help with Fender stage 160

[embotone]

I  agreed to look at my friends amp, but am a little over my head here with a solid state amp.  The amp barley is audible, and only when fully cranked.  The output from the preamp has this problem so I have so far I have only taken voltages from the power supply and preamp.  If anyone could offer to take a look at this and offer some direction, I would surely appreciate it.  Here is a link to the service manual     http://bmamps.com/Schematics/fender/Stage_100-160_DSP.pdf     I am thinking there is a problem with an op amp, but that is a barely educated guess.  The voltages from the power supply seem okay.  I took the measurements with an 8 ohm dummy load and a 1khz, 25mV sine wave at the input.  Here are my voltages: (all the measurements from test points 1-13 are easily within the specified +/-20%, so I won't list them to save some time (except#8 was marginal, at 13.53vdc+, still within 20%).  The signal started getting below what it should be at test point #14.


TP          specified voltages                   measured voltages  (all voltages are mVAC, unless otherwise specified)
1              67.9vac
2              46.2vdc+
3              46.2vdc-
4              18vac
5              18vac
6              22.8vdc+
7              23.6vdc-
8              16.5vdc+                               13.5vdc+
9              16.6vdc-
10            22.8vdc+
11            5.01vdc+
12            25
13            99                                          106
14            11                                          7
15            52                                          40
16            41                                          31
17            60                                          31
18            265                                        144
19            858                                        520
20            160                                        98
21            78                                          85
22            70                                          47
23            65                                          46
24            69                                          67
25            185                                        187
26            185                                        137
27            26                                          24
28            67                                          66
29            67                                          63
30            250                                        192
31            122mvac/1.08vdc+                  76mvac/1.06vdc+
32            122mvac/1.08vdc+                  77mvac/1.09vdc+
33            2.85vac                                   5.13vac
34            45                                           31
35            43                                           64  (measured across r126)

[shooter]

Are you using a meter for the ac readings or a scope?  Following your signal volts there are areas that seem low but like pre-out, TP 28, it’s back to “normal”.  If you’re scoping it, does the signal match up, no distortion, oddities.  Even tp 30, what looks like the last TP before speaker out seems close.  I know that ain’t much help. 
Do all the “functions” seem to react, have you tried 2 speakers in parallel, you should get a volume boost….I think!
Try "smacking" the circuit board around a bit, thump on each if the ops with a chopstck.  look for "hot areas" in the signal path.  When I get time i'll try n match up your numbers better.
good luck.

[embotone]

Thanks for the help.  No, I am not using a scope, just a DVM, I have a scope but can't say I really know how to use it. The schematic says all voltages read with a DVM.  I have a big wedding to deal with this weekend(not mine) so I probably won't get back to this project until Monday.
I have not tried adjusting any functions, just set everything as called for on the schematic.  One or two speakers doesn't seem to make much difference in volume.  I did notice I had a strong signal going into the tone stack, but coming out at TP14 it is low. I have not tried fiddling with the tone pots to see if I can bring that up with a little adjustment.

[PRR]

Well, there's your problem: it has chips in it!!

Chisel them off and put in some good tubes.

> The signal started getting below what it should be at test point #14.

And yet TP28, the output of preamp (input of power amp) is 66/67 correct, spot-on for audio level measures.

Mind-bender power stage. Note how the emitters of the big parallel transistors, which is usually speaker out, is grounded. I (and others) invented that in the late 1980s, but it still bends my mind.

[PRR]

I would be thinking bad contacts in the preamp out / power amp in jacks.

And ANY internal connectors (ribbons, jumpers, etc).

[embotone]

Following PRR's advice (I would be thinking bad contacts in the preamp out / power amp in jacks. And ANY internal connectors (ribbons, jumpers, etc), I cleaned everything.  And now it works.
Thanks

[Willabe]

Good job!


                Brad   

[shooter]

And now you know what to tell your friend next time he asks

[PRR]

> over my head here with a solid state amp.
> thinking there is a problem with an op am
> I cleaned everything.  And now it works.

Take-away lesson: It is usually a connection.

In general, it hardly matters if it is tubes, chips, whale-oil, or dilithium crystals. The sexy stuff has to be connected with mundane plumbing or wires. When you have a problem with a toilet, usually the magic flush-action still works, the problem is a pipe leak under or inside the toilet.

True, connectors evolved (or devolved) with the technology and the times. Fahnestock clips had to be jiggled every few months. Eyelets on board was pretty solid. Elastomeric washers beat leather washers for toilet pipes. Hasty-soldered PCBs and their many-way connectors seem to need attention every few years.

And Out/In loops where signal must pass through switched jacks are always a sore spot. The contacts get plenty of air from the hole. Air tarnishes metal. If a contact is used a lot (such as the guitar hole) the tarnish gets worked off. If a contact is left idle for years, tarnish gets under the finger and breaks the connection.

> #8 was marginal, at 13.53vdc+

R156 and D52 *regulate* a raw ~~22V to a constant "16" Volts. The spec on the diode is 15.2 to 16.6V. The nominal operating current is a little below the spec test current, but I'd expect at least 15V.

And this is an area which on *other* Fender amps has been problematic. These parts run HOT and cook their guts or the PCB. On this design the ratings seem reasonably generous, unless they are tightly packed against other hot parts.

Study TP6 and R156, and the complementary parts on the negative side of that power supply.

Within reason. Ideally you'd lift the PCB and look for cooked/cracked solder joints. But if that is a hassle, and the voltage is "close", and the amp works, and it's for a customer (friend) who does not want to pay for all-day labor, let it go.