[sdiy] TB-303 terror

[WeAreAs1 at aol.com]

In a message dated 12/9/04 6:18:22 PM, mverbos at earthlink.net writes:

<< Is there a bad memory chip?  >>

Yes, this sounds like what might happen if one or more of the bits coming 
from the CMOS SRAM chip have gone bad, and are stuck in either a high or low 
state.  It may not actually be the RAM chip, though -- it might be a buffer IC 
that the RAM feeds into.  I don't have 303 schemos handy, so I don't know what 
kind of buffer IC's (if any) are placed between the memory chip's data output 
lines and the 303's "DAC" (and R/2R resistor ladder DAC, if I recall correctly). 
 If it's a typical late 70's/early 80's Roland resistor DAC circuit, it 
probably has a couple of 4050 CMOS noninverting buffer IC's attached to the RAM 
outputs, possibly with a few buffer stages paralleled for more current drive.  
Those 303 SRAM's (I think it's a 2114, upd444C, or variant) are not easy to find 
these days, but they're out there if you look hard enough.  Start by 
replacing the buffer IC's -- they cheap and very available.