Acoustic 470 channel switching noise

[nsubulysses]

Has anyone experienced this channel switching noise? It ONLY happens when the B channel is engaged, the distortion knob is disengaged (all the way left, clicked 'off'), AND you switch from NORM to DIST/B.

IF you have DIST/B engaged already and you switch between channel A and B it doesn't make the noise. It also does not matter if the distortion knob is clicked on or off -- still no noise. This seems bizarre but is maybe a 'clue' since you are in the same 3 settings which seem to trigger the noise -- distortion knob off, B channel selected, DIST/B selected -- just the order of arrival is different.

90% of the time it makes noise it just fades away to nothing in under 15 seconds. Once in a while the noise hangs around and you gotta flip the switch back and forth so it will go away, which it usually does on the first try. If you think it sounds a little funny when the noise is fading away it is because the tremolo was on during the vid/test.

Not sure if it's a good idea to go checking around the Q211 area or if there is a better place to start. I asked a friend who has a 470 to see if his makes noise under these conditions and he said it didn't I'm not a solid state wiz at this point...... would be nice to waste less time..... this description might be confusing but if you see the vid you'll see what I mean

Any ideas?

Edit: oh yes forgot the schematic: Acoustic 470 Service Manual.pdf

[MarkusBass]

It's hard to read the schematic but if you look at page 22, you can see that the channel switch consists of Q212 and Q213 transistors, C226 and C229 input capacitors and R258 and R259 biasing resistors. I would rather check this area. It sounds like failed capacitor or biasing resistor. It can be also one of the switching transistors, as weel as some capacitor "after" the switch. Do you have +17V power supply voltage there?

Mark

[nsubulysses]

I probed this thing LATE last night and probably have to go back and give it a look but I was gettting very strange voltages around teh Q211 area.

At the 680ohm/2W resistors (R253 and R254) I was getting about 70V/35V, not 33V/14V like the schematic says.
At 2N5457 and 2N4360 I did get 17V just as the schematic shows.
But it does seem like in that area when I am expecting around 33V I am getting about 68-70V.

This B channel was VERY noisy when I got it. I replaced 2N4360 with NTE326 and the noise went away...... until I discovered the channel switching scenario that does yeild noise. Adjusting the trem bias pot makes this noise louder. I assume this is a big clue but I am not sure how to interpret it.

And as an FYI in case it mattes the B channel pilot light was replaced. Is it supposed to act as some sort of voltage regulator?? I am not sure what it was replaced with but it does work, but it is obviously not the same bulb as the channel A light.

R251 and R252 check fine resistance-wise. I wasnt sure if these resistors are supposed to drop the 92V down to 33V, but their resistance is within tolerance anyway so I guess not.

[MarkusBass]

You have to understand how it works. The circuit is supplied with +92V. Then there is one voltage divider: R252, R253, pilot lamp A, Q210 transistor (when channel A is on), and another: R252, R254, pilot lamp B, Q211 transistor (when B channel is on). Pilot lamps are not regulators but they play an important role in each voltage divider. You cannot replace the lamp with "any" lamp. Doing just a basic math show that the lamp should be something like 12V/30 mA. Any other lamp will cause incorrect voltages and maybe this is just a case. A simple test is to exchange A and B pilot lamps (and compare how this influences the voltages in the circuit).

Mark