Arp Quadra synth power supply upgrade

Thread Starter

kevin wiley

Joined Apr 3, 2021
6
Looking at modifying a power supply for an Arp Quadra synth.
I' m replacing caps and any ic that I find that's bad but am concerned about voltage spikes and think it needs some better protection. Any thots along this line? The schematics are on line and I am starting my upgrade to this old synth. Would love to hear from anyone.
Kevin
 

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Delta prime

Joined Nov 15, 2019
840
Hello there :) welcome to AAC!
I' m replacing caps and any ic that I find that's bad but am concerned about voltage spikes and think it needs some better protection.
Referencing the repair service manual
Is always nice to have if you don't have it already. Could you possibly take a photo of the surgery you have already performed? :)
 

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dl324

Joined Mar 30, 2015
12,685
Welcome to AAC!
am concerned about voltage spikes and think it needs some better protection
Where do you think this protection needs to be? The regulator outputs won't be a source of problems. The inputs will handle "spikes" as long as they don't exceed their maximum input voltages.

Better copy of the schematic:
clipimage.jpg
 

Thread Starter

kevin wiley

Joined Apr 3, 2021
6
I have all the schematic diagrams and I purchased a capacitor replacement kit for all caps as well as 4075 filter mods and main cpu oscillator upgrade kit and tempco resistors for the vco's etc. I have yet to begin revamping everything, was looking for some advice about power spikes at power on/off, if anyone could see a need to improve the original ps.
Thank you for responding,
Kevin
 

Thread Starter

kevin wiley

Joined Apr 3, 2021
6
I don't really have the expertise to suggest an improvement, but I'm sure someone here does.
Thank you all for looking,
Kevin
 

dl324

Joined Mar 30, 2015
12,685
did not know the regs kept all the spikes at bay at startup
That's the function of voltage regulators. All three of the regulators are of the linear type. As long as the input voltages stay within the allowed range, the regulators will regulate their outputs.

That said, there could be cases where spikes on the input could perturb the outputs, but we'd need to know the problem(s) that was causing before an appropriate mitigation could be implemented.
 

jlm1948

Joined May 19, 2014
5
Actually, spikes happen at turn-on/turn-off. The regulators cannot avoid them, because they are temporarily out of their normal operation area.
Many of the circuits in synths are not balanced, so at start-up their output voltage goes from zero to their normal operating point, and that is unavoidable.
Now, there are solutions that involve a time delay, during which the output signal is muted, to be unmuted when the other circuitry has reached its permanent regime.
 

anniel747

Joined Oct 18, 2020
435
Looking at modifying a power supply for an Arp Quadra synth.
I' m replacing caps and any ic that I find that's bad but am concerned about voltage spikes and think it needs some better protection. Any thots along this line? The schematics are on line and I am starting my upgrade to this old synth. Would love to hear from anyone.
Kevin
It survived 40 years, why would it need better protection?
 

Thread Starter

kevin wiley

Joined Apr 3, 2021
6
Good point, was intrigued by the last link and wondered about spikes. Thank you all for responding, just doing my best to protect this old synth.
Kevin
 

3D09

Joined Oct 5, 2017
1
4000A Series CMOS digital logic had a 15V Absolute Maximum Rating, and 4000B Series CMOS digital logic had a 18V Absolute Maximum Rating. The 4000A devices would have a 5V to 10V power supply, and the 4000B devices would have a 5V to 15V power supply. The 4000A devices had numerous problems and were quickly replaced by the 4000B and 4000UB series. Usually a 4000A device would fail because of ESD damage. Analog synthesizers typically have patch bays that can direct ESD to an internal circuit. It was common to use 4016, 4051, 4052, 4053, or 4066 Analog Switches to route signals. Running the analog circuits at +13V/-13V versus +15V/-15V may cause some sonic differences, but maybe the Arp Qudras on classic recordings were running at +13V/-13V? I doubt you would notice a difference. When the 4000A devices in Arp Qudras started to fail, a design issue was probably suspected. If they couldn't reproduce the problem, they would have to make a guess. The resistor was changed to drop the supply voltage from 15V to 13V, to provide an Absolute Maximum Rating margin, for the 4000A devices. This was an obvious design flaw and had to be fixed. The 0.01uf capacitors would provide high frequency decoupling for the voltage requlators, so the voltage regulators may have been going unstable during power-up, or from an inductive load being switched on, like a motor. Look at the +13 and -13V lines with an oscilloscope; if you see a spike above 15V, when you switch things on and off, there's an issue. The SA15A Transient Voltage Suppressors (TVS) add overvoltage protection in case of a circuit failure. Some failures would be a dusty trimmer pot wiper or an intermittent Sense line, because of a corroded contact in the P1 connector. Note, the first schematic shows a P1 connector with Sense lines, the second one does not. I think you're on track. I would make the changes called out in Field Change Notice, except for the wrong resistor value. If there's any 4000A devices installed, you could get some 4000B devices to replace them, but it's not necessary. The service manual mentions a 4016 and 4053 in section 3.5.11. And if your playing the synth on a low humidity day, I would touch the chassis to discharge any ESD before I touched the patch bay wires or controls. If the TVSs get hot, turn it off immediately; this means the voltage is above 15V and the TVSs are clamping the voltage somewhere between 16.7V to 18.5V per the data sheet.
 
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