[sdiy] MG-1 revisited, LM337 issues

[Bob Weigel]

Count your blessings.  It amazes me while I understand as well as most 
down to the extreme depths of how things work...virtually nothing I put 
together (simle or complex) works until I am ready to perform stress 
analysis tests on it :-).  THIS is always where I get a strange problem 
that nobody has ever seen in some brand new part or something like 
that.  Prototyping. When I get rich and famous I'll hire this work out 
to people with...better luck in these areas. 
    Anyway yes assume you know where to see if you pot is adjusting the 
bias voltage for the regulator.  If that's moving the output isn't then 
pretty much you have a failed regulator provided the voltage coming into 
it is above the needed headroom. You don't have a scope I take it?  
Would be nice to know if there's a lot of ripple too.  Ripple can play 
various tricks on your meter.  Check for AC voltage with it if you dont' 
have a scope. -Bob

Metrophage wrote:

>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
>
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>		
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