I'm working on a crown IC-150 and there seems to be an issue with one of the op-amps. And ohm readings are off on between the op-amps when I have them out of circuit.
What should I replace them with? They have a compensation pin but I know I can replace with ones that don't have that. Do I need ones with the offset null pins? Do I need to adjust anything in the circuit? I've put in sockets so I can swap new op-amps in and out easily.
The original op-amp is MC1439. Datasheet attached.
What should I replace them with? They have a compensation pin but I know I can replace with ones that don't have that. Do I need ones with the offset null pins? Do I need to adjust anything in the circuit? I've put in sockets so I can swap new op-amps in and out easily.
The original op-amp is MC1439. Datasheet attached.
Attachments
Let me see if I get this. There's nothing wrong with the amp, but you yank out the opamps and with ohm measurements on some pins of thoe opamps you get differences between them?
There's a lot of reasons why you would measure differences if you don't exactly know the connections on the chip and the polarity of the test leads.
Also, in the schematic, there are specific compensation parts specific for that chip.
Don't ruin a perfectly good amp!
Jan
There's a lot of reasons why you would measure differences if you don't exactly know the connections on the chip and the polarity of the test leads.
Also, in the schematic, there are specific compensation parts specific for that chip.
Don't ruin a perfectly good amp!
Jan
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There was an issue though, I just didn't go into details. When I turned one of the treble pots past a certain point, there would be a DC spike woofer would go in and out. Hoping the op-amps are the issue, I at least have to start here because the meter seemed to indicate an issue. (And I'm pretty sure I would have reversed the test leads to measure both ways when testing since I know that can cause differences.)
I half remember replacing compensated op-amps in another circuit and removing some associated components since newer op-amps didn't need that.
I half remember replacing compensated op-amps in another circuit and removing some associated components since newer op-amps didn't need that.
Seems to work with a pair of LM741. They just don't have the compensation pin but that is okay I guess?
And it fixes the issue changing them was supposed to fix.
And it fixes the issue changing them was supposed to fix.
One more issue. If an input that isn't selected is playing, there is loud low frequency distortion coming out of both channels.
Forgot a ground screw! Derp!
I got this thing and the matching power amp (that I have another thread on) back together and playing on proper speakers.
The sound quality was terrible. Like, it sounded like there was a weighted blanket over the sound. Hollow, no highs. A disaster.
I did some searching and tried different op-amps. Pulled the LM741 and found a spare pair of OPA604. Much better!!!
If you ever fool yourself into thinking op-amps don't make a difference, try LM741s in a Crown IC-150!
I got this thing and the matching power amp (that I have another thread on) back together and playing on proper speakers.
The sound quality was terrible. Like, it sounded like there was a weighted blanket over the sound. Hollow, no highs. A disaster.
I did some searching and tried different op-amps. Pulled the LM741 and found a spare pair of OPA604. Much better!!!
If you ever fool yourself into thinking op-amps don't make a difference, try LM741s in a Crown IC-150!
The 741 was designed in 1968, it's 55 years old. It's noisy, has high input offset, low bandwidth, and is very slew-limited in the audio band. Nobody would use one of those for audio, even by the mid 1970s, unless it was a matter of life and death, and the only opamp on hand.
If you were looking for a pair of extremes to prove opamps sound different, those two would be the ones to use.
If you were looking for a pair of extremes to prove opamps sound different, those two would be the ones to use.
I think TL071's would be my go to part here and remove the components to pins 8, 5 and 1
Interesting in original mc1539 ic is resistor between pins 5-6, never seen such variant before, only capacitors for compensation.
I tried those, but they were very noisy in this circuit for whatever reason. But I know them to be a decent part for when I'm not going all out.
I upgraded to OPA627. Be sure to add bypass caps - the layout is really bad. Worked well for me.
That's surprising. I think you would need to remove those three parts though (pins 8,5 and 1) Interesting 🙂
Went back and tried the TL071 with those pins disconnected. Worked well then. But then put the 604 back in, and that was still good. Didn't notice any real difference just through average headphones. I've now sent it home with the matching power amp.
Could probably do more tweaking on this but got to call it sometime.
Could probably do more tweaking on this but got to call it sometime.
The OPA604 was always one of my favourites, a great opamp sonically imo.