Hewlett-Packard 3314A function generator

[tggzzz]

HP 1725A,1715A and 1740 series oscilloscopes all have a 100µF wet tantalum in their first timebase that eats its own negative lead, then the acid leaks out and consumes the track that connects it to the timebase selector switch, the timebase selector switch and the spring detent mechanism for the timebase selector.

On my 1740 the tracks on the timebase selector were eroded by the timebase selector housing when it rotated. Fixed by soldering a wire that lifted the housing by the (small) wire diameter, keeping it off the PCB and track.

There aren't any electrolytic capacitors involved in control timing, the largest capacitor there is a ceramic 100nF in an unusual orange resin-like finish, it looks like those Philips tantalum capacitors, but it's a ceramic. I've already established that the fault is a ramping down of the +15V rail rather than anything abrupt.

Oh, that's interesting. Is it possible to work out an associated time constant, and correlate that with anything shown in the schematic? If you change the value of capacitors, does the slope change proportionately?

That's one of the reasons I think the attention to capacitors is not going to lead to a solution, but given that it has already lead to an apparent worsening of the "resetting to defaults" condition through NVRAM corruption perhaps that is wrong. Maybe the whole PSU is a cunningly disguised analogue computer that controls the digital computer through a series of voltage and current ramps. That's certainly how it feels.

Digital circuits are analogue circuits, with constrained inputs and outputs But you know than.

...I actually love stuff like that...

It's an excellent machine, I'd have given up a long time ago if there was anything else that does as much in a smallish box, even compared to modern fast DAC based arbitrary waveform generators it can do lots of useful stuff rather well.

Better than soduku, in oh-so-many ways

[Zenith]

Better than soduku, in oh-so-many ways

You can do soduku on the train or on a ferry, whereas it wouldn't be so easy to fix a Hewlett-Packard 3314A function generator in such circumstances.

[EC8010]

Sudoku is excellent mental training for technical work. It forces you to think and distinguish between "probably" and "definitely". That's very useful when you're feeling your way in designing something new. I'm feeling smug today because I have got the "I" newpaper's Jigsawdoku right nine days in a row. Pride comes before a fall and I'll probably get sloppy on Monday and muck it up.

[synx508]

I've captured the 15V (via 10:1 probe) and 5.1V (via 1:1) glitch waveforms. The top one is 15V, those ramps are 10ms long and it stays at around 11V for 12ms. The 5.1V resets instantly and recovers as the 15V rises through 14V. Those ramps look so precise, but my reading of the schematic says there's nothing that could deliberately achieve this. If you're wondering why the 5V recovery has that slight "on ramp", it's because the 5.1V rail uses the +15V rail as a reference.

[synx508]

I was staring at the schematic and suddenly noticed a 1µF ceramic capacitor that might be part of a network of resistors that could achieve that 10ms for 4V ramp, so maybe I'll change that capacitor.

[tggzzz]

Sudoku is excellent mental training for technical work. It forces you to think and distinguish between "probably" and "definitely". That's very useful when you're feeling your way in designing something new. I'm feeling smug today because I have got the "I" newpaper's Jigsawdoku right nine days in a row. Pride comes before a fall and I'll probably get sloppy on Monday and muck it up.

I don't disagree, but I get all that from doing electronics/software.

[Zenith]

I've captured the 15V (via 10:1 probe) and 5.1V (via 1:1) glitch waveforms. The top one is 15V, those ramps are 10ms long and it stays at around 11V for 12ms. The 5.1V resets instantly and recovers as the 15V rises through 14V. Those ramps look so precise, but my reading of the schematic says there's nothing that could deliberately achieve this. If you're wondering why the 5V recovery has that slight "on ramp", it's because the 5.1V rail uses the +15V rail as a reference.

What were the circumstances? Did it reboot when the glitch was detected? Anyway, it's quite a glitch.

I never liked those power supplies where several lines work off a single line as a reference. It's as if they were hard up for Zener diodes.

[tautech]

I've captured the 15V (via 10:1 probe) and 5.1V (via 1:1) glitch waveforms. The top one is 15V, those ramps are 10ms long and it stays at around 11V for 12ms. The 5.1V resets instantly and recovers as the 15V rises through 14V. Those ramps look so precise, but my reading of the schematic says there's nothing that could deliberately achieve this. If you're wondering why the 5V recovery has that slight "on ramp", it's because the 5.1V rail uses the +15V rail as a reference.

Is this a SMPS PSU ?
If so I wonder if there is an issue in the +15V feedback circuit ?

[synx508]

What were the circumstances? Did it reboot when the glitch was detected? Anyway, it's quite a glitch.

I never liked those power supplies where several lines work off a single line as a reference. It's as if they were hard up for Zener diodes.

Circumstances were that it was generating functons for a few hours then that occurred on the PSU, which restarted the computer, which restored the default settings as is normal on startup.

The detail of the PSU that I don't like is that the op-amp that regulates the -15V supply takes its power from the -15V regulated supply. Hmm, might have a look at the 1458's datasheet.

[synx508]

Is this a SMPS PSU ?
If so I wonder if there is an issue in the +15V feedback circuit ?

5.1V is a buck converter driven by the same unregulated "24V" that's nearly 29V that feeds the +15V linear regulator. The ramp down in the linear +15V triggers a LM339 comparator turns off the +5.1V supply (by stopping a 555 timer) until the ramp up turns it back on again. The computer restarts.

EDIT: it's actually a transistor with its own 10V zener (CR106) that turns off the 5.1V supply when +15V drops too low.

In a separate development I've noticed that a normal power down results in the occasional incidences of the computer restarting during the power down process, which might be a clue about the NVRAM corruption.

[tautech]

A link to the schematic would be nice....

[synx508]

A link to the schematic would be nice....

Pages 205 and 206 of the service manual from here:

https://www.keysight.com/us/en/assets/9 ... -05762.pdf

[AVGresponding]

Do you have a thermal imager? Might be worth a try, though this sounds so subtle and inconsistent, I'm still tempted to think about dry joints and connectors. A Pimpernel fault, this one.

[tautech]

A link to the schematic would be nice....

Pages 205 and 206 of the service manual from here:

https://www.keysight.com/us/en/assets/9 ... -05762.pdf

Nah, not about to sign up for any furking manual, HPAK can go get stuffed !

Upload it somewhere else.

[synx508]

Do you have a thermal imager? Might be worth a try, though this sounds so subtle and inconsistent, I'm still tempted to think about dry joints and connectors. A Pimpernel fault, this one.

I don't have one, yet… I am not thinking about connectors even though these are notorious for intermittent connectors, this is because I can see those ramps on +15V and that doesn't look like something a connector would do. The pass transistors are in sockets, but what sort of bad connection would make it do that? It doesn't fit with the pattern of the failure or the voltage waveforms, even though the sockets and transistor leads don't look to be in shiny condition.

[synx508]

Upload it somewhere else.

Pfft, okay, just the PSU is here https://mossyvale.co.uk/3314A-PSU.pdf

[tautech]

Looks like an ordinary old school linear PSU.....
OVERVOLTAGE PROTECTION circuitry is where you might focus some thought but normally firing a SCR crowbar can only be reset with a hard power OFF.

[EC8010]

I never liked those power supplies where several lines work off a single line as a reference. It's as if they were hard up for Zener diodes.

Ah, but it makes sense sometimes. If you need +/- supplies, then using one as the reference for the other means that any drift in your reference is applied equally to both supplies and there's a chance it will balance out in the wanted signal.

[synx508]

Looks like an ordinary old school linear PSU.....
OVERVOLTAGE PROTECTION circuitry is where you might focus some thought but normally firing a SCR crowbar can only be reset with a hard power OFF.

I've checked the separate zeners for this and they're correct, the SCR acts on the unregulated side and that isn't affected. Also it resets without that unregulated supply being taken down. I am monitoring the output of the +15V feedback op-amp now, to see what happens during a glitch. My latest theory is that the bootstrap regulator setup is the reason the ramp doesn't drop below about 11.something volts, which suggests the output of the op-amp is dropping to 0V momentarily, possibly due to its own internal short circuited output protection misfiring? Might be an hour or two…

[tautech]

Looks like an ordinary old school linear PSU.....
OVERVOLTAGE PROTECTION circuitry is where you might focus some thought but normally firing a SCR crowbar can only be reset with a hard power OFF.

I've checked the separate zeners for this and they're correct, the SCR acts on the unregulated side and that isn't affected. Also it resets without that unregulated supply being taken down. I am monitoring the output of the +15V feedback op-amp now, to see what happens during a glitch. My latest theory is that the bootstrap regulator setup is the reason the ramp doesn't drop below about 11.something volts, which suggests the output of the op-amp is dropping to 0V momentarily, possibly due to its own internal short circuited output protection misfiring? Might be an hour or two…

+15 and -15 use the same IC....you might replace it to cover both rails...if you have 1 in stock.

[Zenith]

What were the circumstances? Did it reboot when the glitch was detected? Anyway, it's quite a glitch.

I never liked those power supplies where several lines work off a single line as a reference. It's as if they were hard up for Zener diodes.

Circumstances were that it was generating functons for a few hours then that occurred on the PSU, which restarted the computer, which restored the default settings as is normal on startup.

The detail of the PSU that I don't like is that the op-amp that regulates the -15V supply takes its power from the -15V regulated supply. Hmm, might have a look at the 1458's datasheet.

The +15V supply is referenced to the -15V supply and the +5V is referenced to the +15V. Does the -15V line glitch at the same time?

It uses different op-amps in the same package (U100), which I assume is a 1458, to regulate the + and - 15V supplies and its power lines are the regulated supplies. The designers must have worked out what was happening at power-on and found it OK. When the power supply is up and running there should be no problems.

1458 is a dual 741 as I recall. Reliable but very limited by modern op-amp standards. Replacing them with more up-to-date ones may not work out, as the design may rely on the limitations.

Problems caused by dry joints, cracked lands and bad connections can be very frustrating. They can be heat related, they can have apparently random behaviour.

An idea which occurs is to use your own -15V and +15V references from a bench supply, for the purposes of tracking down the problem, but that may not be possible given the layout of the board.

In a separate development I've noticed that a normal power down results in the occasional incidences of the computer restarting during the power down process, which might be a clue about the NVRAM corruption.

They must have made provision for powering down not being mistaken for a power supply fault. How does it detect power supply faults?

[synx508]

I'll probably replace the 1458 soon. The -15V side doesn't glitch but there's no reason to assume it would, really, if it's an output protection mechanism that is operating. If the glitch had been coming from -15V I would changed many things already because at least it's kind of a dead end and causality can go no further.

I've had the DSO watching the output of the +15V side of the 1458 for over an hour and it hasn't glitched, which is a bit disappointing.

[synx508]

I haven't changed the 1458, but I changed a car battery earlier, 5 miles away. It still hasn't glitched.

[synx508]

I disconnected the probe when I last posted, it has now glitched. Coincidence? I don't think so. But maybe it was because the case was slightly ajar and better ventilated when the probe was in place. I'm going to change some 2N3904, 2N3906s and the 1458 tomorrow.

[Zenith]

Well I did tongue-in-cheek mention gremlins.

Take it well away from your property, preferably near where someone you don't like lives. ideally someone with a lot of complex electronic gear. Make sure it's well away from your car when you do this. Give it good spray with holy water, preferably holy distilled water. Take it home, put it in the airing cupboard for a good week. Problem solved. Unless of course you have a particularly persistent gremlin.