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Chip Amps Amplifiers based on integrated circuits

Drop-in replacement for JRC4558
Drop-in replacement for JRC4558
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Old 2nd June 2020, 02:51 AM   #41
Wolfgang Felber is offline Wolfgang Felber  Australia
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Default 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
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Old 2nd June 2020, 06:06 AM   #42
Mooly is offline Mooly  United Kingdom
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Drop-in replacement for JRC4558
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.
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Old 2nd June 2020, 06:27 AM   #43
Wolfgang Felber is offline Wolfgang Felber  Australia
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Default 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
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Old 2nd June 2020, 07:19 AM   #44
Mooly is offline Mooly  United Kingdom
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Drop-in replacement for JRC4558
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.
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Old 3rd June 2020, 10:31 PM   #45
Wolfgang Felber is offline Wolfgang Felber  Australia
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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
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Old 4th June 2020, 06:58 AM   #46
Mooly is offline Mooly  United Kingdom
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Drop-in replacement for JRC4558
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:
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Old 4th June 2020, 07:02 AM   #47
Mooly is offline Mooly  United Kingdom
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Drop-in replacement for JRC4558
Also... does the unit have mechanical 'Record/Play' switches?

If so then they are the number one cause of problems. They look like these, maybe with two or three times as many pins.
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Old 5th June 2020, 11:02 AM   #48
Wolfgang Felber is offline Wolfgang Felber  Australia
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Default Teac C3X

No the Teac C3X does not have a mechanical switch, it is done electronically.
I will do as you suggest and check all the components around the 4558.

Regards
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Old 5th June 2020, 12:40 PM   #49
Mooly is offline Mooly  United Kingdom
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Drop-in replacement for JRC4558
OK Good luck.
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Old 5th June 2020, 01:19 PM   #50
JMFahey is offline JMFahey  Argentina
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Op Amps do not break and heal on their own.

Output voltage closely follows corresponding non inverting input voltage unless itīs 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.
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Last edited by JMFahey; 5th June 2020 at 01:23 PM.
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