SV: Re: [sdiy] VCS3 heat

[Magnus Danielson]

From: karl dalen <dalenkarl at yahoo.se>
Subject: SV: Re: [sdiy] VCS3 heat
Date: Fri, 1 Jul 2005 03:22:16 +0200 (CEST)
Message-ID: <20050701012216.27678.qmail at web25507.mail.ukl.yahoo.com>

> 
> --- 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.

No, it shunts for the full synth, it is just that the PSU is located on the
same card as the amps and reverb-drive. When the PSU is "weak" the lighening
of the envelope-indicator lamp (+12V to -9V) detunes the oscillators.

> 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.

They do. This is why I recommend over-rating them so that it is actually a
physically larger body to radiate from, or even bolt them to some form of
heat sink, such as the Alu-chassi of the Synthi-A.

> 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.

Old and dry bypass caps will break into the audio with a hum as expected. They
should always be checked. Also, the also make the source "weaker" by providing
a higher impedance than the new ones.

> 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.

Indeed.

On the same time, given some time and knowledge, the Mk I PSU can be fixed into
working operation and also a fairly stable one. But for many others I would
say that building a LM317/LM337 PSU it probably a good and sound solution.

When doing that on a Mk I I would also drop the extra transistor that gives
the +11.3V supply for the amps, it is really just a current-bypass hack to go
around the extra current the amps are pulling. Just to prove how bad the
original design is. Ah well, it kind of works! ;O)

> Shunt regulating are something from the past!

Yes, active shunting is certainly so, but I still want to shunt with capacitors
if I may! ;O)

Cheers,
Magnus