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.

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.

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.

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?

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?

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?

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.

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?

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/v2/1...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/v2/1...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/v2/1...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.