Stromberg Carlson Model AU-57 OT Testing

[eurekaiv]

A friend found one of these at the flea market and gave it to me to check out. In going through and testing the transformers to see if they're good, it looks like the output secondary is not wired as it noted in the schematic, which the amp I have seems to match otherwise. Was hoping someone would be a second set of eyes to make sure I'm not making a mistake. The schematic can be seen in the topic linked.

https://el34world.com/Forum/index.php?topic=13654.0

Based on my measurements, it looks like they attached the 16Ω wire color (orange) to the 4Ω tap (yellow) and vice versa. I have a spreadsheet that calculates this for me and have noted the math I'm using in the attached image.

[DeepBias]

Hi eurekaiv,

Not sure to understand what you did, you have injected 1V at primary winding to get higher voltages at secondary? It's usually the opposite, OT converts the high voltage low current signal from the power tubes (primary) into a low voltage high current signal (secondary) to drive the speaker. Maybe you have the results reversed in the title bar ?

If the primary/secondary voltages are reversed, based on the results your math is correct.
 

[eurekaiv]

The secondary voltages are in millivolts, just didn’t label it that way since I know that’s what it should be and this is a doc for myself. I should change it though.

So math being correct, whoever built this transformer screwed up. The amp has a shorted output section and is in miserable condition otherwise but the transformers are all good. I wonder if it was broken thanks to this potential impedance mismatch and then discarded.

[DeepBias]

If the secondary voltages are in mV, here you go :

Yellow  -> Primary : 1V  Secondary : 0,0235V  -> turn ratio : (1 / 0,0235) = 42,55  -> 42,55^2 X 4 = 7242 ohm
Green   -> Primary : 1V  Secondary : 0,0332V  -> turn ratio : (1 / 0,0332) = 30,12  -> 30,12^2 X 8 = 7257 ohm
Orange -> Primary : 1V  Secondary : 0,0464V  -> turn ratio : (1 / 0,0464) =  21,55 -> 21,55^2 X 16 = 7430 ohm

Edit : I wrote A instead of V lol (corrected) ... it's late here 
 

[SEL49]

The secondary voltages are in millivolts, just didn’t label it that way since I know that’s what it should be and this is a doc for myself. I should change it though.

So math being correct, whoever built this transformer screwed up. The amp has a shorted output section and is in miserable condition otherwise but the transformers are all good. I wonder if it was broken thanks to this potential impedance mismatch and then discarded.

All your math is wrong! You must use the same voltage units for the primary and secondary for the math to be valid. IOW, if you use volts in the primary you must use volts (not millivolts) in the secondary.

[eurekaiv]

Thanks!! I figured it would be my mistake, it usually is. Glad I asked.