Vintage Synth Repair

Thanks all,

voltages are ok once the entire system is connected to the PS,
there was a short on the lower board but it is solved now.

I got more issues with the voice card assignments but I'm still 
checking into it




--- In vintagesynthrepair@yahoogroups.com, "Scott Nordlund" 
<gsn10@...> wrote:
>
> >What I mean is, does the tantalum cap get fried because there is a
> >short between the -15v and ground caused at one of the boards, or 
the
> >only possible reason is high secondary transformer voltages which
> >overheat the 7915 regulator ?
> 
> Short answer: Neither.  The cap fried most likely because there was 
too much 
> voltage applied to it (transformer secondary is too high, excessive 
mains 
> voltage, whatever).  If this is in excess of the rated voltage, it 
will 
> fail, it's got nothing to do with the regulator or the load on the 
supply.
> 
> Longer answer: It might be possible that, given excessive current 
drain and 
> leaky filter capacitors, there was a lot of voltage ripple at that 
point and 
> thus the capacitor was placed under undue strain which made it more 
> succeptible to failure.  But I really don't buy that, it's much 
less likely.
> 
> >How do I check the secondary voltages and what should I see 
there ?...
> 
> You'll need an AC RMS voltmeter to directly measure the secondary 
voltage.  
> With no load applied, it will be somewhat higher than it would be 
under 
> normal operating conditions (but the regulated output is still 
regulated and 
> shouldn't vary).  But it's easier to just measure the rectified and 
filtered 
> DC voltage: just measure across the tantalum capacitor.  Since the 
7915 has 
> a 2 volt margin, the input must be at least 17 volts under loaded 
conditions 
> for it to operate properly.  With no load, maybe 18-19.  Voltages 
above that 
> are fine as well, but the excess must be dissipated by the 
regulator: the 
> higher the voltage, the hotter it runs.  The regulator can handle 
up to 35V, 
> something close to that is going to run very hot for even moderate 
current 
> drain (250 mA * (35-15) = 5 W, this is way more than what you would 
want).
> 
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