The HP 8600A Problem has Returned

[Penthode]

The voltage across the resistor solely indicates the current flowing. As Kevin said, the regulator feedback is after the resistor so the minor variation of overall output voltage will have little impact.

What I was looking for was as the unit heats up with the lid closed did the voltage across the sensing resistor reach 0.6 volts or where the roll off in output voltage would begin.

If the voltage across the resistor did rise to 0.6v or more, it could suggest the 5v rail current has risen meaning there might be a problem elsewhere. This could exonerate the regulator altogether.

[Chris K]

Ok that makes sense to me….surprisingly! I’ll go from a cold start tomorrow evening and monitor the voltage across the resistor continuously. I’ll let you know what I find.

[Chris K]

I didn't get a chance to turn the unit off last night (fell asleep ). This morning, after running sealed up and warm all night, the readout was frozen and the voltage was 4.6V. The voltage across the resistor was the same, around 325mV. I froze the 3055 power transistor and the voltage at the test point fell further to 4.1V. When it returned to 4.6V, I froze the resistor and the voltage came up to 5V but it did so slowly. When it fell back to 4.6V, I froze the 723 and it immediately shot up to 5V.

I'm not sure if there's any information that you guys can glean from this but there it is. I turned the unit off. Tonight, I'll do the cold start and measure the voltage across R12 and see what the baseline looks like and we'll go from there I guess??????

[Chris K]

Unit has been on for hours. Voltage across R12 rolled up from zero to 330mV on cold startup and has not gone higher or varied in any significant way since then. Unit is currently working.
This is what has been so infuriating about this. Over the past 3 days, it has performed normally over many hours, then, this morning, it's messed up. Now it's working again. There's a ghost in this machine. How am I going to diagnose an issue that springs up spontaneously and infrequently? I can't do it by ongoing monitoring. I guess at some point, I'll need to connect leads to the 723 pins and measure voltages as Penthode has suggested to figure out what's going on or at least, get closer to an answer.

[Kevin Kuehn]

Did you get any voltage measurements across R12 when you were hitting it and the 3055 transistor with freeze spray?

[Chris K]

No I wasn't measuring. What are you thinking?

[Kevin Kuehn]

Originally Posted by Chris K

No I wasn't measuring. What are you thinking?

If the voltage across R12 remains constant while all this is going on. I totally understand you can't monitor it 24-7. Some kind of voltage recorder/plotter would be handy.

[Chris K]

Would it be diagnostically worthwhile to wait until it fails and monitor the voltage across R12 when I restore function by spraying the 723?

[Kevin Kuehn]

Lets wait for Penthode to chime in. I'm having a heck of a time positing on here lately. I keep getting a "the server is too busy, try again later" message.

[Chris K]

Originally Posted by Kevin Kuehn

Lets wait for Penthode to chime in. I'm having a heck of a time positing on here lately. I keep getting a "the server is too busy, try again later" message.

I think we're all getting that. Dozens of times over the past several days

[Electronic M]

Originally Posted by Chris K

I think we're all getting that. Dozens of times over the past several days

Same here. I wonder what the deal with the servers is?

[Penthode]

Curious that you indicated the voltage popped back to 5.0 v when you cooled the 723.

The current out is .43/.56 = about 0.8 amps. Which appears normal. Is there no way to conveniently measure the voltages around the 723?

[Chris K]

I can…I just need to get set up to do so. The voltages are probably more easily taken off the solder points on the back of the board. Last I checked this morning it was still operating fine. I left it on all day and I’ll go to my shop soon. Will you be able to get any information from taking voltage measurements when it’s working or should I wait until the display freezes?

[Chris K]

Originally Posted by Penthode

Curious that you indicated the voltage popped back to 5.0 v when you cooled the 723.

The current out is .43/.56 = about 0.8 amps. Which appears normal. Is there no way to conveniently measure the voltages around the 723?

Freezing the 723, the original 723, was what led me to replace it. I thought I had identified the issue...a faulty regulator. The unit ran fine for several weeks. Then the drop in output from 5.0V to 4.6V returned. The unit will read and display frequencies until the voltage goes down to about 4.8V DC. Lower than that starts the symptom.

[Chris K]

Voltages on the 723 from the back of the circuit numbered clockwise. The tab on the can is pin 7.



Working normally