PCM expansion card noise.

[z964]

I have both a Polyphon and a TS-6BGA, which are expansion cards that have PCM audio output jacks.

There's a rather severe lack of software support for these boards, so I never really did get that aspect of them tested much, but lately it seems like both of them have broken in exactly the same way.

When I plug in speakers to either of those cards and turn on the computer, I get a squealing noise from the speakers.

This happens even if they're the only cards in the x68000.

It happens in BOTH of my otherwise working x68000 machines (an XVI and an 030, both with CPU accelerators) with either card.  The two machines are connected to completely different speaker setups.  It happens even if there are no other expansion cards in the machine at the time.  Non-sound features of both cards appear to be functional.

It happens before any software loads.  Immediately on power on.  In fact, the squealing is much louder for a second or two upon hitting the power switch.  The squealing does react to the speakers' volume knobs.

I seem to remember being able to connect speakers to either card in the past without this happening.

Both computers have refurbished original power supplies.  I use and have always used the same model of 100v step-down transformers on both computers.

I found a game that supports the sound feature of both cards, and my testing on the TS-6BGA has the game sound almost start for an instant before vanishing.  The squealing noise continues throughout.

Anyone have any ideas?

As a side note, when I have the Polyphon in the machine, the system's main sound output jack on the back has added crackling noise.  This noise isn't present from the internal speaker or the audio that goes through the video port.  This may or may not be related.

[z964]

I tried the cards in a third system, and there there is no squealing noise at all.  So whatever is wrong, it's common to two x68000s, but not a third one. 

[z964]

Mystery solved.

The problem was the power supplies.  The ones with the problem were both refurbished by the same Japanese retro PC seller.  In that past, I was using a different power supply in the machine I tested the cards in - my x68030.  That power supply died on me, though, so I had switched it out.

I swapped in yet another power supply to my x68030 and now the Polyphon's audio works perfectly.

I suppose it's not that surprising for a problem like this to go unnoticed by the seller, since PCM audio cards for x68000 are very rare, and software that uses them even rarer.  They probably use a different power rail than anything else in the system.