Arp Quadra synth power supply upgrade

3D09

Joined Oct 5, 2017
4
Oops. A compressor uses negative feedback to control the volume level, or phase shift of around 180 degrees. Capacitors introduce up to 90 degrees of phase shift, so if there's an open circuited capacitor, you will end up with positive feedback, and have a volume issue. Take a close look at C39 0.33uF.
 

Thread Starter

kevin wiley

Joined Apr 3, 2021
15
Oops. A compressor uses negative feedback to control the volume level, or phase shift of around 180 degrees. Capacitors introduce up to 90 degrees of phase shift, so if there's an open circuited capacitor, you will end up with positive feedback, and have a volume issue. Take a close look at C39 0.33uF.
ok will do thank you!
 

3D09

Joined Oct 5, 2017
4
I read the page from the service manual you supplied. The analog designers of this synth had some impressive skills. I will try to expand upon what it said, because if you can't locate the problem by visual inspection, you will need to know as much as possible about the circuitry to troubleshoot it. You can think of the compressor as an automatic volume control. It's like a person with their hand on a volume knob, and when the sound becomes to quiet to hear the person turns the volume up. When the sound becomes to loud, the person turns the volume down. The expanders do the opposite. So you can visualize the expander as a second person with a second volume control, who turns their volume knob the opposite direction as the first person. The left side of the compressor circuit is the voltage controlled amplifier. Increasing the voltage at the bottom pin of R41 decreases the gain. The expanders are also voltage controlled amplifiers with the control voltage at the bottom pin of R43 and R45. Increasing the control voltage on the expanders increases the gain. The right side of the compressor circuit produces the control voltage. It consists of a full wave rectifier and a filter (envelope follower). The full wave rectifier flips the negative portion of the waveform to the positive side of the zero volt axis, so that you end up with a positive only waveform. This AC waveform is filtered to DC by the Z13B circuit. This DC voltage is the control voltage that tracks the volume level of the compressors output. It is fed to R41, R43, R45, and TP5. If you placed an analog voltmeter across TP5 to ground, the meter would act as an audio level meter. If the compressor is working properly, the meter's needle wouldn't move much. But if you disabled the compressor by lifting the bottom pin of R41 and connecting it to ground, the meter's needle would closely track the volume level of the sound being produced by the synth. When you removed CR1 and CR2, you disabled both the compressor and the two expanders. The control voltage would go to zero volts and compressor would go to maximum gain and the expanders would go to minimum gain. The best way to troubleshoot this circuit would be with a two channel oscilloscope. First I would check the control voltage at TP5 to make sure it's DC and not AC. If it's AC, the filter isn't working. Then I would lift the bottom pin of R41 and connect it to ground. I would verify that the voltage on TP5 tracks the volume level. Next I would connect the 1st channel of the oscilloscope to the input of the compressor (left pin of R26) and the 2nd channel to the output of the compressor (right pin of R28). Then I would lift the bottom pin of R41 and connect it to a variable DC power supply, with the ground lead connected to the synth's ground. I would increase the voltage starting at 0.0V and watch to see if the compressors output waveform decreases. I would do a similar test with each expander. I would increase the voltage starting at 0.0V and watch to see if the expanders output waveform increases. I hope this helps. -Paul
 

Thread Starter

kevin wiley

Joined Apr 3, 2021
15
Thank you 3D09 for your very clear explanation. I have replaced a total of 8 ic's. Everything except for the 3086's ... all other ic's are new and tantalum caps as well. There are a number of square block 10% type caps still original that will be checked out. I will try to go through your troubleshooting suggestions one step at a time. thank you very much!
 
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