HP 54520A Blown Fuse Problem

I will try to search on the web, to find information about checking electrolytics but also the polypropylene HV types. If you have any hints let me know

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My best hint is don't follow this line of investigation. A component failing after a minute with 300+ Volts across it is unlikely to show a problem with a low voltage DVM probe. You would either have to replicate the operating conditions or just replace it anyway. Given their relatively low cost I think replacing is by far the best option. Even if not 'faulty' there is a good chance they are now out of spec after all those years.

Brian.

suggestion:
find an identical 'scope on ebay or a used equipment place
get it for spare parts
if it turns on, but the 'scope doesn't work, you have a new power stage

useless suggestion:
when all else fails, go to destructive testing
replace the fuse with a bigger fuse or a short
this forces a component to be the fuse
then hope that the component(s) that blow up are the bad ones,
and they don't take other things with them
this is not a good method

The output voltages and the crowbar circuit should be checked first.
There are 8 different voltages involved in the crowbar triggering. 7 of them can be adjusted.
The +15.5V output can't be adjusted, and it is the voltage that is shorted by the crowbar.
The purpose of the crowbar is to blow the fuse when there is an overvoltage.

Edit: maybe R7 on the primary side should be used to adjust the +15.5V output.

std_match said:

The output voltages and the crowbar circuit should be checked first.
There are 8 different voltages involved in the crowbar triggering. 7 of them can be adjusted.
The +15.5V output can't be adjusted, and it is the voltage that is shorted by the crowbar.
The purpose of the crowbar is to blow the fuse when there is an overvoltage.

Edit: maybe R7 on the primary side should be used to adjust the +15.5V output.

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What is the crowbar?

What is the crowbar?

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SCR A2, on page 2 and page 7.

There is no R13 and R14 in my circuit, instead the line is connected directly to c3. So I think I am going to desolder the q3 as Brian suggested. Maybe disconnect R12 too.

betwixt said:

My best hint is don't follow this line of investigation. A component failing after a minute with 300+ Volts across it is unlikely to show a problem with a low voltage DVM probe. You would either have to replicate the operating conditions or just replace it anyway. Given their relatively low cost I think replacing is by far the best option. Even if not 'faulty' there is a good chance they are now out of spec after all those years.

Brian.

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Well, I desoldered Q3 completely. I also desoldered R12. I also changed C22 and C23 with other used ones I had.
The PSU still blows the fuse. So thankfully I hope it is something in the HV side and not related to the secondary. I say thankfully, because this is an easier fix.
So first I will replace this cracked thermistor, before trying any other components pulls in the HV side.

What's your thoughts, am I in the right path?

Measure the voltages over C22 and C23 before the problem happens. The center voltage should be stable and half of +V.
If the leakage is too high, the center voltage can slowly drift away and cause problems after a while.

std_match said:

Measure the voltages over C22 and C23 before the problem happens. The center voltage should be stable and half of +V.
If the leakage is too high, the center voltage can slowly drift away and cause problems after a while.

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Ok, but I have replaced these caps with other used ones I had and it still blows the problem. So I do not think this is the fault. But thanks for the hint.

Hello neazoi,
You need to replace the thermistor. It cracked for a reason.
Thermistors are notorious for going short circuit.
Your case is a rare one. They normally short immediately.
After many years, or even within a shorter time, they do change their
molecular structure and no longer work as they should. Varistors
also fail, and sometimes can cause a fire.
Make sure the 4A fuse is a time-delay type (Slow-blow).
Regards,
Relayer

betwixt said:

My best hint is don't follow this line of investigation. A component failing after a minute with 300+ Volts across it is unlikely to show a problem with a low voltage DVM probe. You would either have to replicate the operating conditions or just replace it anyway. Given their relatively low cost I think replacing is by far the best option. Even if not 'faulty' there is a good chance they are now out of spec after all those years.

Brian.

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Hi,
I have disconnected R12 and Q3 (isolating the input of the PSU completely) and the fuse is still blown.
So the problem is somewhere on the input side of the PSU.

Now, I have replaced the thermistor with a new one of the same type, although the new ones I bought seem that have a slightly lower diameter disc. Fuse is still blown.
I have replaced the C22 and C23 with new ones just in case. Fuse is still blown
I have replaced C1 and C51 (the caps near the input common mode choke) with 1.5uF instead of 1uF. Fuse is still blown with a bigger explosion (broken glass). At this point I think I have blown the thermistor (I am not sure) because it opened at half. Since this was not a capacitor problem , I reinstalled the original 1uF caps there.

At this point I need to ark are the materials in the thermistor really toxic especially to breathe, since I accidentally blown it? I was in another room during the big bang, but I came back after the explosion.

Then I replaced the C3. fuse is still blown.


The only components that I have not replaced is the bridge and the varistor. But I measure the bridge in circuit and it seems right. Could one of these this cause the problem of blown fuse after 1 minute or so?

Try to connect an old-fashioned light bulb instead of the fuse. If you are lucky the light bulb will stay off until the problem happens, and it will turn on when the fuse normally blows.
If it stays on, you can try to find the current path by measurements.

std_match said:

Try to connect an old-fashioned light bulb instead of the fuse. If you are lucky the light bulb will stay off until the problem happens, and it will turn on when the fuse normally blows.
If it stays on, you can try to find the current path by measurements.

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I think is is the bridge rectifier. After I deslodered it, no fuse was blown. I have replaced all other parts except the varistor and the fuse was always blown. So it must be the bridge.

This is the first time I have come across to a bridge that delays to fail, even under no load!
That's new to me.

betwixt said:

My best hint is don't follow this line of investigation. A component failing after a minute with 300+ Volts across it is unlikely to show a problem with a low voltage DVM probe. You would either have to replicate the operating conditions or just replace it anyway. Given their relatively low cost I think replacing is by far the best option. Even if not 'faulty' there is a good chance they are now out of spec after all those years.

Brian.

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Hi,
the problem was not the bridge
after a few seconds or tens of seconds it blows the fuse.
I have located the problem in it's switching mode PSU module, because when I disconnect it from the rest of the scope, it still blows the fuse.
Here is a schematic of the very similar model **broken link removed** which you wave sent me through this thread and I have checked it to be very similar inside to mine.

This PSU has a switch as shown, but externally to it, there is another RFI filter can which plugs directly to the mains socket. It is the fuse inside this RFI filter that is blown, not the internal fuse on the PCU module. I have tested this external RFI filter without any load (disconnected from the PSU module) and it does not blown it's fuse, so I do not think it is this filter's problem.
Then I have tried to locate the problem in the PSU module by removing components. The mains side of the PSU (page5) is almost identical to mine. The only differences are that the input mains capacitor in mine is a varistor (275v/20A), and instead of the varistor shown in the schematic, mine has a thermistor. Also R13 and R14 are not presend in mine. All the other things are the same.

Now, I have tried to isolate the HV mains side by disconnecting R12 and q3 completely. The external mains fuse is still blown. So I thought it would be this part of the PSU that has the problem. I changed C22 and C23, fuse still blown. I changed C1 and C51 near the RFI filter, fuse still blown. I changed CR1 with an identical one, fuse still blown. I temporarily removed C3, fuse still blown. I have replaced the thermistor with an identical type, fuse still blown. I have completely removed the varistor, fuse still blown.
Sometimes the thermistor cracks and blows away completely!
Note the fuse shown in the schematic is not blown, it is the fuse inside the external RFI filter can (not shown) that blows away, and the thermistor of the PSU module cracks.

I am starting to get pissed of with this thing, I have almost changed every component and still nothing.
The scope powers up on but after a few seconds/tens of seconds, blows the fuse (and the thermistor sometimes).
Maybe an ingenious man here can give me some useful advice of how to save this scope!

It can be an isolation problem in the common mode choke L1. Remove it and bypass with two wires. If the scope works, get a replacement choke, or rewind it yourself on the existing core..

I would give the power supply to someone who knows how to fix power supplies. If you want to try and fix it yourself, then from what you have said there maybe no fault with the power supply, rather the adjustment of the protection thyristors maybe the problem. Try removing A2 to A5 and see if the fuse still blows.

pjmelect said:

I would give the power supply to someone who knows how to fix power supplies. If you want to try and fix it yourself, then from what you have said there maybe no fault with the power supply, rather the adjustment of the protection thyristors maybe the problem. Try removing A2 to A5 and see if the fuse still blows.

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The crowbar was also my thought in post #23, but since it is on the secondary side and the fuse blows even with the main switch transistor Q3 removed, it must be something else.

std_match said:

It can be an isolation problem in the common mode choke L1. Remove it and bypass with two wires. If the scope works, get a replacement choke, or rewind it yourself on the existing core..

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I have removed the common mode choke and put wires in it's place. Fuse still blown At least the thermistor did not blow, but it may be lust luck.

I remember I had tested the external RFI filter can (not shown in the schematic) alone (without connecting it to anywhere and it's internal fuse did not burn. Maybe under load something is happening on that external RFI filter?

Would that be wise to connect directly the mains to the input of the PSU and completely bypass the external RFI filter can?

But if this external filter is the problem, why the series thermistor blown out, without blowing the fuse shown in the PSU schematic? (it is the RFI can filter fuse that is blown out)

betwixt said:

That would be my guess too Susan.

The fuse looks to be in the AC input to the PSU and as a fault in the output side would probably result in shutdown rather than a blown fuse, my suspicion would be the input filter up to the two big black reservoir capacitors.
It doesn't appear to have a PFC circuit so there is probably a direct connection from the big transistor's collector, through the transformer to the positive side of one of those capacitors. See if you can break the link or failing that, isolate the transistor collector connection (NOT the base or emitter unless you can do both!) or remove it completely and see if the fuse still blows. If it does, move step backwards and see if you can isolate the bridge rectifier by removing it or unsoldering both AC legs.

Brian.

update: I found this on the internet, please note the caution on it. It looks promising it is the same PSU:

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Can it be the gas discharge tubes near the big electrolytics? These are the only components I have not changed in the HV side. If they fail they could short circuit the output and thus blow the fuse.
I read now that these have limited life span ,and since the scope is second hand 30 years old...
But could their short be delayed, after a few seconds or tens of seconds? Have you ever encountered failures in these?

UPDATE
I think I have found the problem. It seems to be indeed these gas discharge tubes. Then I remove both of them completely there is no blown fuse anymore. I have never thought these can fail, but the info I read on the net confirms that they have a finite life span, less than other components!