Archive of the former Yahoo!Groups mailing list: MOTM

While building some 822s, I have found that I could not get an exactly
perfect match between the A and B channels. While the error was too small
to hear, it caused me to realize that I have not allowed enough slop in the
channel matching trimmers to compensate for every possible part tolerance
error.

So, I am recommending some part value changes for 822 and 831 modules for
those that have not yet been built. Now, if you have already built yours
and it calibrated fine, DO NOT go back and change it. This modification
will not make it any better.

However, if you have not built yours yet, or if you ran out of trimmer when
matching the channels B to A on the 822 or B & C to A on the 831, then
please change these values on your schematic. Also, this reduces the number
of resistors that require matching. Here are the changes:

For the 831 (4 parts total):
Change R27 from 499K to 487K (no longer requires matching)
Change R38 from 499K to 487K (no longer requires matching)
R16 remains a 499 K but no longer requires matching.
Change VR11 from 1K to 20K
Change VR12 from 1K to 20K
Note, you must change the trimmers and resistors at the same time

For the 822 (4 parts total):
Change R30 from 402K to 392K (no longer requires matching)
Change R31 from 402K to 392K (no longer requires matching)
R24 remains a 402 K but no longer requires matching.
R25 remains a 402 K but no longer requires matching.
Change VR18 from 1K to 20K
Change VR19 from 1K to 20K
Note, you must change the trimmers and resistors at the same time

I will be changing the documentation and parts list for both 822 and 831 on
line in a couple of days.

Now, if you purchased matched resistor sets from me for your 822 and 831 and
are planning these changes, I will be happy to replace the two fixed
resistors from that set that changed. You are on your own on the 2 trimmers
since I didn't supply them to begin with. Just send me a note that says,
"Larry you scumbag, You sold me these resistors and are now changing the
value. I want my free replacements." They will magically appear in your
mail box. Of course, you have to tell me the total number of 831 and 822
kits you need resistors for. :)

Once again, this is not a required change. But, the last two 822s I built
had a 0.2% error matching to the "A" channel. Granted, that is very small.
But, it was just enough to pi∗∗ me off so I made the change. :)

Other good news. Mouser now stocks a continuous rotation switch that
matches the "large diameter" switch for you 822 builders (for the "STEP"
positions). If you are building in a rack and can use the larger diameter
switches, you can now purchase a switch that requires no modification. I
will add that part number to the parts list when I make the other
corrections.

If you are using the smaller diameter expensive Electroswitches (required
for wood cabinet builders), there is still no one stocking the continuous
rotation switches for the "Step" switches. However, I just bought another
50 directly from Electroswitch. So, I will have them for those that want
them.

Larry Hendry

P.S. Allied is selling the LT1013 chips for about 1/2 what Newark is
getting for them. I just got some for ~$1.20 each.