Worried by 110v reading on 6.3v tap....

[Zekk]

.... on my newly purchased Hammond 379X power trafo.

Measures 110v across one leg of the 6.3v and ground!?? 😱

However, reading across the 6.3v taps is 6.85v.

The HV 425-0-425v is fine.

I'm trying to regulate the filament supply and needs to know which's the positive and negative tap?

Finally, do I have a problem ?

Thanks a lot, Guys!

Zekk

 

[HollowState]

Measures 110v across one leg of the 6.3v and ground!?? 😱... do I have a problem?

Possibly yes, but need a little more information first. Is the transformer installed in a unit? Is the 110 volts AC or DC? If it's AC and the transformer secondaries are essentially unconnected, then you have a big problem. If it's connected to a circuit and the 110 volts is DC, then maybe not.

I'm trying to regulate the filament supply and needs to know which's the positive and negative tap?

The windings are not polarized since it is Ac. Either one will normally serve as either assuming a good working transformer. This is something we must first ascertain.

 

[googlyone]

try putting say a 10K resistor from any point on the 6.3 volt winding to ground, and measure again.

If you are using a high input impedance meter, you can measure AC voltages like this which are simply capacitive coupling from the primary winding.

If with a load such as 10K to ground you STILL get 110V, then be rather more cautious!

 

[Zekk]

It's a new project, trafo just mounted on new chassis. No wiring done other than the filament and HV leads connected to sockets. All AC voltage.

The other 5v also have the same problem... measures 88vac.
But, across the two taps, it's 5.67vac.

Soldered a 10K to ground, no different!?

Thanks Guy for helping out!

 

[Michael Bean]

It's most likely capacitive coupling as mentioned in a previous reply. Just check and make sure that there isn't any continuity between the primary winding(s) and the secondary windings. If it shows infinite resistance, it's good to go.

Mike

 

[TonyTecson]

.... on my newly purchased Hammond 379X power trafo.

Measures 110v across one leg of the 6.3v and ground!?? 😱

However, reading across the 6.3v taps is 6.85v.

The HV 425-0-425v is fine.

I'm trying to regulate the filament supply and needs to know which's the positive and negative tap?

Finally, do I have a problem ?

Thanks a lot, Guys!

Zekk

is that a transformer only measuring or in circuit?
can you post schematics or model numbers we can look at?

 

[KatieandDad]

I'd go with Mr Bean and check the insulation between the secondaries.

 

[Zekk]

Out of desperation, I removed transformer from chassis plus removing the end bells to expose all primary and secondary wires. Everything is in order except the high readings on both filament supplys.

Both center taps are sealed off with shrink wraps and not connected to anywhere.

😕 😡 😕

 

[cnpope]

When you say "Everything is in order except the high readings on both filament supplys," do you mean that you have carried out the resistance measurements between primary and the heater secondary winding, as suggested by an earlier poster, and that you got infinite resistance? (This, of course, is to be done when the transformer is not connected to the mains!) The answer to this question is crucial for determining whether everything is OK or not.

Also, it was not entirely clear from one of your earlier posts whether you had tried the other suggestion, of repeating the AC voltage measurement with a 10K resistor connected between the 6.3V secondary and ground. Again, the information this would provide is important for determining whether there is a problem or not.

Chris

 

[Zekk]

Eureka!

Thanks googlyone for that great advice to solder a 10K to one leg of the 6.3v. This resulted in a 0.62v at the 10K tap and 6.84v at the other tap. 😀

This also apply to the 5v tap.

Now, do I needs to leave the 10K permanently in place?

I'm still very much puzzled by this and it does not happens to the other trafo!

cnpope... Thanks for jogging me out of my slumber to re-analyse the proper approach to the 10K connection. Initially, I did it wrongly.

You Guys are awesome and am thankful for it all.

Regards, Zekk.

 

[Refugee1]

The 6.3 volt heater winding often has a center tap. This is usually earthed to chassis to get rid of the static.
On some equipment there is a 500 ohm pot with the wiper earthed that you adjust to get rid of hum.
The HT rectifier will have enough capacitance between the cathode and heater to get rid of it anyway.
On the other transformer try reversing the mains wires and you may also see the static on there too.

 

[wa2ise]

... This resulted in a 0.62v at the 10K tap and 6.84v at the other tap. 😀

This also apply to the 5v tap.

Now, do I needs to leave the 10K permanently in place?

...

No need to leave the 10K in place. The 5V winding I presume is for a rectifier tube like a 5Y3 or 5U4 or such. If so, it will find itself at B+ voltage. As for the 6.3V winding, you'll probably ground it via a hum pot, or connect it to an elevated DC voltage, something like 40VDC, this reduces hum (having the heater positive compared to the cathode).

Seems you were just seeing some stray capacitance between those windings and the primary. But you did right making sure that it was only that.

 

[Zekk]

Thanks wa21se for the tutorial.

I've already soldered that 10K neatly in and everthing else.

If it hums, I'll use your suggested approach to elevate it to 40v.

Cheers from the other end of the pond.

 

[nigelwright7557]

Also, it was not entirely clear from one of your earlier posts whether you had tried the other suggestion, of repeating the AC voltage measurement with a 10K resistor connected between the 6.3V secondary and ground. Again, the information this would provide is important for determining whether there is a problem or not.

Chris

SMPS transformers suffer from capacitance too.
They usually add a 220nf in parallel with 4m7 to ground to discharge it.