The HP 8600A Problem has Returned

[Chris K]

Got a 2 watt wirewound at 1% for $6. Im not sure the resistor currently on the board is a wirewound resistor. It looks carbon comp to me

[Kevin Kuehn]

Originally Posted by Chris K

Im not sure the resistor currently on the board is a wirewound resistor. It looks carbon comp to me

I thought the same.

[Penthode]

Looking at the circuit, I would not substitute a 7805. I do not think the 723 is necessarily at fault. The circuit is very simple and I am curious if you have performed a few basic tests?

First did you confirm if it may be going into current limiting mode? What would help is when the voltage starts to droop is to measure the voltage at the regulator transistor and put the readings on the diagram.

I suspect the transistor or components around ity may be bad.

[Penthode]

Reading above it looks af if you are going in the direction of current limiting..
.

[Kevin Kuehn]

Originally Posted by Penthode

Reading above it looks af if you are going in the direction of current limiting..
.

I have a hunch, but I'm not an electrical engineer. Simple circuit, but looking at the formulas to calculate resistor values to limit current makes my head spin. Would also be good to look at the unregulated DC coming into the regulators with a scope. Check it for ripple.