Crate Power Block -no sound

[pontiacpete]

The only sound it's making is out of the headphone jack. Connecting the send to the return jack gives nothing and plugging directly into either is the same. Since I have little experience with a class D amp I'm not hopeful about this one, but I'm very curious about it and would like to learn. I don't have the equipment to do SMT at this point. If I double magnify with a headset and a jewelers lamp I can see the components enough to work on it but I have to put he board too close to do anything. Is a microscope the only way?
I was thinking of putting a scope on it and see if I can discover anything. hmm Where to start?

[52 Bill]

The only sound it's making is out of the headphone jack.

The headphone jack is driven by its own chip, so the problem lies in the power amp which according to the schematic is a single chip. Or is the power amp actually a circuit board?

The good news is that the power supply is working.

IC11 is the only thing between the headphone and the power amp in circuit. Check the voltages around it and see if it is passing signal to the input of the power amp.

[pontiacpete]

There is 14vdc and -14vdc on pins 8 and 4. I checked the AC on all the pins and there doesn't appear to be any. So no signal?

[52 Bill]

There is 14vdc and -14vdc on pins 8 and 4. I checked the AC on all the pins and there doesn't appear to be any. So no signal?

What sort of signal are you sending into the input of the amp? The schematic calls for 40mVpp @ 1khz with the controls set up a certain way. Are you doing that? If you are, there should be 4.5 volts on the outputs of the chip.

I looked up the output chip and there is a mute/standby function on pin 6, which is the "Mode" control pin. Check that circuit on the power amp board as well as on the preamp board. The Q1 fet switch needs to work right or the output chip will shut down.

[pontiacpete]

I set the controls the way they say, and send a 1khz 40mV sinewave in and checked the drain of Q1 and nothing there. I use the diode test and there seems to be a short between the source and drain of Q1.

[g1]

Q1 will read low resistance from source to drain without a gate voltage to turn it off.
You should be seeing signal at pin 1&7 of IC11 regardless of the mute/mode circuit.

[pontiacpete]

ok, there's a signal on both pin 1 and 7, looks like the one TP14

[52 Bill]

ok, there's a signal on both pin 1 and 7, looks like the one TP14

Then that leaves board interconnects, the mute circuit and the output chip.

[g1]

That leaves the mode/mute circuit or the output IC. It looks like pin 6 (mode) on the output IC should have some positive voltage (5V or less) after power up. If it stays grounded, then the Fet circuit could have a problem, otherwise the output IC is probably gone (assuming the speaker is ok ).

Edit: Sorry 52 Bill, didn't see your post, sometimes I'm on dial-up

[pontiacpete]

So the output IC is still a suspect?
I have to take the rest of this chassis apart to get a look at it.

[pontiacpete]

IC1 has a burn on what i think are pins 7 - 12.
I wonder if loud has any of these boards.

[AtomicMassUnit]

IC1 has a burn on what i think are pins 7 - 12.
I wonder if loud has any of these boards.

Output is toast for sure!