[sdiy] MG-1 first mods, VC res

[Roy J. Tellason]

On Sunday 31 October 2004 10:39 am, Metrophage wrote:

> The back panel is going to get drilled out like Swiss cheese, and I might
> need to put full 1A regulators in place of the stock 500mA devices. Oh, yeah
> - I replaced the power supply filter caps, and mounted the original
> regulators on larger heatsinks with fresh goop.These things get extremely
> hot!

They shouldn't.

> Might this be because the 58vct transformer is a but much for +12/-15 volts?

I would tend to think so.

> I wonder if the regulators might be happier with something like 35vct.
> Opinions?

Probably.  Measure the DC voltage going in,  subtract what's coming out, and 
figure out how much power you're dissipating as heat.

First time I built something with TTL parts,  probably back around 1974 or 
1975,  I was of the opinion that with current limiting and thermal shutdown 
that those 7805s I was using were fairly indestructible.  That turned out not 
to be the case when I had 6-7 digits worth of LED display,  7490 counters,  
and 7446 decoder/driver chips.  I had the same problem,  they got HOT!  And 
cycled into thermal shutdown,  repeatedly,  until I destroyed the damn 
things.  Then I went to a better power supply (the transformer also didn't 
fare too well),  and more heatsinking,  eventually using two of those 
regulators when before I'd started with one.

I also remember a discrete design that I ran into in some model or other of 
Thomas organ,  where they had just a few parts that needed +5V.  They used a 
discrete transistor and I think a zener or something.  But WAY too much on 
the input side,  and of course no heatsink even though it was a medium-power 
(TO220?) part.  That part would fail due to thermal stresses.  The factory 
had a solution that involved interrupting the input wire to the part and 
putting some 5W power resistor in there,  eating some of the excess voltage 
in the process.

Always gotta leave lots of slack,  there...