CS-80 Resistance Between +15V and GND

[Jonathan40]

With the power off, I am getting a 70 ohm resistance between +15V card riser power rail and ground on my CS-80 - does anyone know if this is this normal?

 

[happyrat1]

You really can't diagnose anything by taking a resistance measurement across active components in circuit.

What issue are you trying to correct? What sort of voltage and current readings are you getting?

Gary

 

[Jonathan40]

It blew the 500mA fuse when I was servicing it and I am trying to rule out any electrical shorts before powering it back up again.

 

[happyrat1]

Try disconnecting the supply from the mainboard, connecting a dummy load and powering up the supply to eliminate any supply issues.

Assuming 500 milliamps at 15 VDC that's approx 30 ohms. Call it 60 to be safe. Use a 10 Watt resistor or better.

Add similar loads to any other taps on the supply. Switching supplies hate to run without a load.

Also do a careful visual inspection of components first to check for any burnt or popped components.

If it passes that test, then so far so good.

Then try hooking it back up again and powering up. If it blows a fuse again then that's what fuses are for and you know the problem is with the main board.

Probably you just ended up shorting a lead with a screwdriver while working on it but better safe than sorry.

Gary

 

[Jonathan40]

I'll be doing a full calibration of the power supply once I dig out some molex pins to connect the +/-15V outputs to the sensing inputs. I can't seem to squeeze alligator clips into the harness to short the pins. Thanks for your help, I'll likely power it up after this and things you mentioned are done. I did a few things that may have caused a short in the system, most notably touching the card riser +15V power rail to the back of the keyboard, where all the PCB's and wires are. Hoping I didn't discharge any cap voltages into the KAS chips. Also jiggled some wires while it was powered up (all oscillators were firing at this point) and heard a popping noise, which may have been the fuse blowing, and then the synth went dead. Was told by a tech to check for resistance from the power rail to ground and got 70 ohms, which made my continuity tester emit a quick beep, which I assumed meant the possibility of a partial short.

 

[happyrat1]

Try reversing polarity on the continuity tester and see what comes up. Should be infinite.

Gary

 

[happyrat1]

Google has a few hits on a CS 80 Service Manual.

https://www.google.ca/search?num=10...nL9GusAXIvZOQBQ&q=yamaha+cs+80+service+manual

http://www.therogoffs.com/cs80/manuals/CS80_Service_Manual/index.html

You may want to check those out.

Gary

 

[Jonathan40]

Thanks, Gary, I'll keep the polarity trick in mind. I have been on the the Rogoff site and printed out the schematics. He also runs the Yahoo CS-80 page. Here are the resistances I'm getting between the power rails and ground:

+15V = 70.3 Ohms

-15V = 64.5 Ohms

+8.5V = 337 Ohms

-6.5V = 878 Ohms
+10.6V = 540 Ohms

I'm thinking the -6.5V could be low, as a friend of mine is getting 3.95 Kohms on his currently working CS-80, as well as 456 Ohms on the +8.5. If anyone can take similar readings on their own CS-80, it'd be immensely appreciated.

Cheers,
Jonathan