Yamaha CS-70m troubleshooting

[error_trials]

Happy new owner of a CS-70m. This one went unplayed for decades apparently. I’ve got a few issues sorted out, but now looking for the cause of a blown fuse after it’s warmed up…

Some background: Hadn’t been plugged in for some 25 years… battery compartment was totally corroded of course, removed for now and will replace soon. A few scratchy pots cleaned up. Main output was intermittent, opened up and cleaned the relay on the output board and now that’s okay. CH 1 was totally out, turned out to be a messed up balance slider pot.

From what I can tell, all the functions and controls now work, it sounds excellent.

But it still blows a fuse once it’s warmed up. Always F2 on the DC board. It blows whether I’m playing or if I leave it untouched, after ~10-20 minutes.

I disconnected all the boards from the PSU, and powered it on. All voltages check right on (+/- 15.0V, + 10.0V, +/-5.0V), and the fuse doesn’t blow, stayed stable for hours.

So I assume that means the PSU is probably ok, and there’s a short somewhere on one or more of the other boards? Or are there more things I should check on the PSU?

There are 3 fuses on the DC PSU board, it’s always F2 that blows, I assume someone smarter than I could narrow things down looking at the schematic.

I plan to test each board one by one with the connections on the PSU and see if one of them blows the fuse and go from there… any other suggestions? Any similar experiences or words of wisdom? Thanks!

[Yekuku]

Hi, finding the fault should be hard.
Since the unit powers up fine, it means that you cant troubleshoot components with power off because everything should measure fine.
When the problematic component is stressed, probably because it heats up , the fuse pops because the unit is drawing more current than normal. When stressed it may be shorting or having a low resistance to ground.
This kind of behavior can be caused by semiconductors , not passive components and the usual suspects are zener diodes, transistors and ICs.
There are a lot of going on after F2 so anything is possible.
My approach would be to disconnect the power on some of the cards and try to find the card that has the component that blows the fuse.
If you find the affected card then by using hot air you can try to find the problematic component , by heating up components and checking if this accelerates the blowing of the fuse.
Wild guess: Disconnect JK board and check if the fuse will pop after a while . I see a voltage generated after F2 that is going to the relays and amplifier IC1.

[error_trials]

Thank you!! Really appreciate your guidance. This makes sense. I'll go about testing power to individual boards and see how it goes... I'll start with your hunch on the JK board! I had been looking there when I was trying to find the source of the intermittent output, cleaned the relay there which resolved it but also bought a backup IC1 in haste, didn't swap it yet though, maybe that's it after all! Will report back. Thanks

[error_trials]

Yekuku I think your hunch was right! Disconnected JK board, and powered on, ran strong for about 80 minutes, with some periodic playing of the keys watching the voice cards light up etc. But then did blow F2... That said that's much longer than typical, and I'm using regular/fast blow fuses in my trials, and technically I think it should be a slow blow in there, at least that's what the others are. Will try some slow blow and see. Maybe there's more than one short.

In any case it does seem that something on the JK board was probably drawing too much current. I see the IC1, a diode, and the relay like you mentioned after the -10V supply. I'll start there. There's also a handful of transistors and a couple more diodes elsewhere on that board (a few caps and some resistors too).

[error_trials]

Knock on wood but the problem seems solved, with a 3A slow blow in there it runs for hours, sounds great. Even with the JK board connected there doesn't seem to be any more issue. I did blow a bunch of air around the boards, and disconnected / reconnected quite a few of the molex connectors.. and actually hadn't had a slow blow in there since I blew the original, before I had worked on a few other issues, so perhaps some lucky combination of things. I've probably jinxed it by typing this but will report back if any further problems... hooray

[Yekuku]

Well done man ! Sometimes the solution is so simple !

[error_trials]

Thanks - simple is good! It's been playing solid all day. Putting it through its paces, hoping to calibrate the VCOs, VCFs and VCAs... can tell one voice's VCA release is quite a bit longer than the others.

I don't see the tuning/calibration procedure in the service manual, or am I overlooking? Anyone know where I could track this down?

Thanks!

[error_trials]

Let myself google that for myself. A few replies down in a youtube video I saw the reference to the "Troubleshooting Guide" for the CS-70m. I had only seen the service manual, owners manual, and servicing guide. Quite the library. The last couple pages of the Troubleshooting guide seem to show how to calibrate things. It's a bit of a visual maze the way they present it, but better than nothing! Just documenting this in case others look at this post trying to do the same thing.

[error_trials]

Ok problem seems to have been instead with the 0.22 uF timing capacitors on that voice, swapped those and behaving much better. I'm not sure, but based on the soldering and consistency I think this one was factory electrolytics on all the voices (and throughout the synth), except one voice (and not the bad one) has tantalums on those timing caps. Extremely interesting. Still could use a full calibration... but it sounds good enough for now!