Hi,
Just joined this forum.
I recently purchased a Marshall 8040 valvestate that has volume on clean channel but no volume on boost channel.
Cleaned pots and jacks.
Put in used 12ax7 tube from working fender amp- no change.
Pin outs on tube voltages:
1 163
2 96.6
3 96.9
4 14
5 14
6 95.5
7 -0.42
8 .988
9 7
I was hoping there might be someone who could walk me through the diagnostics.
I have the schematics and figured someone could look at the tube pin voltage readings and give me some direction.
I would like to fix it myself.
Here is the link to the schematics:
http://www.drtube.com/schematics/marshall/8040-iss8.pdf
Pin 7 voltage on the tube changes with boost on to 0 volts and looks like it becomes Vac.
ic1 is a 5201 switching op amp. If I understand this correctly with boost on we use the B side circuit of the chip pins 6 and 7 input and 5 for output.
Would a bad op amp give a Vac output or should I be looking for a failed diode or cap ??? in that area of the circuit?
Thanks
Hi, welcome.
You tried a different tube and no change, so we assume the tube is not at fault. The voltages you present look like a working tube to me. I have no reason to think the tube is involved in this.
Feed a strong signal into the input jack, set to boost channel. Do the LEDs by IC1 blink with the signal? Pin 5 of IC 1 is output, how much signal is there? AC volts. Should be a similar amount on pin 7 of the tube. How much signal is on the plate of the first triode, pin 6? And on the cathode of second triode pin 3? All AC, not DC. Now at pin 1 of IC2? And pin 5 of IC4?
By any chance, did you replace the input jack? This one has a grounding contact that mutes the boost channel when nothing is plugged into the amp. If you installed some other jack and grounded that line, the channel will never unmute.
The essence of troubleshooting is to isolate the problem. Divide and conquer. The question to ask is not "is it a diode? Is it a cap?" It may not be a bad part at all. It could be a wrong part - perfectly good part, tests OK, but the wrong type for the circuit. This happens when someone tries to do repairs. You could have a broken solder joint or a cracked copper trace. Those are not bad parts either.
The process is to narrow down where the problem is coming from. What areas of the amp work? How far into this circuit will a signal go before it stops?
Thanks so much for responding.
Do the LEDs by IC1 blink with the signal? No they do not blink at all.
Pin 5 of IC 1 : 1.51Vac
pin 7 of tube : 0.593
pin 6 of tube: 15
pin 3 of tube: 14.74
pin 1 ic2: 4.89
pin 5 ic4: 0
A few more observations:
If I have a guitar plugged in I can barely here output but it sounds like it is overdriven (full distortion sound you want) just no volume at all. Like the signal is being fully processed but no volume output.
I put a sine wave on input and tried to match the level on pin 5 of ic 1 that I was getting off the guitar.
Then I took all the readings.
I have not changed any jacks. This amp had this problem when I got it.
The mute thing has me wondering.
I rolled the signal up and did see the LEDs light. Not bright but increased with gain.
Rolled it back and did some more readings.
After the output of ic2 pin 1, I read a Vac through to left side (on diagram) of C19 about 1.25Vac. It becomes 0Vac on the right side.
Well I took a good look at the input jack and the sprung grounding clip used to mute was folded under always making contact. I messed with it a bit free it up and wah-hey I now have a boost channel.
Thanks Enzo for your feedback.
HAH! ALmost like I knew what I was doing or something.
Seriously though, I am glad you got it sorted out.
The purpose of that mute is to prevent the dirt channel from making hiss noises when nothing is plugged into the amp. Not a deal breaker if it doesn;t happen, but a sound killer when it fails as you found.
Yeah that jack is kind of messed up. Both metal springs were bent out of shape. I might look for another jack and replace it.
Next I need to put the amp through its paces and see if the rest of it is ok.
Also might want to look for another bad amp to buy and fix.
Thanks again for all your insight.
AlexT
Enzo is the amp whisperer psychic!! Way to go Enzo!