[sdiy] Siel OR400 power supply busted

[Steve Lenham]

Hi Maeghan,

> I've learned that CMOS chips are super sensitive so I'm just going to
> order replacements for all the CMOS chips.

TBH, I wouldn't really recommend the blanket approach. Not only can it 
be needlessly expensive, but you have to interfere with a lot of 
circuitry which is likely just fine as it is. Whenever one fiddles with 
something there is a small chance of damage or a mistake, so I prefer to 
keep the fiddling to a minimum. You also learn more by diagnosing the 
exact cause of the problem.

On the other hand, sometimes problems are really hard to diagnose, and 
it would be useful to have replacement ICs on hand, so...

> As for the tools; yes! I am equipped with an Oscilloscope.

Good - it will really help. Hope you have figured out how to work it now!

I'm going to rearrange the observations you made:

 > The power supply, when it is unloaded and not powering the synths
 > circuit, it gives off the proper power/voltage.
 >
 > When it is loaded the positive 12 signal jumps up to 15 and the
 > negative jumps down a bit.

That sounds wrong, and indicates that there is still a power rail issue 
somewhere. There is a limit to how good things can get before this is 
resolved, and you should address it first.

It would be worth unplugging the logic, EQ/controls and contacts PCBs in 
turn and in combination, then remeasuring the voltages. If the voltages 
stay OK with one particular board disconnected, you know the problem (or 
one of them) must be there. There is no point in unplugging the 
filter/delay PCB since all power to the rest of the unit goes through 
that, but if there is still a problem with the other three disconnected 
then the filter/delay board may be at fault.

 > As for the LEDs, the LED's that are not mounted on a switch
 > (Animator,LFO, Brass, Piano, Reed, Strings) are very faint.
 >
 > The LEDs that are on the switches: Equalizer, Oct. Down, Animator,
 > and LFO are bright.

The switch LEDs are all driven from the Effects PCB. This uses the +/-6V 
rails, and power must be reaching it.

The other LEDs are mostly located on, or driven by, the Logic PCB. In 
particular, the Brass/Piano/Reed/ Strings LEDs are driven from simple 
flip-flops (ICs 2 and 3), so the fact that they are not working well 
indicates a fairly fundamental problem. That circuitry also uses the 
+/-6V supplies, which we have established are not missing entirely, 
otherwise none of the LEDs would be bright. Try checking that all four 
power rails are present on ALL of the circuit boards - could there be a 
missing ground or power connection between the separate PCBs?

> When I turn it on I hear nothing through the Low output, i hear what
> sounds like some white noise maybe on the High output; also, when I
> press a key I hear a very faint clicking sound.

The signal generation for your synth goes:

Master oscillator > Top Octave Generator > string of Divider/Keyer ICs > 
Filters > Chorus > Mixer

The first three elements all live on the Contacts PCB and, if the power 
rails on that board look roughly OK, you should be able to trace the 
signals through with your scope and see where they stop.

At IC2p1 (pin 1 of IC2) you should see a high frequency (~1-2MHz) 
squarewave master oscillator signal.

On pins 3-8 and 9-15 of IC3 you should see squarewaves of a few kHz or 
so - these are the 12 notes of the top octave.

These go to the 12 divider/keyer ICs, IC5 et al - one for each note. 
When you press a key, the correct divided-down tone is routed through to 
the mixers (IC16). You should be able to see this happening with the 
scope - look at the outputs of IC16 as you press keys.

If some or all of these signals are missing, there's one of your faults. 
If they look OK, but are not getting keyed through, it may be an issue 
with the common key line coming from the Logic PCB (pin 1 of ribbon C).

The little clicks you hear make me think that the keyers are trying to 
route signals through when you press a key, but that the signals are 
missing.

> When I turn the Equalizer on, the white noise increases in dynamics.

That's probably a good thing - shows that something is happening.

I hope this gives you a few things to try. Unfortunately I'm going to be 
away for a while and unable to keep in touch, but hopefully others will 
chime in when they see your results.

Good luck! One way or another, this synth CAN be fixed.

Cheers,

Steve L.

Steve Lenham
Benden Sound Technology
www.bendensound.co.uk