[sdiy] MG-1 revisited, LM337 issues

[Metrophage]

It amazes me how I make large circuits which I barely understand, and
they almost always work as they are supposed to. Whenever I make a
circuit which appears to be so simple, one can hardly imagine how it
wouldn't possibly work - it doesn't!

Last year I did a lot of experimentation on mods and extras for my
MG-1. So far there is an extra board with switchable filter caps, a
real VCF summer/gain amp, and an envelope follower with trigger and
gate extractor. This stuff all works quite well, the only problem being
that the stock MG-1 power supply used 7812 and 7915 regulators which
ran way too heart-attack hot. It was barely passable for the existing
circuitry, but certainly not enough for more current.

First things I did were toss the crumbling transformer, in favor of a
nice 36vct unit I had laying around. Replaced the diodes with some
which can handle more current. Replaced the electrolytic caps. Replaced
the heatsinks with beefier ones. Replaced the regulators themselves.
Then it ran only marginally less hot, still quite bad. Since I had a
lot of extra stuff I wanted to add, I put it aside for later.

So getting back to the bench for a while, I took a scrap of perfboard
and put standoffs on it in the pattern of the MG-1 heatsink holes, and
made a simple 317/377 voltage regulator board of it. The MG-1 power
input goes to the inputs of the new regulators, the MG-1 ground
connects to the adjusting trimmers, the regulator outputs are not
connected to the MG-1, so I can tweak the voltage. Firing up the MG-1,
there were no sparks, no smoke, no hot components. I connected my lame
meter to the 317 side and easily tweaked it to +12v. Then I tried the
-15 side, and the trimmer does not do anything. This side always
outputs about 22 volts. The boggling part is that I have looked over
everything there thoroughly, and I cannot find any problem. No wrong
connections, no cold joints, no inadvertent bridges. There does not
appear to be anything wrong with it... so far as I can see. 

All I can speculate for now is that it is an NTE curse. The 317 is a
"real" 317, but the 337 is one which I had labelled as such - it is an
NTE957. The pinout and general characteristics appear to be the same as
a 337, but I don't have any "real" 337 to compare with. I think
whatever place was out of them when I tried to get some. Of course I
guess it is more likely that this is my own stupid fault.

I am wondering whether or not there is any special reason to use
negative regulators for bipolar supplies. Why not just use two 317s? Or
two 78XX? I suppose there must be reasons since I never see this done.

I'm off to play with the baby. Maybe the board will look different
tomorrow...
CJ




		
__________________________________ 
Discover Yahoo! 
Find restaurants, movies, travel and more fun for the weekend. Check it out! 
http://discover.yahoo.com/weekend.html