Question on reading a Fender Stage 160 Schematic

[Jam]

I have a Schematic of Fender Stage 160 and I dont understand some of the test points. Im looking at test point 23 which is U6 a op amp and it says 732mVAC and under it says 589mVAC both for TP23. When I test all I get for the test point 23 is 0.2 and I have my meter on DCV. Thanks for any input

[Enzo]

Did you read the notes at the bottom of the drawing? Many drawings have notes explaining test conditions, as well as other useful data. SOme also include adjustment procedures and other good stuff.

First, if the test point says volts AC, then don;t expect a proper reading an a meter set to DC.

Read note number 6, which specifies for TP8-28 that there will be a certain signal level at the input, a certain output load, and how the controls should be set. If you have no test signal applied, then signal voltages - which is what those AC TP readings are - won;t be present. A couple exceptions, the AC readings in the power supply are transformer voltages, not signal.

Now specifically TP23. And if you look, all the TPs from 23-28 will have two TP readings. Look at each place and see the Note 10 indication. Now go down and read note 10, which explains the different readings are for different models. The Stage 100 and Stage 160 will be a little different.

The test points are ballpark figures. It may say something real precise like 732mv, but really, anything close is fine. WHat they did was take a unit and measure what was going on in it. Due to component variations and changes in mains voltages, all amps won;t have those exact readings. So the point of that 700mv reading is that under the stated conditions you should see ABOUT that much, as opposed to 20mv or 3000mv.

[Jam]

Thank you so much for the info I had one more question if Im testing the U6 a OP amp and on pin 1 I get 0.2 DC does it mean its bad? Thanks

[52 Bill]

Because I didn't search out the schematic, this response will be full of assumptions, so read with caution.

Assuming that pin 1 is an output pin of an audio stage circuit in this amp which has a bipolar power supply, 0.2vdc is not anything to worry about.

What is the problem with the amp?

[Jam]

On the clean channel once in while when you play anything low it will crackles or buzz and it will come out of it. Its only the clean side and so far I looked and I dont see any bad solder joints?

[Chuck H]

Has it always done this? It's possible that it's just the nature of the circuit. It could be that high input signal or the high current of big low end is boffing the bias condition of a stage in the clean channel. I can't find a good schematic online, but I would try raising the low pass knee or just trimming low end through the circuit.

[Jam]

Its a Solid-state amp

[Enzo]

Schematic?

Fender web site:

http://support.fender.com/schematics..._schematic.pdf

[Chuck H]

Even transistors need to have a stabile relationship between the emitter and collector.

[52 Bill]

On that series of amps, I'd be looking for a bad solder joint or a broken component lead.

Can you induce the problem by mechanical stimulation? Turn off the reverb, feed a signal into the amp and give it a good Enzo whack. Can you make the noise happen? If the chassis is out of the case try tapping on the board with the handle of a small screwdriver. Anything? Try pushing on the two large filter caps, as they often come unsoldered.