- Joined
- Nov 29, 2005
- Posts
- 4,275
Had a MV4F board that came in with a Z80 error and a rapid clicking sound in the speakers. Strange thing is that as the speakers clicked the picture on the monitor flickered.
Little did I know this thing would kick my ass.
Replaced the Z80 CPU to fix the Z80 error. For some reason SNK put a Z80 in there and the board is labeled for a Z80A (higher speed) CPU.
Now the system booted but flickered like mad. Audio ouptut was horribly distorted.
With the audio down the system worked fine except the audio output IC was getting very hot. Recapped the audio section. No help. Replaced the audio output IC. No help. Replaced the volume control. No help.
For some reason there was an oscillation happening in the audio circuit. This oscillation was causing the audio output IC to suck up large amounts of current. This was causing dropouts in the +12v output of my bench supply. These dropouts matched the screen flicker and the clicking of the speakers.
Checked every trace in the audio section - all were good. Replaced the 3 op-amps. No help. Checked all the film caps (the non-electrolytic caps)... all checked good.
All voltages looked OK but were a little lower than another MV4F board but they were within reason (+12v was a little low so the other voltages around the chip were correspondingly lower)... On a whim I removed both of the .1uf film caps on the speaker outputs. Audio was distorted but amplifier was stable.
Next I put them back in circuit one at a time. The one on the right speaker was the cause of the problem. Replaced the cap and problem was still there. It looks like there's an issue with the ground trace inside the board. Removed the cap and ran the board for an hour at 3/4 volume with no problems.
Whew. What a bitch.
Hope you don't run into a problem like this one. It took a LOT of time to figure out.
RJ
Little did I know this thing would kick my ass.
Replaced the Z80 CPU to fix the Z80 error. For some reason SNK put a Z80 in there and the board is labeled for a Z80A (higher speed) CPU.
Now the system booted but flickered like mad. Audio ouptut was horribly distorted.
With the audio down the system worked fine except the audio output IC was getting very hot. Recapped the audio section. No help. Replaced the audio output IC. No help. Replaced the volume control. No help.
For some reason there was an oscillation happening in the audio circuit. This oscillation was causing the audio output IC to suck up large amounts of current. This was causing dropouts in the +12v output of my bench supply. These dropouts matched the screen flicker and the clicking of the speakers.
Checked every trace in the audio section - all were good. Replaced the 3 op-amps. No help. Checked all the film caps (the non-electrolytic caps)... all checked good.
All voltages looked OK but were a little lower than another MV4F board but they were within reason (+12v was a little low so the other voltages around the chip were correspondingly lower)... On a whim I removed both of the .1uf film caps on the speaker outputs. Audio was distorted but amplifier was stable.
Next I put them back in circuit one at a time. The one on the right speaker was the cause of the problem. Replaced the cap and problem was still there. It looks like there's an issue with the ground trace inside the board. Removed the cap and ran the board for an hour at 3/4 volume with no problems.
Whew. What a bitch.
Hope you don't run into a problem like this one. It took a LOT of time to figure out.
RJ