Valve Power Transformer testing (my sanity)

[fusepop]

Hello there all ;

A friend brought over a vintage DIY amplifier that he purchased with a "bad" Power Transformer . He had a schematic as it was built from a magazine article and had previously heard it playing a year before.

Making  resistance checks, I unsoldering some wires and found that the primary of the PT read at 2.7 Ohms ! Wow ! that's close to a dead short ! Should be around 15-25 Ohms I'm thinking.   (Got to be the problem right ?)

Testing the HV secondaries  I found them to be 2.1 Ohms across both ends !(around 1 Ohm from each end to the CT) Should that not read somewhere around the 500-ish Ohms area?! (this is supposed to supply 400V each side of the CT !)

I found the filament windings and they measured 2.6 Ohms ?! Isn't that high ?!

Did this transformer implode? I have never seen this before.

 This is a Thorndarson 22R35, very popular but I have not been able to find winding resistance specs. Maybe I'm losing my marbles.

Wondering if this is not the original PT?

Any thoughts or advice greatly appreciated....

Rick

[font=Arial, Arial, Helvetica]You can also test a power transformer by measuring the resistance between leads. A healthy Hammond 270AX 240-0-240v power transformer measured: 14 ohms between primary leads, 223 ohms between the secondary center tap and HT wire 1 and 250 ohms from center tap to HT wire 2, 0.3 ohms between the 6.3v leads [/font]
[font=Arial, Arial, Helvetica]You can also test a power transformer by measuring the resistance between leads. A healthy Hammond 270AX 240-0-240v power transformer measured: 14 ohms between primary leads, 223 ohms between the secondary center tap and HT wire 1 and 250 ohms from center tap to HT wire 2, 0.3 ohms between the 6.3v leads [/font]

[K O'Connor]

Hi Rick

As a reference, touch the leads of the ohmmeter together to see what your meter says is "zero", then subtract this from low-R readings you make. Obviously, if the reading is in tens of Ohms or more, then the probe resistance is insignificant.

The winding DCR on large PTs can be pretty low. According to the single data sheet I could find this PT is about 262VA total. The plate winding is 190VA and with solid-state rectification could support a 70-80W amp.

What is the recent history of the application? Is this in a restored PSU using oil-filled or paper caps? The latter are complete garbage with limited life and should be replaced with plastic if one wants to avoid electrolytics.

Of course, you should use a Power Limiting Safety Socket to test this unit. The PLSS uses lamps to limit power,; a variac provides no protection at all. Test without tubes initially, and a low-wattage lamp. Pre-connect the meter to high-voltages prior to powering up. A PT with an open primary can often pass the unloaded test and provide output voltage, but then even a tiny load will show up its flaw. You are measuring low resistances, so your concern would be more of winding-to-winding shorts. This might happen with a paper-bobbin PT that has been in a moist environment.

The PT is not necessarily the problem here, so just be methodical and control the power input while doing baby-step-tests.

Have fun

[fusepop]

Hey, Thanks for the reply !

Last night, I got my hands on a Power Transformer from a Healthkit Mono tube amp and proved what you're saying is correct.

The primary winding read 3.2 Ohms and I know it is a perfectly good PT. 
I based my original readings comparing to a Hammond transformer which I think read in the 20 Ohm range.

I've attached a pic of the amplifier for context. I also should have mentioned that when powering up the amplifier initially raw, with no tubes in the sockets the 3A primary fuse shredded instantly. Hence raising questions about a shorted PT.

Thanks again,  and this shows that it's always good to get advice and do further research when possible. 
The quest continues

[K O'Connor]

Hi Rick

What is the amp model? The pic does not expand clearly.

Seems a little odd to have a supply-referenced zener instead of ground-referenced. I guess they want to maintain the difference rather than maintain a specific voltage?

Have fun
Kevin

[makinrose]

What I think is missing here is why we are suspicious of the power transformer in the first place?  What is it doing?  Are you blowing fuses?  Do you have the correct voltages??   As Kevin mentioned this should be done with a limiter.  A meter can help you determine if the transformer is grossly bad. 

GeoFX has a good run down of simple power transformer test you can perform here: Power Transformer (geofex.com)