[sdiy] Siel OR400 Transformer needed

[Tom Wiltshire]

Hi Maeghan,

That sounds pretty good. If there's two red wires and one white wire, it's reasonable to assume the white wire is the centre tap, and the two red wires are the two outputs. So you'd need to measure between each of the red wires and the white wire. If you did that and got 16.8V, that's good. That's about what you'd expect from a 15-0-15V transformer, so it doesn't look like the transformer is damaged. So the good news is that you don't have to replace the transformer.

The next step is the voltage across the rectifier.

The easiest place to get to that might be across those two large 1000uF caps, Ca1 and Ca2. They're grounded in the centre, so you need a probe there. Then you can test the other contact of each cap and you should see 20V across them (marked on the schematic). The voltage should be DC since it's been through the rectifier. Alternatively, you could test straight from top to bottom and you'd expect to see 40V.

If the rectifier is dead/damaged or those caps are shorted, you'll see less, probably a lot less.

Incidentally, those caps are a good bet for a failure, in my view. They're only 25V rated and they've normally got 20V across them, so they're fairly close to their limit. Given the age of the synth, they're probably getting pretty feeble. Achim spotted them too, and suggested replacement with 35V or 50V-rated caps if they fit, and that's not at all a bad idea. It's likely that new caps *would* fit since modern capacitors get more capacitance into less space. Some people would replace electrolytic power supply caps as a matter of course to prevent future failures ("Recapping").

Hope this helps,
Tom


On 26 Jul 2013, at 20:19, M Maeghan Skala <synth.diy at pulsewidthmod.com> wrote:

> Dear Tom,
> 
> FOrgive me cause I'm a newbie … the transformer's got three wires coming from it (that go to the power supply board). 2 red and one white … the two red wires both read 16.8, the white wire reads 0 … from my understanding the secondary voltage is the wires coming from the transformer that go to the power supply … is the white wire supposed to read something?


> On Jul 26, 2013, at 1:47 PM, Tom Wiltshire <tom at electricdruid.net> wrote:
> 
>> Well, that doesn't look *too* awful. That schematic you posted shows +/-6V and +/-12V supplies.
>> 
>> You've got +5.9 and -6, so that's probably the +/-6V supply, and it looks like it's working. The other voltages aren't so good. +9.5 is a bit low for +12V, and -10.3 is not -12V either, so those aren't so hot. If you didn't have the main synth boards connected when it went bang, you're lucky, 'cos you unlikely to have damaged irreplaceable. Power supplies are just power supplies, and anything that gives the right voltage at the right amperage will do.
>> 
>> If you're happy poking around in mains powered equipment ( and I can see why you might not be after your recent experiences) you should test the AC output from the secondary of the transformer (NOT the primary). It's AC, so don't test it with a DC meter (my usual error - forgetting to switch the multimeter to AC).
>> 
>> If the secondary voltage there is ok, probably the regulators are dead, or possibly there's something shorting/ed and dragging the output down. If the secondary voltage is not ok, then you definitely need a new transformer, and once you've replaced that you can repeat your test and find out if you got lucky and that's all that's wrong.
>> 
>> Good luck, and be careful.
>> 
>> Regards,
>> Tom
>> 
>> 
>> 
>> On 26 Jul 2013, at 17:54, M Maeghan Skala <synth.diy at pulsewidthmod.com> wrote:
>> 
>>> So I decided to test the poet supply once more so I connected everything back up and did some readings … out's for each of the IC"s are as follows:
>>> 
>>> IC1:		-10.3
>>> IC2:		9.5
>>> IC3:		-6
>>> IC4:		5.9
>>> 
>>> Just to make sure I tested at the Ribbon cable out:
>>> 
>>> 5.9
>>> 0
>>> -6
>>> 9.5
>>> 0
>>> -10.3
>>> 
>>> So the synth is giving a constant noise, it's also modulating it's pitch. When I hit the keyboard, it does nothing. I guess when the power supply short circuited … it arced over to other components. This is a really really sad day for me. I can't afford to send it off to get to get fixed. It's all over most of tracks :(
>>> 
>>> I don't know what to do.
>>> 
>>> I purchased the schematics from music parts.com … :( I don't know what the hell I'm doing.
>>> 
>>> 
>>> 
>>> 
>>> 
>>> On Jul 26, 2013, at 6:16 AM, Steve Lenham <steve at bendentech.co.uk> wrote:
>>> 
>>>> On 26/07/2013 10:39, M Maeghan Skala wrote:
>>>> 
>>>>> So, I"m looking for a 30VA transformer, center tapped … which has two
>>>>> secondaries rated at 15VAC.
>>>>> 
>>>>> Please correct me if I am wrong.
>>>> 
>>>> That's about right.
>>>> 
>>>> To be absolutely clear, a transformer will only be described as centre-tapped if it has three secondary pins (two ends and one centre tap). It may also be described as, say, 15-0-15 or 30VCT.
>>>> 
>>>> Here in Europe at least, pretty much all transformers now seem to be made with two separate secondaries, i.e. four secondary pins. These are not strictly centre-tapped. They can be wired centre-tapped as I described, but can also be used in other configurations which is why the modern parts are made that way. They may be described as 2 x 0-15 or 2 x 15V.
>>>> 
>>>> BTW, it is OK to use a higher VA-rated transformer if it is easier to source and will physically fit.
>>>> 
>>>> HTH,
>>>> 
>>>> Steve L.
>>>> 
>>>> 
>>>> 
>>>> 
>>>> 
>>>> 
>>> 
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>> 
>