I’m restoring a non-working Ensoniq SD-1.
The previous owner had the battery replaced and it still has the low battery message.
Powers up and calibrates the keyboard, but no audio signal, just hum, buzz, and noise.
Found some blown components.
The attached schematic is part of the Maid board / Audio board.
I don’t understand the circuit. Is it a further part of power regulation and filtering?
C152 was exploded and VR3 is shorted input to ground.
I’m trying to understand what could have caused this before I replace the parts.
Any thoughts?
I should add - the power supply tests all good unloaded.
When connected to the main board everything is fine except the -12v pin reads -4v. (I’m assuming because VR3 is shorted to ground?)
Ok I solved the problem.
Here’s an update in case this helps anyone else in he future:
Turns out the 7915 voltage regulator was good. (VR3)
What caused the problem was a tantalum capacitor (C92) had failed closed, shorting the -15v rail to ground which took out C153. Thankfully it didn’t fry the DAC (U41)
I replaced C92 and C153 and that solved all problems.
It powers up, calibrates the keyboard, and is ready to play quickly.
Audio sound is perfect.
(Battery message gone also)
Thanks.
I did replace it with a low (-ish) ESR electrolytic.
I read somewhere that the present day tantalum’s are better than they were 30 years ago. Any reason to avoid using tantalum’s for that TDA1541 DAC chip?
Or rather - why do you think they used tantalum caps in that part of the circuit originally?
Size and low ESR?
Just curious - as I’ll go with electrolytic regardless.
There are two DAC’s (for left and right). I’ll replace the other tantalum filter cap also while I’m in there.
Thanks again,
One of the most common causes of failures in today laptop motherboards are shorted capacitors (although not sure all of them are tantalum type) so IMHO, better to avoid them as there are better alternatives (mcc ceramic types for instance).
The problem with tantalums is that they short suddently, where an electrolytic will degrade progressively (leaking, higher ESR).
Tantalum capacitors have indeed some qualities like low ESR, capacitance stability, making them perfect for RC timing networks and a good size/capacitance ratio…but this was 20 years ago.
Today, Kemet company for instance manufactures high reliability, military/space rated polymer-tantalum capacitors, not a true ‘old’ tantalum however.
On the bad side, tantalum’s are polarized and highty sensitive to reverse polarity and overvoltage, and awful in the audio path (needs high negative biasing).
