9720 VCO trouble shooting
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Author:  jamesveeder [ Tue Feb 08, 2011 1:55 pm ]
Post subject:  9720 VCO trouble shooting

So, i have power going to the VCO now, but it has a few problems that I can not figure out for the life of me :/

i'm still trying to get the modulator working atleast.

The Modulator:
This part seems to only want to work sometimes when i turn it on, and it only cycles when the attack is at a certain point. (Anything above the 8 o' clock position, the led freezes and the modulation stops.) The release effects it, but when turned all the way up, it does NOT take very long to dim back down. And with the output all the way up, i get a few semitones of change (if that) when the mod out isn't patched to anything. when it's patched to P1 of either oscillator or PW it gets a good amount of modulation, though. LED seems to turn on randomly every now and then when playing with the oscillators

Oscillator 1:
This seems to be tuned very low, oscillator also seems to jump pitches quite often, it's very unstable. LFO seems to work when the knob is pass the 3 o clock position. All waves work like this and are the same pitch. Sine wave is also distorted

Oscillator 2:
This one seems pretty stable for the most part, it doesn't jump pitches and if it does its not much. Pitch knob gives it a few semitones of pitch, not octaves though. Pulse width works great. All waves work and are at the same pitch.

I'm getting a lot of bleed from the oscillators.

I've checked all of my solder joints and they seem fine, everything appears to be grounded correctly, and everything appears to be in the correct place. I'll go over it once again to make sure its facing the right way/in the correct place. I have yet to check the resistors, I'll do that too.

Any suggestions and help would be GREATLY appreciated.

Author:  PAiA-Scott [ Tue Feb 08, 2011 3:55 pm ]
Post subject:  Re: 9720 VCO trouble shooting

Check for the power supply voltages from the two three-pin voltage regulator ICs.

Maybe one of the 14pinICs is in place of another??? The TL 084 op-amps can fail if there is an accidental short-circuit between pins (the bottom of the board touching the chassis metal or a tool while powered).


9720ts.pdf [51.89 KiB]
Downloaded 1450 times

Author:  jamesveeder [ Thu Feb 10, 2011 12:30 am ]
Post subject:  Re: 9720 VCO trouble shooting

Okay, Scott, i hate to be THAT customer.

here's a video showing exactly what the VCO does.

I still have yet to find where i've misplaced anything. all of the solder joints look good. I'm pretty new to electronics so forgive me for my lack of knowledge in the subject.

I've read over the troubleshooting and here's some of the dvm measurements i've come up with... most of them are off so i've obviously done something wrong.
group 1
q5 base 0
q5 collector 11.36

q6 base .67
q6 collector .12

q4 base 11.38
q4 emitter 12.04
q4 collector 1.19

d5 anode 12.05
d5 cathode 11.56

ic8 pin 9 5.01
pin10 .58
pin 8 11.18
pin 12 .65
pins 13/14 1.08

group 3
q5 base .31
q5 collector 11.35

q6 base .62
q6 collector .16

q4 emitter 12.04
q4 base 11.38
q4 collector 1.53

d5 anode 12.04
d5 cathode 11.56

IC8 pin 9 5.12
pin 10 5.44
pin8 fluctuates between negative and positive voltages
pin 12 8.44
pin 13/14 fluctuates a lot

This is very frustrating. I really hope i can figure out what's going on. I feel like i'm spending hours staring at this thing and finding nothing.

Author:  PAiA-Scott [ Thu Feb 10, 2011 1:28 pm ]
Post subject:  Re: 9720 VCO trouble shooting

Thanks for posting the video. It seems like it's close, but there shouldn't be the interaction between the A and B sections when the Pitch A is adjusted, and, the AR Release should have more affect on the time and the Attack shouldn't cause the cycle to stop.

The variations in voltages you list don't really tell so much and could be attributable to the condition that's causing the interaction between sections and AR operation not being completely right.

Looking at the assembly manual illustration supplement pages, figs 1 and 2, notice that the AR modulator circuitry is at the panel end of the board and the panel connections for the VCO outputs are very near where the board meets the panel. Maybe a wire or terminal or a wire extending through a terminal is contacting one of the parts on the board.

Another way things can interact when they shouldn't or cause a section to malfunction or be compromised is if a panel point-to-point wiring path soldering connection isn't soldered, or, the solder did not flow to all wires or component leads in the joint (or a terminal/lug). Nudge the bare wire links on the panel wiring watching for any movement in the joints--a sign that the joint needs more heat/solder.

Figs. 3, 4, and 5 are useful for confirming the panel connections. Look to be sure the pots for Rs 203, 204, and 206 have the same markings, and, Rs 200 and 201.

Inspect closed-circuit connectors shown with the shunt terminal (X) for a possible bulge or deformation in the black plastic body--it melting from heat during soldering can cause the shunt to not make it's normally closed contact with the Tip (T). J10 is a connector like this for normally connecting the modulator output to the VCO B Pulse Width control voltage input. J11 works to link the VCOA Pitch control to the VCOB pitch when there is not a plug inserted to substitute a CV for the VCOA P1 control input. J3 is another, used for the VCO A PW cv input.

Sometimes if I can see photos clearly showing the panel connections and top and bottom sides of the board, I can reply about something that looks out of order or possibly in need of attention.

Let me know if you'd just prefer to get the trouble corrected and I'll send the pdf service authorization/information form for sending it here.

Author:  jamesveeder [ Mon Feb 14, 2011 1:06 am ]
Post subject:  Re: 9720 VCO trouble shooting

I may need to request that form, but firstly I'm going to put all of the modules together and test them. I will definitely let you know.

Author:  jamesveeder [ Thu Mar 10, 2011 4:39 pm ]
Post subject:  Re: 9720 VCO trouble shooting

Hello Scott, I'm definitely going to need that service PDF file, I'm having a rough time with this and other, less knowledgeable hobbyists can't really figure out what's going on with it either.

I realize that this is time consuming and I would have absolutely no problem paying for you/partner's services.

I'd greatly appreciate it if you got back to me, I really hate sitting on projects and this one has been staring me in the face for months.

Author:  PAiA-Scott [ Fri Mar 11, 2011 1:01 pm ]
Post subject:  Re: 9720 VCO trouble shooting

I'll get that pdf emailed today.

Thanks. --Scott

Author:  jamesveeder [ Mon Mar 14, 2011 3:09 am ]
Post subject:  Re: 9720 VCO trouble shooting

I never did end up receiving that email. Was it sent to

Author:  jamesveeder [ Mon Apr 11, 2011 1:20 am ]
Post subject:  Re: 9720 VCO trouble shooting

Scott, I was just wondering you guys have received my package and if maybe I could get a status update?

Author:  PAiA-Scott [ Mon Apr 11, 2011 1:22 pm ]
Post subject:  Re: 9720 VCO trouble shooting

Yes, it made it here, but I haven't had the chance to have a look yet.

I saw your email and was working my way to it, but... I'll reply to it when I know more, later this week.

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