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/v2/1024x768q70/924/Wm5E76.jpg

https://imagizer.imageshack.com/v2/1024x768q70/923/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.

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/Vishay/109D106X9025C2?qs=p6VZ%252BklCkRRcHLZPR8h4mg%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.

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/KEMET/T322B685K006AT?qs=sGAEpiMZZMtZ1n0r9vR22SxAQVzr7x7a 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?

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.

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

https://imagizer.imageshack.com/v2/1024x768q70/922/JJIvHR.jpg


https://imagizer.imageshack.com/v2/1024x768q70/924/kaEliE.jpg


https://imagizer.imageshack.com/v2/1024x768q70/922/iKsFpK.jpg


https://imagizer.imageshack.com/v2/1024x768q70/922/6agCdX.jpg


https://imagizer.imageshack.com/v2/1024x768q70/924/g3GVYc.jpg

Here is the schematic a bit bigger


https://imagizer.imageshack.com/v2/1600x1200q70/922/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-content/uploads/2021/02/7805-5V-Fixed-Voltage-Regulator.png

https://www.eleccircuit.com/wp-content/uploads/2021/03/LM7805-pinout.png

https://www.mouser.com/ProductDetail/STMicroelectronics/L7805CV?qs=9NrABl3fj%2FqplZAHiYUxWg%3D%3D

1.5a

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-content/uploads/2021/02/7805-5V-Fixed-Voltage-Regulator.png

https://www.eleccircuit.com/wp-content/uploads/2021/03/LM7805-pinout.png

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.

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.

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

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..
.

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.

I am an electronics engineer and have designed regulated power supplies using the 723 since the '70s. 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.

You can tack solder extension wires onto any circuit board points you want to measure with the board plugged in. Such as across the current sense resistor.

The two connections across the 0.56 ohm resistor are current limit and current sense. When the voltage across the resistor reaches 0.7 volts, the current limiting begins at the output voltage will begin to decrease if the current is exceeded.

You need to accurately measure the 0.56 ohm resistor. If it has increased even slightly, you will see the problem you are exhibiting. But measure the voltage across it first for if the resistance increases, the voltage across it will increase. Right now it is designed to limit at just over 1 amp.

If the voltage across the resistor exceeds 0.6 volts, there is one of two problems: the resistor has increased in value and needs to be replaced or there is a problem with the circuitry downstream eg a leaky capacitor.

If the voltage remains below the 0.6v and the output voltage drrops, then there is a problem with the 723 circuit. If you have replaced the 723 with a known good one, I cannot see that there is a problem there.

Right now it is designed to limit at just over 1 amp.


Can you predict how much headroom they would design in, for example 500ma, or would they run much closer to the limit?

Can you predict how much headroom they would design in, for example 500ma, or would they run much closer to the limit?

There should be a knee at just over 1 amp. So the voltage should stay at 5.0 volts up to the knee. I would suggest 0.6v is a good maximum where the roll off begins. In normal operation, it should be running with no more than 0.4 v or so across the resistor

Have the top of the 8600 open and it is operating normally. Could be it's not getting hot in this configuration but I'll keep it on for as long as it takes. It's held steady for about 30 minutes. The difference in voltage across the R12 resistor is 0.357V DC in normal operation right now.

2 hours and 45 minutes and we’re still working. As I said, all of this is with the top metal panel off so any heat can vent. There’s some significant warmth coming off the chassis mounted power transistor for this circuit and physically it is almost directly under the 723 and R12. My intent tonight was to let the 8600 fail and then freeze either the transistor or R12 but no failure so far. We’ll see what running overnight does and I’ll check it in the AM.

What about putting something like this on it?

That's a possibility. It was holding steady at 5.003V DC this morning after about 10 hours of running. I put the top panel back on and left for work so we'll see what's going on when I get home.

That's a possibility. It was holding steady at 5.003V DC this morning after about 10 hours of running. I put the top panel back on and left for work so we'll see what's going on when I get home.

How long did it usually take to act up with the cover on? Hopefully the 723's current limiting will keep things in check until you return.

How long did it usually take to act up with the cover on? Hopefully the 723's current limiting will keep things in check until you return.

It took maybe 10-15 minutes to lock up. The symptom was/is a frozen frequency readout. When I cooled the 723, I monitored the voltage rise from around 4.6V to a bit over 5.0. Function returned but as the voltage on the test point dropped below 4.8 or so, the readout froze again. I could repeat this over and over. My plan was to freeze the 3055 and see what that did but, as stated, the 8600 never lost function last night. During lockups, I did turn the trimmer so it would maintain 5V DC. Last night with the trimmer maxed and normal function maintained, the voltage was around 5.3V. That scared me so I adjusted it to 5.003V DC. We'll see what I find in a few hours with the cover on all day. My attempts at doing a comprehensive alignment on the RCA, probably one of, if not the, most challenging early era TVs to get just right, even with an enormous amount of help and guidance from Penthode, were so compromised by faulty equipment and then issues with this piece, I had to abandon that project. I'm hoping I can get this working reliably again so I can finish the RCA.

When you freeze the 723, is it possible you're also cooling a few surrounding passive components? I remember initially you were able to randomly fix this by banging the case or bending the circuit board. So don't rule out the possibility there's still an intermittent passive component or connection in the mix.

When you freeze the 723, is it possible you're also cooling a few surrounding passive components? I remember initially you were able to randomly fix this by banging the case or bending the circuit board. So don't rule out the possibility there's still an intermittent passive component or connection in the mix.

Yes you are correct and I was thinking about that last night. The readout freezing is the only issue now. Back then, there were additional problems of frequency readouts all over the place etc. I replaced a burned out resistor, a shorted film capacitor and cleaned all of the board connection traces on all of the boards I could easily remove. That seemed to resolve everything but this persistent issue. Another thing I noticed is there's evidence of many IC chips being replaced on one particular board. When you look at the unit from the front, the board is on the left almost up front (at work and do not have the unit or schematic in front of me). It's almost exclusively 14 pin ICs and there's a lot of variation in manufacturers and lots of flux around the pin solder points on the back or bottom if it was horizontal. I've been looking for a replacement for around 6 months now but nothing has shown up. That's funny because Penthode suggested in the RCA thread I look at finding an 8600/8601 sweep generator combo and I bought one on eBay in about an hour!..Since then, no more 8600s.

Home and still OK. This kind of intermittent issue drives me crazy. Voltage rock solid at 5V. Now, I did have the top of the cabinet just laying on top. I just installed it correctly and screwed it down so the unit is sealed. Let's give it a few hours tonight and see what happens.

Did the voltage across the current limit resistor rise when it got hot?

Did the voltage across the current limit resistor rise when it got hot?
I don’t understand the question my friend. I did measure the voltage at 0.36V last night when it was running for several hours and not failing in fact, 24 hours since I started this and it hasn’t failed. Now that the cabinet is sealed up, maybe it will fail and when it does I’ll measure the resistor

R12 is within the voltage regulation feedback loop. If it's resistance increases as it warms up, the regulator will compensate until the point of current limiting kicks in. So I think the question is, is the voltage drop across R12 still the same after running 24 hours?

R12 is within the voltage regulation feedback loop. If it's resistance increases as it warms up, the regulator will compensate until the point of current limiting kicks in. So I think the question is, is the voltage drop across R12 still the same after running 24 hours?

No it isn't. It was 0.357V or 357mV and right now it's 328mV. The unit is still holding and reading frequencies actively however, since the cover has been on, the voltage has dropped from 5.0V to 4.96V. Probably insignificant but I haven't seen anything other than 5.0V on the nose until I just measured it after taking off the top.

I think we were expecting it to increase with temp.
I'm not sure how to interpret that. :scratch2:

Yeah I think I effed up. The difference yesterday was measured when the trimmer was maxed and the voltage was 5.3V and not the 5V it is now. So I don’t think the voltage difference on the resistor yesterday vs today really provides any information. If it’s still working fine by the time I go to bed I’ll turn it off and then back on tomorrow morning and test the resistor for a startup baseline. Then we’ll see what’s up when I get home from work

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.

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.

I didn't get a chance to turn the unit off last night (fell asleep :boring:). 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??????

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.

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

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

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.

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

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.

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 :dammit:

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

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

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?

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?

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.

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

https://imagizer.imageshack.com/v2/1024x768q70/924/wCRxSv.jpg

Working normally

It looks to me like the tab should be pin 10, because pins 7 and 8 should have the unregulated DC input on them.

Those are just arbitrary numbers I put on the pins starting at one at near 12 o’clock. They aren’t labeled as you would count them on the transistor. It continued to work fine. What typically happens with this is there’s really no resolution and I button it back up, it works for a while and then starts with issues again. I think what I’m going to try is to mount a small cooling fan on the side blowing into the cabinet. If heat buildup is the issue, maybe that’ll solve it. I am going to replace the resistor with a new 1% 0.56 ohm wire wound resistor as it is supposed to have. As long as I have it open I’ll replace it.

Buttoned it up...put it back on the shelf, just waiting for the fan. this morning I came down to the shop and turned it on along with the 8601 and this is what greeted me on the display.

https://imagizer.imageshack.com/v2/1600x1200q70/924/2A27tD.jpg

All voltages were fine. I'm just about done with this. I pulled out the counter board, the one with all the IC chips on it. It has lots of replacements. This board (and no doubt this machine) looks like it drove someone nuts in the past just as much as me. When I put this board back in place, It started working.

https://imagizer.imageshack.com/v2/1600x1200q70/922/N5VdDo.jpg

This is typical for this instrument. Days and days of normal operation...it gets put in service and bang...it stops working. It's just a turkey. I think I'll sell it as non working and wait for another to come up for sale.

Corroded edge card connectors. I seen that problem in broadcast equipment as well, particularly at stations down on the coast.
But shouldn't this thread be in the Test equipment forum?

Probably but by extension, this instrument issue is connected to, and an extension of, a restoration thread on the alignment of an RCA KCS28 chassis TV that went on for months and months. The issues with this instrument came to dominate the thread and I began this one here because I imagined the same folks involved in the original thread would be interested in the update.

When someone has done poor re-work like that on a two sided circuit board, there's no telling what they did to the feedthroughs when they removed the IC's. Board needs a thorough cleaning and inspection. Make sure the IC pins are soldered both top and bottom wherever traces feed between sides. Besides the edge connector, the socket it plugs into needs to be cleaned. There could be a film of solder flux on some of those areas. I use lacquer thinner to carefully clean the flux off. Beware lacquer thinner can melt some plastics.

When someone has done poor re-work like that on a two sided circuit board, there's no telling what they did to the feedthroughs when they removed the IC's. Board needs a thorough cleaning and inspection. Make sure the IC pins are soldered both top and bottom wherever traces feed between sides. Besides the edge connector, the socket it plugs into needs to be cleaned. There could be a film of solder flux on some of those areas. I use lacquer thinner to carefully clean the flux off. Beware lacquer thinner can melt some plastics.

What about denatured alcohol Kevin?

What about denatured alcohol Kevin?

Usually works on fresh rosin based flux. But as it ages it's harder to remove. It appears a paste flux was applied to some of that. And it sure looks like there's a corrosive thing going on with the gray or blackening in some areas. Acetone is another solvent that will remove flux.

OK...I'll do a rehab on the board and see what that does. I think that'll be it for this piece. I'll just wait for another to come along.

I thought you had two of these, but maybe I'm confused with some other test gear.

I thought you had two of these, but maybe I'm confused with some other test gear.

I have the complimentary sweep generator. The 8600 and 8601 generator are a matched set and really cannot be used to do alignments with other equipment. I did have to go through 2 8601 generators before I got one that worked. Like I said, these 2 pieces of equipment have been very very challenging to my mental health. :screwy::screwy::screwy::screwy::screwy:

I have the complimentary sweep generator. The 8600 and 8601 generator are a matched set and really cannot be used to do alignments with other equipment. I did have to go through 2 8601 generators before I got one that worked. Like I said, these 2 pieces of equipment have been very very challenging to my mental health. :screwy::screwy::screwy::screwy::screwy:

:D

https://www.google.com/url?sa=t&source=web&rct=j&opi=89978449&url=https://www.youtube.com/watch%3Fv%3DO_1ruZWJigo%26pp%3D0gcJCdgAo7VqN5tD&ved=2ahUKEwifrZvp_IyNAxUd8MkDHQEBE-4Q78AJegQIFBAB&usg=AOvVaw3LRCzLmqQm9SJjvreiAQW9

Sorry Chris that you ended up with a dud 8600a. I was luckier with mine. I have used it now for over 20 years and the only problem I had was a broken marker push button.

If you go for another, keep your eyes out for the later version which uses the 8 segment led displays. I believe that will prove more reliable.

Sorry Chris that you ended up with a dud 8600a. I was luckier with mine. I have used it now for over 20 years and the only problem I had was a broken marker push button.

If you go for another, keep your eyes out for the later version which uses the 8 segment led displays. I believe that will prove more reliable.

Thank you my friend. I was not aware there was another version of this with led displays. Do you know if it happens to be the same model number?

Here's a video of an LED version.

https://www.youtube.com/watch?v=ZHUJa2NbTgc

Model number is still 8600A. It's a production run change

Model number is still 8600A. It's a production run change

I'll give ya $5,000 for it! :yippy:

Just kidding of course...well kinda'

Who me? Sorry, I don't have one. I just noticed the LED version is the same model.

Ahhhhh. Got it

Started working on the cleanup of the counter circuit board. 90% isopropyl is working well removing the old dried flux residue. I've ordered NOS original Texas Instruments ICs and I'll be replacing the suspect chips soon. I'll post pictures of the board when it's done. Of course, I'm hoping this solves the issue once and for all!!!

The best solvent for flux residue is a 50-50 mix of denatured alcohol and "xylol" from the paint dept of Menard's or Lowe's etc. "Xylol" is actually the hydrocarbon xylene (mixed isomers),

Of course, I'm hoping this solves the issue once and for all!!! Hopeful is good, but don't rule out the possibility of more than one issue. It's hard to know if the display lockup is being caused by the low 5v supply, or a symptom of.

Oh yes I have put this piece of equipment in the “lemme see if there’s anything else I can do” category while I continue to look for another one. The 5V freezing display problem was accompanied by the return of the crazy frequency jumping on the display. In other words both of the original symptoms have returned now that I’ve had it in use again for a significant amount of time. I have little hope for this fix Kevin and the way the piece has tortured me in the past, I’ll cleanup the board, replace the suspect ICs, put it in service, declare it’s been working reliably for weeks…and then it’ll act up again. The guy I bought the sweep generator from has offered to fix it and his skill set indicates he’d be successful so there’s always that route. Just want to exhaust what I’m capable of doing before I spend a couple hundred dollars more on repairs and shipping.

In spite of the freezing and jumping display, It's too bad we didn't get to the bottom of the sagging 5 volt power supply. The flux mess on the display board is something that's good to clean up regardless if it fixes anything.

Looks like it’s working fine after the cleanup. I’ve had it on for short and long timeframes over the past 5 days and no issues. Gonna do some endurance and accuracy tests over the next month or so before I’ll be confident the fix is completed

Failed again. Frequency readout on CW Counter freezing with the same low 5V issue however, if I switch the instrument to scope with the generator in CW/no sweep, it reads the frequency perfectly. Tracks with it just fine and stable. WTH?

A little reading saves months of stupidity. I read through fault modes in the service manual in particular, the ones specific to the regulator board and I'm embarrassed to say all of this was caused by a bad 2N3055. Freezing the regulator 723 simply broadened its regulation range so it was able to pull it in to 5V despite the 3055 being bad. It is rock solid 5.000V DC on my DMM and has been that way for days. I wanted to be sure before I posted. Pretty darn happy and pretty damn embarrassed I'll say.

Thanks everyone for the tremendous support. Now I'll get back to that gosh darn RCA!!!