VIC20 with NOS 6561 sound problem

[jjne]

I just bought a new VIC chip for a used VIC20 with black screen (but otherwise working - tape working, responding to keyboard) fault.

This works but I've found that there's a volume fault on this rig. Setting volume 36878 with bit 0 set but bit 3 not set (ie volume 1, 3, 5, 7) results in muted sound.

The volume is increasing (ie volume 8 is higher than 6 is higher than 4 etc) but there's clearly an issue here.

I'm thinking this is most likely the new VIC, but is there anything else that could be causing this? Flaky memory for instance?

Any thoughts appreciated.

[jjne]

Actually I got the above completely wrong, it's actually 2 and 4 that are mute. The full output chart is:

15 1111
14 1110
13 1101
12 1100 slightly quieter than 11
11 1011
10 1010 slightly quieter than 9
9 1001
8 1000
7 0111
6 0110 much quieter than 7
5 0101
4 0100 dead
3 0011
2 0010 dead
1 0001

Overall this VIC (I think it's the VIC) sound is very non-linear and I can't see this being a memory issue. The issue is also present with a 35K RAM expansion (which I think pages in the cart memory over the built-in SRAM unless anyone can say different) so I believe that rules out memory anyway.

It looks from the above that the volume is implemented as a resistor ladder possibly although the symptoms don't seem especially consistent. Possibly bit 1 (second least significant) has an issue I'm not sure.

I've asked the seller of the VIC chip what happens next. I think a lot of folk might be prepared to live with this as most games on the VIC tend to just have the sound at full pelt most of the time, but I'm not happy really.

[Mike]

Even though bigger memory expansions shift things around on the VIC-20 (notably, BASIC memory and text/colour RAM), the behaviour and position of I/O (VIC and VIA) remains unchanged.

a 35K RAM expansion (which I think pages in the cart memory over the built-in SRAM unless anyone can say different)

This is strictly incorrect, there do not exist any select signals for the address range of the internal RAM on the expansion port.

Your RAM expansion merely fills all possible slots for memory expansions on the VIC-20, $2000..$7FFF (BLK1..3), $A000..$BFFF (BLK5) *and* RAM1..3 ($0400..$0FFF), which gives +35K in total. The internal RAM is placed at $0000..$03FF and $1000..$1FFF. See the sticky post in the Programming section, VIC-20 Memory Layout.

It looks from the above that the volume is implemented as a resistor ladder possibly although the symptoms don't seem especially consistent. Possibly bit 1 (second least significant) has an issue I'm not sure.

You might want to check against this post: https://sleepingelephant.com/ipw-web/bu ... &start=531

I've asked the seller of the VIC chip what happens next. I think a lot of folk might be prepared to live with this as most games on the VIC tend to just have the sound at full pelt most of the time, but I'm not happy really.

Quite some tunes in newer productions use either volume modulation or even 4-bit digis from the volume register, so this defect will lead to noticable distortion in those cases.

[tokra]

I had a VIC-chip as well that was defective on bit 3 of the volume-register. This went in to the bin, luckily I had replacements back then. I agree that the defect is subtle and you only notice it with certain games/demos. But still it is a defect.

[jjne]

Thanks for the replies. I got a replacement VIC and all is well now.

Now on to my next projects - why does this VIC's space bar not work even after servicing (suspecting bad trace at this point - swapped the rubber domes around and cleaned all contacts and all keys working apart from space), and a deeply frustrating and saddening problem with a Plus/4 which I replaced the power jack for a VIC/C64 Style one which brought the machine up - until I screwed it back together after initial test and now black screen. Not PLA (I have a Super PLA from a C64 reconfigured for 264 and still nothing), all voltages OK, no overheating. Have bought a TED and a C64-to-C16 CPU adapter board to rule those out.