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Old 15th Oct 2022, 3:34 pm   #21
Uncle Bulgaria
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Default Re: HP 5300B Overcurrent

Here are the waveforms at the anode of A1CR2 and the cathode of A1CR4 (the junctions between the diodes and T2). Channel 1 (upper) is A1CR2. x10, 0.2V/div, 20µs/div.
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Old 15th Oct 2022, 4:30 pm   #22
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Default Re: HP 5300B Overcurrent

The restoration of the -5V line is definite progress. The original diode is likely to be a fast recovery type. As the voltages on the output rails are now correct it's possible that there is an error in the manual concerning the 10V p-p waveform. The overshoot isn't anywhere near as bad as it was either, judging by the photo.

The next thing to do is to determine if the regulation circuit is working properly. Monitor the +5V line and increase the 6V supply slowly to 7 or 8 volts. If all is well, the +5V should remain close to the voltage it was when fed with 6V. Expect it to rise by a few millivolts but not much more than that. If it rises by a lot (don't get too close to 5.9V as the Zener will be damaged), there is a problem in the regulation circuit and that will need to be tracked down.

If the 5V stays constant, you can safely increase the voltage further, towards a maximum of 22V. Continue monitoring the 5V with your DMM and monitor the voltage on the collector of Q17 with your 'scope. Stop before the peaks reach 66V or if the +5V output rises above 5.5V. Note the supply voltage at this point.

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Old 15th Oct 2022, 6:56 pm   #23
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Default Re: HP 5300B Overcurrent

Thanks awfully frsimen. This is really giving me the impetus to continue.

Increasing to 8V, the +5V rail went from 4.99V to 5.00V. This is maintained as I increase the voltage further. However, I wanted to clarify if you meant the ringing peak was what I should be monitoring on A1Q17's waveform, or the main peak of the square wave.

I attach a photograph of what I mean. At 11V supply, the ringing peak is ~60V (x10, 1V/div.), though the main square wave is lower.
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Old 15th Oct 2022, 8:06 pm   #24
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Default Re: HP 5300B Overcurrent

That's a good question. The regulation seems to be working well. Maybe the 68V diode is intended to clamp the voltage spike. I was referring to keeping the spike below the Zener voltage. I don't know if the spike is correct or not, it may get smaller at higher supply voltages, but that's possibly wishful thinking.

Perhaps the way to go is to increase the supply voltage, keeping an eye on the supply current and switching off to check the Zener's temperature from time to time. Stop if the Zener is getting hot or the current becomes excessive.

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Old 15th Oct 2022, 8:28 pm   #25
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Default Re: HP 5300B Overcurrent

Quote:
Originally Posted by Uncle Bulgaria View Post
5.9V axial zeners appear to be unusual. I replaced CR6 with a 5.6V zener, and still have +0.7V on the -5V rail. It's lucky I have thick hair.

Edit: crossed post. Yes, CR6 was the right way round (original Motorola part - checks out as a diode on the component tester, but haven't checked the zener voltage). Continuity checked between it and neighbouring components.

Indeed, a very good idea to check the test equipment! Quis custodiet ipsos custodes?
They do like using odd value Zeners in HP frequency counters, I did my best to test a bunch of 6.2V Zeners, to find some that were close to 6.04V for a 5216A counter, sadly it's still broken due to shorted primary on the mains transformer (had mains wire trapped underneath the transformer from the factory).

To test them I used a bench PSU and decade resistor to set & limit the current to 5mA or 10mA (depending on wattage) and measured the voltage across the Zener after a few minutes, to allow for warm up.

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Old 15th Oct 2022, 9:10 pm   #26
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Default Re: HP 5300B Overcurrent

I wasn't too concerned about A1CR10 as I have a score of 68V zeners bought since every foray into this unit caused one to blow!

Gingerly increasing the supply to 22V showed the spike remaining at the same amplitude. At 22V I noticed something peculiar. +5V was +5.01 but -5V was +4V. Slightly twisting the transformer T2 (still mounted on raised wires after removing it for insulation testing) returned the reading to -4.5V. I guess there's a loose connection there, and the lower voltage is from the 5.6V zener rather than the 5.9V required. Other than that, the voltage rails are within specification, so perhaps I should dare to plug it into the mains...
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Old 15th Oct 2022, 10:57 pm   #27
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Default Re: HP 5300B Overcurrent

I suggest you sort out the mounting of T2 and check the connections between it and CR4 before going any further. I hope it's just a poor connection and not something more serious that caused the odd voltages.

The 5.6V Zener diode is probably a +/-5% tolerance component. That will give a Zener voltage of between 5.33 and 5.88 volts. Below the Zener voltage a Zener diode will draw very little current, so it isn't the cause of the voltage on the -5V rail being -4.5V. This power supply design only checks and regulates the +5V supply. With the other rails, you get what you get and that will depend on how the various rails are loaded. I think all seems to be about right, odd problem mentioned above aside).

Once you are happy that you have sorted out the connection issue, just run it up one more time from the external supply. If all seems well it ought to be safe to try from the mains supply. Double check that you have the supply set for 220/240V, and not the 115/120V tapping if there is one.

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Old 16th Oct 2022, 8:28 pm   #28
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Default Re: HP 5300B Overcurrent

I've repaired the holes under T2 with eyelets. There seemed to be something funny with pin 4 of T2, but now working. Voltage rails at 22V applied over A1C6: 3.45 on +3.5V, 4.92V on +5V, -4.3V on -5V, 17.3V on +17V and -18.4V on -17V.

I have been using a Farnell SB 30/10 to ensure enough current. With both halves plugged together (the 5302A counter) the display lights and the units cycle when the function knob is turned. It's been on while typing this with nary a flicker, though the PSU doesn't half make an ultrasonic whine. I don't think it's loud enough to be the fault the flowchart refers to, but perhaps the HP engineers had rather less HF hearing!

Plug in mains, puff, there goes A1CR10. I have to surmise a rectifier problem. I tried bringing it up on the variac, and the diode blew at 160VAC input.

P.S. I don't understand your explanation of the effect of the zener. I thought it was there to clamp the voltage to its own value, so the zeners were in the supply to reference the rails to 0V.
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Old 16th Oct 2022, 9:41 pm   #29
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Default Re: HP 5300B Overcurrent

The 22V from the mains power supply will have several volts of ripple on it. The peaks will be somewhat higher than 22V but with 160V supply from the variac you shouldn't be getting anywhere near a voltage that will cause trouble. Try measuring the AC voltage that is applied to the bridge rectifier with the power switch off, that will explain what is going on, I think. I would expect the voltage to be at least 17V but not more than 21V when fed from 240V.

The rest of the circuit works quite well down to just 6 volts on the 22V line. Check carefully the wiring around the voltage selection switch, it may have been wired incorrectly by someone, so that it is wired permanently in the 120V position.

The Zener diodes are used for in two ways in this circuit. The voltage reference is provided by A2CR4 and that is fed by a resistor to control the current through the Zener.

Many of the other Zener Diodes are being used to provide overvoltage protection. A2CR5, 6, 7 and 8 all are used for that purpose. You will notice that their voltage rating is rather higher than the voltage which normally appears across them. They don't have a current limiting resistor either. If the voltage across them becomes excessive, they will conduct and pass a lot of current as there is no limiting resistor. They will fail short circuit and that should shut the power supply down. They play no part in regulating the output of the supply.

The unfortunate A1CR10 protects Q17 from overvoltage in a similar manner.

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Old 16th Oct 2022, 11:22 pm   #30
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Default Re: HP 5300B Overcurrent

Ah! I'm groping towards enlightenment now: A1CR4 in combination with the tap from T2 is creating the +5V reference, rather than A2CR5 against ground.

At 240VAC with the unit off, there is 21.5VAC at the input to the rectifier. Output is nominally 22VDC, but on the 'scope there is a lot of ripple. The picture shows x10, 1V/div. Zero is the centre of the graticule, so it's peaking at 30V.
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Old 17th Oct 2022, 8:42 am   #31
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Default Re: HP 5300B Overcurrent

HP was a manufacturer of fast schottky diodes, and some of their early switch-mode power supplies were designed around them. Too-slow rectifiers can leave spikes which fast ones would have clamped and turned into useful output.

Also, zeners are not normally fast at all and don't on their own make fast clamps. To make a fast clamp, use a very fast diode into a zener with a big capacitor across it, and a resistor to bias it up.

I've not had anything to do with the 5300, so this is no more than a general observation.

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Old 17th Oct 2022, 7:28 pm   #32
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Default Re: HP 5300B Overcurrent

The ripple is less than I was expecting given the values of C6 and R8, but that will have no effect on the operation of the rest of the circuit. The supply voltage is a little higher than I stated yesterday but the service manual says it should be 24V+/-2V.

The service manual states that the output voltages when measured at A1J1 should be:


+5.2V +/-0.25V at contact 1 (4.95V) Your reported voltages are in brackets
-5V +/-0.25V at contact 2 (-4.3V)
-17.5V +/-1.7V at contact 3 (-18.4V)
+17.5V +/-1.7v at contact 4 (+17.3V)
+24V +/-2V at contacts 25 and 50 (when connected to the mains).

All your measured voltages are correct apart from the -5V rail which is 8.75% lower than the nominal voltage. The allowed tolerance is +/-5%.

I wonder whether David (Radio Wrangler) has come up with the cause of the trouble. At this stage a number of components have been changed, possibly something that isn't suitable has been substituted somewhere leading to a problem. It would be worth checking what is actually fitted and noting anything that isn't as described in the parts list in the manual.

If you can find the reason for the low -5V output you will probably have found the cause of the A1CR10 failures. I hope it isn't a fault in T2, such as a shorted turn associated with pin 4.

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Old 18th Oct 2022, 12:11 am   #33
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Default Re: HP 5300B Overcurrent

Thanks both. The rectifier diodes look like original ITT 1N4004s. The T2 output diodes are the originals. I don't know about A2CR3 (the unit was sold on here after the previous owner had given up, so some components were changed). The original problem, before me, was the +5V rail.

That 'scope picture is just after the rectifier with the unit off, so C6 isn't in the picture yet.

If the unit's running with DC applied to C6 and CR10 is not expiring, then doesn't that mean the switcher's OK and therefore the overload is caused by the mains side of things?

I have a Taylor 68A signal generator. Should I simply connect the Belling-Lee output to one of the HP's BNC input channels and see if any of the rest is working?
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Old 18th Oct 2022, 4:54 pm   #34
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Default Re: HP 5300B Overcurrent

I'm wondering now if you measured the voltages with the 5302 module plugged in or with it removed, while feeding the power directly to A1C6.

I had assumed the latter, but if you made the measurements with the module plugged in, that changes thing. If the module was in place, the low -5V rail could be caused by a problem in the module. Can you remove it, then measure the voltages again. If the measurements were made with the module removed anyway, you could repeat them with the module in place. The whistle from the supply could be indicative of an excessive load somewhere, of course.

Applying a signal to the module may help to identify if there is a problem on that, but only if the voltages are still in limits when it is plugged in.

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Old 18th Oct 2022, 5:52 pm   #35
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Default Re: HP 5300B Overcurrent

The 1N4000 series are slow diodes. The higher voltage end of the range are actually PIN structures and barely fast enough for 50Hz! 1N4004 would be expected to be inefficient in a switcher.

Switchers vary mark/space ratio to regulate their output. As there is only one parameter to control, they can only get one output right, any others either get a free ride and have to work with whatever they get, or are wound for a higher voltage and use a 3-terminal reg on the lower power output.

With no secondary reg, loading up the main output widens pulse widths as the switcher tries harder, and the unregulated output goes up.

There were funny circuits using magnetic amplifier post regs to get several outputs under control but not in small boxes!

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Old 18th Oct 2022, 7:14 pm   #36
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Default Re: HP 5300B Overcurrent

I think the 1N4004 rectifier diodes that are referred to are used in the bridge fed from the mains transformer. The photos show different types on the switcher outputs.

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Old 18th Oct 2022, 11:42 pm   #37
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Default Re: HP 5300B Overcurrent

Ah, that's better!

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Old 30th Oct 2022, 2:54 pm   #38
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Default Re: HP 5300B Overcurrent

Yes, the mains rectifier diodes are the ITT 1N4004s. The ones on board A2 are the originals (including one striped glass body one that I don't recognise). The only one replaced is the CR10 68V zener, for which I've not used a special type.

With 5302 disconnected, the readings are very similar. With a 22V supply across A1CR6, I have +3.8, +5, -4.5V, +17.4, -17.5.

Add mains and 5302...there goes CR10.
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Old 31st Oct 2022, 12:40 am   #39
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Default Re: HP 5300B Overcurrent

As the mains supply output voltage has been measured to be around 22V, there seems to be a fault in the 5302 module that is causing the trouble. The service manual describes linking pins 25 and 50 on A1J1. You can then run the unit with the module unplugged. If it then works without damaging the 68 volt Zener diode, that would confirm that there is a problem in the 5302 module.

Have a look on the -5 Volt rail for a low resistance. Possibly there is a tantalum capacitor that has become leaky in the module.

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Old 4th Nov 2022, 12:11 am   #40
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Default Re: HP 5300B Overcurrent

Alas, with 5302 disconnected, pins 25 and 50 bridged on the multi-pin connector and mains applied, CR10 glows...then croaks and the fuse blows. Why this doesn't happen with 22V across C6 I don't know.
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