Electronics > Repair

Replacement for old Intech or Fairchild op-amp.

(1/8) > >>

bdunham7:
This is a preamplifier for the low voltage range of a DMM.  The input range is 0-120mV plus some headroom, with a gain of 10X.  I haven't fully diagnosed the problem yet, but so far it looks like the op-amp is bad.  The requirements are low bias current, DC stability and compatibility with this circuit with as little modification as possible.

The service manual calls this out as a Fairchild ADO-26B, for which I found a brief release note but no datasheet.  The actual part installed is an Intech A-1001, for which I could not find any info.  I've attached the schematic, a photo, the release note.  Ideas welcome.





Kleinstein:
There seems to be external pot for the fine trim. So one could get away with a low offset OP and ignoring the trimmer for the OPs offset.
The DC performance of the original does not look that great ( < 2 µV/K drift) so one could get away with a normal, non AZ op, like OPA140 or OPA145 (slightly slower, but may be higher offset to start with).  The change would be ignoring the ground and offset trimer for the OP. I would add a capacitor (or 2) for local decoupling, just in case.
The fast OPA140 might like some capacitance in the FB to slow it down.

The OPA140 may be available as DIP for an easy dead bug fix. A SO8 version could use a SO8  to dip adapter board and use wires from there instead of the pins.

I would first check the supply - just in case the zeners at the supply could have failed and this could damage a replacement.

bdunham7:

--- Quote from: Kleinstein on July 10, 2021, 12:01:45 pm ---There seems to be external pot for the fine trim. So one could get away with a low offset OP and ignoring the trimmer for the OPs offset.
The DC performance of the original does not look that great ( < 2 µV/K drift) so one could get away with a normal, non AZ op, like OPA140 or OPA145 (slightly slower, but may be higher offset to start with).  The change would be ignoring the ground and offset trimer for the OP. I would add a capacitor (or 2) for local decoupling, just in case.
The fast OPA140 might like some capacitance in the FB to slow it down.

The OPA140 may be available as DIP for an easy dead bug fix. A SO8 version could use a SO8  to dip adapter board and use wires from there instead of the pins.

I would first check the supply - just in case the zeners at the supply could have failed and this could damage a replacement.

--- End quote ---

Zeners are OK, the op amp has a total supply voltage of ~32VDC.  With inputs and outputs disconnected from the rest of the unit, the output is ~-13VDC with an input of ~+0.49VDC, which is just the output/10, limited by the input protection diodes.  So it looks like the op-amp is the issue.

I will probably have to use a modern device without the COM and TRIM connections, but I'm wondering if I'd be better off bodging something into the original design or just making new board.  The OPA140 actually looks pretty good except I think I need to lower the supply voltage a bit.  I'm wondering if its initial offset will really be good enough.  I might want to revise the circuit using existing components, perhaps using the balance pot in the existing zero circuit somehow.  My initial idea was to replicate the original hybrid design by adding JFET inputs to something like a 741, but that probably is silly.

Kleinstein:
The moden JFET OPs like the OPA140 are really good - the only point missing is often the possibility to trim the offset. It would be very hard to get close performance (dirft and offset) with a old stype design with seprate fet and the low frequency noise is also not so easy to beat. It would likely need 2 step trim for offset and TC.

There is the external offset trim (likely from the front panel). With a divider 1.3 M and 2.7 Ohms this would not be very much trim range (e.g. +-30 µV), but it could be just enough if the offset is at the typical value and not much higher.

What is the resolution of the meter ? So how good does it have to be ?


There are also OPs (like OPA191/192) with lower offest, though slightly higher noise that could be used. 1.4 µV_pp noise for the 0.1 - 10 Hz range is still not that bad.

The OPA134 is an audio OP, but still relatively good LF noise and it still has offset trim pins. There may be others too - but the choice is not that large anymore.

Another possible option would be a small board with an AZ OP like ICL7650 or similar. This would however need a reduced supply (e.g. +-6 V).
A +-2 V ouput range would still be possible even with an AZ OP with only a +-2.5 V supply and thus a low voltage type.
There are AZ OPs (e.g. LTC2057, OPA187, ADA4522) than can run with a 30 V supply, but these tend to have a relatively high bias (more like 50-200 pA range).

The diodes on the board shown are accross the OPs inputs. So the actual input voltage can be higher than 500 mV.  To test for the amplifiers offset one should have a short at the input, not an open input.

bdunham7:

--- Quote from: Kleinstein on July 10, 2021, 06:08:26 pm ---The moden JFET OPs like the OPA140 are really good - the only point missing is often the possibility to trim the offset. It would be very hard to get close performance (dirft and offset) with a old stype design with seprate fet and the low frequency noise is also not so easy to beat. It would likely need 2 step trim for offset and TC.

There is the external offset trim (likely from the front panel). With a divider 1.3 M and 2.7 Ohms this would not be very much trim range (e.g. +-30 µV), but it could be just enough if the offset is at the typical value and not much higher.

What is the resolution of the meter ? So how good does it have to be ?

--- End quote ---



It's an old Fairchild 7000A that I got from user Runco recently.  It really doesn't have to be particularly good because it is more a collectors item than anything else--although I may end up using it just because I like to use old stuff.  It is a 12000 count meter and this preamplifier is for the 100mV range, so the LSD is 10uV.  The mV offset is on the front panel and is a less-than-single-turn pot.  I could modify the trim circuit and use a multi-turn, I suppose.  I don't know how stable it was in the 100mV range originally, but on the other ranges it is surprisingly good for very ancient (late 60's) technology.  It autoranges smoothly and correctly measures the DCR of inductors (I had to check, of course).  :)


--- Quote ---There are also OPs (like OPA191/192) with lower offest, though slightly higher noise that could be used. 1.4 µV_pp noise for the 0.1 - 10 Hz range is still not that bad.

The OPA134 is an audio OP, but still relatively good LF noise and it still has offset trim pins. There may be others too - but the choice is not that large anymore.

Another possible option would be a small board with an AZ OP like ICL7650 or similar. This would however need a reduced supply (e.g. +-6 V).
A +-2 V ouput range would still be possible even with an AZ OP with only a +-2.5 V supply and thus a low voltage type.
There are AZ OPs (e.g. LTC2057, OPA187, ADA4522) than can run with a 30 V supply, but these tend to have a relatively high bias (more like 50-200 pA range).

The diodes on the board shown are accross the OPs inputs. So the actual input voltage can be higher than 500 mV.  To test for the amplifiers offset one should have a short at the input, not an open input.

--- End quote ---

I assumed that since the input voltage was opposite the maxed-out output, the op-amp was bad.  You're right, shorted would be better as it would eliminate the possibility of me mistaking the polarity, so I tried again--same result of -13ish volts with inputs shorted.

I'm considering the LF155AH as a drop-in replacement.  It's old, but I think I can get one and these were designed specifically as a replacement for JFET hybrid module op amps.  The offset TC is a bit worse than what the ADO-26B was advertised at, but it might be worth a try--as long as I can zero it, I think it meets the original design goal.  Any reason it wouldn't work?

Navigation

[0] Message Index

[#] Next page

There was an error while thanking
Thanking...
Go to full version