Should a Split/Tran(tm) DST-7-48 Signal Transformer get really hot?

Thread Starter

treehouse3911

Joined Feb 6, 2016
10
Hi all;

I'm trying to diagnose a power supply that includes two Split/Tran(tm) signal transformers: a DST-7-36 and a DST-7-48. When connected to the circuit boards it powers (from a TAS 101 Telephone Network Simulator), the power supply is blowing its 6A/250V fuse.

When the power supply is isolated from the circuit boards, I've noticed that the DST-7-48 transformer gets quite hot, but the DST-7-36 does not get warm at all. I left it powered up only for about 60 seconds and within that time, without any load, the DST-7-48 warmed up significantly.

Would this be indicative of a fault in the DST-7-48 transformer? A shorted winding perhaps? Is there a diagnostic technique I could use to assess its condition (I could remove it from the board, if necessary)?

Many thanks,
Tim
 

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BillB3857

Joined Feb 28, 2009
2,506
Although it could be a bad transformer, I would first disconnect ANY load. That would include disconnecting the rectifiers from the secondary. A shorted diode in the bridge, or a shorted capacitor down stream could also overload the transformer causing it to heat up.
 

Thread Starter

treehouse3911

Joined Feb 6, 2016
10
Although it could be a bad transformer, I would first disconnect ANY load. That would include disconnecting the rectifiers from the secondary. A shorted diode in the bridge, or a shorted capacitor down stream could also overload the transformer causing it to heat up.
Hi Bill,

Thanks for your quick response. In looking at the board, I'm not sure I can readily identify any rectifiers, but a little more disassembly might be required. If I am able to isolate the transformer, what tests might I perform that would confirm it normal status? Is it a question of checking for continuity on each of the coils? A resistance of the coils?
Tim
 

Dodgydave

Joined Jun 22, 2012
8,572
Measure the output voltage of the transformer, if you know what they should be, or if not remove the secondary side one at a time and monitoring the primary current, my guess is a faulty tantalum capacitor.
 
Last edited:

BillB3857

Joined Feb 28, 2009
2,506
The rectifiers I would check are CR7,CR8,CR9 and CR10. They comprise the bridge rectifier that will feed the filter capacitors. Trace the foil paths and two paths will lead to the transformer. Look for a trace that ties two diodes together at the banded end. That should tie to the PLUS side of a capacitor. Also, there should be a tie between the other two diodes un-banded end. That should tie to the NEG side of the capacitor. If any one of the diodes are shorted, the transformer will get too hot.
 

Thread Starter

treehouse3911

Joined Feb 6, 2016
10
Thanks to both, Dave and Bill, for the pointers! That at least gives me some hope of salvaging this unit.
Will let you know what I uncover.

Tim
 
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