electro-music.com   Dedicated to experimental electro-acoustic
and electronic music
 
    Front Page  |  Radio
 |  Media  |  Forum  |  Wiki  |  Links
Forum with support of Syndicator RSS
 FAQFAQ   CalendarCalendar   SearchSearch   MemberlistMemberlist   UsergroupsUsergroups   LinksLinks
 RegisterRegister   ProfileProfile   Log in to check your private messagesLog in to check your private messages   Log inLog in  Chat RoomChat Room 
go to the radio page Live at electro-music.com radio 1 Please visit the chat
  host / artist show at your time
today> Sorry, postponed until next week Chez Mosc
 Forum index » DIY Hardware and Software
uA726 - a word of advice
Post new topic   Reply to topic Moderators: jksuperstar, Scott Stites, Uncle Krunkus
Page 2 of 2 [32 Posts]
View unread posts
View new posts in the last week
Mark the topic unread :: View previous topic :: View next topic
Goto page: Previous 1, 2
Author Message
LeBon



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

PostPosted: Wed Jan 05, 2022 9:11 am    Post subject: Reply with quote  Mark this post and the followings unread

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.


s-l1600.jpg
 Description:
 Filesize:  135.65 KB
 Viewed:  122 Time(s)
This image has been reduced to fit the page. Click on it to enlarge.

s-l1600.jpg


Back to top
View user's profile Send private message
nuipb



Joined: Oct 06, 2009
Posts: 47
Location: France

PostPosted: Wed Jan 05, 2022 12:12 pm    Post subject: Reply with quote  Mark this post and the followings unread

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.
Back to top
View user's profile Send private message
LeBon



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

PostPosted: Wed Jan 05, 2022 3:38 pm    Post subject: Reply with quote  Mark this post and the followings unread

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!

Last edited by LeBon on Thu Jan 06, 2022 10:27 am; edited 1 time in total
Back to top
View user's profile Send private message
nuipb



Joined: Oct 06, 2009
Posts: 47
Location: France

PostPosted: Thu Jan 06, 2022 3:32 am    Post subject: Reply with quote  Mark this post and the followings unread

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
Back to top
View user's profile Send private message
LeBon



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

PostPosted: Thu Jan 06, 2022 5:44 am    Post subject: Reply with quote  Mark this post and the followings unread

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.
Back to top
View user's profile Send private message
nuipb



Joined: Oct 06, 2009
Posts: 47
Location: France

PostPosted: Thu Jan 06, 2022 6:07 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks.
LeBon wrote:
6V2 is a generic 6.2V zener diode, BZX79C6V2 or 1N753.
So I understand my first link was right.
Back to top
View user's profile Send private message
synthRodriguez



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

PostPosted: Mon Jan 30, 2023 3:49 pm    Post subject: Reply with quote  Mark this post and the followings unread

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?
Back to top
View user's profile Send private message
Display posts from previous:   
Post new topic   Reply to topic Moderators: jksuperstar, Scott Stites, Uncle Krunkus
Page 2 of 2 [32 Posts]
View unread posts
View new posts in the last week
Goto page: Previous 1, 2
Mark the topic unread :: View previous topic :: View next topic
 Forum index » DIY Hardware and Software
Jump to:  

You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
You cannot attach files in this forum
You can download files in this forum


Forum with support of Syndicator RSS
Powered by phpBB © 2001, 2005 phpBB Group
Copyright © 2003 through 2009 by electro-music.com - Conditions Of Use