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HP 54510A Power Supply Smelling

Paul Ramsbottom

Aug 12, 2016
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Yes the clicking returned when I reconnected that rail.

I will go through what has happened so far.
Scope working but smell from power supply. Resolved by replacing faulty caps on primary side.
Then worked for several hours over several days. New fault. It would run for a few minutes then cycle on and off. Arcing noticed between C16 and cathode fixing of Cr24 due to leaked electrolyte. C16 replaced and board cleaned. No effect on cycling fault.
Replacement of all electrolytic and some tants started.
No improvement. When C19 replaced it would not start up. Clicking noticed and fan twitching st same time.
C30 31 replaced again and then no output at all from 15.5v rail. CR24 replaced. No improvement. Traced to broken link between caps and L4. Link restored and clicking fault resumed.

Low ESR caps ordered for C30 31 and 19.

A thought is that I could disconnect the crowbar again and see what happens to the voltage. I could either disconnect R63 and see whether the caps remain charged. Or I could use the new link to measure the current in the rail with no load, which should just be that through R63. I am very far from being an expert in this so comments on that would be welcome.

I could also just look for a new scope on eBay but that seems a bit defeatist!
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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The clicking comes from a gross overload and the SMPS shutting down. When it restarts, it gets a gross overload and shuts down again. Each click is as a result of a cycle of this.

If you can, disconnect the rectifiers and capacitors on the low voltage side one by one. When the clicking stops (if it does) you have found the secondary circuit with the problem (essentially a short).

Obviously, turn the power off before you connect or disconnect anything.
 

Paul Ramsbottom

Aug 12, 2016
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Thanks Steve. I think that the fault is in the pins 17,19,20 rail as the voltage on that is only around 1.5v. The -5.2 is also down to 3.5. The others are near their values but a little low and varying presumably due to the switching on and off of the SMPS.

I tried removing C19 whose replacement seemed to cause a problem but that had no effect.

I think the fault is past C30 and 31 as they charged and remained charged when the link to L4 broke. I have replaced C33 so does that mean that CR 37 snd14 are suspect? Thinking about what you said earlier about ripple on the rail causing the crowbar to trip I disconnected the SCR. This brought the voltage up on the rail but the clicking continued. Using my old scope on the output shows a sawtooth wave, presumably reflecting the output from the mains side. Would those diodes be the next to replace?
 

(*steve*)

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What voltage do you see on C30 and C31?

Measure the resistance across all the filter caps (in circuit, power off and discharged). The resistance should keep on increasing until it gets reasonably high. Do any remain at a low value?
 

Paul Ramsbottom

Aug 12, 2016
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The voltage on C30 and C31 fluctuates but is around 11vwith the link in place. It rises to around 16v with the link removed. The resistance on that rail only rises to around 107 ohms but I assume that is due to R63. As far as I can tell the resistance rises ok on the other rails. I tried disconnecting CR27 in order to disable the -5.2 rail but the clicking continued. It is much fainter with no load on the power supply but is still there. I assume I should expect it to work silently.

I am beginning to think we a flogging a dead horse here here and I should think about getting another 'scope. We have done our best for the power supply but there is something somewhere that is just refusing to work.
 

Paul Ramsbottom

Aug 12, 2016
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After writing the above posting I thought what the hell and decided to connect the main 'scope up with the crowbar disabled and it worked. I would like to get it working properly to be on the safe side. I assume the first place to go would be a new SCR but there are no markings on it as far as I can tell. Can you suggest a suitable replacement?
 

(*steve*)

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Sorry about the delay getting back to you.

  1. Did you disconnect the SCR or the connection from the protection circuit to it?
  2. Is there still any clicking?
  3. Can you get a picture of the SCR? I don't imagine it's overly critical.
 

Paul Ramsbottom

Aug 12, 2016
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No problem at all, Steve, I am very grateful for your help.

I disconnected two of the legs of the SCR as it wasn't apparent how to disconnect the protection circuit.

There is a very quiet clicking when the power supply is not connected to the rest of the 'scope, but it is impossible to tell when it is connected due to the noise the fan makes

Here is a photograph of the SCR

Thank you

Edit. When I looked at the photo I realised I was looking at the back of the SCR. I have since removed it and it is an MCR 72 3.

Paul

PaulPower supply SCR.jpg
 
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(*steve*)

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Here is the datasheet on that SCR.

Remove that SCR completely and measure the resistance between all combinations of all three leads (there should be 6 measurements. Do this on the diode test range on your meter.

the 6 tests are red on pin 1, and black on pin 2, then 3 (2 tests). red on pin 2, black on pin 3. Then repeat with the leads reversed.

I'm most interested in the readings involving both pin 1 and 2, but the others will be useful too.

MCR72's are easily available in the UK. RS will sell 10 of them to you for 80p each. This device is simply a sensitive gate SCR, you don't necessarily need to get this one, but don't buy another type without checking with us. This all assumes that there is something wrong with the SCR.
 

Paul Ramsbottom

Aug 12, 2016
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Thanks Steve. All the tests show open circuit except Pin 1 Black and Pin 3 Red which shows 593.

Paul
 

(*steve*)

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All the tests show open circuit except Pin 1 Black and Pin 3 Red which shows 593.

In that case the problem is not just that the SCR is dead.

The fault may be in the protection circuit.
 

Paul Ramsbottom

Aug 12, 2016
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Might it be one of the zeners or is it likely to be more complex than that? The 'scope doesn't get a lot of use and I am tempted to run it with the protection circuit disabled unless there is something fairly straightforward I can try.
 

(*steve*)

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I'm still inclined to think that it may be due to excessive ripple on the power rails, but that may be because I'm not greatly familiar with the protection circuit.

Having the protection circuit disabled isn't the end of the world. I wouldn't leave it on when unattended, and beware that a subsequent power supply fault could take out the scope.
 
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Paul Ramsbottom

Aug 12, 2016
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I suppose that now it is working again I could use it to look at the ripple on its own power supply, Leave it with me and I will try to get some pictures.

In the meantime, many thanks for all your help and patience with this problem. Similar thanks to Edd.

Paul
 

Paul Ramsbottom

Aug 12, 2016
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I wasn't sure whether I could upload videos onto here so I put them on youtube. They are numbered and follow the pin order on page 6 of the PDF. I think you will be able to get them if you search for MrPaulfrazer on youtube.
 

Paul Ramsbottom

Aug 12, 2016
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The 'scope is still working ok without the protection circuit but I am a bit concerned that it is vulnerable to a fault in the power supply. Just a post to ask whether there are any further ideas about finally sorting it out?

Thanks

Paul
 

(*steve*)

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Re-reading the thread to familiarize myself with the problem again...

My understanding is that the SCR has been completely removed from the circuit, and that the power supply operates.

The last image you posted showed 2 leads of the SCR unsoldered, but the remaining lead very close (shorting) to a nearby resistor. I presume that is an optical illusion.

As shown in that image, the pins, if read from left to right go cathode, anode, gate.

If the problem recurs with just the left two pins connected, then I would suspect the SCR. They're pretty cheap and any of the MCR72 variants will work.

If it still triggers with a new scr,, and disconnecting the gate (far right pin) stops it from triggering, then the fault is definitely in the protection circuit.

Faults in the protection circuit are best diagnosed with the SCR removed or disconnected, and we would have to take a good look at the schematic again.
 

Paul Ramsbottom

Aug 12, 2016
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Thanks Steve

It was an optical illusion. In any case I completely removed the SCR. I have ordered a new one and will report the results.

Thank you

Paul
 

Paul Ramsbottom

Aug 12, 2016
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Replacing the SCR hasn't helped so I have taken it out again. I suppose I could just continue to use it without the protection. It doesn't get a great deal of use and I have replaced many components in the supply side so it is unlikely that it will fail by supplying too great a voltage.
 

(*steve*)

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Ok that's great. It means the fault is with the protection circuit.

All we need to do is figure out why.
 
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