VCO not working / D25 DC 0V

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975262801
 
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VCO not working / D25 DC 0V

Post by 975262801 »

Hi, my x0xb0x is not working. The SEQ works (there is signal at R146), but there is no signal on all other section. The J4 power supply is fine. I got 5.33V and 12V goes to the main board. I found something wrong in the VCO section (that's why there is no signal goes to the VCF). The junction point of D25 and Q27 I got DC 0V. The junction point of D25 and Q25/Q24 are all DC 0V reading too. Other DC voltage seems fine except the D25 and it's junction point. I have already changed some new transistor on Q27/Q24/Q25 and new diode on D25. I got something around 6V when I changed the new transistor. However, when I turn the switch to keyboard mode or random mode try to play the x0xb0x, the D25 and it's junction points drop back to DC 0V. I don,'t know what's going on. D25 is not short to ground, also R101 seems to have the right reading. I've spend whole night and I still can't think of what cause D25 and the junction point have 0V. Anyone please help. Thanks

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Last edited by 975262801 on Wed May 30, 2012 7:40 am, edited 2 times in total.

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antto
 
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Re: VCO not working / D25 DC 0V

Post by antto »

i think this is part of what generates the sawtooth
and if your sowtooth is broken there will be no square either (since the square is waveshaped from the sawtooth)

if replacing the nearby parts didn't help.. i don't know
maybe check the surrounding parts for propper values (resistors and such) and propper orientation
and check that there isn't some tiny connection to the ground :?

975262801
 
Posts: 30
Joined: Wed Feb 01, 2012 2:06 am

Re: VCO not working / D25 DC 0V

Post by 975262801 »

Hi, thanks for ur reply. Just got back home and I just switch on the power again. I got reading of 6V on D25 for 30 seconds and it drop back to 0V reading again. That's so weird. Then, I have taken out the D25 diode and the junction point of Q25 and Q26 drops to 4.12V from 8.1V. It goes back to normal when I put the diode back, so I 've tried to lift the Q27 middle leg up. I got correct reading on the left and right pins, but I got no reading on Q27 middle pin too.

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antto
 
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Re: VCO not working / D25 DC 0V

Post by antto »

i also have 0V on D25
i played the x0x, and when i put the probe on D25 - the vco mutes
when i remove the probe from there - it takes a small amount of time (about one second or less) till it comes back

*shrug*

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mmm
 
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Re: VCO not working / D25 DC 0V

Post by mmm »

975262801 wrote:Hi, my x0xb0x is not working. The SEQ works (there is signal at R146), but there is no signal on all other section.
If there is signal at R146 it means that the Gate signal is generated by the digital part, true. But I wouldn't jump to the conclusion that then the VCO must be at fault.

Is a proper CV generated at all? Do you get useful values at pin 5 and 7 of IC11?

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