The HP 8600A Problem has Returned
 

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.

https://imagizer.imageshack.com/v2/1...924/Wm5E76.jpg

https://imagizer.imageshack.com/v2/1...923/P84ZUo.jpg


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?

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

Sounds tough to find and replace!

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?

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

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

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.

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!!!!

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.

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

A 3 pin to replace a 10 pin?

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.

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!

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?

Can you share the diagram?

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

Sure give me a little bit. It will be up shortly

https://imagizer.imageshack.com/v2/1...922/JJIvHR.jpg


https://imagizer.imageshack.com/v2/1...924/kaEliE.jpg


https://imagizer.imageshack.com/v2/1...922/iKsFpK.jpg


https://imagizer.imageshack.com/v2/1...922/6agCdX.jpg


https://imagizer.imageshack.com/v2/1...924/g3GVYc.jpg

Here is the schematic a bit bigger


https://imagizer.imageshack.com/v2/1...922/JJIvHR.jpg

Well, it was supposed to be bigger!

Basically, you would remove the 723 and pass trandsistor.

7805 "IN" connects where the pass transistor collector was
"OUT" connects where the pass transistor emitter was

GND goes to ground



https://www.eleccircuit.com/wp-conte...-Regulator.png

https://www.eleccircuit.com/wp-conte...805-pinout.png

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

1.5a

I'll give it a try. Thanks so much for the time and effort in detailing the instructions! It's very appreciated.

Couple of questions. What would be the problem solving strategy behind this replacement given the failure of the original and replacement 723? Is the 7805 more robust so whatever is damaging the 723 wont affect it or is the logic I just got a bad out of the box replacement 723 and I might as well replace it with something more modern and reliable? I have both on order so should I just replace the 723 again, give it some time, and if it fails go with the 7805? I know what I would do but I'm just checking if my thoughts align with what you are thinking.

Yes, 7805 is more robust. Also, this eliminates issues with the pass transistor or any of the other 723 support components.

BTW you can click 'post reply' rather than 'quote' to avoid duplicating all the images. Or you can edit them out of your post.

Thanks

The 723 is used for a reference with a series transistor for higher current capacity. I would have to guess there's something more wrong in the circuit besides the 723. The 723 is likely doing what's designed to do, protect the power supply in the event of a over current.

That’s what I figured. So would compensating for a fault by installing a more robust regulator cause harm? I suppose it depends on what’s causing the issue.

My concern would be how HP has the current limiting set, vs how a stock 7800 series regulator will provide whatever max current it can. If there's something down stream loading the power supply, you don't really want to provide unlimited current...

On the regulator circuit board that toasty looking .56 ohm resistor is the current sense resistor(in series with the output voltage). The 723 monitors voltage drop across that resistor through pin 1(current sense), which in turn limits current to some set value. Off hand I don't know how to calculate the current limit. The pass transistor for that 5 volt supply is supposedly mounted on a heat sink attached to the chassis. The two transistors on the circuit board are for the + - 15 volt supplies. How big physically is that series pass transistor?

I’ll get on that and back to all of you midday tomorrow. I don’t know how much drift from the 0.56 ohm the regulator function tolerates and my comment earlier that it is “spot on” isn’t exactly accurate. My test leads have around 0.3 ohms of internal resistance and I’m getting around 0.9 ohms when I measure it so accuracy to within 0.01 ohms isn’t something my DMM can do.

I've studied the circuit and HP has the 723 5 volt regulator configured with fold back current limiting (quite aggressive pullback). I think if the charred looking R12 .56 ohm current sensing resistor has drifted high in value(possibly more so when hot), it could very well be the cause your 5 volt supply voltage backing down. In the parts list R12 is specified as wire wound, .56 ohms, 5%, 2 watt. Personally I would try a new resistor before attempting the 7805 conversion. Attached a really good article from MODERN ELECTRONICS / February 1987 explaining the capabilities and design setup of the 723 regulator.

Chris,

If you by chance have an accurate reading ESR meter, that would be ideal for reading such a low value resistor value. The one I have allows you to zero out the test leads resistance.

I think the resistor is a good place to start. I saw one on Mouser at 5%

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

I thought the same.

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.

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.

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?

Penthode, I did think measuring unregulated voltage into the 723 vs the voltage out would be useful but with the board plugged in, it’s going to be physically difficult and I don’t want to buy an extension for the connection slot just yet. Makes me appreciate point to point wiring in tube gear!

I am an electronics engineer and have designed regulated power supplies using the 723 for many decades. I wanted to see voltage readings around the 723. The 723 is an old and reliable device.

I think this will be a simple problem to resolve. Why do you not first measure the voltage directly across the 0.56 ohm resistor as this will indicate the current from the +5v supply and immediately identify if the 723 is going into current limit mode.