poly800 repair (help with schematic/PSU) - Page 1
Forum section: electronics / repair

author: algorithm
author_rank: Regular Contributor
timestamp: December 23, 2015, 08:34:17 am
content: Hello! I have a korg poly800 mk2. Was given to me and I have had it open for over a year, I would really like to put it back together for good. Now I have been solely focusing on it basically.This keyboard uses a 9v dc wall wart with reverse polarity, so at first I expected the problem was the wrong power source got plugged in. This keyboard took a drink of beer at some point.So I started off replacing Q1, a nec 2SB44A, as well as a zener diode (as I read this being common, and Q1 tested bad.) this did not resurrect it. If I remember correctly, I was seeing +5v, but I wasn't seeing -5v. I recall reading something about a hartley oscillator being used, q18/q17. Im not really sure how to proceed… Could anybody explain how this power section is working?If anybody could take a look at the schematic and chime in, that would be incredible.It is on the page after 18, titled "circuit diagram" cut halfway off scan.(just noticed I referenced the wrong page late this morning,)http://www.synfo.nl/servicemanuals/Korg/KORG_POLY-800II_SERVICE_MANUAL.pdfThanks in advanceCheers

author: algorithm
author_rank: Regular Contributor
timestamp: December 23, 2015, 09:56:57 pm
content: here is a photo in case people don't want to visit the .pdf

author: singapol
author_rank: Frequent Contributor
timestamp: December 24, 2015, 04:20:12 am
content: Briefly...You have been blindly replacing components based on other's experience which may not be same as your's. As usual as per service manual, check for +5 and -5V as these are the main voltages used in the circuits. Look at the power schematic at lower left hand bottom of main pcb KLM 1032. Look at box designated "A" and "D", it stands for Analog and Digital. The negative is tricky, it's marked box "E" at C11 10uf. 16V. There's a box "H" too. If these voltages are present and correct then go on to the CPU IC18 UPD65010. Check +5 supply at pin 64. Also power45 reset "A" at pins 1,2,4,5,7. Lastly check clock signal at pin 63. You will need an oscilloscope for this or at least a frequency counter. This is just a start, after this examine the block diagram and the trouble shooting tips in the service manual. Good luck.One more thing you mentioned 9V reverse polarity..how did did know? That is, which is positive the center pin or outer barerel of the connector? Please check carefully before inserting. Examine the pcb to confirm.

author: algorithm
author_rank: Regular Contributor
timestamp: December 25, 2015, 09:49:54 am
content: Quote from: singapol on December 24, 2015, 04:20:12 amI did mention q1 tested bad before replacing it, right?Quote from: singapol on December 24, 2015, 04:20:12 amIt is 3 in the morning, so I will have to test tomorrow, but I can recall what I saw a few days ago...I saw +5v on the the collector of q3. it seemed like q1 was switching on, which made me think d4 was working as well. I did not see -5v but maybe I was looking in the wrong places… I would like to mention, d2 (protection diode) is good, and I don't imagine a improper wall wart could have got past that. Also, you may have noticed the power switch in the schematic, that is working, sending power to the power section.Quote from: singapol on December 24, 2015, 04:20:12 amThank you for the advice, it is very helpful. Strangely the schematic Ive been looking at does not have letters like in the picture posted. I was confused about where 6-8A was located. I think I was confused about what rd-4 was, might have been mixed up. Either way seemed like the schematic was showing something I couldn't see on the board.Thanks again, I will test the next chance I get and report back.Quote from: singapol on December 24, 2015, 04:20:12 amI have an old junky oscilloscope, made by BK precision.. no longer supported by BK precision, so Ive never seen the manuals for it.Quote from: singapol on December 24, 2015, 04:20:12 amI know because I researched the original transformer and read they are center pin negative. So I modified a 9v wall wart to be right in correspondence to the poly800. I tested the output and made sure with a DMM.Thanks again, it is appreciated.