HP 54520A PSU repair

[akimpowerscr]

On the schematic of DeepLInk, I see C7 (and not C17) in serie with R4 (22R 5W)

[sv3ora]

Ok we are talking about the same capacitor thanks. Note the schematic I have reverse engineered at the previous post.

My S1 reads 1K by the way in circuit.

Can I temporary remove RT1 thermistor from the mains of the PSU? I am running out of them, so I want to place them there when the PSU finally works.

[SoundTech-LG]

Shotgun replacement of CNY21 (if you find one) is possible (but unlikely) to solve the issue. What did you do to test the part, and determine it was bad?

[sv3ora]

I did not test the opto isolator, I just read what other members here suggested about possible fault.

I replaced C46 and C17 with the same type and value of capacitors (one of them is tandalum). I also temporarily removed the mains inrush thermistor, which was blown every time the fuse was blown, because I am running out of them.
I powered up the scope and it worked for half a minute without blowing up anything. Good that the inrush current is not much to blow out the fuse without the thermistor in place.
Then I checked immediatelly for very heated parts in the PSU and I could not find any. The Q3 and the R16 are heated a bit though, but nothing worrying.

The fan on the scope passes air through the PSU enclosure to cool the heatsinks, so I do not want to operate the psu a lot of time without enclosing it and place it as it should inside the scope to allow for air flow. But I can point a fan onto it as it is now (without any enclosure, just the bare PCB and when/if the fuse blows again, I could check with my finger for overheated parts.

Will this help?

[SoundTech-LG]

No need to be concerned with heat. It can run a long time cool, without forced air before it gets very hot at all (not an issue) at least when it is operating properly. The CNY21 is easy to test. Remove it (carefully) and use your diode tester on your DVM. The LED will conduct one direction with a voltage drop 1-2 volts or so. The photo transistor will test open unless you light up the LED. You should also find an open ckt. between the two components (no leakage). If these conditions are found, it is unlikely it is bad.

[sv3ora]

Ok after the changes I did, mentioned in the previous post, and with an external fan blowing air into the PSU, the fuse was still blown after 4:34 mins, of course without blown thermistor, as there is no such now.
The Q3 is cool ok.

I am going to check the optoisolator the way you proposed, although it seems to me unlikely that this causes the problem after such a delay.

[SoundTech-LG]

Here are some photos I snapped of the creeping Electrolyte under, and into the main transformer. This was a real dog to find, and I almost gave up. It was not so readily apparent with it mounted to the board, but certainly tested leaky.
Oh, and it had the same symptom as yours, but instead, blew the TiPL755A after about 4-5 minutes.

[sv3ora]

Ok you convinced me. I am going to remove the transformer and check.
Wait for my update.

[SoundTech-LG]

Can't hurt to have a look. BIG soldering iron. Metal strips coming off transformer to ground plane. Also plastic insulators on the pins standing off the transformer to board, easy to lose. Before you remove it, you should test leakage across primary to secondary. Should be practically unmeasurable resistance. You may have to suck the solder out of the connections to really be sure. Good luck.

[sv3ora]

It's true that the board is very clean and I cannot see any signs of fluid or corrosion by looking veeery carefully in different light angles and in backlight under the PCB. I look carefully underneath the transformer and I cannot notice anything. and the board by the sides of the transformer is very clean and shiny.

How did you fixed yours?

[SoundTech-LG]

I replaced both transformers. Both had leakage across primary to secondary. You need to confirm little, or no leakage. If tests good, then no need to remove them.
Got the transformers from a donor board.

[sv3ora]

I am not sure, but I checked with the multimeter in the auto range all the pairs of the primary/secondary pins and I find no leakage (high resistance value) in both transformers.

The only place I can't check for liquid, is at the base of some secondary capacitors, cause these have silicone glue on their base.
I quess I have to remove it or them and check.

I will also check this optoisolator the way described tomorrow and let you know.

[SoundTech-LG]

You are ok if you measure no leakage. You certainly want to replace ALL the 1000 uF 35V caps at the very least. They are the ones which really leak out. Sometimes it is hard to see the electrolyte on the board. It depends how long it has been there. I have seen it clear and really hard to notice (until it starts corroding everything).

[akimpowerscr]

No need to be concerned with heat. It can run a long time cool, without forced air before it gets very hot at all (not an issue) at least when it is operating properly. The CNY21 is easy to test. Remove it (carefully) and use your diode tester on your DVM. The LED will conduct one direction with a voltage drop 1-2 volts or so. The photo transistor will test open unless you light up the LED. You should also find an open ckt. between the two components (no leakage). If these conditions are found, it is unlikely it is bad.

It is quite impossible to detect a loss of gain of an optocoupler in this way.
However, it is precisely this defect that could cause the problem that sv3ora notices.

Anyone with a bit of experience repairing devices with optocouplers would start by replacing the optocoupler without hesitation.

[akimpowerscr]

I am not sure, but I checked with the multimeter in the auto range all the pairs of the primary/secondary pins and I find no leakage (high resistance value) in both transformers.

The only place I can't check for liquid, is at the base of some secondary capacitors, cause these have silicone glue on their base.
I quess I have to remove it or them and check.

I will also check this optoisolator the way described tomorrow and let you know.

The insulation faults of a transformer must be measured with a megger, under voltage of 250V or 500V, not with a multimeter.

The multimeter measurement only makes it possible to detect obvious faults, such as a clear short-circuit between primary and secondary and is therefore totally insufficient.

[sv3ora]

I suspected it so, but it's too dangerous to measure when live!
I will order a new optoisolator and replace it just to make sure. If it isn't at least we would have excluded this to be the fault.
I will keep you updated.

[akimpowerscr]

If it is an earth leakage due to a faulty transformer insulation, the earth current protection of your electrical installation should cut off the current to your electrical network.

Do you have such protection (10 or 30mA) against earth leakage currents?

Try powering your oscilloscope with an isolation transformer and check if the fuse blows again or not.

[sv3ora]

I am not sure I have, but if regulations here specify a must have, I should.
Problem is that I do not have an isolation transformer of that power. All these tests (isolation transformer, variac, scope) require instruments that I currently don't have. Perhaps it is a good investment to buy some in the future.
so simple things like the multimeter, capacitance/inductance meter, frequency meter is all I have now. I used this scope for measurements, but since it failed...

I am ordering another optoisolator.
Oh by the way, can I use another type of optoisolator temporarily as a test, like for example a 4N25, or I have to stick with this particular model?

[akimpowerscr]

It doesn't seem logical to burn out fuses and NTCs and not damage transistor Q3.

The overcurrent therefore seems not to pass through Q3.

There remains one point that has not been verified: the VR1 and VR2 spark gaps.

If an arc bursts on one of the spark gaps, the other will also strike and there will therefore be a dead short circuit of the dc power supply, which would immediately burn the fuse.

Replace CNY21 by the same model.

[sv3ora]

I have changed these spark gaps, they are of the same type, brand new. I have also changed the big electrolytics after the rectifier bridge. All brand new.

[akimpowerscr]

If we add your lack of measuring instruments and equipment with everything that has already been replaced, we gradually evolve into a problem without solution.
 

[sv3ora]

There are always solutions.
One will be replacing everything, although the cost would be high
Even if this can't be fixed, there may be a member who has a failed scope and wants to sell his PSU.
Anyway I will try this opto isolator to see if this is the problem.

[akimpowerscr]

When I talk about a problem with no solution, it refers to finding the fault in this power supply and fixing it.

Apart from this, of course, there are always solutions, for example, to buy another oscilloscope.

Replacing everything is not that easy, for example, the transformer certainly should not be easy to find.

There is essential information that you are not able to provide.
1) is the overcurrent which causes the fuse to burn a current between phase / phase or phase / neutral or a current between phase / earth?
2) how is your distribution network configured in Greece?
It is, I imagine, 220V or 230V, but is it 220V between phase and neutral or 220V between phase and phase?
3) Does your installation have protection against earth leakage currents? Find out more or go see your electrical panel.
4) Have you checked your mains voltage? Are there no transient overvoltages?
This information would be essential in trying to understand what is going on ....

Intermittent failures that occur in random times are very difficult to diagnose.

And C2 and C47, are they really X2 capacitors? Have they never been replaced?

Remove them to see if the fault continues or not without these capacitors.

[akimpowerscr]

I still don't understand how it is possible to burn out multiple fuses without damaging transistor Q3.

A fuse does not normally protect a transistor.

The only semiconductors that can be protected by a fuse are diodes, thyristors and triacs, and special, ultra-fast fuses are required with an I²dt at least 30% lower than that of the semiconductor to be protected.

[DeepLink]

Replacing components from one end - is not the proper way of repairng

Have you measrued the output voltage of any of the outputs?

I suspect that it could be an overvoltage on the output, which triggers the SCR (A2) and kills the +15,5V
As the supply runs for a few minutes before it blows the fuse, it could be that the output voltage creeps slowly up
which could be a worn out opto coupler (CNY21) with low CTR, or an overload on the +5,15V from the scope itself

Again the best way to repair this is to do it out of the scope (this way you dont destroy the rest of the scope when something goes wrong)

Plan your measurement
Attach your DMM
Turn on
Record/observe
Turn off
Evaluate
Plan your next measurement
....