No Sound

Symptom: No sound output.
Target : C-64 (all models)

Possible sources of failure

fuse -> no 9V (new board)
modulator
output transistor
Z-diode CR7 (new board)
SID

Analysis

Fuse ok?

If you have got an old C-64, skip this step; a broken fuse would also lead to a black screen.

In actually all books/magazines/whatever you will find the following method to check a blown fuse:

"Check the C-64's internal clock by typing ?TI,TI,TI" (many books even write TI$) "; if the all numbers printed are equal, the fuse is blown; replace it with a new one."

This is definitely incorrect, in other words: W R O N G ! Neither can you detect a missing 9VAC voltage with these commands nor will a blown fuse make the TI$-watch stop! The TI$-clock is in no way related to the 50/60Hz triggered real time clocks in the CIAs, which would indeed stop without the 9VAC. The variable TI$ is set during the interrupt routine which is called periodically by a system clock driven CIA counter.

Simply have a closer look at the fuse or check it with a continuity tester and replace it if necessary.

Modulator ok?

Connect the AV cable's audio output to your amplifier and check if the sound works now. If if does, replace the modulator.

Transistor 2SC1815 / 2SC945 / PN2222 ok?

Replace transistor 2SC1815 (or 2SC945 or PN2222 or BC237C) with the same type or with a BC547; on old PCBs, it is located on the left of the modulator, between modulator and SID (or PLA on some boards), on new PCBs, it is located below the modulator, between modulator and SID. If it still does not work, continue with the NeXT step.

Zener diode CR7 ok?

This test is only applicable for the new boards. First, check the voltage of the SID between pin 14 (GND) and pin 28 (Vdd). It should be around 9V. If it is not, check the voltage above the Z-diode CR7 (on the right side of the keyboard connector), it should be around 9-10V. If not, replace it with a diode of the same type (or ZPD9.1 or BZX55-10 or similar).

SID ok?

Check whether the SID becomes really extraordinary hot (The SID is ALWAYS hotter than other chips). If yes, it is very likely that it is damaged. If not, you should replace it anyway. If that still does not help, check for hair cracks.

Updated: May 18th, 1999
Created: January 1997
Status : NOT VERIFIED!

This document is part of MJK's Commodore 64 & LCD Page Brought back to life by Peter Schepers, Dec 10, 2005 because I really liked this site!