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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 7th June 2020, 08:48 AM   #51
Wolfgang Felber is offline Wolfgang Felber  Australia
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Default 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?
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Old 7th June 2020, 09:48 AM   #52
Mooly is offline Mooly  United Kingdom
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Drop-in replacement for JRC4558
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.
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Old 8th June 2020, 11:55 AM   #53
Wolfgang Felber is offline Wolfgang Felber  Australia
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Join Date: Aug 2012
Location: Australia
Default 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
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Old 8th June 2020, 12:48 PM   #54
Mooly is offline Mooly  United Kingdom
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Drop-in replacement for JRC4558
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.

Quote:
Separate three-position bias and equalization lever switches are provided for normal, CrO2, and metal tapes. These work in conjunction with a preset/adjust pushbutton that permits the user to optimize the record bias and 0-dB record-level indication on the meters for specific tapes using four (two per channel) front-panel screwdriver-adjustable controls. A LED indicates when the adjust option has been selected. The adjustment procedure requires the use of an external audio generator (Teac makes a suitable accessory, Model TO-8) and is a little more complicated than most such "optimizing" techniques, but because the built-in dbx noise-reduction system is sensitive to frequency-response variations the procedure should be mastered if you intend to use a wide variety of tape types.
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