Help understand schematics - Page 1
Forum section: electronics / beginners

author: taran2ula
author_rank: Contributor
timestamp: July 26, 2023, 12:54:34 am
content: Hello friends. I am hoping someone can help me understand a few things regarding a schematic for a vintage groovebox type synthesizer I own from 2003. I have tried to include the PDF service manual in this post but it is too large so I left the link to it. (https://www.manualslib.com/manual/1361231/Yamaha-Rs7000.html). There are several circuit boards in this unit but the ones in question are PNSW, DM 1/2, DM 2/2 (power supply board very top above DM 1/2) and PNVR-2You can view a general layout, with labels, of the circuit boards on page 13.All of these circuit boards and their detailed connections can be seen on pages 96 & 97. (Note to zoom in hold ctrl and roll mouse wheel. Same to zoom out)Circuit board PNSW is the very bottom board with all the push buttons (some not working even after soldering in new) for the various tracks.Circuit board DM 1/2 is the next board above PNSW and shares connections with multipin connectors on the board below it, PNSW.____________________________________________________________________________Specifically the shared connections are:DM 1/2 (CN4 8 pin second connector upper left of board) to PNSW (CN1 8 pin upper left)DM 1/2 (CN8 16 pin upper part of board third connector from right)DM 1/2 (CN5 11 pin upper left of board third connector from left) to PNSW (CN3 11 pin far right second from top)DM 1/2 (CN7 9 pin fifth connector from far left upper) to PNSW (CN4 9 pin far right last connector on bottom right)______________________________________________________________________________1.) On the bottom of circuit board PNSW there are 16 buttons labeled K-Z. Each button has a diode on the right side of the button. The connections to the diodes on buttons K - R come from an 11 pin connector on the right side of circuit board PNSW labeled CN3 and the connection comes from pin 2.The connections to the diodes on buttons S - Z come from the same 11 pin connector on the right side of circuit board PNSW labeled CN3 but come from pin 3.QUESTIONS: Is this connector wire coming from CN3 pin 2 & 3 the positive leads? Is the current traveling from connector CN3 in the same direction that the arrow of the diode is pointing? Or in opposite of the diode from the negative to positive?The fat thick black wire with the slanted connectors on the other side of the buttons (opposite the diode), is this the negative wire or rail?All of these buttons have LED's above them and are activated by pushing the track buttons while holding down the select button, first button on left of track button K. How does the LED get power? It does not seem to have any connection to those buttons other than the negative lead? Perfect example is the record button. It does not seem to have any connection with the buttons yet when you push record the LED above it lights up. How?2.)On board DM 2/2 the power supply is located. I see regulators changing the input current to 5v and 3.3v but no indication where they go to. How can I tell how many volts go to the buttons on PNSW? And how can I tell how much current or resistance? If I had two I could use ohms law for the rest. In other words I need to kniow how many volts to apply to those buttons and LEDS to test them and how many amps.3.) Last question is how do you test tactile or push buttons when they are not connected to power to see if the switch is good before you solder it in?Apologies for such a convoluted post. Here is the link to the service manualhttps://www.manualslib.com/manual/1361231/Yamaha-Rs7000.htmlI am trying to fix buttons L & T, Section button E, F2, Record, shift, arpeggio on, octave up, mute/scene button, and the store button right next to mute/scene. All located on circuit board PNSW. Also the WAVE button on circuit board PNVR-2Every other button works and I can see that they are wired in parallel.So what I want to do is connect my DC power supply with correct voltage to the connectors on PNSW and see if I can get the buttons to power on. I have continuity from all the buttons (before the diode) to their respective connectors.Well that's it. A novel and I do apologize for such a bloated post but this is driving me crazy. I don't knw enough about reading schematics to figure out what the voltage and current is going to those buttons and LED's and if I knew I could take my tests to the next level. I hope someone can help me with this. Cheers folks and I really want to thank you guys fior you help and expertise. It amazes me how complex electronics can be. I seem to understand it fairly well so far, but certain things confound me badly. I am learning everything from books and the books don't always go into enough detail. Anyhoo. Cheers and have a swell evening folks.

author: Kim Christensen
author_rank: Super Contributor
timestamp: July 26, 2023, 01:11:19 am
content: Quote from: taran2ula on July 26, 2023, 12:54:34 amEdit your post to include the PDF... Sometimes it doesn't get attached. If it's too big, just provide a link to it.

author: taran2ula
author_rank: Contributor
timestamp: July 26, 2023, 09:31:40 am
content: Yes, good idea and thank you for that. Brain freeze. Lol.https://www.manualslib.com/manual/1361231/Yamaha-Rs7000.html

author: Kim Christensen
author_rank: Super Contributor
timestamp: July 26, 2023, 10:36:19 pm
content: Quote from: taran2ula on July 26, 2023, 12:54:34 amCan't call it "the positive leads"... It's a scan output from an IC. What they do is multiplex the switches into a matrix (Saves wires. ie: 4x5 (9 wires) would let you check 20 switches). The diodes are just there to prevent conflicts if the user pushes multiple buttons in the matrix at the same time.QuoteThe switches are inputs to the MCU (Mini computer). The switches signal to the MCU what the user wants to do. The MCU does it's thing and turns on/off the appropriate LED. Check out the block diagram on page 14.QuoteThey use labels and arrows to show where it goes. For example the pin 3 of IC86 is the +5V output of the regulator which goes to a arrow pointing right with the label "+5A". That means if you see anything else labeled "+5A", such as the DAC (IC84) circuit located right below it, this means that it's connected to the same point as if a line had be drawn. Makes the diagram easier to read.Just use an ohm meter directly across the switch. It should read less than an ohm or two when pressed and much higher when released. Don't put power into anything. You'll blow something up. LEDs rarely fail so if they don't work, it's probably something else. You can put your meter into diode check, and if you have the leads the right way around, a LED will glow dimly.

author: taran2ula
author_rank: Contributor
timestamp: July 28, 2023, 10:28:58 am
content: Well thank you kindly indeed. I think that is going to be far beyond my capabilities to work with at my novice level of experience. That's amazing how you can understand this. I want to get there too, and I won't give up, but I need much more time and experience in the books first, obviously. I have used what little knowledge I have, and a bit of common sense to solve the elementary electronics problems with other devices I own so I guess I got the nerve to try and diagnose these issues, lol. I am a self-taught precious metal caster with my own fully equipped studio lab and there are many dangers working with molten metal that I have discovered the hard way, because I have learned the hard way. Today I am fully functional and equipped. I am, of course, equally cautious in my endeavors with electronics. I have been very curious about electronics since I bought this vintage synth (in case you didn't figure it by now, I have a home recording studio with many synthesizers as well), and it didn't work correctly. I am going to read as much material about scan outputs to try and better understand how they work, exactly. All of the buttons and LED's used to work when I bought it, although some needed to be pressed a bit harder than normal. I bought new ones and soldered them in, and this started happening. I did find a couple broken traces and repaired them, but I don't see any more. I would venture to guess that the problem lies in a few bad buttons since I bought them as a bundle. I was not sure how to check these types of buttons with a meter so thank you for that too. I did indeed notice that my meter lit the LEDs dimly, I believe when I did diode check for these buttons. THANK YOU! My god your level of knowledge is amazing. Very grateful for your time and input. Have a wonderful day.

author: taran2ula
author_rank: Contributor
timestamp: July 28, 2023, 04:21:33 pm
content: I know you stated not to apply power to anything for fear of frying something and I certainly will not do so unless you say it is ok, but should I put the appropriate amount of V and I across the LED contact points to see if they are ok? If not then the perhaps the problem would likely be the buttons? Or perhaps one of the traces leading to the buttons since they are scanned by that IC, although I have not seen any broken traces using my jewelers high powered magnifiers? Is that scan output located on the same board (PNSW)as the switches? How much power would a red LED use in V and I? So, am I correct in thinking, from your statement that the scan output monitors these switches and when you push a switch, it informs the MCU, and the MCU decides what button or LED circuit to open up for current? I thought when you pushed the button, according to the diagram, that this would close the circuit. I guess I was trying to figure the circuit out and put current to specific areas (the connectors on the circuit board, or the LED, button contact points directly) and see if the buttons and LED's would function, or not. If it didn't work, I thought I would test the buttons, but I didn't know how (thanks to you I do). Especially without any power going to them because it was too touchy to connect all the wires between the boards and then apply AC voltage while pushing buttons and balancing circuit boards precariously without shorting something or potentially shocking myself. One last question I hope you wont be angry. I understand the conventional and electron current flow. In a circuit such as the area of the circuit board where my button array is located, does the current, when opened by the MCU, travel to the button or LED from negative to positive as in electron flow? And if so, does it travel in the same direction as the arrow in the diode symbol, meaning the side with the diode is negative? I note the buttons are non polarized. Anyway, you are a very nice person and I appreciate your help. I can't thank you enough and please forgive me if my questions sound stupid, elementary, or just plain off to you. I am only learning. To me, because of my burning desire to know how circuits work, and the itch to learn, your information was gold. I will take it and run with it. Cheers and have the happiest of days, friend.

author: Kim Christensen
author_rank: Super Contributor
timestamp: July 28, 2023, 08:33:06 pm
content: Quote from: taran2ula on July 28, 2023, 04:21:33 pmJust use your meter on diode check because that's the safest thing. If they glow dimly, then they are OK. (Try the leads both ways. One way it will light)QuoteIt comes from another board. You'd have to trace out where on the PDF diagram which will take a bit of work. But if you do it, then you'll be able to check that both the switch AND the traces to it are OK. If I was working on this, I'd print out the schematic on the largest pieces of paper my printer could handle and tape them together to form one big drawing. Then I'd highlight the circuit paths I'm interested in and make pencil notes on it.Lets take a look at the REC button (SW6). Note that it has a diode in series with it, and the anode, along the anodes of those other diode-button pairs, goes to a wire named SB8... Notice the other end of the REC button goes to a wire named SD(1) Lets trace both wires out. SB8 goes to CN3 pin 9. CN3 connects to CN5 on the DM-1/2 board. It most likely goes pin for pin, so it's probably connected to CN5 pin 9. Double check with a ohmmeter.The other wire SD(1) goes to pin 2 of CN4 which connects to CN7 on the DM-1/2 board and probably connects to pin 2 of CN7. Double check with a ohmmeter.Now you put your meter in diode check. (remember there is a diode in series with the REC button) We know that the anode of the diode connects to CN5 pin 9 on the DM-1/2 board so we connect the red meter lead to there. Then connect the black meter lead to pin 2 of CN7 on the DM-1/2 board. Press REC button. You meter should beep and display like it's checking a diode every time the REC button is pressed.You'll notice a pattern with these connections. So the remaining ones should be easier to figure out. The first few are the most tedious.(It's possible I messed up tracing these wires since it's hard to do on a computer screen and PDF file. That's why I print big spralling diagrams like these)QuoteTypically a red LED would run at 1.8-2.0V @ 10mA... Always use a resistor with LEDs and do the math. Say you had a 5V supply then:5V - 1.9V (midrange LED voltage) = 3.1V across resistor. Since we need 10mA to flow... 3.1V / 10mA = 310 ohms. Next highest standard value = 330 ohms so that what we'd put in series.That's quite a bit of typing and it's lunch time. I'll come back later for the rest.

author: Kim Christensen
author_rank: Super Contributor
timestamp: July 28, 2023, 10:08:55 pm
content: Quote from: taran2ula on July 28, 2023, 04:21:33 pmRemember the X-Y wire matrix that I mentioned? A switch bridges each X-Y cross point. What the MCU does is output a voltage on one of the X output lines while monitoring all the Y input lines. If it sees zero volts on Y, it puts the voltage on the next X line and keeps scanning through all the X lines waiting for a connection between an X line and a Y line. It does this a 100 times a second or so. If a button gets pushed, then there is a connection between one of the X and one of the Y lines. From this, the MCU code can determine which button has been pushed.QuoteYou can see from my explanation above that this won't work.QuoteHistorically, when these diode and transistor symbols were first created, people thought electrons flowed from positive to negative, when in reality, it's in the opposite direction. But it doesn't really matter when looking at schematics. It's OK to think it of as flowing from positive to negative (In the direction of the diode arrow) and being blocked by the diode in the opposite direction. (When the cathode  is more positive than the anode [fat arrow end] almost no current flows in the diode)

author: taran2ula
author_rank: Contributor
timestamp: July 28, 2023, 10:18:11 pm
content: wow thank you so much. I copied all of your response and will go through the whole thing with a fine tooth comb just the way you explained. It is fascinating to learn about the switch matrix and the monitoring of button presses. I never would have dreamed that such a thing took place, and this is old tech from 2003. Man do I have a lot to learn but honestly this is how I learn best, and then study the things intensely that I do not understand wholly. Many, many thanks to you for being so generous to me in your answers, but especially the effort you made to trace things down like that. I will indeed go over it myself and follow up. YOU, are the best! Thank you eevblog and Dave for starting it.