Hey folks,
I need some guidance here.
I have some experience on class a/b amps, have troubleshooted and restored a lot of pioneers, marantz, realistic, etc... but this is my first Carver.
This is not a magnetic field unit. Not the TFM 35x either (a lot of similarities though).
I have replaced all electrolytic caps, and a few faulty components: one of the ICs in the input board that was intermintently shorting, blown fusible resistors and bad diodes.
I have also replaced all transistors in the amp circuit.
Everything seems fine and stable but there is a DC voltage in the right channel that I am not able to get rid of. It is 0.8V and it won't budge no matter what.
I tried replacing back old output transistors and pre drivers one by one and did not make any difference.
I HFE matched the predrivers and also no change.
There is no bias offset adjustment on this amp, just IDLE current adjustment trim pots, but the TP testing points are not showing anything usable.
I need a peer to review and help me to troubleshoot this issue.
I am not comfortable with this DC voltage coming out of the amp.
DMM, o-scope, thermal camera, audio SG are available...
I need some guidance here.
I have some experience on class a/b amps, have troubleshooted and restored a lot of pioneers, marantz, realistic, etc... but this is my first Carver.
This is not a magnetic field unit. Not the TFM 35x either (a lot of similarities though).
I have replaced all electrolytic caps, and a few faulty components: one of the ICs in the input board that was intermintently shorting, blown fusible resistors and bad diodes.
I have also replaced all transistors in the amp circuit.
Everything seems fine and stable but there is a DC voltage in the right channel that I am not able to get rid of. It is 0.8V and it won't budge no matter what.
I tried replacing back old output transistors and pre drivers one by one and did not make any difference.
I HFE matched the predrivers and also no change.
There is no bias offset adjustment on this amp, just IDLE current adjustment trim pots, but the TP testing points are not showing anything usable.
I need a peer to review and help me to troubleshoot this issue.
I am not comfortable with this DC voltage coming out of the amp.
DMM, o-scope, thermal camera, audio SG are available...
The right channel has all of the stereo/mono switching, make sure that switch is clean, exercise it a few times. What's the DCV on pins 1 and 7 of IC101?
Craig
Craig
thanks for the reply
IC101
pin 1 = 0.016vdc
pin 7 = 15.22vdc
yes, switch is clean and tested for continuity
IC101
pin 1 = 0.016vdc
pin 7 = 15.22vdc
yes, switch is clean and tested for continuity
Pin 7 is the Right channel, should be 0VDC, there's your DC offset problem. Is that the opamp you replaced?
And what voltages do you see on IC101 pins 5 and 6?
I'm surprised with that with 15V on pin 7 that there isn't more than more than +0.8VDC at the output.
I'm surprised with that with 15V on pin 7 that there isn't more than more than +0.8VDC at the output.
Sorry folks, I apparently I cant count pins.
IC101
pin 1 = 0.016vdc
pin 7 = 0.015vdc
pin 8 = 15.22vdc
IC101
pin 1 = 0.016vdc
pin 7 = 0.015vdc
pin 8 = 15.22vdc
Is excessive offset the only problem? i.e. are sine waves reproduced cleanly?
Check for any voltage across R116. Ideally, should be 0mV.
Check for any voltage across R116. Ideally, should be 0mV.
Can't quarrel with R116 voltage.
Would you measure voltage re ground on IC101 pins 5 and 6 and the other end of R114 that routes through connector MC102 pin 4. This latter voltage should appear at the right channel speaker output. If it doesn't, trace continuity. Relay RY302 would deserve extra scrutiny--- is it energized?
Would you measure voltage re ground on IC101 pins 5 and 6 and the other end of R114 that routes through connector MC102 pin 4. This latter voltage should appear at the right channel speaker output. If it doesn't, trace continuity. Relay RY302 would deserve extra scrutiny--- is it energized?
IC101
pin 5 = -0.65vdc
pin 6 = -0.66vdc
MC102
pin 4 = -0.864vdc
Which matches the speaker output voltage.
So, yes, the relay is energized. I think the trigger for safety is about 1vdc.
pin 5 = -0.65vdc
pin 6 = -0.66vdc
MC102
pin 4 = -0.864vdc
Which matches the speaker output voltage.
So, yes, the relay is energized. I think the trigger for safety is about 1vdc.
After I posted, I realized relay was surely active. 😳
Is IC101 the original opamp? Perhaps a substitute? I've posted a link to PDF of the data sheet. According to the data, opamp's input bias current is 50 to 200nA. This times R104 value of 100K suggests IC101 pin 5 voltage should be +5mV to +20mV. This opamp has PNP input stage, so bias voltages would be positive rather than negative.
https://www.mouser.com/datasheet/2/294/NJM4565_E-1917662.pdf
Is IC101 the original opamp? Perhaps a substitute? I've posted a link to PDF of the data sheet. According to the data, opamp's input bias current is 50 to 200nA. This times R104 value of 100K suggests IC101 pin 5 voltage should be +5mV to +20mV. This opamp has PNP input stage, so bias voltages would be positive rather than negative.
https://www.mouser.com/datasheet/2/294/NJM4565_E-1917662.pdf
It is not original, but it is this exact same model NJM4565L, same datasheet you shared.
Since I bought 10 of these I have replaced it by another one with the same results, just to be sure the new part is not problematic.
Also, note that I have replaced IC501 and IC502 too.
Since I bought 10 of these I have replaced it by another one with the same results, just to be sure the new part is not problematic.
Also, note that I have replaced IC501 and IC502 too.
Where did you purchase the part? Who is manufacturer? I know counterfeit parts are rampant these days...
Maybe there's something else going on, though I haven't a clue what.
Ostensibly, the other channel is the same. Would you measure the same points in the other channel for comparison?
Thanks!
Ostensibly, the other channel is the same. Would you measure the same points in the other channel for comparison?
Thanks!