SV: Re: [sdiy] VCS3 heat

[karl dalen]

--- Magnus Danielson <cfmd at bredband.net> skrev:

> 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.

Its shunt only for synth for AMP and rev there its pass transistor.
However we should regard it as shunt but schematic states 100mA
if that is the real case then the 5W shunt pass resistor shouldent
go hot, not even barely mild to the finger tip. However old and dry 
wet bypass caps from old age can fail to regulate at all so then you
would have great ripple wich causes the 5w resistor to go hot, check 
the zener as well if its hot too.

However i agree with Magnus , this is a bad design, my recomendation 
as well as Hinton says at:
http://www.hinton.demon.co.uk/ems/emsmods.html#psu
replace the power supply with a LM317, and a LM337 design.
Shunt regulating are something from the past!

Reg
KD


 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
>