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

[GRAHAM ATKINS]

Hi Gil,

I'm wondering now what voltages you have from the rectifier
bridge. Fundamentally no bridge is a perfect AC to DC
convertor, we always get some ripple or modulation of the
DC output related to supply freq, 60Hz. We then "smooth"
this out using a suitable C across the bridge O/P. Look on a
scope for excessive ripple. First though could the bridge be
faulty and is your capacitor suitably rated for the job. If the
SC circuit used 10v tants, is your transformer secondary the
same as theirs ?.

Graham
On Nov 7, 2007, at 17:30, Gil W. wrote:

> Hi, thanks .
>
> This particular v3.2 I got without its bottom/rear
> panel. I build the power supply myself, following the
> schematics, so you can imagine yourself that the
> powering system is not completely compatible to the
> designed one (I used a polysix transformer).
>
> One thing for sure - with regular electrolytes used as
> main filter caps, I get perfect voltages. Could it be
> that the tantalum is too sensitive for heavy filtering
> applications ? I might consider taking it to a friend
> once I complete restoring it, and scope it. What
> should I ask him to look at ?
>
> Thanks very much.
>
>
> --- GRAHAM ATKINS <gatkins at blueyonder.co.uk> wrote:
>
>> Mmm,
>>
>> I always think if the "designed" component is
>> failing it
>> suggests as you say an underlying problem before
>> this
>> stage. It does not pay to cover up problems which
>> could
>> lead to more serious damage to the keyboard later.
>> If it
>> was my Prophet 5 I would want to ensure it was
>> safe.....
>> PCB's not readily available !!!!!!.
>>
>> I'd like to get a scope across the rails.
>>
>> Graham
>> On Nov 7, 2007, at 16:23, Gil W. wrote:
>>
>>> I asked myself the same question - does any
>> problem
>>> actually *exist* or is the tantalum just not good
>>> enough for this task ? I think the original cap
>> was a
>>> 10uf/10v tantalum...
>>>
>>>
>>>
>>> --- GRAHAM ATKINS <gatkins at blueyonder.co.uk>
>> wrote:
>>>
>>>> David,
>>>>
>>>> The electrolytic may be more tolerant than the
>>>> tantalum but
>>>> the underlying problem with high ripple off the
>>>> rectifier may still
>>>> be there, are you sure it is OK to connect to the
>>>> rest of the circuits.
>>>> Are you just covering up the problem ?
>>>>
>>>> Graham
>>>> On Nov 7, 2007, at 9:46, Gil W. wrote:
>>>>
>>>>> Hi,
>>>>>
>>>>> The problem has been solved by replacing the
>> tant.
>>>>> with regular 10uf/50v cap...
>>>>>
>>>>> Thanks all !
>>>>>
>>>>>
>>>>> --- "Cornutt, David K"
>>>> <david.k.cornutt at boeing.com>
>>>>> wrote:
>>>>>
>>>>>> Possibly either the rectifier has an open
>> diode,
>>>>>> or something else is causing the supply rail to
>>>>>> oscillate.
>>>>>> Tantalums don't like a lot of ripple current.
>>>>>>
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>>>>>
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