Computer and Electronic Repair image


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.

  As most people with a Centaur know if the echo disappears from the sound it's usually down to the SAD4096 ic (at position U2 on the "Say-it-again" board). My circuit is based around an equivalent ic to the Reticon SAD4096 called an "MN3205" made by Matsushita (Panasonic). I believe these are more reliable least in as much as mine's still working. The MN3205 is not cheap and not that common although there are some websites selling them and at the time of writing there's a guy on Ebay (U.S.) with some. The main differences between these ic's are the operating voltages and the pinouts.



MN3205 Circuit image


 (Courtesy of Reticon)

 (Courtesy of Panasonic)

SIA Circuit
 (Courtesy of Bally)
See pdf files =====>
Vero Topside image  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!!!!
Vero Underside image Veroboard In Situ image
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  (with lots of other great projects too).

Good luck , Mark.
Addendum:  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.