[sdiy] VCS3 heat

[Magnus Danielson]

From: mark verbos <mverbos at earthlink.net>
Subject: [sdiy] VCS3 heat
Date: Wed, 29 Jun 2005 22:49:17 -0400
Message-ID: <42C35DAD.8060605 at earthlink.net>

> Hi everybody,

Mark,

> After 10 years of having only one side on my VCS3, I have found and 
> replaced the amplifier chip. The channel now works but I am worried by 
> one thing still. Should the power resistors at the top of that board be 
> getting so hot that they burn my hand when touched? The board shows 
> burning around the solder points. What would cause that? Is it normal? I 
> just don't want to be right back to a blown chip next week.

The power-supply of Mk I VCS3 and Synthi-A runs hot, since it uses shunting
regulation. In that powersupply the resistors run hot. My best recommendation
is to mount either larger resistors (more mass and heat-radiating surface) and
make sure it has alot air-contact for convection.

Whatever you do, make sure that the solder-points are fresh. Mk I PSUs all
suffer from heat-related stress so you can see that there is tubes in the
solderjoints and the resistor leads is becomming more and more "just inserted"
into those tubes. Scary stuff.

They redesigned them for the Mk II. There also exists a few variations on the
Mk I theme. On a Synthi-A Mk I we moved the initial transistor to the aluminium
frame and that helped to distribute the heat from concentrated heat to
distributed form. On that Mk I there was an additional transistor, but the
extra alu-flange on the PSU board is *BAD* since it creates a local "heat
pocket" under which all heat-radiating transistors and resistors all sitt in
reduced convection and creating their own micro-climate of considerable heat.
Moving the transistor and resistor of-board to the Alu-frame and remove the
heat flange severly improved the state. Beware of copper-strips comming of the
board or having micro-cracks. I've seen a VCS3 being totally dead due to a
micro-crack on the PSU board.

Fun machines, but the PSUs can be a head-acke to repair, and so far all the
ones I have done have had different failure-modes.

Cheers,
Magnus