HP 3478A Transformer Winding

[salvodi89]

Hello everyone!
I'm Salvatore from Italy and I'm new in the forum.
I bought an used HP 3478A multimeter from ebay and today it arrived... but... It doesn't work!
I think it's a problem with the transformer becouse when I try to measure the primary winding I get an Open Circuit!
Can anyone give me what he measure?
I need 220V winding...

Thanks everyone!

PS: The forum and the video are wonderful!

[ddavidebor]

Ciao, domanda non scontata, hai misurato direttamente sul trasformatore o sei passato per i vari circuiti di filtraggio e protezione?

Hi, do you have measured right on the transformer or on somewhere else?

[salvodi89]

Yes! I measured directly on the transformer (I'm not so stupid  ).
Anyway there is no mouch filtering on the primary side of the transformer.


I can add more details...
- 46-50Ohm on the 100V tap (between 6-7 on primary side of the transformer).
- 50-55Ohm between 8-9
- Anywhere else open! 

[madshaman]

Hi, funny story, I've also just purchased an HP 3478A from eBay that doesn't work and am trying to fix it (post in this topic).

I'm wondering what kind of failure would cause the primary to become an open circuit inside the transformer?

From what you're saying, between 7 and 8 you've got an open circuit (and I assume of course it's an open circuit between 6 and 9, which is what you want)?

One suggestion, but maybe not a smart one given there might be damage inside the transformer, is if you've got a variac, you could try to drive the 100V tap.  I'd probably try to remove the whole transformer to test it.  I hope I don't have a similar problem as there are a whack of secondary taps (looks like six).

I'm wondering how easy it would be to find a replacement for that transformer; doesn't look hopeful.  Personally I'd be loathe to operate anything from a transformer that looks like something bad happened inside.  If you're feeling brave you could always tear it apart and try to rewind it ^^'.

I'm going to try and video myself as I try to troubleshoot and repair mine, can't make any promises on how long that would take me to edit and post though.

Good luck!

[alm]

Transformers often contain an internal thermal fuse. Not sure if this applies to the 3478A, but it's worth a shot. The fuse is usually user replaceable with some care.

[kripton2035]

I had the same problem in an elv 8500 battery charger I bought intentionnally broken.
I measured the primary and get infinite ohm
you must dismantle the transformer slowly, and there is a fuse on the primary side after some turns.
change the fuse and everything comes back.
there are videos on youtube to explain how to open a transformer. you must first remove one metal sheet after that all the sheets come out
good luck.

[madshaman]

Nice, learned a couple things already today.  Did some digging on that transformer as well, apparently its NSN is 5950-01-166-2928; might even be able to find a replacement.

Question: if the fuse blew in the first place, what are the odds that another problem in the meter is what caused it and that the fuse would just blow again right away?

[ddavidebor]

Fuses can blow without any reasons, is not unusual.

Sometimes, you turn on something that has not been used for a long time and the fuses blow... But when you change it all work fine.

[salvodi89]

From what you're saying, between 7 and 8 you've got an open circuit (and I assume of course it's an open circuit between 6 and 9, which is what you want)?

Yes, you're right. I need 220V winding.

One suggestion, but maybe not a smart one given there might be damage inside the transformer, is if you've got a variac, you could try to drive the 100V tap.  I'd probably try to remove the whole transformer to test it.  I hope I don't have a similar problem as there are a whack of secondary taps (looks like six).

Yes, I will do tomorrow! For now I haven't a variac adn it's too late... I don't know if I will buy a variac or a 220V to 100/110V transformer...
For now I tryed with 24V on 100V winding of the transformer and I had the correct output at the secondary.

[lowimpedance]

Measured 110 ohms between pin 6 and 9 directly, 46 ohm on the 100v tap pin 6-7.

AC volts on the secondary pins. (refer to schemattic)

pin 12 - 11.4V , pin 13 - ground ref. , pin 14 - 11.4V.

flying lead 2 - 23V,
flying lead 6 - 11.4V
flying lead 1 - 0V ref.
flying lead 6 - 11.4V
flying lead 2 - 23V
Above volts all measured on my 3478a with 240V mains (nominal).

[salvodi89]

Measured 110 ohms between pin 6 and 9 directly, 46 ohm on the 100v tap pin 6-7.

AC volts on the secondary pins. (refer to schemattic)

pin 12 - 11.4V , pin 13 - ground ref. , pin 14 - 11.4V.
flying lead 2 - 23V,
flying lead 6 - 11.4V
flying lead 1 - 0V ref.
flying lead 6 - 11.4V
flying lead 2 - 23V
Above volts all measured on my 3478a with 240V mains (nominal).

Thanks a lot!
I resolved my oroblem just 10 minutes ago with a small hack!
Tomorrow i will check the outputs and post some photo. For now I'm very happy 

[madshaman]

You shorted 7&8 didn't you , just a guess.

[salvodi89]

You shorted 7&8 didn't you , just a guess.

More o less: I shorted 7 and 8. Now I have about 110Ohms between 6 and 10 (240V winding) as said lowimpedance.
So I can use 240V winding instead of 220v.

Here is a photo:



The first problem I found when it arrived at home was that the 6 pin of the transformer was not connected to the board (probably due to the shock during the transport).

Now on the unregulated 5V i measure 9.5 - 10V. What do you think, is it normal?

[lowimpedance]

 I tried the same arrangement as you , ie shorted pins 7 and 8 and measured the resistance from pin 6 to 10 (240v tap) and indeed get 110 ohms,
just as if you were using the 220v tap!. So it looks like these windings are still good for now!!.
Just an aside have you checked to make sure there is still good resistance insulation to the laminations ?.
I wonder since you had mechanical damage to pin 6, perhaps the open winding is also a mechanical fracture (near the terminations??).
Any way I measured the unreg 5v rail at 14,5 odd volts on my 3478a. I would be still quite comfortable with the values you measured.
Im sure you have checked the 5v rail for value and no ripple.

[salvodi89]

I tried the same arrangement as you , ie shorted pins 7 and 8 and measured the resistance from pin 6 to 10 (240v tap) and indeed get 110 ohms,
just as if you were using the 220v tap!. So it looks like these windings are still good for now!!.

I hope It will work for long time!

Just an aside have you checked to make sure there is still good resistance insulation to the laminations ?.

Even if I don't think this is a measure to do with the multimeter and the transformer in place I tried and it's ok!

I wonder since you had mechanical damage to pin 6, perhaps the open winding is also a mechanical fracture (near the terminations??).

No they are well connected, I made all resistance measure near the winding soldering point.

Any way I measured the unreg 5v rail at 14,5 odd volts on my 3478a. I would be still quite comfortable with the values you measured.
Im sure you have checked the 5v rail for value and no ripple.

Thanks for your measures. In the schematic there isn't any information about the voltage output from the transformer.
Yes, no ripple!

[retiredcaps]

A somewhat related repair video.

[salvodi89]

Something has gone terribly wrong!
After some weeks I repaired the transformer, the fuse is blown!
When I opened the multimeter the transformer was very hot, primary winding is shorted.
Any idea? Transformer replace?

A somewhat related repair video.

It's terrifying when he measures the main with so long tips!

[madshaman]

I actually put on safety glasses when I'm probing around anything that could draw a high current (like accidentally shorting mains to ground), I don't relish the idea of molten copper landing on the surface of my eye.

[TangentAudio]

That's actually my repair video.  Glad I could strike terror into the hearts of my fellow engineers.   Those long probes are insulated all the way down to the very tip.  I actually chose them because of that feature, versus shorter probes which had more exposed metal.  I felt perfectly safe doing it, otherwise I wouldn't have done it.  Just made sure I wasn't too jittery from coffee (or lack thereof).

Also, for what it's worth I wear glasses.  They're not as good as safety glasses, but they are always on my face and do provide some protection.