Hi,
I don't believe it is a bad connection problem. Nothing leads me (in my unit) to that conclusion. Yours may be different.
Well, changing the battery for a new one might be good (it's old for sure), but somehow doubt that will resolve any offset problems. I think the recall-able factory CAL settings are stored in a non-volatile memory, so battery swaps only destroy user settings. Pretty sure. It will be interesting if you try it to know the results. May as well put a new battery in at the same time if you do it.
One thing I tried is diconnecting, and isolating U600 pin 6 from the transistor buffers on the output. Check to see if the trace has zero volts, or close to that, and put zero volts on the trace if it's close. Those 2n3904, and 2n3906 transistors blow easily, and short, check them. Also, one of the traces there is extremely close to the transistor pin pad holes. Then look at those inputs on pin2, and pin3 again. U600 is pretty hardy, not seeing issues with it. The output amp should also have close to zero DC offset now.
Ultimately (in my case), the real issue causing offset was not U600 circuit, or the output amp, but the offset coming from U702 AD834 over the coax from the upper board. The open collector outputs on it, require a bias voltage higher than it's main supply bias. That voltage comes from the lower board in the output amp over the two coax. R615, and R616 supply the voltage from U602 6.8V Reg. to U702 AD834 collector outputs. I found that lowering that voltage on either collector output (at the coax) with a resistor to ground of about 600 ohms, and then removing it, would reset the offsets, and then all good for days on end. Ultimately creeping back again, only to trip into error once more.
I also swapped out several op amps, and U100 bi-lateral switch in that OUT_OFF circuit to no avail. Also suspecting, but never changing the D/A converter U109 in that ckt. Very expensive, obsolete, and ebay only replacements.
Good luck in your TS efforts!