OP AMP question

[bsco]

I have a question.......While working in the lab late the other night, I found an IC chip in an old Hammond organ and there was a 10K resistor tack soldered between the input and output pins of the IC....When I removed te resistor And checked out the board, there was no sound.....I scoped the signals through and found that there was a signal at the input pin but nothing at the output.....When I installed the resistor back again, the sound worked.....SO did somebody just bypass the audio around the bad chip or did this because it was in a remote location and never had the part with them??? I am thinking this is what had happened....I never seen that before....SO, if it works with this unit, could you bridge a 10K resistor across the in and out pins of other op-amps in general to see if you have a bad IC??? I am very curious....
Cheers

[52 Bill]

....SO, if it works with this unit, could you bridge a 10K resistor across the in and out pins of other op-amps in general to see if you have a bad IC???

The answer would be maybe. It would really depend upon the circuit design.

Most opamp circuits use a resistor connected from the input to the output as part of the feedback loop that is used to control the gain of the stage. Adding another may or may not prove anything.

Are you certain that the chip in the Hammond is indeed an opamp?

In any case, I would think that a capacitor would be a better choice for signal bypassing, as it would block any dc that could cause all sorts of problems when doing this test.

[bsco]

The answer would be maybe. It would really depend upon the circuit design.

Most opamp circuits use a resistor connected from the input to the output as part of the feedback loop that is used to control the gain of the stage. Adding another may or may not prove anything.

Are you certain that the chip in the Hammond is indeed an opamp?

In any case, I would think that a capacitor would be a better choice for signal bypassing, as it would block any dc that could cause all sorts of problems when doing this test.

Yes..It is an RCA Op AMP from back in the 70's which is no longer available.....I can't remember the number off hand...I would have to check it out over at the other location...I will be there tomorrow.....So let me see if I have this right.....you would suggest that by connecting a cap like a .1 uf coupling cap you could determine if an IC chip was not passing a signal....and this technique would be one way to determine if the chip is suspect....I will have to try that experiment tomorrow.......Thanks for that idea......
Cheers

[Mick Bailey]

If I had an op-amp where the supply voltages were correct, and there was an input signal and no output I would say the op-amp was bad. Unless the gain-setting components were open/shorted, or the output was shorted to ground. My next check after measuring the supply at the IC pins is to test for DC on the output of an audio stage.

Bridging the input to output with a capacitor seems to be a way to draw false conclusions or complicate things. Maybe there's something in this I haven't spotted.

[bsco]

If I had an op-amp where the supply voltages were correct, and there was an input signal and no output I would say the op-amp was bad. Unless the gain-setting components were open/shorted, or the output was shorted to ground. My next check after measuring the supply at the IC pins is to test for DC on the output of an audio stage.

Bridging the input to output with a capacitor seems to be a way to draw false conclusions or complicate things. Maybe there's something in this I haven't spotted.

You are probably right.....It's just that somebody had done this for a reason...maybe they were in a remote area and didn't have the part...(this organ came in from a 7 or 8 hour ride...)or maybe they didn't want to be bothered to remove the board and did it this way instead....I was just curious if this procedure could be used as a tool in basic trouble-shooting procedures...sort of like a quick and dirty test just to confirm if something is bad....sort of like bridging a suspect capacitor with a known good one......I just thought I would ask.....
Cheers

[J M Fahey]

I think you are extrapolating wildly based on an unconfirmed hypothesis
a) the Op Amp (IF it's an Op Amp) is bad and somebody tried to bypass it.
If so, why not just a piece of wire?
Why think about caps when obviously there is no DC (or DC problem) involved?
2) somebody bypassed it with a resistor
why do you think it's not original?
why think it's "bypassed" ?
Op Amps DO have resistors connected from output to input ... and stop working if removed .... from that observation to conclude that the resistor is there to bypass the Op Amp makes me remember a childhood (cruel) joke:
- The amateur biologist was "experimenting" with a trained spider, called Anne.
He ordered: "Anne!!! ... Jump!!!!" .... and the spider would jump.
Then he tore out a leg, and repeated the order ... the spider jumped again, just not that much.
He repeated the experiment, taking notes on his Lab notebook.
When just one leg was left, he repeated his order ... Anne twisted around, trying to jump, unsuccessfully.
Hmmmmm!!!!! , said the experimenter, this is becoming interesting !!!!!!
He pulled the last leg, repeated his order, but Anne didn't move a millimeter.
Smiling, he wrote the result in the notebook:
"When insects lose all their legs, they become deaf ."

Before rushing forward at top speed (and crashing against a wall) it would be interesting to know what IC are we talking about and see the schematic, at least the part around said IC.

Another detail is that Op Amps have two inputs ... which one is being used here? ... not the same.

3) not sure what's happening here, I would hesitate on recommending as a general purpose tool in other Op Amp problems ... we are not sure it works here ... at least as described.

[rjb]

The amateur biologist was "experimenting" with a trained spider, called Anne.
|
Smiling, he wrote the result in the notebook:
"When insects lose all their legs, they become deaf ."

What a silly conclusion!
Everyone knows that spiders are arachnids- not insects.

[bsco]

I think you are extrapolating wildly based on an unconfirmed hypothesis
a) the Op Amp (IF it's an Op Amp) is bad and somebody tried to bypass it.
If so, why not just a piece of wire?
Why think about caps when obviously there is no DC (or DC problem) involved?
2) somebody bypassed it with a resistor
why do you think it's not original?
why think it's "bypassed" ?
Op Amps DO have resistors connected from output to input ... and stop working if removed .... from that observation to conclude that the resistor is there to bypass the Op Amp makes me remember a childhood (cruel) joke:
- The amateur biologist was "experimenting" with a trained spider, called Anne.
He ordered: "Anne!!! ... Jump!!!!" .... and the spider would jump.
Then he tore out a leg, and repeated the order ... the spider jumped again, just not that much.
He repeated the experiment, taking notes on his Lab notebook.
When just one leg was left, he repeated his order ... Anne twisted around, trying to jump, unsuccessfully.
Hmmmmm!!!!! , said the experimenter, this is becoming interesting !!!!!!
He pulled the last leg, repeated his order, but Anne didn't move a millimeter.
Smiling, he wrote the result in the notebook:
"When insects lose all their legs, they become deaf ."

Before rushing forward at top speed (and crashing against a wall) it would be interesting to know what IC are we talking about and see the schematic, at least the part around said IC.

Another detail is that Op Amps have two inputs ... which one is being used here? ... not the same.

3) not sure what's happening here, I would hesitate on recommending as a general purpose tool in other Op Amp problems ... we are not sure it works here ... at least as described.

At first, I thought it might have been a mod that Hammond had come up with for some sort of issue....but I don't think that was the case....I can get the schematic over the weekend and post it here for you...and the number on the IC.....I only asked because I never ran into this before....When I worked in the gaming industry we used to have to do that sort of thing from time to time depending on the issues....and the release was from the manufacture....so, at first I thought that was the case here...but it looks like it was not......so this is the reason I had asked.....I'll get that info for you over the weekend so you can take a look....Thanks for replying.....
Cheers

[J M Fahey]

Oh, we are all used to guessing in this Forum, and "fill in the blanks" (literally) all the time, but on amplifiers (or supplies or some pedals) we more or less can imagine what is needed depending on function .

Now, on "a Hammond organ" ... what might that (presumed) Op Amp be doing?
Amplify? ... filter ? ... oscillate? ..... is it part of a tone control? etc.

I remember old unobtainable RCA "experimenter ICs" , a couple were real Op Amps, others just glorified transistor arrays, I bought the book and built as many projects as I could.

If I remember right, those ICs had code numbers similar to K or KD**** (similar to K1118 and such) while the commercial ones were CA3*** of which I actually used the CA3140

[olddawg]

How Do Operational Amplifiers Operate? | Analog content from Electronic Design

[Enzo]

When I read your initial report, my first reaction was that a 10k feedback resistor somewhere went open, and instead of removing the board to replace it, they simply tacked a new one across the IC directly.

[J M Fahey]

Yes, or the tacked one, on solder side, in parallel with another still there on the component side, to, say, lower gain.

FWIW I cofess I'm guilty of the same sin: if you open one of my amps today, very often you'll see same thing, typically because customer brings a too hot active guitar or bass or goes through a very hot pedalboard, and I have to adjust headroom, it's something very common.

You'll also see some small capacitor (think 100pF) tacked here and there after assembly to kill some nasty FM radio getting through or instability.

But let's wait for the hand drawn schematic
Otherwise a proper answer is impossible (besides wild guessing)

Dear bsco, sorry if I looked rude, I knee jerked like if I saw somebody almost stepping on dog droppings, pushing him aside even before talking was an automatic reaction

[bsco]

Oh, we are all used to guessing in this Forum, and "fill in the blanks" (literally) all the time, but on amplifiers (or supplies or some pedals) we more or less can imagine what is needed depending on function .

Now, on "a Hammond organ" ... what might that (presumed) Op Amp be doing?
Amplify? ... filter ? ... oscillate? ..... is it part of a tone control? etc.

I remember old unobtainable RCA "experimenter ICs" , a couple were real Op Amps, others just glorified transistor arrays, I bought the book and built as many projects as I could.

If I remember right, those ICs had code numbers similar to K or KD**** (similar to K1118 and such) while the commercial ones were CA3*** of which I actually used the CA3140

I am going over there today.....When I get back home I'll send you the info...The IC in question is an old RCA CAxxxx I can't remember the number off the top but I will get it for you.....
Cheers

[bsco]

How Do Operational Amplifiers Operate? | Analog content from Electronic Design

Thanks for that...I'll have a look....
Cheers