Well what do you know, the -3 opto's work just like the original designer intended them to! 
Basically the -4 was over compensating the loop and there was too much current "steering" around R20 for a given incremental adjustment after U4 starts to drive U5. I don't know why someone decided to make that change, however it can't work unless you either change the gain of the U4 diff amp circuit or change the opto emitter resistor, R23 to a 2K or something close. But why bother, there is no value. Bottom line, if you have a "reissue" and can't attain bias temperature, this is most likely your issue. Also, both channels adjusted nicely now 50.2C on the right, 49.6C on the left. Using a 2K pot with 4 turns also improved adjust-ability. Sweet amp
Basically the -4 was over compensating the loop and there was too much current "steering" around R20 for a given incremental adjustment after U4 starts to drive U5. I don't know why someone decided to make that change, however it can't work unless you either change the gain of the U4 diff amp circuit or change the opto emitter resistor, R23 to a 2K or something close. But why bother, there is no value. Bottom line, if you have a "reissue" and can't attain bias temperature, this is most likely your issue. Also, both channels adjusted nicely now 50.2C on the right, 49.6C on the left. Using a 2K pot with 4 turns also improved adjust-ability. Sweet ampAttachments
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