I'm new to the list and just acquired an HP 5370B time interval counter from eBay. This instrument is nostalgic for me as I had one on my bench working for HP out of college in 1981. The listing for the instrument said that it powered on but they could not test it. My experience is that it powers on, tests the LEDs in the display, shows 0.0 for 10 seconds, and then gets an Err 02. Combing through the list I knew to check a few things. I reseated all of the PCBs and I checked the relay K1 on A1 and found it in good condition and working when I apply 24v. I find that when I put in signals to Start and End, the trigger LEDs flash but once Err 02 is displayed, the machine is unresponsive until I power cycle it. From what I understand, Err 02 means that a measurement was triggered but it's completion timed out at around 10 seconds. So I have a few specific questions: 1) What should the counter do when powered on with no inputs? I'm guessing that if there is no trigger on the start channel, no measurement will commence and there should be no Err 02. 2) If I change the function to Frequency or Period, shouldn't the error clear and a new measurement commence? 3) I have verified all of the voltages and that the OCXO is outputting a decent sine wave. I have tried switching in an external reference and providing one from a signal generator and that did not change anything. Is there any other basic tests to perform? Thanks, Marc