Vintage Synth Explorer Forums

You are making me feel old, there is no way i can read the pic.
Please have in mind that I am no electronics or repair guru, I just enjoy repairing synths.
You should remove the slider from the pcb in order to test it properly.
In many cases when maximum volume is set , the slider should work at near 0 ohms, lowering the volume should raise the resistance up to near 10kohm for a full mute.Sometimes it is exactly the opposite. Anyways, the point is that the slider should travel smoothly from 0 to 10k.
Now since you dont get 0 ohms, I suspect that the carbon rails or wiper are just dirty or corroded. Also maybe the wiper does not make full contact on the carbon rails, you can try softly bending it but be careful because it might break since it is a vintage slider.
Some times the carbon rails get worn out, in such case the slider works but with noise.
As I said, a good cleaning should should solve your problem hopefully.

I removed the pot, cleaned it, made sure the small wipers were making contact with the rails, and then tested it while it was disconnected. I got readings of 0.4 ohms open and 10.97 kiloohms closed. The readings I'd done a few days ago were wrong -- the values were reversed because I had the underside of the cover off and upside down, so the pots were oriented opposite of what I thought. So anyway, the correct readings suggest the pot is functioning normally -- close to zero resistance open, and around 10K resistance closed. However, when I reassembled everything, the keyboard still gives the same weak sound output, no different from before.

I am sorry to hear this, let me check the schematics and I ll get back to u.

So, I am assuming that you are using the HIGH output jack and that you have the schematics
At the output section, first of all check that both output jacks have proper ground. Broken ground connections here can cause low volume.
Here are the schematics:

As I see the slider that you have cleaned is CA1. Check the signal before the CA1 (114) - (115), it should give you max volume. if it is not, check cables/connection/joints of (114) - (115).
You can use headphones with alligator clips and a needle to listen to the audio signal inside the circuit.
If you still dont have max volume, check first the output and then the input of the preamplifier , to determine if the preamp is working right.

Yes, I'm using the high output jack. I have the schematics, also.

"You can use headphones with alligator clips and a needle to listen to the audio signal inside the circuit." What kind of needle do I use, and where does it go?

Actually what I described is a poor man's audio detector, proper way to do this is using an amp and a speaker, there was a radio shack product about this but I have never seen it or used it.

You need 2 cables with alligator clips, use one cable to connect the headphones jack "SLEEVE" to the RS101 ground.
Then connect the other cable to the jack's "TIP" and the other end to the needle, this is actually your audio probe. You should be able to monitor the audio signal via the left speaker of the headphones.
Dont touch the +15V,-15V lines because you are going to blow the headphones.

red dots are the test points I have mentioned earlier, connect the probe (needle) there.
Did you check for broken ground connection on the output section ?

I just need to buy a few alligator clips and I'll be ready to test, first for a broken ground, then the audio signal. I probably won't be able to for the next day or two, but I'll let you know how it goes!

For broken ground connection , use a multimeter in continuity test mode, while the synth is turned off.
If you have an old headphones set that you dont need anymore, just cut the trs jack and use sleeve and tip just like I have mentioned.
Be careful not to short anything, good luck!

Good news! Using the audio tester you described, I've narrowed down the problem to the preamp section of the Modulation PCB. I've attached an image of the schematic, showing all the points I tested. I think it's either a transistor or a resistor in the preamp that's causing the weak sound.

Unfortunately I can not view the picture properly , it is a bit small, but as it seems you did a good work.
Resistors can get up in value or get open, first check in circuit and if in doubt pull it off and test it.
About transistor testing, it depends on the transistor type, you 'd better google it.
good luck, let us know how it goes

Ohh... so that funny resistor thing with the hieroglyphs is an opto-coupler? That makes sense.

I reckon the problem is there because judging by your diagram you're getting good signals on the emitters of both pre-amp transistors, which would suggest they're ok. Instead, it looks like the signal coming out of the opto-coupler is weak. I found the data sheet of the coupler in question: http://www.datasheetarchive.com/dlmain/ ... 77-750.pdf.

It's the old fashioned 'Vactrol' type - essentially just an LED and a photoresistor. I would replace it; I doubt you'll find an exact replacement but some types of vactrol are still produced that are probably suitable. I'd try the VTL5C4 from Thonk BUT I'm not guaranteeing it will work - others may offer better advice.

Would also be worthwhile replacing the electrolytic coupling capacitor that sits between the pre-amp circuit and the opto-coupler.

Yes, I'll look for a new opto-coupler. I tested all the resistors in the preamp section and they all have readings very close to their original specifications. And last month I replaced that capacitor you mentioned, when I replaced all the electrolytics on the Modulation PCB).
I've been comparing the P873 opto-coupler with the VTL5C4 vactrol. Which characteristics should match (or be similar) between them? The Forward Drop Voltages are 3.0V in both, for example. Any other values that I should focus on? Thanks.

I replaced the photocoupler with a VTL5C4 vactrol. It works, but I'm still getting the same weak outputs at the same points as before. The problem must be in some other component. I also replaced the 100K trimpot (R101) that adjusts the modulation output. Again, same results as before. Could the problem be with one or more of the resistors (R108-112)? I tested them in-circuit, and their values are all normal, but maybe in-circuit testing of resistors doesn't tell the whole story?

Proper way to test a resistor is off circuit, but since they read ok in circuit,most chances are that they are good.
Finally I have managed to save and magnify the picture you have uploaded.
i think that the 3 pink dots should give you the same output, but as you have reported it is not. please confirm.

Have you tested transistor Q23 ?

Two of the pink dots are not giving the same output: the leftmost one and the rightmost one. The leftmost one gives a strong, unmodulated signal (a clean signal with no ensemble effect), and the rightmost one gives a strong, modulated signal. I haven't checked the signal at the center pink dot (the base of transistor Q23), but I will do this and tell you the result.
I haven't tested transistor Q23. It's brand new, I replaced it about a week ago.