How to fix the "Say-it-again" board without using an SAD4096
|Firstly the disclaimer: I take
absolutely no (that's zero) responsibility for any damage incurred
whilst attempting to fix the SIA board."Damage" being to either the board, the machine or
YOU! These machines have potentially lethal voltages (200v DC) in them and if you're not sure what you're doing find
somebody who does. (That goes for the soldering skills too).OK. now that's out of the way, good luck and
trust me the Centaur sounds much better with an echo.
|I've cobbled together my efforts on a piece of vero board (wrongly assuming the ic had a 16 pin spacing, it doesn't, it's 14 pin). So don't pay too much attention to my board or you'll end up having to bend the ic's legs as I did! Basically, design your own layout. Please note the use of screened wire (grey leads) on the audio in/out signals. The leads are so short that it's probably okay using normal wire (better to be safe than sorry I thought). Grounded at one end only (vero end). Please also note none of the components require heatsinks, if any of your components are getting hot, turn if off, something's wrong!!!!|
|HOW IT WORKS: Essentially the MN3205 is the same as the SAD4096 (Analogue BBD) but runs on a lower voltage (9v max) so the 12v supply needs reducing to 8 or 9v. This is done by the 7808 regulator (a 7809 would have been ok but I only had a 7808 at the time). If you choose to use a 7809 the resistor values will need recalculating. Eg. for the 7808...........12v x 10k/(4k7+10k) = 8v approx. The resistor divider networks bring the clock voltages (pin 2 and 16) down to a useable level and give the 14/15 Vdd supply for Vgg.(pin 8). The audio input and outputs need no alterations as they are well below the 8v supply and do not clip/distort. As a further note I believe the link F-F has to be cut/open on the "Squawk and talk" board otherwise there ain't ever gonna be an echo!!!|
|I'm sure this design can be improved upon
using a smaller etched pcb and pins soldered straight to the SIA board
but you can't see it in situ, so I'm happy. Hope this helps ..................oh and
any musicians that are reading, this'll probably sort out your effects
pedals with the dodgy SAD4096 ics in them.
PS. I'm not supplying kits for this unless there's an overwhelming demand for it........which I doubt will happen somehow.
PPS. For a digital alternative please see Big Clive's website http://www.bigclive.com/centaur.htm (with lots of other great projects too).
|Good luck , Mark.|
Please change pots R15 (10k), R29(50k) on the SIA board and R69(1k),
R70(1k) on the "Squawk-and-talk" board, preferably with a sealed type of
preset, as they're over two and a half decades old and give much
trouble. Then set them up as per the manual. The caps on my SIA board
tested just in spec. with an ESR meter but I changed them anyway.