a) I might get a solution
> b) it's an example of getting an additional feature "for free"
> c) It's an example of what can bite you when you try 2. above.


Interestingly, the typical bias current and maximum offset current are
about 3:1 for the LM324/224 so the original method would be expected
to work often - as it indeed did. Alas, as you note, there is no
minimum bias current specified.

1. LM3900/2900 may be suitably different enough to prove better, or
worse, in practice.
Amplifiers from different manufacturers may prove better or worse.
Even date coded parts from an earlier era may do the job if the
problem has only occurred recently. A die shrink or process change may
have 'improved' the product and contributed to the previously
successful kluge now sometimes not working.

2. Don't try this at home / YMWV:    To add to the conniptions of the
prior commentator you could consider adding a 'blob' of a 'suitably
high resistance when set' compound at an appropriate point in the
circuit - depending on the number of channels being used for this
purpose and which amplifiers in the package were used and polarities
etc it could be between inverting and output or non inverting and
ground or ... .  Something with 10's to 100's of megohm/square at an
application thickness of semi-random and final characteristics
dependant on temperature, humidity, phase of moon and proximity of
deceased fish would add to the excitement and naughtiness of such a
solution. And it might even be able to be made to work reliably enough
to be useful. Or not.

3. Add on resistors would be less exciting. Just maybe these could be
suitably sized devices added at locations where there were not
intended to be resistors present. Such a solution approaches the
naughtiness of the prior one. Some fish may be harmed in its
execution.



                 R

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