The HP 8600A Problem has Returned

[Chris K]

The HP Digital Marker continues to cause headaches. The same issue has returned. The newly installed 723 regulator can't hold the 5V DC required voltage again, dropping to 4.5V even with the trimmer maxed out. Works when it's cooled with freeze spray going up to 5.3V but begins to drop once it warms up. I think something is affecting this can IC other than them just being bad out of the box. The new one worked for about a month and then began having the exact same symptoms.






Does anyone have any suggestions as to where to look next? I tried to measure the electrolytic caps but only one would give me a value. There's something printed on them besides uF and maximum voltage. It says "1500 +-10%? Anybody know what that refers to? Should I be looking at the Circuitrim 500 ohm trimmer?

[bandersen]

Sprague 150D series wet slug tantalum capacitors. 10% tolerance.

[Chris K]

Originally Posted by bandersen

Sprague 150D series wet slug tantalum capacitors. 10% tolerance.

Sounds tough to find and replace!

[bandersen]

You can get them, but expect to pay $45-$100 each!
https://www.mouser.com/ProductDetail...ZPR8h4mg%3D%3D

Good thing is they rarely go bad. A modern low ESR electrolytic will work as well in most applications.

That big resistor looks toasty - does it check OK?

[Chris K]

The resistor is OK...1.0 ohms.
Would a "lead core" tantalum work ok? They are about $6 USD

[bandersen]

Originally Posted by Chris K

The resistor is OK...1.0 ohms.
Would a "lead core" tantalum work ok? They are about $6 USD

Not familiar with that term, but I assume you mean the more common solid core tantalum caps.

Yes, they'll work but are not as robust. When they go bad, they typically short.

https://www.mouser.com/ProductDetail...wT4GZejpmX4%3D

The reason these are used at all is low ESR over wide frequency and temperature. -55C to +125C! You typically see them in test equipment and military grade gear.

[Chris K]

The resistor is measuring 0.6 ohm. It is a 0.56 ohm resistor.

[Chris K]

Yeah that's what I was trying to talk about. I don't even know if they might be involved in this issue. Could the power transistors be damaging the regulator if one of them isn't doing what it's supposed to do? I need to look at voltages and sources going into the regulator and the trimmer.

This machine is cursed!!!!

[Electronic M]

If the regulator circuit fails a third time I'd be tempted to engineer a new circuit. 7805/7905s are easy to make work but might not have the current handling capacity.

[Penthode]

I was going to suggest the same to install a 7805. Or use the 723 to drive an outboard transistor.

[Yamamaya42]

Originally Posted by Electronic M

If the regulator circuit fails a third time I'd be tempted to engineer a new circuit. 7805/7905s are easy to make work but might not have the current handling capacity.

there are ways to add a load transistor to beef up the current output of a 78xx 79xx.

[Chris K]

A 3 pin to replace a 10 pin?

[bandersen]

Yes. The 723 is a swiss army knife regulator that can be configured in a wide range of operating modes. Since all you need is a fixed 5 volts, a 7805 will do nicely.

[Chris K]

As little as I know about tube gear, I know almost nothing about solid state. Seeing the board in the above photos, where do you suggest I start to figure out how to do this? I have a schematic of the board but I wouldn’t know where to start!

[Chris K]

So 7 of the 10 pins on the 723 are redundant in this application? I would need to find the collector/base/emitter points on the board, mount the 3 pins on the 7805 to them, maybe slap on a heat sink and it should work?