Newly purchased UTC A-48 Hybrid Transformer- It it busted? [DCR measurements listed]

[poppaflavor]

Hi All,

I wasn't sure if I should post this here or in Magnetics forums; The Lab seemed to be about troubleshooting so here I am

I recently picked up a UTC A-48 Hybrid Transformer and I think it is faulty based on DC resistance measurements and trying to pass audio through it.

But since hybrid Xformers are a bit atypical and I'm fairly new to the DIY space I am hoping that you all might be able to give feedback; I don't want to contact the seller and indicate it's busted until I've exhausted all potential checks.

I tried to check historical posts here but unfortunately all the images are no longer working so I can't assess expected results. For example: UTC A-48 Hybrid Transformer

In a nutshell, there is a continuity with low ohm resistance (~0.1 Ohm) between windings that I do not think should have continuity based on the schematic present on the Xformer case. Please see below:

For reference here is a schematic of a Hybrid Transformer all set up for action (from wiki):

Here is the UTC A-48 schematic which indicates that all of the windings should be separated and non-conductive (the windings are within the dashed line box but the inter/intrawinding connections are outside the dashed line box):

Here are my DC resistance measurements along with a more cogent schematic. To cut to the chase see the "Expected" column. It is the 5-7 connection that is baffling, and the connections associated with it. In particular the 5-8 winding connections do not respond as the 9-12 windings connections do, and the latter respond exactly as expected from the schematic.

Any pair of connections for which a resistance is not explicitly listed in the table below showed no continuity (open lead).

I'm a hare's hair away from contacting the seller for a refund unless someone can indicate that these results are somehow consistent with an intact UTC A-48 hybrid Xformer.

Thanks in advance for any thoughts.

 

[Whoops]

Hi,
by your DCR measurements it seems to me there's a problem between 5 and 7 connectors,
it seems 5 and 7 are shorted somewhere.
If they are actually shorted then your DCR measurements are correct,
check photo below

 

[poppaflavor]

Hi,
by your DCR measurements it seems to me there's a problem between 5 and 7 connectors,
it seems 5 and 7 are shorted somewhere.
If they are actually shorted then your DCR measurements are correct,
check photo below

View attachment 108530

Aha! Lightbulb moment. Thank you for drawing out the short and the straps.

That really helped me visualize what you're describing. I was having trouble imagining the connections which might head to these DCR results, and this makes it clear now. While it's a bummer to get this result, I'm happier knowing what's going on even if it's not good news rather than having a mystery on my hands.

Much appreciated!

 

[CJ]

You might try and resolder the terminals that are giving you trouble.

A common problem with utc's is the internal lead wires slipping out of the terminals when someone hooks external leads to the transformer.

Look to see if you can spot the internal leads under the solder on the terminals. Sometimes you can see a few strands sticking out.

 

[poppaflavor]

You might try and resolder the terminals that are giving you trouble.

A common problem with utc's is the internal lead wires slipping out of the terminals when someone hooks external leads to the transformer.

Look to see if you can spot the internal leads under the solder on the terminals. Sometimes you can see a few strands sticking out.

Thanks CJ, I'll give this a try- there are some internal strands poking out from the hole.

I've actually noticed this on a couple of other transformers that the internal lead strands seem to have fallen back inside.

Wouldn't some strands need to be external to the hole for a re-soldering to restore the connectivity? And is there any way to fish them out of the hole when there is none exposed?

 

[CJ]

You can pop the terminal plate and rewire the terms, I think the a- 48 has a soldered header iirc, which is a pita,

If not you can drill the pipes in the four corners and glue the lid back on.

The hot tar bubbles into the solder joint during the unsoldering process making life smelly and difficult.

 

[poppaflavor]

You can pop the terminal plate and rewire the terms, I think the a- 48 has a soldered header iirc, which is a pita,

If not you can drill the pipes in the four corners and glue the lid back on.

The hot tar bubbles into the solder joint during the unsoldering process making life smelly and difficult.

Oof! I was thinking you might say something along those lines. Thank you for a perspective on how that goes. At this point I can't see myself doing that. Reading through some of your teardown threads has given me an inkling on how that might go. A little beyond where I am at the moment, but definitely an aspirational goal.