Acoustic 470 PA

[Enzo]

The output jacks are supposed to be isolated from ground. Look at rhe schematic. See the "speaker -" at the lower right? That is the - side of the speaker, and it does not return to ground, it returns through that resistor. I can;t read it, R433? Looks like .05 ohms, 5 watts? Therefore the speaker jacks cannot be grounded. That is where it tells you that.

If that resistor is open, you get no output from the speakers. You also get no feedback from the output from that point, so the amp will try to compensate.

If you think the output cap is bad, clip in almost anythig in its place. Right now you are chasing no output, right? That is totally different from distorted output. We are not interested in freq response, tone, or distortion right now, all we care about is getting sound.

So:
Is the output bus stable at about half V+? (45v more or less)

If so, is there output signal on the otuput bus? Look on R430,R431. If there is no signal here, it won't likey be on the speaker either. If there is no signal here, everything after this doesn;t matter - this is the important question then.

From there, the output signal goes off board to that inductor and switch, and back on. Is signal present at both exit an re-entry of the board?

If it gets that far, then is it getting to the + end of C409 output cap? If it gets to one end of that cap and doesn't come out the other, then try a different cap. It doesn't matter what cap you try as long as the voltage is enough. All we want it to do is verify the cap as the problem, OR NOT. tacking a 470uf cap there would tell us if that made signal come out the speaker or not. It wouldn;t have much power or low end, but it would tell us if the cap was the problem.

[gbono]

Enzo,

Thanks for the reply. I did see the output jack connection at R433 (.03 ohm 5W) but isn't the other side of R433 connected to power ground?

I'll keep chasing the signal through the amp and see what I find.

Thanks,

george

[Enzo]

Yes, one end of that resistor is grounded. The other end is the speaker return. It may be a low resistance, but it is not ground, thus the insulated jacks. The speaker current drops a voltage signal across that resistor. That voltage signal is then fed back into the amp as a sample of the output.

This is classic troubleshooting. If you have signal at the amp circuit output, then track it through all that inductor, switch, output cap, jacks, etc. If you don;t have signal at the circuit output, then none of that stuff matters until you do.

[gbono]

Okay there is 45V at R431 but the wvaeform at this point is very low and non-symetric(see below). The signal at the base od Q403 looks like this

http://i284.photobucket.com/albums/ll33/bonosurf/DSCF05211.jpg

. The voltage at the node of R410 is about .6V actaully so is the collector of Q402. I've posted a table of the other node voltages in a previous post. Doesn't make sense to me.

[gbono]

Okay it looks like Q404 isn't turning on with 51V on the base and 46V on the emitter - I've checked all the resistors from R417,418 and 419 and they are okay. The trim pot (R406) also seems to work but it has no effect on the base volatge of Q404. Stumped.

[Enzo]

Unless Q403 pulls down on the base of Q404 enough to get that 51v lower than the emitter volts, then 404 surely won;t turn on. Q403 controls what is opn Q404. I'd be a lot more suspicious of Q403 than 404. At least from here.

[gbono]

Yup - Q403 has .6V on the base and .6V on it's emitter and of course 52V on the collector. If I test the diodes across the BE of Q403 I get conduction in both directions - Mmmm either the BE is shorted on Q403 or both diodes are damaged - probably Q403 since I don't have one in stock (i.e. MJE344).