Need to repair a Casio PT-20.

[Biffa]

Hello folks. I am new here, so please forgive me if i mess up. 

My grandfather had bought my father a PT-20 when he was a teen. It is a very sentimental object to him. As he had been trying to get it back working a few years ao, he used a higher voltage adaptor, burning out (hopefully) a internal fuse circuit.. I hope on restoring the synthesizer, and gifting it to him on his birthday. Any advice? 

Will be happy to provide any resource that might help restore it. 

[Biffa]

Looking around in other threads, with a person with somewhat of a similar issue, i think i can start diagnosing the issue. CYBERYOGI talks about how it is possible to burn out the cpu with overvoltage. Let's hope it is not the case. Will send photos of it assembled and disassembled

Edited March 11, 2023 by Biffa

[pianokeyjoe]

Sadly I have the issue with my 2 PT20 units that they do not work. They both turn on, but one unit has strange sounds and not quite working and the other just sits there dead but the power LED is on but no response from the keyboard or buttons. I do have experience with people plugging in the wrong power supply. The Casio PT20 uses 7.5V DC NEGATIVE CENTER POLARITY adapters. If you use a 9VDC negative center adapter you should be fine as many of my Casio keyboards with the 7.5VDC worked fine on 9VDC for short periods of time. Now 6VDC which is what the Casio PT1 and VL1 uses, is a different matter. You can use a 7.5VDC adapter for short periods of time with those but not 9. If the PT20 was powered on with a 12VDC or higher adapter, then it's goose IS cooked. Likewise, if you used ANY power adapter with the standard most common POSITIVE CENTER POLARITY on an Early 80s Casio then you have a possible dead keyboard too. Some newer KORG and ROLAND synths use this same NEGATIVE CENTER barrel pin polarity and those power adapter are more expensive and hard to find so there are many of these keyboards getting fried by those that do not have the correct adapter to begin with and just go trying the keyboard with the first thing they have on hand only to burn the instrument up. I fix these and collect them since I was a teen so I have enough experience to say. Sadly if your father used possibly the wrong polarity adapter and then a higher voltage, then that would be a double whammy for the poor thing. Dead batteries left in the Casio keyboards is also a common thing I see when I buy these used and sure enough, dead batteries left inside means corrosive material has leaked all inside the keyboard ruining the circuits inside like indeed is the case with BOTH my PT20 keyboards. IF that is not the case for your father's PT20, have him try batteries to see if it turns on and plays correctly.. Thats step one.. Step two, open her up and take pics and post here so we can determine the failure IF we can.

[Bala]

I am very happy to have discovered the forum CASIOMUSICFORUMS.COM and this is my first post. Right from the 80's, I have been a fan of CASIO keyboards. Since one of my hobbies is electronics, I am naturally interested in troubleshooting keyboards.
Recently, I got a PT-20 from a friend. It was damaged by battery leakage and had stopped working. Disassembled the entire keyboard, cleaned all parts with isopropyl alcohol and put them back. The corrosive liquid from the batteries had damaged a small area of the solder mask on the PCB, but the tracks were intact. As many of the battery compartment spring contacts were corroded, I connected a 7.5V adapter to power it up, but there was no sign of life in the keyboard and even the red LED didn't light up. Later I noticed that the D734 NPN transistor was getting unduly hot when power was supplied. Desoldered the D734, checked with a multimeter and found nothing wrong with it. Soldered it back and checked all the capacitors and diodes. Couldn't locate any defective part. YouTube videos on repairing PT-20 didn't give me any clue to solve my problem. 
Then I came across the post by Pianokeyjoe which clearly stated that Casio PT20 uses 7.5V DC NEGATIVE CENTER POLARITY adapters. This information came as a shock to me. Till that time, I was checking with a Positive Center Polarity connector. No wonder, that transistor became extremely hot. 
Thanks a million Pianokeyjoe, for that crucial information you provided. I made the center pin of the connector Negative and supplied power. The red LED lit up, giving me lots of hope. Barring a few mechanical problems like two white keys have to be pressed hard, now the PT-20 Is functional in all aspects. Luckily for me, the reverse polarity in the power supply didn't permanently damage any component.
Big thanks to this forum, I read many other posts on repairing PT-20 and learned quite a bit of useful info from them. If any more details are needed from my side, I will be glad to provide them. 

[Brad Saucier]

Welcome to the forums @Bala.