The HP 8600A Problem has Returned

[Chris K]

The power transistor is a 2N3055 and it is mounted on the chassis next to the regulator board. I have not done any testing other than looking at the voltage on the test point and observing the symptom. The construction of this instrument makes doing some of the suggested testing very difficult. I imagine if I study the schematic a bit more there would be places to measure the voltage easier than directly on the 3055 which would entail removing the bottom of the unit. If I understood what happens in the regulator when I use the freeze spray and it restores function for a while until it gets back to operating temperature I might be able to help figure this out. Why does freezing the 723 make it regulate properly?