Map the output of the op-amps and other signal levels from the input RCAs to the input of the amplifier stage. Are they the same in both amps?
Confirm that the RCA input shield has no signal on both amplifiers.
Are all pots and switches in the same position in both amplifiers?
Confirm that the RCA input shield has no signal on both amplifiers.
Are all pots and switches in the same position in both amplifiers?
Hear is the picture of the op amps. The left 4562D is the right channel. The right one is the left. The NE5534N is for the Sub/Lowpass section. The sub switch is in the out position. I realized after I made all my initial measurements, one of the channels on the good amp was clipping. There is obviously a big difference between the two amps. I will try and make more measurements, but I will need to try tracing the circuit to know what the other components are doing.
Bad amp
Left op amp
Pin Reading.
1 7.3vP-p inverted
2 0v
3 0v
4 -17.3v
5 168mv p-p inverted
6 168mv p-p inverted
7 720mv p-p
8 17.26v
Right op amp
Pin Reading
1 7.24p-p
2 600mv p-p
3 600mv p-p
4 -17.3v
5 130mv p-p
6 130mv p-p
7 528mv p-p
8 17.26v
Good amp
Left op amp
Pin Reading.
1 7.4vP-p inverted
2 0v
3 0v
4 -17.8v
5 6.0v p-p inverted
6 5.92v p-p inverted
7 32v p-p (Clipped)
8 17.26v
Right op amp
Pin Reading
1 6.5vp-p
2 450mv p-p
3 450mv p-p
4 -17.8v
5 5.32v p-p
6 5.4v p-p
7 24.8v p-p
8 17.26v
Bad amp
Left op amp
Pin Reading.
1 7.3vP-p inverted
2 0v
3 0v
4 -17.3v
5 168mv p-p inverted
6 168mv p-p inverted
7 720mv p-p
8 17.26v
Right op amp
Pin Reading
1 7.24p-p
2 600mv p-p
3 600mv p-p
4 -17.3v
5 130mv p-p
6 130mv p-p
7 528mv p-p
8 17.26v
Good amp
Left op amp
Pin Reading.
1 7.4vP-p inverted
2 0v
3 0v
4 -17.8v
5 6.0v p-p inverted
6 5.92v p-p inverted
7 32v p-p (Clipped)
8 17.26v
Right op amp
Pin Reading
1 6.5vp-p
2 450mv p-p
3 450mv p-p
4 -17.8v
5 5.32v p-p
6 5.4v p-p
7 24.8v p-p
8 17.26v
The two amp boards are the same. There are some different manufacturer transistors, but same base part number. Caps and resistors all seem to be the same values.
I have been spending my evenings trying to create a schematic for them. One thing I found is that there is a n channel JFET between the output of the first opamp in the preamp circuit and the second opamp. This appears to be used as a mute to allow the power supply to come up before signal is allowed through. Basically the jfet is grounding the signal, not allowing it to pass until it is switched on. If the fet is bad or not turning all the way off that could cause the lack of signal from the second opamp.
I have been spending my evenings trying to create a schematic for them. One thing I found is that there is a n channel JFET between the output of the first opamp in the preamp circuit and the second opamp. This appears to be used as a mute to allow the power supply to come up before signal is allowed through. Basically the jfet is grounding the signal, not allowing it to pass until it is switched on. If the fet is bad or not turning all the way off that could cause the lack of signal from the second opamp.
If it's for muting, pulling it shouldn't cause any problems except possibly a turn-on/off pop. Typically, the gate terminal has to go to -15v to switch off (for J108/111 type JFETs).
Post or email me whatever you have for the diagram and I'll see if I can find something close.
babin_perry@yahoo.com
Post or email me whatever you have for the diagram and I'll see if I can find something close.
babin_perry@yahoo.com
I pulled the jfets, and got nice clean high power outputs. The jfets that actually mute the preamp section are fine. The jfet that drives them is bad. When I put my meter on Neg to gate and Pos to source or drain, I get 37k ohms the other two give OL. I have some 2N5639 JFET on the way. Hope this fixes it.
Replacing the bad 2N5639 JFET worked. I now get full output.
This is the Left channel driving a 4 ohm load just before clipping.
Right channel (negative) into a 4 ohm load just before clipping.
Right Negative
It sounds good with full range audio playing. No turn on or turn off pop.
Thanks Perry for the help.
This is the Left channel driving a 4 ohm load just before clipping.
Right channel (negative) into a 4 ohm load just before clipping.
Right Negative
It sounds good with full range audio playing. No turn on or turn off pop.
Thanks Perry for the help.