Sound trouble

Wonder-Boy · Post by **Wonder-Boy** » Sun Sep 24, 2006 5:05 am

Hi, I bought a VC-20 off ebay fairly cheap. I suspect there is something wrong with the sound. When I e.g. play the cartridge game Omega Race, the volume goes up and down. The volume was low before I started to move the ship, but when I pushed the joystick forward the volume went up and then it kept changing. Also I bought te VC-20 because I was so impressed with the demo Robotic Liberation, and I copied it to a disk and loaded it on the VC-20. The music sounds bad and not like in the videos on e.g. Youtube. Notes appear to be either too high or too low in pitch, which makes the music sound out of tune.

Does anyone know what the problem is? Is there a program that you know of that I could use to test the sound?

PS The RF modulator didn't work so I am using a video cable that I use for the C64

Poly-poly man · Post by **Poly-poly man** » Sun Sep 24, 2006 6:37 am

Code: Select all

poke 36875,175
poke 36878,15

Tell us what it sounds like

poly-p man

Wonder-Boy · Post by **Wonder-Boy** » Sun Sep 24, 2006 9:16 am

It sounds like when you turn on both the hot and cold water tap at the same time and the pipes scream, but pretty low pitch and not loud, distorted

Poly-poly man · Post by **Poly-poly man** » Sun Sep 24, 2006 1:58 pm

odd...
Open it up and check all of the connections (even inside the little metal box).

This probably won't help, but are you PAL or NTSC?

poly-p man

carlsson · Post by **carlsson** » Mon Sep 25, 2006 2:19 am

Aberdeen, UK sounds like PAL country to me.

The video cable you use for your C64 is one with a 5-pin DIN for composite video? The picture is stable and crisp? I'm considering whether it could be some mix-up in the cable, monitor or that your VIC is partly failing. Is it the older model with a two-prong power connector, or the cost reduced model using a round DIN connector?

Wonder-Boy · Post by **Wonder-Boy** » Thu Sep 28, 2006 2:18 pm

Checking all connections seems too difficult. I have tried pushing all chips down into their sockets. It is PAL. This is a VC-20 made in West Germany, but it looks like it was sold in Norwich judging from a sticker on the bottom. It is the older model with a two-prong power connector. The picture is stable and crisp. It is a five pin din video cable that splits into four connectors for the TV, of which I have connected two for video and audio.

I found this program on the web that I tried

linuxelf wrote: Actually, it'd be a poke statement. peek would read a memory address. Poke would set one. (heh)

I remember the pokes for the VIC-20 sound. The VIC had 4 voices, 36874, 36875, 36876, and 36877. 36877 was the 'noise' voice, nice for doing explosions. 36876 was the voice that I usually used. Then you had 36878 which was volume. Each of these could hold one byte, so you'd set their value from 0 to 255. Setting 36878 to 255 would be max volume. So, something like this:

10 FOR T = 200 TO 150 STEP -1
20 POKE 36876,T
30 FOR R = 255 TO 0 STEP -1
40 POKE 36878,R
50 NEXT R
60 NEXT T

would sound something like hitting piano keys. The sound would hit nice and loud, and then get softer and softer until it was silent, then a slightly lower note would hit.

But it just sounds terrible on my computer. It seems like the sound channels are garbled up, and the volume command does not work at all.

Seller claims "everything worked when I sent it" so there is no guarantee. I suspect he is tone deaf, though

carlsson · Post by **carlsson** » Mon Oct 02, 2006 6:10 pm

Ah, sold in Norwich. That explains everything!

No, I don't have a good idea. According to the schematics, there are a bunch of components: at least three capacitors, two resistors and one transistor on the way from the COMP SND output from the 6561 chip to the audio/video connector. Perhaps some component on the way has gone bad, but in that case, which one would result in distorted sound?

See the rightmost, upper-middle part of this hand drawn schematics of an early PAL VIC-20:

ftp://ftp.zimmers.net/pub/cbm/schematic ... 4001_1.gif

Wonder-Boy · Post by **Wonder-Boy** » Tue Oct 03, 2006 12:41 am

Thank you, carlsson. I have now won an auction for another Vic-20 so I hope that that one works. And I plan to use it to find out what is wrong with the one I have so I can repair it. I will certainly check the components you have pointed out. I read also on Ray Carlsen's page that it may be IC 7402 (UB9) that is faulty as the symptoms are similar.

6502dude · Post by **6502dude** » Tue Oct 03, 2006 7:52 am

Wonderboy wrote:it may be IC 7402 (UB9) that is faulty

I would not suspect the 7402, as it is used to supply clock to the 6560/6561. Since your video is stable, this indicates clock is ok.

I would be suspect of C16 based on conditions described.

If you have a high impedance set of headphones, have a listen to output of pin 19 of the 6560/6561 and then at emmitter of Q5. If sound is okay in these spots, then C16 is bad.

Wonder-Boy · Post by **Wonder-Boy** » Wed Oct 04, 2006 1:25 pm

Thanks, 6502dude, you sound like you know what you're talking about. It will be too expensive to buy high impedance headphones just for this, so instead I think I will try to replace this capacitor to see if that helps.

Wonder-Boy · Post by **Wonder-Boy** » Thu Feb 01, 2007 2:14 pm

*UPDATE*

Hi, after quite some time I replaced capacitor C16 yesterday, but it did not help. So working back from what 6502dude said I removed the 6561E chip today and put in the one from my working Vic-20. This solved the problem.

Ray C wrote: EARLY VERSION BOARDS: ASSY #324003 FAB #324002-01 REV D 1980

UB7 6560-101 VIC VIDEO/AUDIO
White or blank screen, garbled or no video, screen full of (or few random)
"garbage" characters. "Blind" disk commands may still work.
Partial failure: Dark or smeary image, loss of color, garbled or no sound,
game paddles or light pen doesn't work.

So now I have to source a replacement part... But it still works though as long as no sound is needed. Thank you guys for your help.

Mike · Post by **Mike** » Fri Feb 02, 2007 2:47 am

Wonderboy wrote:I found this program on the web that I tried
linuxelf wrote:I remember the pokes for the VIC-20 sound. The VIC had 4 voices, 36874, 36875, 36876, and 36877. [...] Setting 36878 to 255 would be max volume. So, something like this:
Code: Select all
 would sound something like hitting piano keys. The sound would hit nice and loud, and then get softer and softer until it was silent, then a slightly lower note would hit.[/quote]
But it just sounds terrible on my computer. It seems like the sound channels are garbled up, and the volume command does not work at all. :([/quote]
The volume resides only in the bottom 4 bits of the register. The top 4 bits define the auxiliary colour in multicolour graphics.

You should try this instead:
[code]1 POKE 36876,192
2 FORT=15 TO 0 STEP -1
3 POKE 36878,T
4 FORS=0TO100:NEXT
5 NEXT
6 POKE 36876,0
Greetings,

Michael