Fender Bassman 150

[goldtop5]

I have another one for you. This is a Fender Bassman 150 solid state bass amp. This was a project that I've been beating myself up for months trying to figure out. The output transistors kept shorting and I later discovered that the gold heatsink compound I was using was conductive. So I cleaned it off the transistors and used the white silicone-based compound and the short went away. Now, I don't have a any signal on the main speaker output. I powers up fine, and I get a signal in the pre-amp section, but it does not pass to the output. The schematic for the power amp section is attached.

[Enzo]

So plug a signal into the FX return, sound? Plug signal into the AUX IN jacks, sound? Get a power on thump?

Make sure the ribbon cable from the rear jack board is intact and on the pins right.

Verify the main power supply voltages are getting to their circuits. The schematic is full of voltage test points, compare yours.

Apply a signal to the FX return, then trace it back to the main board, through IC7 IC9 and into the power amp.

[goldtop5]

So plug a signal into the FX return, sound? Plug signal into the AUX IN jacks, sound? Get a power on thump?

Make sure the ribbon cable from the rear jack board is intact and on the pins right.

Verify the main power supply voltages are getting to their circuits. The schematic is full of voltage test points, compare yours.

Apply a signal to the FX return, then trace it back to the main board, through IC7 IC9 and into the power amp.

I plugged a signal in to the return and got nothing. Nothing with the aux ins. All the cables seem to be intact.

IC7 seems fine, but when I look at IC9, there dc voltages on pins 1, 2, and 3. There is hardly any AC voltage on pin 7. Could this be a possible cause? Could IC9 may have gone south?

[g1]

U9A is over in the C8 area of the schematic. It is running the mute LED, so DC is expected there.
U9B pin 7 AC voltage will depend on the master setting, is it turned up? Q3 is the muting Fet for the master. If it fails, it will kill the signal at U9 pin7. You can remove it to see if it is the problem.

[g1]

Also, note #9 says jumper W1 (C4 area of schematic) needs to be removed to test using the FX return jack.
Can someone explain?

[DrGonz78]

What is the current status of the speaker? I mean sure keep looking at the other parts of the amp too, but have we tested the speaker yet? Also, yes IC9 pin 7 is suspect, but have you pulled Q3 J113 mute B. I am pretty sure that would be something to look at as well.

Edit: I guess I had this page open for a while before I posted... I did not see G-one had posted, sorry for repeating the same thing as him about Q3.

[Enzo]

Look at the circuit W1 disables, it is a limiter across U9b. By removing W1 jumper, it disables that function. You can test the power amp without doing that, but if you want to check limits of the amp, the jumper comes out.


It doesn't say the jumper has to be removed to test using the FX return jack, it only says you should remove it for the test they propose. I'd ignore that, your problem is a gross loss of signal, not a shade off on peaks.


Um, have we checked to make sure the MUTE button is not now pushed in? Check the gate of Q2 Q3, is there about -15v there?


Watch the schematic, don't just look for part numbers, follow the signal path.

[goldtop5]

I replaced Q3 and it brought the signal back to the main speaker out. I thought I was finished with it but noticed the line out xlr jack doesn't work. I could've sworn it was working before. Could something else be muting that out as well?

[Jazz P Bass]

Have you turned the Line Level pot up?

[goldtop5]

Yes

[Jazz P Bass]

Did you try the pre & post switch?
I have attached that portion of the circuit.
Signal tracing time. You can use a meter set to read volts ac.
There should be a signal on pin 1 & 7 of U13 and 2 & 3 of the XLR.

[goldtop5]

Update...I got the problem fixed. I pulled the preamp board and immediately spotted a solder bridge between pins 2 and 3 on the xlr output. Darn! I can't believe I missed that!!! The amp is working fine now. Thanks!