Hi Daniel,
a couple of thoughts that may help, having recently acquired a dodgy sound card I've been reading up on this stuff!
- have a read of the World Cup sound card manual. I have a recollection that the test button functions slightly differently with the toggle switch in the notes or synthesised sound different positions. This may not be relevant. Reading between the lines I got the feeling the older sound roms were a bit more primitive in the test routines (Phoenix doesn't have test capability?). Not stopping producing a constant tone when button is pressed sounds normal behaviour to me. Maybe try the prom from your brothers' Time Warp?
- assume all the links are correctly set for the prom / rom you have installed?
- I'd stick a logic probe or scope on the 1408 input lines - pins 5 to 12. The idea behind this chip is that the pia puts a digital value on these input pins and an output current (proportional to the input value) is generated on pin 4, so I'd measure pin 4 as well. BTW the 1408 looks like it can be replaced with a DAC0808. This info should tell you if the fault is in the digital or analog section, by the sounds of it (note very poor pun) your problem is in the analog section.
- if the digital section proves to be all ok then there is not much in the analog. When you say you've replaced all caps, did you mean all electros or ALL caps? Depending on what bits you have around I'd replace C13 first, next C8 and C9 then Q3, Q4 and the TDA2002 audio amp all pretty cheap and easy to do. That's the stuff in the top right of the schematic
Hope this helps.
