Dear all,

I have bought a used HP 54501A scope from the USA.

It won't turn on. Then i checked the fuses. The 110V fuse was blown and the 250V fuse was missing.

I put a 250V fuse back into it but it still won't turn on.

Is this because it needs both fuses to turn on, or am I in trouble?

Here's a photo of the fuse socket, there is a fuse on each side of it.



Thanks a lot for your help.

Paul

 

Did you put the fuse module in the 230V position before you applied power?
There is a troubleshooting section in the service manual attached.

 

Did you put the fuse module in the 230V position before you applied power?
There is a troubleshooting section in the service manual attached.

Yes I did.

I didn't hear anything when I first turned it on, so I don't think the first fuse went with me.

 

Since I need to turn the fuse socket upside down to use 250V, then I guess it's not that I need both fuses for it to work. Bad news. I have a new 250V fuse in it but it still won't turn on.

Could it be another fuse inside the power supply?

 

Take the covers off and give it a good inspection for any charred bits (your nose may help too).
Then, if you have a multimeter, follow the troubleshooting in the service manual.

 

Could I have damaged the scope in case the fuse did blow with me ? Actually when I first turned it on I remember it was marking 110V. I put 250V on. I didnt realize. But I didn't actually hear any noise and I was very close to it.

If the fuse blew when I put 250V through it, then the fuse would have protected it right? Or could damage have occured?

This is so sad, I waited 2 weeks for this scope!

 

For the brief time that the fuse survived there would be much greater voltages than normal on the secondary of the mains transformer so yes, damage is certainly possible. Semiconductors generally blow faster than fuses.

 

Bad news. I have a new 250V fuse in it but it still won't turn on.

Applying twice the expected voltage on the transformer primary wold have doubled the secondary voltage which could have blown other fuses and/or damaged components.

 

Could I have damaged the scope in case the fuse did blow with me ? Actually when I first turned it on I remember it was marking 110V. I put 250V on. I didnt realize. But I didn't actually hear any noise and I was very close to it.

If the fuse blew when I put 250V through it, then the fuse would have protected it right?

!

Probably not! - at 110V, you need twice the current to get the same total power, the fuse will be rated accordingly - that's why there's one for 110V and another for 220V.

It looks like the sort of thing that might have a SMPSU, but I didn't find a post that said which.

An iron cored transformer would probably die slowly, hum loudly and probably let the magic smoke out.

An SMPSU will die instantly - usually quite a few destroyed components, repair can be daunting - but not as daunting as getting hold of an original mains transformer.

 

The fuse is to protect against overcurrent in case something fails during normal operation.
It's not designed to protect against applying 240V to the 120V input so, yes, I'm afraid you likely fried something before the fuse blew.
Not hearing a sound doesn't mean anything since most devices will fail quietly.

Do you have a schematic for the oscilloscope to help in troubleshooting?

 

LOL IT JUST TURNED ON !!!!

MAGIC ???


HAPPY !!!!!!!!!!!!!!!!

UAHUUUUUUUUUUUUUUUU

 

It seems this scope will not turn on without a ground connection,

Any explanations for this ?

When I used a plug without ground it didnt turn on.

Then I changed to a grounded plug and it turned on!

 

It seems this scope will not turn on without a ground connection,

Any explanations for this ?

When I used a plug without ground it didnt turn on.

Then I changed to a grounded plug and it turned on!

AC/DC TVs & radios could end up with a live chassis - many engineers disconnected the earth lead in the scope mains plug so it didn't go bang when they applied the scope earth clip.

It could be a safety feature designed to encourage engineers to fork out for a mains isolating transformer.

 

AC/DC TVs & radios could end up with a live chassis - many engineers disconnected the earth lead in the scope mains plug so it didn't go bang when they applied the scope earth clip.

It could be a safety feature designed to encourage engineers to fork out for a mains isolating transformer.

Sorry but I didn't understand what you said. Could you please explain again ? Why would they disconnect the ground if it is supposed to help in case the case gets hot?

 

Sorry but I didn't understand what you said. Could you please explain again ? Why would they disconnect the ground if it is supposed to help in case the case gets hot?

Why did you?

Sometimes ground loops cause noise in the system when the scope ground is connected to the system ground so the test engineer isolates the scope ground to eliminate the noise.

 

I think he was talking about using the scope in AC mains. I am using it for DC electronics, hence why I didnt understand what he said.

Anyway I didn't disconnect the ground, I was using an adaptor that didn't have a ground, then when I used my UK plug that did, it worked. I have no idea why it would work with a ground and not without.

 

Sorry but I didn't understand what you said. Could you please explain again ? Why would they disconnect the ground if it is supposed to help in case the case gets hot?

The original AC/DC sets just had a dropper resistor and rectifier, one of the mains wires was tied to chassis - with a 2 pin plug; there was a 50/50 chance of a live chassis.

The earliest SMPSUs were non isolating - the chassis always had the negative half cycles coming out of the bottom of the bridge rectifier.

Any discussion of constructing these types of circuit would not be permitted by this forum terms of use.