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Old 25th Nov 2021, 10:42 pm   #1
mole42uk
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Default HP 5300 power supply fault

Hi guys,
I have two HP 5300 Measuring System boxes - a 5300A and a 5300B. They both have 50MHz frequency counter modules. One has never worked since I got it, the other has behaved perfectly until a couple of days ago when it failed. The evidence was a blown mains input fuse. Replacing that with a slightly higher rated fuse rewards me with two sets of results.

1. If the 5300 board is disconnected from the counter, 20v ac is present at the bridge rectifier and 22v dc across the main smoothing capacitor.

2. If the counter board is connected, the dc voltage drops to about 2v and the rectifier diodes get hot. The ac voltage is as before.

Now, the DC path is bridged across the counter module, so if that is disconnected there is no load and the DC supply is okay. It seems that there is a shorted component in the remaining supply section which provides ±5 and ±17, amongst other voltages. I would expect there to be a fault in the "A2 Power Supply Regulator Assembly" so I wonder if anyone has an intimate knowledge of these HP beasts and knows if there's a likely culprit?
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Old 25th Nov 2021, 10:57 pm   #2
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Default Re: HP 5300 power supply fault

I don't know these units, but look for tants!
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Old 25th Nov 2021, 11:05 pm   #3
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Default Re: HP 5300 power supply fault

The only tants will be the metal cased axial variety, they can still short though, had one fail in a HP 1821 scope timebase.

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Old 27th Nov 2021, 11:02 am   #4
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Default Re: HP 5300 power supply fault

I have an HP 54510A 'scope. The power supply failed on that and it turned out that every electrolytic capacitor was leaking. There was a lot of them! I think that the tants were ok.
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Old 27th Nov 2021, 11:12 am   #5
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Default Re: HP 5300 power supply fault

The 54xxx scopes are a couple of decades later than the little 5300 snap-together instrument system and are completely different. The 5300 has several different power supply designs across the different members of the family. Some switchers, some linear. I've not had anything to do with the switchers, so can't be much help.

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Old 27th Nov 2021, 3:00 pm   #6
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Default Re: HP 5300 power supply fault

I've only got the 5300A with 5301A 10MHz lower half.

This guy on the eevblog has bought a whole selection and posted a video last year;
https://www.youtube.com/watch?v=ua5AySUoSdA
I seem to remember from that video, that some of the later lower halves are incompatible with the older 5300A.

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Old 27th Nov 2021, 5:32 pm   #7
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Default Re: HP 5300 power supply fault

Richard, I don't quite understand your statement in post 1, 1). If the 5300A (top unit) is disconnected from the 'plug on' counter (bottom unit), then there is no path for the unregulated d.c. from the bridge rectifiers to the main PSU regulators. The bottom unit links pins 25 and 50 of the main Cannon D connector and, without this, all you have is an unregulated and virtually unsmoothed 22V d.c. supply. You need the two pins linked before this voltage is fed back via the on-off switch part of the Sample Rate pot. So the main smoothing capacitor C2 is not even in circuit until you fit a 'plug-on' and switch on.

Maybe you mean something else by 'the counter' in 1, 1); so can you clarify exactly what you are measuring and under what condition? Also, do you have the 5300A Service Manual which includes the description of operation, circuit diagram and basic fault-finding charts?

My 5300A PSU contains metal clad tantalum capacitors and my 5300B dipped tantalums. The metal tantalums tend to be quite reliable, so I would not immediately blame one of them for going short circuit.
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Old 28th Nov 2021, 2:33 pm   #8
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Default Re: HP 5300 power supply fault

It's not clear in the first post which one is being repaired, the A or the B version

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Old 28th Nov 2021, 2:45 pm   #9
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Default Re: HP 5300 power supply fault

Apologies, I tried both versions and neither worked. I have been using the 5300B permanently to monitor my GPSDO until one day it exhibited the fault condition described above. I realised that I was using the circuit diagram for the A version which has a rather different regulator section. Once I had grabbed a circuit for the B version the next diagnostic steps became apparent.

I removed the Regulator Assembly and found the rectifiers still getting warm, so, having narrowed the problem area down to the main PCB I spotted a power transistor in the power control circuit. Measuring across C-E showed a short circuit, but B-E and B-C measured as good diode junctions. Checking the circuit revealed a 68v zener across the transistor C-E and visually, it looked distressed. Unsoldering one leg enabled me to prove that the zener was short and that to power transistor measures perfectly well. I haven't moved any further yet because I don't have a 68v zener in stock but confidence is high.

As a reward, I have persuaded the A version into some sort of life, it shows a display which didn't happen before, but it won't measure frequency using either of the 5302 plug-ins that I have. Since one of those does work, I suspect the 5300A head which I'll attend to after the B head is repaired. Another reward is that the B version that I have is the 5300B-001 option which has an on-board 10MHz OCXO.
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Old 28th Nov 2021, 5:34 pm   #10
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Default Re: HP 5300 power supply fault

I have to say the 5300B pdf on the web is not the finest quality, hopefully that zener is the only problem, my stock of 400mW/500mW zeners runs out at 56V.

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Old 28th Nov 2021, 6:54 pm   #11
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Default Re: HP 5300 power supply fault

You can put two zeners in series.

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Old 28th Nov 2021, 8:20 pm   #12
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Default Re: HP 5300 power supply fault

I thought of that but I only had 47v & 10v in stock, so rather than use three zeners I just ordered some 68v 5w ones.
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Old 28th Nov 2021, 8:37 pm   #13
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Default Re: HP 5300 power supply fault

Have a read of this webpage, scroll down to the 5300A;
https://emperoroftestequipment.weebly.com/#H

Their 5300A seems to have had a similar failure, i.e. blowing mains fuse, shorted 68V protection zener, the cause was the potted transformer having a short between primary & the secondary.

Oh and I had quick look inside my 1980ish 5300A, uses a similar looking switcher to the B model, also noted two slightly hidden RIFA madness smoke bombs attached to the back of the mains inlet/voltage selector block, now on the do not use pile waiting a round tuit.

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Old 28th Nov 2021, 8:53 pm   #14
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Default Re: HP 5300 power supply fault

Quote:
Originally Posted by mole42uk View Post
I thought of that but I only had 47v & 10v in stock, so rather than use three zeners I just ordered some 68v 5w ones.
You do realise it was a .4W zener (CR10 1902-3381) and looking where it is, you may struggle to fit a 5W part in it's place.

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Old 28th Nov 2021, 9:26 pm   #15
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Default Re: HP 5300 power supply fault

Quote:
Originally Posted by factory View Post
Quote:
Originally Posted by mole42uk View Post
I thought of that but I only had 47v & 10v in stock, so rather than use three zeners I just ordered some 68v 5w ones.
You do realise it was a .4W zener (CR10 1902-3381) and looking where it is, you may struggle to fit a 5W part in it's place.

David
Yes but I’m ingenious and I don’t want it to fail again!
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Old 28th Nov 2021, 9:28 pm   #16
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Default Re: HP 5300 power supply fault

Quote:
Originally Posted by factory View Post
Have a read of this webpage, scroll down to the 5300A;
https://emperoroftestequipment.weebly.com/#H

Their 5300A seems to have had a similar failure, i.e. blowing mains fuse, shorted 68V protection zener, the cause was the potted transformer having a short between primary & the secondary.
I’ll test that transformer on my B version. So far I’ve found no fault except the failed zener but there’s no harm checking.

Okay, tested the transformer- no shorts. Primary & secondary are quite separate. Maybe the zener died of old age!
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Last edited by mole42uk; 28th Nov 2021 at 9:45 pm.
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Old 8th Dec 2021, 9:20 pm   #17
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Default Re: HP 5300 power supply fault

Update time: having inserted the 5W zener, not a difficult task since HP used HUGE thp holes for the component leads.....I was not further forward. Looking at the circuit for the 5300 power supply, the part I am interested in is exactly similar between the A and B versions. A number of waveforms as published in the Service Manual didn't match to the ones I was observing. I tested several semiconductors in the switcher module and found them all to be behaving, except for my doubt about the UJT so I ordered some of those and replaced. No change.

Further diagnosis by applying 22V DC from my bench power supply revealed that the 5300 was drawing too much current. The supplies are fed from the SMPS into the instrument through some large chokes, and disconecting all of those to see where the fault might be, then reconnecting one by one revealed that ±17V were okay, -5V was okay, +3.5V was okay, but +5V not so good. SMPS waveforms around the main switching transistor were according to spec. Removing the 160µF smoothing capacitor on the 5V rail seemed to give some relief - the supply functions, all the voltages are within limits but there's still a fault somewhere. I would not recommend disconnecting sections of a switcher since many times the input is regulated by one of the outputs, but under controlled conditions this course of action can work as a diagnostic tool.

Now, if I use the mains input to the power supply, the 160mA fuse remains intact but the voltage on the main smoothing capacitor is only 12V or so, it should be 22V. Obviously more work to do, I suspect the bridge rectifier. There is still an anomoly in the 5V power rail, I was looking to see where else it connected, but with the 5300 top board only in action, the 5V rail goes nowhere.
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Last edited by mole42uk; 8th Dec 2021 at 9:26 pm.
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Old 10th Dec 2021, 8:01 am   #18
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Default Re: HP 5300 power supply fault

After spending some time with the 5300B last evening, I could have the supply working and producing the correct voltages and then suddenly just going into overcurrent shutdown, after switching off and ramping up the supply again it'd be back to proper voltages. I'm inclined to suspect a breakdown in the HF transformer as referenced by David in post 13. I've put both 5300 counters back together and chucked them under the bench where they'll probably sulk until one day in the Spring when I'll throw them both into electronics recycling.
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