Fender M-80 Bass Amplifier making distorted output
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elnaldo
Lunatic Member
Are you following the schematic? How is the signal at U4 PIN6 ? At pin 7 you have a switch "Delta Comp", make sure it's OFF. When it's ON, it connects to all that feedback network going back to pin6, perhaps with undesired effects.
Edited.
I was referring to the driver transistors at the output, Q19 Q20
Edited.
I was referring to the driver transistors at the output, Q19 Q20
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buck1108
New Member
The voltage at pin 6 is zero (flat line).
In the schematic, there is a circle at the contact point of pin 6. At several other points in the circuit (all at IC's), there is this circle symbol, and at each of these, the voltage is zero. Does the circle indicate a 'detachment' of some sort?
In the schematic, there is a circle at the contact point of pin 6. At several other points in the circuit (all at IC's), there is this circle symbol, and at each of these, the voltage is zero. Does the circle indicate a 'detachment' of some sort?
elnaldo
Lunatic Member
Voltages are OK. (I wanted to discard some "offset" of the signal, causing that assymetrycal clipping)
So, just confirm if the signal is OK at the input (U4, pin 6), and if so, I'd say: Signal is OK, voltages are OK, the 1st suspect is a bad OP amp...
Make sure the "Delta Comp" is disabled, check you don't have continuity from U4 PIN7 to diode CR1 or resistor R29 before testing the signal at pin6.
I see that blank circle, it seems to indicate the "+" signal input to the OP amps... Perhaps is just a convention of the drawing software, indicating the OPamp positive input.
So, just confirm if the signal is OK at the input (U4, pin 6), and if so, I'd say: Signal is OK, voltages are OK, the 1st suspect is a bad OP amp...
Make sure the "Delta Comp" is disabled, check you don't have continuity from U4 PIN7 to diode CR1 or resistor R29 before testing the signal at pin6.
I see that blank circle, it seems to indicate the "+" signal input to the OP amps... Perhaps is just a convention of the drawing software, indicating the OPamp positive input.
buck1108
New Member
When I measure the input (U4, pin 6), the scope just shows a flat line at zero. I've had the scope on "AC" mode, but still no voltage reading.
When "Delta Comp" is disabled, there is continuity from U4, pin 7 to diode CR1/resistor R29, but that checks ok with the schematic. The schematic shows what must be "normal disabled" signal path. I also checked the "Delta Comp" for continuity between CR40 and R129 and there is none when "Delta Comp" is disabled.
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elnaldo
Lunatic Member
I suspect there is something wrong at that point (not 100% sure tough, it could be the normal operation, but I don't think so). You traced the signal, and up to U3 output it was OK. So at U2 pin7 the sine wave is OK. That U2 pin 7 output pass through C17, R26, and goes to U4 pin6. I'd check again at both sides of that capacitor to confirm the signal is OK.
I'd narrow the search around all that feedback network at U4, diodes and transistors, perhaps swapping them to see if the clipping flips upside down. Perhaps one of them is failing. Test the resistors too. My understanding of that part of the circuit is that the positive half goes through CR2-Q2, and the negative half through CR1-Q1. I'm not an expert in this networks anyway.
If you have 2 spare NPN transistors, you could replace Q1 and Q2, just in case. They are NPN, 60V, 600mA, 625mW, EBC pinout. Any 60V or higher transistor will serve for a test. Just check the pinout.
Also, check the signal switching the DELTA to "on"...The idea is to disconnect that feedback network.
I'd narrow the search around all that feedback network at U4, diodes and transistors, perhaps swapping them to see if the clipping flips upside down. Perhaps one of them is failing. Test the resistors too. My understanding of that part of the circuit is that the positive half goes through CR2-Q2, and the negative half through CR1-Q1. I'm not an expert in this networks anyway.
If you have 2 spare NPN transistors, you could replace Q1 and Q2, just in case. They are NPN, 60V, 600mA, 625mW, EBC pinout. Any 60V or higher transistor will serve for a test. Just check the pinout.
Also, check the signal switching the DELTA to "on"...The idea is to disconnect that feedback network.
buck1108
New Member
Thanks - I've only had time to test C17, R26 so far. C17, both sides has a clean sine wave. R26 has a clean sine wave on one side and no wave at all on the other lead. I tried taking the Volts/Div up and down, but could get no wave. However, there has to be signal there, as that is the only link to the rest of the circuit. Any suggestions as to why there is no wave on that side of the resistor?
Also, I'm staring to work on testing all resistors and other components in the feedback network around U4, as well.
As Q1 and Q2 are both NPN transistors, would the swapping of them (per your suggestion) be to isolate the potential fault in one of the transistors, rather than the U4 chip? Thanks.
elnaldo
Lunatic Member
Can you just replace U4? perhap's it's cheaper and easier than removing and testing all the rest. At this point, with no signal after R26, the 1st suspect is that OP amp shunting the signal to ground...
Did you try that "Delta comp" switch, to disconnect the feedback network?
Anyway, I'd replace U4 before any other test, just to be sure and trust that component. I'd clean and check that R128 potentiometer too...
You are right, swapping the transistors was an idea to see if the "flat" clipped wave turned upside down.
Did you try that "Delta comp" switch, to disconnect the feedback network?
Anyway, I'd replace U4 before any other test, just to be sure and trust that component. I'd clean and check that R128 potentiometer too...
You are right, swapping the transistors was an idea to see if the "flat" clipped wave turned upside down.
buck1108
New Member
I've replaced U4, and the output remains the same (power amp test point); however, when I monitor pin 7 of U4 and tun the Delta Comp switch off and on, I'm seeing a variation I hadn't seen before. Pin 7 shows a clean wave when the Delta Comp is on; however, when Delta comp is off, Pin 7 shows a wave clean on the top, but flat on the bottom.
Do you think that the transistors mentioned before (Q1 and Q2) might be suspect, as they are in the 'Delta Comp - off' feedback network?
Thanks.
Do you think that the transistors mentioned before (Q1 and Q2) might be suspect, as they are in the 'Delta Comp - off' feedback network?
Thanks.
elnaldo
Lunatic Member
you are rignt, those transistors let the signal pass, positive and negative halves (follow the transistor's arrow). So probably one of them is saturating, or it's biased too high... But the fact that disabling that network restores the proper signal, tells that something is not OK there.
Did you swap them finally? (since now you have a flat bottom, not a flat top...)
I'd check those resistors and replace both transistors with new parts. I think it's easier to replace them than test them in some test circuit and so.... If you have some spare transistors you could use them, just check the voltage and pinout.
Did you swap them finally? (since now you have a flat bottom, not a flat top...)
I'd check those resistors and replace both transistors with new parts. I think it's easier to replace them than test them in some test circuit and so.... If you have some spare transistors you could use them, just check the voltage and pinout.
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buck1108
New Member
Update: I replaced both Q1 and Q2 transistors, but the wave is still the same.
When monitored at TP6 (pin 7 of U4), the wave has a flat bottom when the Delta Comp is off.
When monitored at TP6 (pin 7 of U4), the wave has a round top and bottom (clean wave) when the Delta Comp is on.
When monitored at TP15 (pin 1 of U4), the wave has a flat top when the Delta Comp is on.
When monitored at TP15 (pin 1 of U4), the wave has a round top and bottom (clean wave) when the Delta Comp is off.
When monitoring the leads of Q1 and Q2, the wave shape is very un-sine like. In fact, there is some difference in the appearance of the wave from Q1 and Q2.
Do you think the next step is to replace the resistors?
elnaldo
Lunatic Member
so at TP 15 YOU HAVE A CLEAN WAVE with "delta comp. off", THAT'S ok !!! I'd check the output under this condition.
That feedback network we were checking seems to be asymmetrical, the resistors are all different, I really can't tell what's the normal operation. Check resistors just in case, and the diodes, or reflow the solder, but if the output is OK, I'll leave this alone by now
That feedback network we were checking seems to be asymmetrical, the resistors are all different, I really can't tell what's the normal operation. Check resistors just in case, and the diodes, or reflow the solder, but if the output is OK, I'll leave this alone by now
buck1108
New Member
Thanks.
I've unsoldered the legs of the three resistors in the feedback circuit loop, tested them with an analog volt meter, and they each test accurately within 1k ohm. However, I wasn't sure of the tolerance, so I may need to go back and re-test. Also, I couldn't get the precision of a digital volt meter, so I may try that next.
I recently tested each of the diodes with one leg unsoldered, and each tested ok (with a digital volt meter).
I wonder if just replacing the diodes and resistors (in the feedback loop) would be a good next step, as I see no readable evidence in testing?
Thanks
buck1108
New Member
Thanks
Yes, the signal is distorted at the output.
TP15 changes, according to whether the Delta Comp is on or off (please see my quote above).
TP6 is the opposite as TP15, in that it is clean when TP15 is dirty (please see my quote above).
elnaldo
Lunatic Member
I understand the signal is OK at TP15 when Delta Comp is OFF.
In that setup, with a clean wave at TP 15, I'd keep tracing the signal to the output. If I remember it right, TP15 is the last point before the power amplifier.
"Delta Comp" is a compression, so I'm not surprised it distorts the signal in some way. We need to read a bit about that feature to understand what it's supposed to do.
But when it's OFF, the signal must be undistorted.
In that setup, with a clean wave at TP 15, I'd keep tracing the signal to the output. If I remember it right, TP15 is the last point before the power amplifier.
"Delta Comp" is a compression, so I'm not surprised it distorts the signal in some way. We need to read a bit about that feature to understand what it's supposed to do.
But when it's OFF, the signal must be undistorted.