uA726 - a word of advice

LeBon · Joined: Mar 23, 2015 Posts: 18 Location: USA

https://www.ebay.com/itm/143981901715

The 10n capacitor is optional. Some synths have it, others don't. The 49k9 resistor runs from pin 6 to ground and sets the temperature of the chip. Its value changes from one synth to another, depending on the design. This is why it MUST be placed on the VCO board and not inside the uA726.

The minimoog has a 49k9 resistor from pin 6 to ground. The System 100 has a 82k resistor on the same place. I don't have the service manual for the RSF. You don't need to add anything to the circuit.

However, I suggest you to replace the 82k resistor on the System 100 for a 174k ± 1% metal film resistor. That's the correct value to get 1V/Octave. I'm pretty sure you won't get proper tracking with a 82k resistor, it's a design flaw.

Most synthesizers from the 70's and early 80's are infested by design flaws. It was an epoch of trial and error, and the result was inevitably... error.

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nuipb · Joined: Oct 06, 2009 Posts: 49 Location: France

Thanks for the link to the sockets.
Of course correct me if I'm wrong but your explanation reads a little like "if it's there do leave it there, and if it's not there don't add it", therefore I don't really feel the need to say it, so why would you mention it ?
except of course in the case of the System-100 since I understand it's an original error...
For the record the RSF Expander has a 200k on pin 6.

LeBon · Joined: Mar 23, 2015 Posts: 18 Location: USA

As a rule of thumb, leave everything where they are and don't add anything else to the circuit, unless you know exactly what you're doing.

For the RSF, in order to calculate the resistor value, I need to know what is the voltage supply to pins 8 and 5.

For the minimoog, the initial copies of the new VCO board (the new design incorporating the uA726) came with a 75k resistor. They soon realized that it was wrong and quickly replaced it for the correct value of 49k9. Ironically, 75k is the value that appears on the datasheet example. My guess is that the VCO designer (which wasn't Bob Moog) didn't bother to calculate the resistor value and simply copied it from the datasheet.

For the System 100, replace the damaged uA726 with my circuit and calibrate the VCO as per manual. If it refuses to tune after the 3rd pass, replace the 82k resistor for a 174k and try again. If it still doesn't tune, then there is probably something wrong somewhere else in the circuit. If everything goes ok, post pictures of your synth board with the new replacement. We're making history here!

nuipb · Joined: Oct 06, 2009 Posts: 49 Location: France

Thanks for everything, note however that it might take a few months before I have one implemented and can send you some feedback as I never had a PCB built and I have to understand the process and make no mistakes.
To start with you uploaded a .lay6 file and the PCB manufacturer I was recommended to asks for "Allowed KiCAD (.kicad_pcb) file, an Eagle .brd file or a single .zip or .rar file containing one Gerber (X or X2+) or ODB++ dataset."
In Sprint Layout I found a Gerber-export feature but it creates 4 .gbr files. (copper_bottom; outline; silkscreen_top & soldermask_bottom)
Does that make any sense ?

Also, one of your parts is called 6V2 is this the one :
https://www.mouser.fr/ProductDetail/onsemi-Fairchild/BZX79C6V2?qs=sGAEpiMZZMsVgcksf1EMUlGoem01Bwuo

or
https://www.mouser.fr/ProductDetail/Vishay-Semiconductors/BZX85B6V2-TR?qs=sGAEpiMZZMukxKgYRb08uKWNVKc6F87MopUD%2FQYNXqU%3D

LeBon · Joined: Mar 23, 2015 Posts: 18 Location: USA

In addition to the 4 .gbr files, you must send to the PCB manufacturer the drill file. File -> Export -> Drill data. Use the default parameters.

6V2 is a generic 6.2V zener diode, BZX79C6V2 or 1N753.

nuipb · Joined: Oct 06, 2009 Posts: 49 Location: France

Thanks.

synthRodriguez · Joined: Jan 30, 2023 Posts: 1 Location: Piqua, OH

Thought I would chime in.

Working on a Korg Trident mkII that I thought had a bad UA726; turned out to be a broken via between the upper and lower layer. At any rate it does use a 75k heater resistor in its design.

It got me thinking though. I've worked on a lot of synths, but the Korg is my first with a 726. I do see a lot of people online finding dead ones/searching for replacements. How does the UA726 typically fail? Does the heater section die? One of the transistors?

mikefiction · Joined: Jan 23, 2013 Posts: 3 Location: Circleville, OH USA

nuipb · Joined: Oct 06, 2009 Posts: 49 Location: France

[quote="mikefiction"]

mikefiction · Joined: Jan 23, 2013 Posts: 3 Location: Circleville, OH USA

nuipb · Joined: Oct 06, 2009 Posts: 49 Location: France

I see what you mean and as far as I was aware of it I didn't "mirror" the drawing. Of course I could have made a mistake that day but doubt it. However I certainly hoped after delivery that the traces would be the other side around at least, so that the parts remain accessible.
I also thought of condensing the PCB once it fully works. My first suggestion would be to reduce the diameter of the UA726 imprint and make it the same diameter as the original. The current diameter is quite bigger and is a pain to assemble...
I'm certainly interested in your results. Please do keep us informed !