HP 3403C Repair - Wrap-up

There seems to be an issue with the thermopile protection circuit but all the components and traces test out correctly - Except for some custom HP parts - I'm going to need to either acquire a new unit or those parts to replace and test.

Unfortunately that means that for now I'll be setting this aside and will hopefully get back to it later.

[TheDefpom]

I would be replacing all the caps in the suspect area, just to rule those out completely first.

[ElectrolabAg]

Sometimes the cap will read good ESR but may break down on working voltage, you should replace them as a rule of thumb.

[JerryWalker]

Have you tried adjusting the offset null on the AC feedback amplifier? If the problem is at low signal levels then it could be as simple as a small offset that needs nulling out.