Thema:	Vectrex buzz reduction - mods.zip (00/10)
Von:	ccwest@ix.netcom.com (Bill Seiler)
Datum:	Sat, 13 Mar 1999 01:53:55 GMT

Here are my modifications to reduce the buzz in my Vectrex.

I add an OPAMP mixing circuit to the Logic board to increase the
signal level sent the Power board.  I reduced the gain of the audio
amplifier on the Power board to match the higher level signal now
coming from the Logic board.

Detail Steps
1) Open your Vectrex and locate C323(4.7uF) and R327(33K).  They are
on the edge of the Logic board opposite the cart connector.

2) Remove C323(4.7uF) and R327(33K).  

3) Air wire the C323 to the back side of the Logic board.  Connect
C323's plus side to the original plus side via hole marked with a "+"
in the Logic board.

4) Air wire the R327 to the back side of the Logic board.  Connect one
end R327 to the negative side of C323.

5) Connect the other end of R327 to R327's original via ini the Logic
board near R346.

6) Connect a wire from the junction of C323 and R327 to the negative
input of the OPAMP circuit.  On my TL082C OPAMP this is the white wire
going to pin 6.

7) Connect a wire from the OPAMP circuit output pin to other
unconnected via for C323 on the Logic board.  On my TL082C OPAMP this
is the gray wire coming from pin 7.

8) Connect the +5V, -5V, and GND from J204 on the Logic board the the
OPAMP circuit.

9) Add a resistor R403(1K) in parallel with R105 to the back side of
the Power board.

Enclosed is a ZIP file mods.zip which contains the folling files.
audio.jpg    - OPAMP schematic added into a the Vectrex circuit. 
opamp.jpg - Photo of OPAMP circuit stuck to back of Logic board.
mod1.jpg    - Photo of the Air wired C323 and R327.
mod2.jpg    - Photo of R403 added to back of Power board.

Parts required
R400	100K
R401	200K
R402	33K
R403	1K
C400	0.1uF
C401	0.1uF
U1	General OPAMP (TL082C, 741 etc.)

Bill Seiler 
750 University Ave, Suite 270 
Los Gatos, CA 95032 
408 395 4697 Direct 
408 395 6487 FAX 
ccwest@ix.netcom.com



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