Teac C3X
Hi all, I think I have a faulty op amp, it is a JRC4558DF, it is in the line amplifier section. It is also intermittent.
It is a dual op amp, pins 7 & 1 are the output, when working the output from the pins are 15.72 volts, when the channel goes faulty and stops recording, the output jumps up to 22.5 volts.
Does this sound like a faulty op amp, or should I be looking at the surrounding components?
If it is faulty, what can I use as a replacement.
Thank you
Hi all, I think I have a faulty op amp, it is a JRC4558DF, it is in the line amplifier section. It is also intermittent.
It is a dual op amp, pins 7 & 1 are the output, when working the output from the pins are 15.72 volts, when the channel goes faulty and stops recording, the output jumps up to 22.5 volts.
Does this sound like a faulty op amp, or should I be looking at the surrounding components?
If it is faulty, what can I use as a replacement.
Thank you
The 4558 is very common and you should have no problems finding replacements. Just make sure you get the correct package such as 8 pin DIL. Pretty much any dual opamp could be used to replace it such as a TL072 or even NE5532.
Is it faulty... I would be very very surprised indeed if it were. We would need to see the circuit diagram showing how its configured to advise better. As you say the output is at 15.72 volts normally then that suggests single rail operation.
Measuring ALL the pins when working and not working might give a better clue.
Is it faulty... I would be very very surprised indeed if it were. We would need to see the circuit diagram showing how its configured to advise better. As you say the output is at 15.72 volts normally then that suggests single rail operation.
Measuring ALL the pins when working and not working might give a better clue.
Teac C3X
Thank you for your reply, I will test the electrolytics around it. There are 4,
one that goes to the non inverting input + side, there is one that goes to the inverting side - side of the input, this is part of the negative feedback coming from the output, pin 7. There is also one on the output side going to the bias setting variable resistors. they are all 10uf 16 volt electro's.
I will try and test the resistors, a bit hard whilst they are in circuit.
Thanks
Thank you for your reply, I will test the electrolytics around it. There are 4,
one that goes to the non inverting input + side, there is one that goes to the inverting side - side of the input, this is part of the negative feedback coming from the output, pin 7. There is also one on the output side going to the bias setting variable resistors. they are all 10uf 16 volt electro's.
I will try and test the resistors, a bit hard whilst they are in circuit.
Thanks
Try and diagnose a fault by voltage measurement rather than randomly pulling parts.
The voltages on all the pins of the chip will give a good clue as to whether it is faulty or not. Opamps obey very definite rules and if we know all the voltages and can also see a diagram then we can say if there is a problem or not.
The voltages on all the pins of the chip will give a good clue as to whether it is faulty or not. Opamps obey very definite rules and if we know all the voltages and can also see a diagram then we can say if there is a problem or not.
Hi from Australia, I have done what you suggested. Please see below:
Left Channel recording
Pin 1 11.70 volts
Pin 2 11.71 volts
Pin 3 11.55 volts
Pin 4 0.010 volts
Pin 5 11.54 volts
Pin 6 11.62 volts
Pin 7 11.69 volts
Pin 8 23.31 volts
Left Channel not recording
Pin 1 11.70 volts
Pin 2 11.75 volts
Pin 3 11.55 volts
Pin 4 0.010 volts
Pin 5 11.57 volts
Pin 6 7.46 volts
Pin 7 22.76 volts
Pin 8 23.51 volts
I have tried to attach the circuit diagram, it is from a Tascam 122 which
does not record on the left channel. It is an intermittent fault, actually
the unit records fine for about 10 to 15 minutes and then starts dropping
out. When it drops out you can hear a sound like a lightning strike on the
radio and the needle deflects to the right and then it stops completely.
Regards
Wolfgang Felber
Left Channel recording
Pin 1 11.70 volts
Pin 2 11.71 volts
Pin 3 11.55 volts
Pin 4 0.010 volts
Pin 5 11.54 volts
Pin 6 11.62 volts
Pin 7 11.69 volts
Pin 8 23.31 volts
Left Channel not recording
Pin 1 11.70 volts
Pin 2 11.75 volts
Pin 3 11.55 volts
Pin 4 0.010 volts
Pin 5 11.57 volts
Pin 6 7.46 volts
Pin 7 22.76 volts
Pin 8 23.51 volts
I have tried to attach the circuit diagram, it is from a Tascam 122 which
does not record on the left channel. It is an intermittent fault, actually
the unit records fine for about 10 to 15 minutes and then starts dropping
out. When it drops out you can hear a sound like a lightning strike on the
radio and the needle deflects to the right and then it stops completely.
Regards
Wolfgang Felber
The left opamp with pin 5, 6, and 7 show something possibly amiss.
Pin 7 is actually doing what it should based on the input voltages. Pin 5 (non inverting input) is high with respect to the inverting input and so the output has swung hard over to the positive rail. The opamp is doing what it should up to this point.
There are two possible causes. First is the opamp itself but that is the least likely but don't discount it altogether.
Most likely is a fault in the feedback around the IC.
Compare the circuit for L and R and look at what connects between opamp output and the inverting input.
Is something intermittent or is there a break somewhere?
Is there a cap that could be pulling the input pin down intermittently?
It would help to see the circuit of that area but the above are the most likely:
How to attach images to your posts.
Pin 7 is actually doing what it should based on the input voltages. Pin 5 (non inverting input) is high with respect to the inverting input and so the output has swung hard over to the positive rail. The opamp is doing what it should up to this point.
There are two possible causes. First is the opamp itself but that is the least likely but don't discount it altogether.
Most likely is a fault in the feedback around the IC.
Compare the circuit for L and R and look at what connects between opamp output and the inverting input.
Is something intermittent or is there a break somewhere?
Is there a cap that could be pulling the input pin down intermittently?
It would help to see the circuit of that area but the above are the most likely:
How to attach images to your posts.
Good luck.
Op Amps do not break and heal on their own.
Output voltage closely follows corresponding non inverting input voltage unless it´s configured to have DC voltage gain, which does not seem to be the case.
*Some* faults come from "bad parts", some come from bad connections (faulty switches, cracked tracks or solder, oxidized contacts, etc.) and then you can test or even replace ALL parts and not catch them.
WHEN you post schematic, might suggest something specific.
Output voltage closely follows corresponding non inverting input voltage unless it´s configured to have DC voltage gain, which does not seem to be the case.
*Some* faults come from "bad parts", some come from bad connections (faulty switches, cracked tracks or solder, oxidized contacts, etc.) and then you can test or even replace ALL parts and not catch them.
WHEN you post schematic, might suggest something specific.
Last edited:
Teac C3X
Hi , I have managed to download the circuit diagram, it is U104 which is the 4558df chip/op amp.
I have discovered something really weird, I have measured one of
the resistors in the feedback circuit of the OP amp, in circuit, power on,
the resistor measures open circuit or infinity. When you check it against
the working side that resistor measures as per specs. When you turn the
power off, the resistor that measures open circuit, measures OK 3.9k. Go
figure. Can a resistor go open circuit if current runs through them? I have
removed the questionable resistor and it measures OK.
Replaced the questionable resistor and it still does the same thing. no power measures fine. turn it on after a while and records OK for 15 minutes and then stops. resistor 148 goes open circuit.
This is very confusing, should I look at the electro's there are 4 x 10uf 16 volt electro's around the op amp C146,147, 149 & C150. I use a ESR meter on them in circuit and they seemed to be OK a couple were down to 8uf, is that within the 20% tolerance?
Hi , I have managed to download the circuit diagram, it is U104 which is the 4558df chip/op amp.
I have discovered something really weird, I have measured one of
the resistors in the feedback circuit of the OP amp, in circuit, power on,
the resistor measures open circuit or infinity. When you check it against
the working side that resistor measures as per specs. When you turn the
power off, the resistor that measures open circuit, measures OK 3.9k. Go
figure. Can a resistor go open circuit if current runs through them? I have
removed the questionable resistor and it measures OK.
Replaced the questionable resistor and it still does the same thing. no power measures fine. turn it on after a while and records OK for 15 minutes and then stops. resistor 148 goes open circuit.
This is very confusing, should I look at the electro's there are 4 x 10uf 16 volt electro's around the op amp C146,147, 149 & C150. I use a ESR meter on them in circuit and they seemed to be OK a couple were down to 8uf, is that within the 20% tolerance?
Attachments
Measuring resistors in circuit, and particularly high value ones is very unreliable... not only are there other circuit elements in parallel to the part you are measuring but also even slight residual voltage from the PSU can confuse the meter. If you do get the expected reading in circuit then its most likely to be OK, if you don't then isolate one lead of it and try again.
So pin 5 is at correct voltage. It is biased through the 150k from a divider across the rails. That all looks good.
I think your problem will be, in order of most likely suspect...
1/ C148 failing very leaky or short circuit.
2/ C149 failing as above.
3/ The opamp.
4/ R186 failing high in value.
Edit... just looking at the numbers (your voltage readings) a total short on C149 would give your voltage readings or a leak in C148 of about 18k. I still think that little cap is the more likely but its a coincidence that a dead short of the electrolytic would give the voltages you measure. It might be wise to replace both unless you find a definite issue with either simply because the heat from soldering could appear to 'fix' the cap temporarily.
So pin 5 is at correct voltage. It is biased through the 150k from a divider across the rails. That all looks good.
I think your problem will be, in order of most likely suspect...
1/ C148 failing very leaky or short circuit.
2/ C149 failing as above.
3/ The opamp.
4/ R186 failing high in value.
Edit... just looking at the numbers (your voltage readings) a total short on C149 would give your voltage readings or a leak in C148 of about 18k. I still think that little cap is the more likely but its a coincidence that a dead short of the electrolytic would give the voltages you measure. It might be wise to replace both unless you find a definite issue with either simply because the heat from soldering could appear to 'fix' the cap temporarily.
Teac C3X
Hi Mooly, I did as you suggested and it managed to get through the whole side of the tape without failing in the record mode. Do I put this down as a success?
If yes I have you and a few other people to thank. I shall continue testing over the next few nights. Once we have established that it is OK, can I ask a big favour, I would like to set the deck up for TDK SA tapes, I assume I can do this by pressing the adjust pre set button. I am just not 100% sure on how to do that, I need advice from the expert.
Regards
Wolfgang
Hi Mooly, I did as you suggested and it managed to get through the whole side of the tape without failing in the record mode. Do I put this down as a success?
If yes I have you and a few other people to thank. I shall continue testing over the next few nights. Once we have established that it is OK, can I ask a big favour, I would like to set the deck up for TDK SA tapes, I assume I can do this by pressing the adjust pre set button. I am just not 100% sure on how to do that, I need advice from the expert.
Regards
Wolfgang
That's good 
it sounds like a success so far. Did you replace both caps?
I'm not familiar at all with the TEAC operationally but can see it has a manual adjust option, apparently requiring the use of an external signal generator.
TEAC TO-8 - Manual - Test Tone Oscillator - HiFi Engine
There is a circuit diagram in the attached pdf.
it sounds like a success so far. Did you replace both caps?I'm not familiar at all with the TEAC operationally but can see it has a manual adjust option, apparently requiring the use of an external signal generator.
TEAC TO-8 - Manual - Test Tone Oscillator - HiFi Engine
There is a circuit diagram in the attached pdf.
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