"False center tap" blown, why?

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radardoug

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If you are blowing fuses, its something serious. Disconnect all transformer secondaries and see if you blow a fuse. If you do, its the transformer. Then connect up your filaments. Then your ht, testing in between. This is servicing 101.
 

Bowie

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No, it shouldn't. I think you have to completely disconnect the xfmr secondaries and start at square one. Does the fuse hold the open current?

Would it be worth checking for an interwinding short between heater and HT ?
You would think leakage between primary and heater should cause your mains supply to trip out instantly at the breaker.
If your heater winding is disconnected and you still blow a primary fuse it would seem to point to a transformer fault , although the fact that the heater winding itself measures good compared to another new transformer of the same type would seem to rule out shorted turns in the heater winding itself .

With both HT and heater circuits fully dissconnected if the transformer still blows a primary fuse when energised it has to be the transformer .

Theres a simple explanation , you just havent found it yet .

If you are blowing fuses, its something serious. Disconnect all transformer secondaries and see if you blow a fuse. If you do, its the transformer. Then connect up your filaments. Then your ht, testing in between. This is servicing 101.
Thanks all for the help. I found that the two HT leads measure closed (when removed from circuit). As reference, an unused 269JX reads around 160r between those two leads.
It measures open between one of the HT leads and their CT. A new 269JX measures around 65r between either lead and CT.
I can't find anything unusual about the heater tap. Doesn't have any conductivity to other areas of the xformer.

Knowing this, what thoughts do you all have? I'm trying to get my head around what the series of events was here. My initial thought is a heater CT resistor went bad and damaged the HT tap before throwing the fuse. But, I've not dealt with this before so I'm learning on the job here. Thanks.
 

abbey road d enfer

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Knowing this, what thoughts do you all have? I'm trying to get my head around what the series of events was here. My initial thought is a heater CT resistor went bad and damaged the HT tap before throwing the fuse. But, I've not dealt with this before so I'm learning on the job here.
It does not look likely. You must do a post-mortem. Not easy when the xfmr is impregnated.
 

Tubetec

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Id be inclined to suspect the insulation between one leg of the ht winding broke through to the heater winding , that would have provided a low resistance to ground , which allowed enough current to flow to burn out the ht winding , in other words the the burnt resistors were a symptom of the fault not the cause itself . Could you return the unit to Hammond as faulty , maybe get them to disect it and just send you a replacement ?
 

Bowie

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Id be inclined to suspect the insulation between one leg of the ht winding broke through to the heater winding , that would have provided a low resistance to ground , which allowed enough current to flow to burn out the ht winding , in other words the the burnt resistors were a symptom of the fault not the cause itself . Could you return the unit to Hammond as faulty , maybe get them to disect it and just send you a replacement ?
Thanks, I'll try that. I went through this once before with a problematic xformer and Hammond was cool about it. That time I posted a quick vid of the issue (mechanical buzzing) to youtube and they took that as evidence enough to send a replacement. I'll talk to them about sending this one in.
 

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