Hello guys, after starting to build a MidiboxSID V2, with a rev 3 PCB some months ago I had other commitments and I put a halt on it. I have picked it all up again from where I left off. My problem is that I seem to have a very small output signal from the audio out pin; I have the test tone app running, and my SID is getting 4.99V Vcc, and 8.97V Vdd and is properly grounded 'n all.
If I hook up the audio out to a hifi amp I can hear the test tone but very very quietly with amp gain up full (I tried with a mixer mic amp, plus a seperate SSL mic amp, loads of things and I can only just hear it with stacks and stacks of gain [+ all the noise of course]). If I lift the audio output pin away from the IC socket, carefully, so it’s effectively isolated from the output circuit of the MBSID design, and AC couple the pin with a cap to the amp input, the signal has a bit more power so my first problem is that the output transistor stage is doing something strange by loading the signal down. I can see how it provides protection for the output of the SID, but I’d have thought with the collector up at 9V and a small signal on the base, there would be some degree of voltage gain of the signal appearing across the 1K res from emitter to ground? I could easily be wrong though, as I often am! I take it the 470pF provides some LP filtering for supersonic stuff that you don’t really want appearing there? Unless I have a faulty BC547 I cannot see how this stage is giving me headaches! It is built correctly, I followed it all round with an ohmeter, though signal at the base of the NPN is OK, at the emitter is quieter and a lot noisier, although this transistor checks out on a tester OK with an Hfe of about 250, pretty typical of a BC547. At first I couldn’t see how this circuit would work i.e. how the transistor is biased, but I guess it is so by the DC output of the SID; if there isn’t one, or too small a voltage, it won’t work, this is a possibility of what is going on here.
*update*: since I’m using an 8580 device, I believe it has an internal pull down and thus doesn’t have an open drain output, so is R2 really needed? The DC offset of the 6581 audio output is about 6V as opposed to the 4.75V of the 8580; both have a p-p output of about 3V I THINK..
The thing I really wanted to know was what should the resistance be between SID audio output and ground? If I measure the chip out of circuit, I get about 2.4K, with chip in circuit this goes in parallel with the 1K pull down so you end up with about 690R, I wondered if that is a sign that my SID 8580 is dead? Or is there an internal pull down of 2.4k etc? The original datasheet states you should measure 4.75V DC between the output and ground, I get more like 800 mV ish. I don’t suppose there is any other test to find out if the little fella is well and truly dead, except put the C64 back together and load up a game with some bangin’ tunes on!
If anyone has any ideas great stuff, if nay, no problemo, I thought I’d ask 
It’s doing my head in! Sorry for the lengthy-ness, I’m trying to cover all aspects to get to the fix.
David_