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 9720 VCO assembly problem 
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Joined: Sat Mar 07, 2015 6:23 pm
Posts: 10
Post 9720 VCO assembly problem

First post. Assembling the VCO. On page 19 of the "Assembly and User Manual" on step 4):

Listen to Ramp outputs of OscA and OscB. - we had no signal from the speaker.

Measure voltages of outputs -
OscA: DC 220-230mv (expecting 200mv), AC 13-16V (expecting 1.25V)
OscB: DC 220-230mv (expecting 200mv), AC 15-16V (expecting 1.25V)

We checked the solders on the mentioned components and swapped out the transistors. It seems to be a general problem across both oscilators.

What can we do to diagnose/fix this problems?

Thanks in advance,
Nic Roche

Sat Mar 14, 2015 7:20 pm
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Joined: Sun Nov 09, 2008 2:49 pm
Posts: 726
Post Re: 9720 VCO assembly problem
Below are some checks and tests. If you can send photos ( of the board soldering, the parts installed on the board, and the panel connections, I might be able to spot the trouble and reply with advice. For missing ramp outputs (basic to the other waveform outputs) from both VCOs, it seems like it might be power supply related, two or more of the ICs in place of one another, or a panel wiring connection (ie the voltage to the panel pitch controls).

9720 VCO checks/tests Start by measuring the DC voltages on the outputs of the voltage regulators, ICs 1 and 2. IC1 is the 7912 negative regulator and IC2 is the 7812 positive voltage regulator. The pins on these parts read, l-r looking at the markings on the face of the part with the legs down through the board, 1, 2, and 3. On the 7812, the voltage goes in on pin 1 and comes out on pin 3. Pin 2 is the 7812's 0V/ground connection. On the 7912, the voltage goes in on pin 2 and comes out on pin 3. Pin 1 is the 7912's 0V/ground connection.

Put the tester 0V/Common/(-) black probe to G and use the red DC V probe to measure for +12V DC. If the tester has a digital readout, it will indicate negative DC polarity with a (-) preceding the reading, but if it is a meter movement or needle that swings across, the probes should be reversed (put red on ground and measure with the black). or a polarity switch set to the -DC setting. Measure for -12V DC.

An easy spot to get to these circuits is on the power supply input pins on the TL 084 or LM 324 ICs. Pin 4 should have the V+, +12V supply and pin 11 should have the V-, -12V supply. The pins of the IC count around the part in a ccw direction from the notch as viewed from the top of the part. On these fourteen pin ICs, count down one side 1-7 and then proceed straight across and count up this other side 8-14. Pins 4 and 11 are the middle pins on each row of 7.

Other voltage readings that would be useful include the ones on each end of Rs 1 and 2. This will give us an idea of the current consumed on the board. It should be about 33mA on the positive supply and 42mA on the negative supply. To get the current, divide the difference of the readings at each end of R1 and R2, ie 0.73V/22ohms=33mA; 2.35/56ohms=42mA.

VCOA dc voltages (VCOB will be comparable except at pins 6 and 1 of IC6B where readings are 1 to 6 and 1.78, respectively).

IC4 Pin 1 Pins 5 & 7 Pin 8 Pin 13 Pin 16 (-)10.5 2.44 1.34 0.01 (-)10.5

IC7A Pin 3 Pins 1 & 2 (-)5.78 (-)6.04

Q10 Emitter Base Collector (-)11.06 (-)11.11 (-)5.79

IC6D Pin 10 Pin 11 Pin 13 (-)5.9 (-)7.83 (-)11.64

IC7B Pins 5 & 6 Pin 7 (-)6.04 (-)1.65

IC6C Pin 8 Pin 9 Pin 14 4.57 2.75 (-)8.69

junction R86,R87 R87, R95 (-)3.7 (-)2.07

IC8B Pin 5 Pin 6 Pin 7 (-)0.8 (-)0.9 (-)0.1

Scott Lee

Tue Mar 17, 2015 6:15 pm

Joined: Sat Mar 07, 2015 6:23 pm
Posts: 10
Post Re: 9720 VCO assembly problem
Hi Scott,

Thanks. I've sent the photos. I'm slowly working through this (allocating a few hours each week).

With paragraph 4 I'm not sure why I should find the TL 084. Cant I just test the legs of the voltage regulators (IC1 & IC2)?


Sat Mar 21, 2015 10:10 pm
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