SMPSU transformer failure - UK Vintage Radio Repair and Restoration Discussion Forum

crestavega · 22nd Feb 2020, 11:00 am

Been doing battle with the multivoltage SMPSU on the back of a Sailor R2022 receiver (12/24V in, +5V, +12V, -18V, +18V +24V out). it was "working", that is, the correct voltages were appearing but the PSU was pulling high amps and the main chopper FET was overheating.

I appear to have diagnosed a malfunctioning main transformer, which is bad news because I don't think Im going to be able to replace it.
but I am suspictious of my diagnosis. both primary windings on the transformer have DC continuity but appear to be short-circuit at 60kHz. The only thing I can think of is that the trafo has got a shorted turn on it somewhere. does this really happen in practice? Ive seen plenty of little trafos fail open-circuit but never seen a shorted turn "in the wild". does it happen?

also would have expected to see a hotspot on the transformer resulting from a shorted trn (I have a cheap thermal camera for problem solving) but I cannot see one (admittedly I cannot drive it for too long because the chopper will expire)

this trafo has 2 sets of primary windings, one of which appears to be redundant in this application. I have pulled the trafo out of the board and applied drive to either winding with the same overcurrent symptoms.
also on the primary side there is a strange "other" wire coming out the core which is grounded in the design. would this be a shield layer between pri and sec?

do shorted turns really happen?

Ed_Dinning · 22nd Feb 2020, 11:13 am

Hi, yes shorted turns can and often do happen. Very common on instrument amplifiers that are left on for a long time. There it will reduce the power output to the speakers.

It will be a case of stripping the transformer and rewinding it. It could also be a short between windings.

Ed

MrBungle · 22nd Feb 2020, 11:20 am

I’ve rewound a couple of SMPS transformers recently and they’re not as bad as the standard mains ones to do. If it’s low voltage it’ll probably not have many turns as well which is probably a bonus.

orbanp1 · 22nd Feb 2020, 1:37 pm

Those transformers are usually ferrite, and the halves are glued together with epoxy.
How do you part them without destroying it?

Regards, Peter

kalee20 · 22nd Feb 2020, 2:57 pm

Putting I a pan of hot water and bringing to boiling (to reduce thermal shock) may soften the ferrite sufficiently to break the bond. Or put in oven, raise to 120°C, take out with gloves and immediately try to separate.

The winding getting saturated with water doesn't matter. You're going to strip that anyway.

MrBungle · 22nd Feb 2020, 3:25 pm

The ones I did were clipped together and the glue had disintegrated respectively. The plastic had gone brittle on one bobbin and I managed to crack it slightly so be careful there. The whole job only took about 30 mins for each. Make sure you get the phase of the winding correct!

Edit: if it’s an E core you can probably just buy a whole new bobbin and ferrite from RS. Depending on the topology of the power supply it might not be too fussy about what material is used.

22nd Feb 2020, 11:00 am	#1
crestavega Pentode Join Date: Apr 2014 Location: Kirkwall, Orkney, UK. Posts: 165	SMPSU transformer failure Been doing battle with the multivoltage SMPSU on the back of a Sailor R2022 receiver (12/24V in, +5V, +12V, -18V, +18V +24V out). it was "working", that is, the correct voltages were appearing but the PSU was pulling high amps and the main chopper FET was overheating. I appear to have diagnosed a malfunctioning main transformer, which is bad news because I don't think Im going to be able to replace it. but I am suspictious of my diagnosis. both primary windings on the transformer have DC continuity but appear to be short-circuit at 60kHz. The only thing I can think of is that the trafo has got a shorted turn on it somewhere. does this really happen in practice? Ive seen plenty of little trafos fail open-circuit but never seen a shorted turn "in the wild". does it happen? also would have expected to see a hotspot on the transformer resulting from a shorted trn (I have a cheap thermal camera for problem solving) but I cannot see one (admittedly I cannot drive it for too long because the chopper will expire) this trafo has 2 sets of primary windings, one of which appears to be redundant in this application. I have pulled the trafo out of the board and applied drive to either winding with the same overcurrent symptoms. also on the primary side there is a strange "other" wire coming out the core which is grounded in the design. would this be a shield layer between pri and sec? do shorted turns really happen?

22nd Feb 2020, 11:13 am	#2
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,194	Re: SMPSU transformer failure Hi, yes shorted turns can and often do happen. Very common on instrument amplifiers that are left on for a long time. There it will reduce the power output to the speakers. It will be a case of stripping the transformer and rewinding it. It could also be a short between windings. Ed

22nd Feb 2020, 11:20 am	#3
MrBungle Dekatron Join Date: Jun 2016 Location: London, UK. Posts: 3,687	Re: SMPSU transformer failure I’ve rewound a couple of SMPS transformers recently and they’re not as bad as the standard mains ones to do. If it’s low voltage it’ll probably not have many turns as well which is probably a bonus.

22nd Feb 2020, 1:37 pm	#4
orbanp1 Heptode Join Date: Nov 2009 Location: London, Ontario, Canada Posts: 675	Re: SMPSU transformer failure Those transformers are usually ferrite, and the halves are glued together with epoxy. How do you part them without destroying it? Regards, Peter

22nd Feb 2020, 2:57 pm	#5
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,081	Re: SMPSU transformer failure Putting I a pan of hot water and bringing to boiling (to reduce thermal shock) may soften the ferrite sufficiently to break the bond. Or put in oven, raise to 120°C, take out with gloves and immediately try to separate. The winding getting saturated with water doesn't matter. You're going to strip that anyway.

22nd Feb 2020, 3:25 pm	#6
MrBungle Dekatron Join Date: Jun 2016 Location: London, UK. Posts: 3,687	Re: SMPSU transformer failure The ones I did were clipped together and the glue had disintegrated respectively. The plastic had gone brittle on one bobbin and I managed to crack it slightly so be careful there. The whole job only took about 30 mins for each. Make sure you get the phase of the winding correct! Edit: if it’s an E core you can probably just buy a whole new bobbin and ferrite from RS. Depending on the topology of the power supply it might not be too fussy about what material is used.