Power supply choke

[w5rkl]

I've been doing some calculation on determining the minimum choke value in a choke input power supply. The supply in question is in the Heathkit DX-40. I don't have my DX-40 yet, I have one coming so all my calculations are based on values found on the DX-40 schematic and in the part list.

I hope I'm doing the math right:

The voltage output of the DX-40's supply with the transmitter tuned to maximum DC input power, 72.5 watts, is 580VDC at 125ma.

580 x .125  = 72.5 watts

Minimum choke inductance is determined using the formula:

Lmin = E / I / 1000

Using the above formula along with the voltage and current, I get a minimum choke value of 4.6 Henrys

580 / .125 = 4640 / 1000 = 4.6H

Heathkit uses a 7 Henry choke that has a current rating of 150ma.

The DX-40, just like the DX-35, appears to have a history of power transformer failures and I was wondering if the source of the failures is caused by the choke value and the current rating of the choke? I don't know what the maximum current ratings of the power transformer's secondary windings are or when the transformer fails, what windings cause the failure?

73
Mike W5RKL

[KE6DF]

The 7 HY choke sounds OK to me.

I don't think changing the choke would make any difference to the life of the power transformer.

Most likely power transformer problems are just that -- problems with the power transformer.

Too low a rating, running too hot without enough ventilation, shorts of failures down stream of the transformer, or anything else that puts higher than rated load on the supply for a period of time.

Dave

[N4LTA]

A choke input supply has a critical inductance to keep the supply voltage from soaring at light loading (when the key is up).

You need to know what the transmiter including the bleeder pulls when you are not transmitting in order to calculated the minimum inductance. When the input inductor is greater than L critical, the power supply voltage will remain close to loaded voltage even when not fully loaded.

If I remember correctly, L critical is    =    output voltage fully loaded / non loaded current) /  1000


So a 1000 volt supply that had a 50 mA bleeder would require a 20 H choke stabilize the voltage from no load to full load. Often you can get by with a little less choke, because the choke is rated at full current and at light loads they swing a bit and have a higher inductance.


I don't remember formulas correctly all the time so look it up.

Pat

[KA2DZT]

Those 7hy chokes were made by Chicago xfmr and should be good forever.  Like other have said, xfmrs usually go bad for any one of a number of reasons.  Mostly from overloading which creates too much heat in the core.  If the xfmr hasn't been used for a long period of time there may be some moisture in it.  Sometimes you made need to run the xfmr at reduce voltage to generate some heat in the core to drive out any moisture.  OTOH if the xmtr has used recently I wouldn't worry about moisture.

Fred

[w5rkl]

I did some additional research and talked with Gary who rewinds transformers (see following link)

http://members.tripod.com/tubes_tubes_tubes/transformerrewindingservice/

I found the HV secondary winding current rating is 150ma. Gary says this is adequate for the transmitter. Gary did say he has found from experience rewinding DX-40 transformers that the internal insulation between windings is poor. He also said that with increased primary AC voltage today this contributes to early filter capacitor failure (higher voltage across the filter capacitors). Gary rewinds the DX-40 so the primary windings are rated at 122VAC instead of the original 117VAC.

The DX-40 also does not have a fuse in the primary. If my calculations are correct, using the original 117VAC at 175 watts (maximum input power) that would be 1.49 amps. I don't know why Heath didn't include a fuse, maybe a cost saving measure. I intend on adding a fuse in the primary with a 2 amp fuse before any testing of the transmitter is conducted.


73
Mike W5RKL

[KM1H]

The DX-40, just like the DX-35, appears to have a history of power transformer failures and I was wondering if the source of the failures is caused by the choke value and the current rating of the choke? I don't know what the maximum current ratings of the power transformer's secondary windings are or when the transformer fails, what windings cause the failure?

That rumor appears to come from one person who didnt bother to investigate why but simply created a web site on a sample of 1.

The filter caps used are a major problem as they were leaky to some degree from new and got worse. This is also true with several other Heath products. Long term excessive current then deteiorates the insulation which then gets blamed. Those people will never become Forensic anthropoligists 

I regulary suggest using a bucking transformer on all vintage electronics if the line voltage is above 118-119V. Operating at 107-113V will reduce heat, drift, and preserve many components.

Carl

[w5rkl]

Thanks Carl for your input. I talked with Gary from Transformer Rewind Service and he said he has done quite a few DX-40 rewinds. He said one of the problems that cause transformer failure in the DX-40 is the poor insulation used. He also said the primary windings are rated at 117VAC so he rewinds the transformer with 122VAC rated winding (turns per volt ratio). He also said the HV current rating of 150ma is quite adequate for the transmitter. I have new 47ufd 450VDC caps in stock to replace the older original caps plus new 20K 10 watt resistors (2 x 20K) across the 2 filter caps in series at the supply's output. Always good to have spares on hand if needed.

When my DX-40 arrives I intend on doing some test with various power levels, starting at 117VAC and working down to the levels you mentioned using my variable AC transformer (variac). I did that with my Marauder HX-10 which has a primary voltage rating of 117VAC, same as the DX-40. If I run it at 121VAC the transformer does get quite warm after an hour and a half, portions of that period is operating at 90 watts output on CW. If I reduce the power to 117VAC the heat build up in the transformer is less, also take longer for the transformer heat to occur so I agree with your assessment of operating at a lower primary voltage.

73
Mike W5RKL
 

[WQ9E]

The higher line voltage definitely heats up some of the marginal transformers and some of this ham gear was really built right on the margin.  In addition even though the VF-1 doesn't draw very much power if the DX-40 is truly on the edge of failure adding even this slight additional load could be the straw that breaks the camel's back.

If you check the transformer temp in the Drake R-4 series receivers it gets quite hot but apparently the insulation in these is up to the task since failure seems quite rare and a lot of these were operated continuously over contest weekends.  My B and C lines both saw a lot of 48 hour stretches without issue but you could cook food on the transformer.  I never took the cover off an operating AC-4 to see if those are also hot boxes.

Did Gary state whether the insulation issue with the DX-40 transformer is with the rectifier filament winding (like the Hallicrafters HT-32 and HT-37)?  If that is the case then a properly implemented solid state rectifier replacement would be advisable (which allows disconnecting the 5 volt winding and using it to buck the primary if you wish) but this is only a reasonable approach IF you account for both necessary voltage dropping and inrush current limiting.

[w5rkl]

Gary said the failure's are due to very poor insulation and today's higher primary voltage levels and failures with the filter capacitors. I took that to mean the insulation between the windings was poor, higher primary voltages (higher than 117VAC) plus failure in the filter capacitors, all contributed to transformer failure. Gary didn't mention the lack of a fuse in the primary. Gary also commented about HV secondary winding current rating of 150ma. He said the current rating of the HV secondary winding was actually more than what the schematic says.

I'm aware of the hot power transformer in the Drake R4B. That's something I've always wondered about but as you say, it keeps on working. I have 2 R4C receivers and the transformer's in both R4Cs, after a few hours of use, are only slightly warm to the touch, nothing close to the R4B transformer.

I appreciate all the comments everyone's given on this subject. I'll just have to do some testing once the DX-40 arrives.

73
Mike W5RKL

[KM1H]

Gary's main line is always to blame the insulation since that is what he sees. However since the same insulation has been used for decades by all transformer manufacturers it rarely holds up under scrutiny.

Sometimes the lack of iron is at fault as with many Hallicrafters, some Nationals and a few other brands. They worked fine at 117V but are into partial saturation at 123V and the heat rise is far from linear once that starts. These MUST have a bucking transformer for reliability. The HT-32/37 problem is due to insufficient insulation but that is a fairly unique case

In most cases I am very much against going SS as it not only causes more current to flow, stressing tubes and other components, but the near instantaneous switching of those diodes is also hard on a transformer compared to a tube. Having to add a series wirewound resistor to reduce voltage back to original is a ridiculous answer as it is now a new localized heat source in an already hot chassis bottom, tube heat is on top and easily dissipated. The removal of the rectifier tube is such a small part of the overall current consumption that it barely registers and the SS actually negates that savings and adds more of its own.
These old tech high impedance transformers are a completely different design than the new ones.  Run them cooler and they will last.

If you want to save 5W then replace a 5U4 with a 5R4 which will also increase the voltage drop a bit. You can also rebase and use in place of a 5Z3 as I do in my SX-28's and SX-17.

There are exceptions which I use. In a TX such as the DX-100 it does become beneficial to convert the LV B+ to SS and then change to a choke input by moving one wire. This now reduces the output to a lower value resulting in longer tube life while still having sufficient grid drive for 110-120W on 10M. Heck, I even drop the VFO (I use a 6AH6) to a 108V OB2 on the screen, and zener to 160V on the plate and still have plenty of drive.

Carl

[WZ1M]

No Carl. thats not my main line, but close. There are several causes of failure, as you know. Back in the day they were using single coated enamel magnet wire and age took its toll. Even some of the wire in early radio transformers were cotton covered, thats all. Higher voltages from todays suppliers is another factor. Lack of Iron is one more reason for transformer failures at todays primary voltages. And as mentioned, lets not forget the filter caps in the power supply and lets go a little farther here. Any leaky cap on the B+ line will be a problem with transformers. The list goes on and on.
Regards,
Gary