[sdiy] Prophet 5 -15v rail is shorting... Need advice

[John Henson]

Hi All,
Over the years I have found Tantalum Bead capacitors slung across power
rails to be problematic. They don't take overvoltage or high inrush current
very well, and they go short.
Of course, if you do a resistance check across the rails in the power off
state, you won't usually see a problem, but power the unit up and the main
regulator will shut down due to the short.
Tantalums (tantala?) are great in their element, timing in LFO's and Lag
generators etc. but in these circuits there is a relatively high resistance
feeding into them, so reverse voltages and high inrush don't occur.
In a triple rail PSU, the sequence of voltage rises and time to stabilise
may have some wierd transient effects due to inductances, and parasitic
capacitances in the rectifiers and the possibility that one regulator may
pull against the other for a short time on power up.
Regards
John.
I replace power decoupling Tants with electrolytics, modern ones are small,
have low ESR and take abuse more gracefully.
I wonder if Tants have a lifespan like electrolytics, I know there is no
liquid electrolyte but they are a polarised component.
On a separate note, I'm currently working on a Eurocard sized PCB layout of
Jurgen Haible's discrete SSM2040 filter clone, single sided so I can etch it
at home.
Curious serendipity that it should be such a hot topic at this time.
----- Original Message ----- 
From: "Gil W." <gil_we at yahoo.com>
To: <synth-diy at dropmix.xs4all.nl>
Sent: Saturday, November 03, 2007 11:05 PM
Subject: [sdiy] Prophet 5 -15v rail is shorting... Need advice


> Hi all,
>
> I'm trying to solve a problem with a P5 I just got.
>
> It looks like the -15v rail on the main board is
> shorting.
> Each time I'll power it up, the 10uf tantalum
> capacitor which filters the -15v is getting fried and
> open (shorting) immediately.
>
> Any ideas where to start looking at ? Any common parts
> on the P5 with are known to short over time ?
>
> Thanks for the help !
>
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