Roland Alpha Juno 1 -- no stereo operation

[timrichter9]

Ok.
Roland Alpha Juno 1. Some case screws missing so someone was inside it previously. No obvious signs of burned parts or non-factory soldering.
Here is the symptom. When I plug a 1/4" mono cable into the Mono, and put it on a violin patch, I get a strong signal that registers about 30mV on my scope (volume all the way up). When I move the cable to the other side, I get the same thing. However, when I plug both cables in (one into MONO jack, one into STEREO), the STEREO side has no output and the mono side is normal (30mV)

Ok...

1.) Does 30mV seem low for a keyboard output with volume all the way up (p-p, scope set to 5mV/Div, and got 6 divisions peak to peak).
2.) Does this indicate one "side" is blown?
3.) Looks like the signal is mono until the CHORUS effect is added (from the block diagram).

http://www.cem3374.com/docs/Manuals/.../Juno-1_SM.pdf is the service notes.

Any thoughts on what to do to verify it? This is my first Keyboard diagnostic. I usually work on tube amps with no ICs or Transistors. Thanks

[Enzo]

Who brought you this? We just had a thread on this exact question within the week.

[Enzo]

Ah, I just looked, it was YOU.

OK so you have a dead output. The jacks are normalled together, so plug into either ONE and you get the mono signal. The headphones runs on the same signal, is one side missing there too?

BY far the most likely fault is the volume slider itself. You got signal on the top of both halves of the slider? Max the slider, you got signal on both wipers? The bottom has not been pushed out of the slider has it? If the signal is not even getting there, then on the main board there is a mute transistor for each channel, one could be shorted or stuck on.

The ssynth itself is mono, the thing only splits into stereo for the chorusing at the end of the trail. Since you have a good output, the signalo generation is working. SO if your problem is before the mutes, there is just a few op amps in a row to check.

Get the complete schematic here:
Lothar and the Hand People - It Comes on Anyhow - YouTube

[timrichter9]

Enzo! Thank you. I wasn't even CONSIDERING the volume fader! Duh!! Going to open it tonight and experiment with the scope. Is 30mV at full volume too small for a synth? I have guitar pickups that can output 250mV easily!

Uhh? That link goes to a youtube vid .... am I missing something or was it a cut and paste error?

[Enzo]

HAH!!! I can't believe I did that, I grabbed the wrong link. Lothar was known for using theramins, but that wasn't why he was there.

I think I was going for this:
SYNFO - THE 20th CENTURY SYNTHESIZER DATABASE

Find your Juno there, and the missing schematic pages will be yours.

You reported in post #1 you got that level, but I was not sure what it sounded like to you. Does it come out an amp OK, or is it real weak?

In general, peak to peak is not what we go by, signal is more likely measured in RMS volts, which at 30mv p-p is about 10mv of signal. Is your probe on x10?

One thing at a time, get both channels going, then we can worry about level.

[timrichter9]

OK!!! Thanks to the wonders of an O-scope and a wee bit of knowledge garnered from this board and people willing to help, I believe I have isolated the issue.
I have no signal on 1/2 of VR1 (the volume potentiometer)! So....If I was being adventurous and could not get the 10kx2 Slider pot required, I could set the volume at
full blast by bypassing the pot entirely with jumpers, right?

What is the likelihood that I will be able to find a correct PCB mount 2x10K pot for a 1980s keyboard???

[Enzo]

You missing it on the top or on the wiper. If you have both channels on the tops, then yes, a couple jumpers should work. To keep things as they were, you might also put a 10k resistor to ground from your jumpers to represent the slider resistance.

Part number is 13359353, call Roland and ask if they still have them. Probably not.

SLiders come in standard sizes, like 30mm 45mm 60mm 100mm whatever. Some brands like the legs across the ends, others like them on the sides. Yours look more like the side type. I seem to recall them as small, so maybe 30-45mm.

Determine what is wrong with the old one. Open resistor stripe? Wiper not making contact? You can carefully disassemble it and maybe fix it if it is a wiper problem. Little bifurcated feeler might have picked up some debris. SOmetimes I have a broken off feeler, so I take the shuttle out of some other silder from the same brand and put it in the old slider body.

[timrichter9]

A little bit of DeOxit and now I have intermittent contact. Proof that it works though.
Still seeing about 300mV P-P on the waves. I suppose that is ok. Sounds ok through a little practice amp (Roland Cube).
Thanks for all the help guys. Will try to get part from Roland, if not, maybe bodge a fix

You missing it on the top or on the wiper. If you have both channels on the tops, then yes, a couple jumpers should work. To keep things as they were, you might also put a 10k resistor to ground from your jumpers to represent the slider resistance.

Part number is 13359353, call Roland and ask if they still have them. Probably not.

SLiders come in standard sizes, like 30mm 45mm 60mm 100mm whatever. Some brands like the legs across the ends, others like them on the sides. Yours look more like the side type. I seem to recall them as small, so maybe 30-45mm.

Determine what is wrong with the old one. Open resistor stripe? Wiper not making contact? You can carefully disassemble it and maybe fix it if it is a wiper problem. Little bifurcated feeler might have picked up some debris. SOmetimes I have a broken off feeler, so I take the shuttle out of some other silder from the same brand and put it in the old slider body.