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Locked Re: HT-37 transformer failure due to rapid STBY-->OFF-->STBY

 

Halden
It was not reported that a short circuit involved the common ground tap so I am not sure that this is of concern.? When one 5 volt heater winding is at +800 RMS volts, the other is at -800 RMS volts.? So we have 1600 RMS volts between the two 5 volt windings or about 2300 volts peak to peak.

Only one 5 volt winding can have a short to the 800 volt B++ winding which is below it since the other 5 volt winding is connected to the innermost 800 RMS volt winding which is physically far away, near the transformer's core.?

Assume that the heater copper winding is 4 times the diameter of the B++ copper winding and that each wrap?of wire around the core induces half a volt.? The 5 volt winding will then have 10 wraps around the core while the B++ winding directly below it will have 40 wraps or 20 volts induced across those windings.? So you can have 5 volts in parallel with 20 volts, worst case.? Changing which 800 volt winding is connected to which 5 volt winding should only make matters worse.? The short circuit still exists.

If the rectifier heater winding is completely disconnected then this short is no longer an issue since the 5 volt winding is now essentially floating with no other reference to the B++ winding except where the single short between the two windings exists.? This is why solid-stating the rectifiers ignores the short circuit between the two windings and the power transformer will still do the job, in spite of the short circuit between the two windings.

Does all of this make sense?
Regards,
Jim
Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy


On Sunday, July 21, 2024 at 01:43:09 PM CDT, HF via groups.io <incorridge@...> wrote:


Hi Jim,

Thanks for your explanation of the typical layering sequence on transformers for radio equipment of this era!?

I¡¯m not sure I understand the part in bold where you say the 5V winding can see almost 2.5 kV.? If this is true, then 2000 VDC isn¡¯t enough for my hi-pot test.? So I¡¯d like to post my reasoning and ask if I got it right and then ask a question.

The center tap of the HV secondary is grounded, so the ends of the HV winding give 800V rms with no load.? That¡¯s based on my measurement of the HT-37 transformer Randy gave me.? Thus, the peak voltage should be about 1130 V relative to ground, alternating polarities.

When the original rectifier starts to conduct, the 5V winding will be near ground and then charge to + 1130 V less a few tens of volts due to the drop in the rectifier and a bit more due to the drop due to current flow in the bleeder resistors.? I¡¯ll estimate that at +1100V.?

The HV winding endpoints will continue to swing back and forth between -1130 and +1130 V.? When at -1130 V, there will be 2230 V between the HV winding endpoint and the 5V winding on top of it because the latter is at +1100 V relative to ground.? If that¡¯s what¡¯s going on here, then I should test this at 2300 V or maybe 2500, not 2000.

You wrote that the 5V windings are probably side-by-side atop the HV winding.? Thus, the insulation between each 5V winding and the HV winding below it would be the same (except for the fault under discussion).? If that¡¯s the case, then a ham with an HT-37 that failed in this way could change the HV rectifier filament to the other 5V filament winding, depriving the lower B+ rectifier of its filament.? Then, swap out the lower HV rectifier for a pair of silicon diodes and keep the HV rectifier as-is.? Wha¡¯ d¡¯ya think?

Cheers,

Halden VE7UTS
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