[sdiy] replacing a transformer, was:Re: (no subject)

Rutger Vlek rutgervlek at gmail.com
Tue Mar 7 14:19:32 CET 2017

Hm, I like that idea of simplifying the PSU and making the expression pedal
more durable. I already learned the unit can't work without the
pedal, there's no normally closed contact that substitutes for the pedal in
connecting audio to the output socket. I'll check if the 6V is wired as the
schematics suggest.


On Tue, Mar 7, 2017 at 1:54 PM, Roman Sowa <modular at go2.pl> wrote:

> It looks like 6V is used only for the lamp in expression pedal that is
> used for photosensor.
> So replace it with good bright LED and skip the 6V part entirely.
> Or leave original lamp and fit inside the pedal any DC/DC converter to go
> down from 12 to 6V. But the lamp wil burn someday anyhow, so why not some
> slight upgrade now...
> You should better confirm that by following the cables if it's really just
> the pedal lamp.
> Roman
> W dniu 2017-03-07 o 13:27, Rutger Vlek pisze:
>> Good point Ingo!
>> Towards the end of the service manual
>> (http://www.vintageshifi.com/repertoire-pdf/pdf/telecharge.p
>> hp?pdf=Yamaha-A-3-Service-Manual.pdf)
>> the PSU schematic is displayed. There's a full PSU diagram, and an
>> overview diagram. The latter shows a straight coupling between both DC
>> grounds of the 12V and 6V lines, however the first diagram does not!
>> Confusing, I'll measure this in the actual unit to be sure.
>> Assuming both grounds are connected, what you wrote is applicable here.
>> So it means I'll have to start looking for a transformer with dual
>> independent secondaries for 6.3V and 13V (or 12.6V). Anyone knows one?
>> At least 7VA total.
>> Regards,
>> Rutger
>> On Mon, Mar 6, 2017 at 6:06 PM, Ingo Debus <igg.debus at gmail.com
>> <mailto:igg.debus at gmail.com>> wrote:
>>     > Am 06.03.2017 um 14:00 schrieb Roman Sowa <modular at go2.pl
>>     <mailto:modular at go2.pl>>:
>>     >
>>     >>
>>     >> Is there any harm in using the centre tap for the 6.3V and the
>>     end taps
>>     >> for the 12.6V supply to the PSU? It means the coil between one
>>     end and
>>     >> centre tap gets a little more loading than the other side. The same
>>     >
>>     > No problem at all, just make sure that added load from 6V and 12V
>>     circuits doesn't exceed current rating for that winding.
>>     Careful. This won’t work if the two secondary windings of the
>>     original transformer are isolated from each other, and they are
>>     feeding two bridge rectifiers, and on the DC side of the bridge
>>     rectifiers the grounds are connected.
>>     With two half-wave rectifiers there’ll be no problem however.
>>     Ingo
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