[Heathkit] DX-40 Pwr Xfmr

[Gary]

After rewinding a dozen or so of these transformers, this is what I have 
found.

After a rewind I do a series of tests. One is to bring up the new 
transformer on a variac with no load to monitor the current draw on the 
primary. At around 90vac the current starts to increase rapidly and by the 
time the primary voltage reaches  117 volts the current draw has doubled and 
reaching its full saturation point. So, if 125vac were applied to the 
primary the transformer would be well into full saturation. Every DX-40 
transformer that I have done the transformer was totally cooked. Not just 
one winding but the primary and the hv winding. The hv winding wire is very 
small gage compared to the primary winding thus most failures are do to hv 
windings shorted. The culprit in all of this is NOT ENOUGH IRON. To insure a 
longer life, I rewind these for 122 volt primary. The volts per turn remains 
the same or very close to it and they run a little cooler because an 
increase in primary winding helps to reduce the saturation level.
Regards,
Gary
----- Original Message ----- 
From: "john" <johnmb at nc.rr.com>
To: "Gary" <xfrmrs at roadrunner.com>; "Bill Stewart" <cwopr at embarqmail.com>; 
"heathkit" <heathkit at mailman.qth.net>
Sent: Monday, January 30, 2012 7:14 PM
Subject: Re: [Heathkit] DX-40 Pwr Xfmr


> Hi Bill and Gary
>
> That 125 V is within 5% of a nominal 117, so I doubt that's the reason, 
> since that's a typical line voltage tolerance... I'd guess 55 years of 
> aging might be more of a factor.  :-)   Are the caps ok?
>
> John your neighbor to the north  K5MO
>
>
>
> At 06:49 PM 1/30/2012, Gary wrote:
>>Hi Bill:    With a line voltage of 125 volts. That pretty much says why 
>>the
>>transformer failed.
>>Regards,
>>Gary
>>----- Original Message -----
>>From: "Bill Stewart" <cwopr at embarqmail.com>
>>To: "heathkit" <heathkit at mailman.qth.net>
>>Sent: Monday, January 30, 2012 6:32 PM
>>Subject: Re: [Heathkit] DX-40 Pwr Xfmr
>>
>>
>> > The verdict is in...its shorted. I took it out and put 60vac on it and 
>> > its
>> > now nice and toasty. Looks like a trip up north for the ole tranny.
>> >
>> > One question: those who have had transformers re-wound, do you have 
>> > them
>> > re-wound to today's line voltages? or keep 'em to the old spec...mine 
>> > L/V
>> > is in the 125v range.
>> > Tnx for all the suggestions...73 de Bill K4JYS
>> >
>> > ----- Original Message -----
>> > From: "Bill Stewart" <cwopr at embarqmail.com>
>> > To: "heathkit" <heathkit at mailman.qth.net>
>> > Sent: Monday, January 30, 2012 2:55:09 PM
>> > Subject: [Heathkit] DX-40 Pwr Xfmr
>> >
>> >
>> >
>> >
>> >
>> > Good afternoon:
>> >
>> > My trusted DX-40 is blowing its fuse. After some initial checks, I have
>> > disconnected the HV leads and made the following measurements: Total HV
>> > winding resistance: 312 ohms
>> >
>> > Outside-to-ct: 181ohms/344 vac....
>> >
>> > Outside-to-ct: 131 ohms/274 vac. The ac measurements are made with 
>> > 60vac
>> > at the plug.
>> >
>> > The xfmr gets warm even tho the on time is short.
>> >
>> >
>> >
>> > Would ya say this resistance/ac voltage imbalance would indicate a 
>> > tranny
>> > problem?
>> >
>> > Thanks, 73 de Bill K4JYS
>> >
>> >
>> >
>> >
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