltage at both ends (i.e. is the fuse good or blown)? If you have your 27V at the fuse out terminal, then either the PCB track is damaged or the 7815 has a dry solder joint. Take out the fuse and check its continuity with your meter. With power off check you have continuity from the fuse terminal to the 7915 input (centre pin) and also to the capacitor's -ve terminal (I assume it's the capacitor nearest the 7815). Can't think of anything else to suggest right now... Quote Link to comment Share on other sites More sharing options...
Rookierafa Posted March 26 Author Share Posted March 26 (edited) Hi IanB. All the fuses are OK. After detailed examination, I decided to repair some traces that where dubious around the 7815. The result is not the prettiest but it worked, finally I got readings: Pin20/yellow = -15.18V Pin19/brown = -15.22V I get the same readings in the tip of the cables that feed the main board so they have continuity. But everytime I reconnect the cables to the main board, brown and yellow cable return to 0V, and no sound. I checked again and one of the op amp is reading +15 in both sides but the other is reading +15 in one side and in the other is oscillating near 0V and very small values. Edited March 26 by Rookierafa 1 Quote Link to comment Share on other sites More sharing options...
IanB Posted March 26 Share Posted March 26 OK so that's progress. As I said previously, the 7815 will shut down if it sees an overload/short on its output (they are well designed to avoid blowing up). So likely there is a short on the main board somewhere. This might be a physical short circuit, or may be a failed capacitor, transistor or some other component. I'm about to head out to work but I'll take another look at the circuit diagram this evening when I get home. The power readings on the main board will be doing strange things because the negative is basically disconnected when the 7815 is in shutdown. Quote Link to comment Share on other sites More sharing options...
CYBERYOGI =CO=Windler Posted March 28 Share Posted March 28 (edited) Measuring -15V on a positive voltage sounds very malicious. My Nokia 417TV (low emission SVGA CRT monitor with high resolution PAL TV) had an atrocious design flaw the destroys irreplaceable special ICs. Namely in the power supply of the TV PCB it contains a positive and a negative voltage regulator. The positive regulator LM2940CT has an awful feature because it was designed to be also suitable for battery chargers, and so for safety (to protect against wrongways battery insertion) turns itself off if during power-on it detects any negative voltage residue on its positive voltage output. So if e.g. by moisture or leaky capacitors that line goes only slightly negative, it will selfdestruct because it output no positive voltage, and so current through the rest of the circuit pulls it even further negative, destroying the RGB amp IC and TV section microcontroller (which is impossible to find). Edited March 28 by CYBERYOGI =CO=Windler 1 Quote Link to comment Share on other sites More sharing options...
Rookierafa Posted March 28 Author Share Posted March 28 (edited) I whish you all a good Easter with your families, and thank you again for keeping helping me on this adventure to repair the Casiotone. I already checked for visible defects, found a capacitor with bad soldering bed and repaired it, but the power still stops somewhere. Made a continuity test in the main board for the ground rail and everything is grounded. Started to check the negative rail, but when arrives to the main board it splits in different directions and since the board is big I loose myself some times. I looked into the schematic and found that some transistors are labeled with "base, emitter and collector" but there are 3 of them that are labeled with "ground, supply and drain". I suppose this mean some difference exists between them. Can anybody enlighten me? Anyway I restarted to measure some stuff and found that the negative rail have power in some places on the main board, I found -15V on the connector with the Yellow/20pin cable and it is feeding pin 1 and 3 of the STK020 power amp and have 2 caps connected on the negative side to the ground. But the connector with the brown/19pin cable doesn't show values is just 0V. The first components I found in the path are some resistors and 2 transistors T08 and T27. And that's it. I am still waiting for a component tester I ordered, maybe next week is here. And I hope I can count with your help to figure it out because I don't have enough knowledge or experience to do it by myself. Edited March 28 by Rookierafa 1 Quote Link to comment Share on other sites More sharing options...
IanB Posted March 28 Share Posted March 28 A good Easter to you also @Rookierafa and I'm sure we'll get this sorted. Will post more thoroughly later (just got home from work, rather tired) but two types of transistor are "ordinary" bipolar ones with the base etc and FETs with drain etc. The power amp has its own supply that doesn't run through the 7815/7915 regulators. It's the outputs at the top of the PSU diagram on the right labelled +VA and -VB on the diagram (but apparently +VA and -VA on the actual PCB. 1 Quote Link to comment Share on other sites More sharing options...
Rookierafa Posted April 2 Author Share Posted April 2 (edited) Hello again. Hope everybody had a nice Easter, I am still in digestion mode. Today I disconnected and removed the main board from the Casiotone to check it better. I looked into the NJM4558 Op Amps datasheet and saw an YouTube video on how to use the multimeter to check Op Amps. I didn't remove them from the board, so I don't know if the results help in anyway to determine the problem or to narrow the extension of it. I made a draw with the 4 NJM4558D chips and respective reads. https://imgur.com/gallery/YR7zTTb Edited April 2 by Rookierafa 1 Quote Link to comment Share on other sites More sharing options...
IanB Posted April 3 Share Posted April 3 Hi @Rookierafa are those resistance readings? This isn't something I'd really be checking at this stage. I'm speculating at the moment that there is a short circuit somewhere on the main board causing the negative regulator to shut down. Likely culprits would be either a physical solder "bridge" somewhere (have you soldered anything on the main board?) or a capacitor or possibly transistor which is short circuited. Try doing a continuity test on the main board, while it is unplugged, between the incoming regulated voltage rails and ground. If either is shorted (it "beeps") that will be useful although it probably won't be that simple. It's a bit strange that this all started with the PSU suppression cap blowing but that might be a red herring; possibly blew open circuit years ago. Quote Link to comment Share on other sites More sharing options...
IanB Posted April 3 Share Posted April 3 So here is what I'd likely do: connect your meter, measuring voltage, securely to the output of the 7815 voltage regulator- you may have or may need to obtain crocodile clips that slip onto the multimeter probes. Starting with the main board disconnected, turn the power on. You should be reading -15V. Now plug the main board in and see if the voltage drops straight away to zero. Quote Link to comment Share on other sites More sharing options...
Rookierafa Posted April 14 Author Share Posted April 14 Hi IanB I will do it when I return home I am a traveler worker, so I don't have the Casiotone with me now. As soon as I return I will do the experience and post the outcomes. Thank you for the advise. 1 Quote Link to comment Share on other sites More sharing options...
IanB Posted April 15 Share Posted April 15 Keep us posted @Rookierafa🙂 Quote Link to comment Share on other sites More sharing options...
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