Amiga Audio Issues

A little while ago I bought an Amiga A500. After having lots of fun cleaning it up and putting in a new ROM (Kickstart 2.04) I tested it thoroughly by playing...er...I mean running the demo of Frontier Elite II. I also played lots of music through OctaMED.

It was during the music that I noticed something a bit off. The audio sounded OK but was a bit muffled. It then occurred to me that the Amiga's audio filter was switched on. But according to OctaMED it wasn't.

For the uninitiated, the Amiga has a hardware low pass audio filter built in which can be controlled by software. It was, allegedly, included by Commodore to improve the sound of sampled audio which was quite low quality (compared to today - in those days it was awesome!). Basically, if you turn the filter on it effectively hides some of the artefacts or 'graininess' of the sampled sounds. In most uses it's turned off and this is indicated by the Amiga by dimming the power led a bit.

So, in OctaMED I tried turning the filter on and off an noticed that it made no difference to the sound of the audio, even though the LED dimmed like it should. Fortunately, RetroGameModz on youtube has a whole 50 minute video on troubleshooting the audio filter. It's worth a look if you are into your Amiga hardware (probably give it a miss if not..). Find it here. Time to get that top off..

Amiga Audio Testing Studio - also known as my desk.

To cut a long story short I followed the advice given in the video and found that the chip U38 looks like it's having 'issues'. When the filter is on then the voltage at pin 2 of the chip should be around 0 volts and at pin 3 should be about 12 volts. This is pretty much what I get give or take a tenth of a volt.  With the filter off the voltage at pin 2 should be near 5 volts and the voltage at pin 3 should be -9 volts or so. In my case though, the voltage at pin 2 is about 6 volts but at pin 3 there is still 12 volts (11.85 to be precise). This is, as they say, a bit of a problem.

Voltage at U38 Pin 2 - Filter On

Voltage at U38 Pin 2 - Filter Off

Voltage at U38 Pin 3 - Filter On (Correct)

Voltage at U38 Pin 3 - Filter Off (Argh! Incorrect. Should be -9v)

I don't pretend to fully understand the circuit but, in simple terms, by having 12 volts from pin 3 of U38 then the transistor Q301 has 12v at the base and at the emitter, meaning that the transistor is off. Because of this, the other side of the circuit has -12v since there is nothing to pull up the voltage. This results in -7v (ish) at the gates of two JFETs resulting both JFETs being 'cut-off' so current is forced to take the path through the low pass filter.

Audio Filter Schematic (part)

I've also tested this on a known working A500 (it's an A500+ actually but the circuit is the same) and the voltage results I get from this more closely match those on the YouTube video since the MC1488 chip is behaving as it should.

So. Is it worth replacing the chip? Probably. They're about 40p from Farnell so it's not like they are obsolete or difficult to get like the A1200 video DAC for example. The only danger is that I damage the board as I try to remove the chip, bearing in mind that these boards are over 25 years old. This particular motherboard is marked as being made in 1989 so he's actually 27!

Old

I do have a concern that the switching voltage at pin 2 of U38 seems a bit high at 6v. I traced it back to the CIA and it all seems OK.

CIA (Odd) - _LED Comes from Pin 3

I suppose it could just be a variation of the components and the different motherboard designs or even possibly a symptom of the faulty MC1488  - although it is a simple single input device. I have a few spare CIAs so I might put one of those in just to verify that the voltage is the same no matter what. Looks like an order to Farnell is also on the cards.

To be continued...