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 Post subject: Hallicrafters Power Supply Problem Help Sought
PostPosted: Dec Fri 01, 2017 4:39 pm 
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Joined: Jan Fri 08, 2016 1:19 am
Posts: 27
Location: mapleton sd
Bought a derelict sx25 hallicrafters for parts for another sx25 I was restoring and decided to
try to get the derelict working. Had it working but suddenly the receiver stopped working. When I investigated the cause I noticed the filter choke had melted and cooled wax underneath the laminations. I also noticed that I could see a tiny bubbling of (varnish?) between the paper surrounding the laminations. The receiver is dead but I hear low to moderate level hum in the speaker but certainly not that which the two 6F6s are capable of putting out. There is no "click" when I switch the bandswitch or pull out a tube as there usually is.
All the electrolytics have been replaced with new. All the out of tolerance resistors have been replaced with new as well as the paper capacitors. I have checked and doublechecked the wiring many times and am convinced it is as it is supposed to be. It also is identical to my other sx25 that works fine.

So I first thought my focus should be on the filter choke. I measured it out of circuit and it is 9.9 henries and 385 ohms. When it is in circuit it draws 650 miliamps. So then I measured the good choke in the other (good and working) sx25 and it measured 8.8 henries, 295 ohms and draws only 120 miliamps. But then I put the good choke from the working sx25 into the nonworking sx25 and it drew 650 miliamps. So something must be shorting or leaking.

The 80 rectifier tube is fine and puts out its rated voltage. I've checked the 6F6s for shorts and found none. The power transformer indicates proper voltages on all terminals.

I can't post the schematic to this forum because the compression required by bandwidth will render it unreadable so I have uploaded it to link site. The link is: http://docdro.id/BSRRhy7

I am interested in anyones ideas because maybe Ive been over this so many times I am missing something obvious.


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 Post subject: Re: Hallicrafters Power Supply Problem Help Sought
PostPosted: Dec Fri 01, 2017 5:11 pm 
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Todd,

You have a short or component failure on the low current B+ bus given your symptoms. Use resistance checking to find the problem before you destroy the power transformer (and good choke, your other choke may have been stressed enough to consider replacing it after you find the problem).

The SX-25 is set up like many receivers where the high power output stage takes its power before the filter choke since a high degree of filtering isn't needed for that stage compared to the low level/high gain stages, oscillators, etc. The output stage draws the lion's share of current in a receiver so the draw through the choke should be fairly low by comparison.

You could have a failed tube causing excessive current draw, an inadvertent short from insulation failure or a component lead, or a failed component. Since you have a second SX-25 for comparison it will be pretty simple to compare resistance readings and narrow the issue.

Do NOT power this receiver up any more until you locate the issue or it is likely to suffer a failed power transformer also and I wonder about the fuse since that level of current draw on the B+ bus should have opened a properly sized primary fuse.

Rodger WQ9E


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 Post subject: Re: Hallicrafters Power Supply Problem Help Sought
PostPosted: Dec Sat 02, 2017 5:10 am 
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Joined: Sep Thu 01, 2016 3:56 am
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Location: San Jose, Ca.
Todd, I echo all that Rodger said. If you can not find the short without powering it up, be sure to use a variac and start out very low. Just bring it up to where the short shows up, but be sure to not heat up the choke, transformer, or anything. If you don't have a variac, try to borrow one.

Sounds like the choke got quite hot - I hope it's still usable.

Regards, Larry


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 Post subject: Re: Hallicrafters Power Supply Problem Help Sought
PostPosted: Dec Sat 02, 2017 2:47 pm 
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Location: Hudson, MA
Seems like the filtered side of the choke has a dead short to gnd. 0.65 A & 385 Ohms = 250V across the choke , = 163 Watts :shock:

For the good one 0.12 A & 295 Ohms = 35V across the choke , = 4.2 Watts :D

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 Post subject: Re: Hallicrafters Power Supply Problem Help Sought
PostPosted: Dec Sat 02, 2017 4:50 pm 
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Joined: Jan Fri 08, 2016 1:19 am
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Location: mapleton sd
Thanks Rodger for the guiding hand. I am following your suggestion and have been checking/comparing
resistances. So far the only discrepency has been that my volume control is leaking 15 ohms to ground when it is shut off. I've checked and doublechecked the tubes involved with the part of the circuit prior to the choke (audio output 6f6, 1st audio 6sq7 and phase detect 6sq7) hoping to find internal shorts but none found. The only wild card is the audio output transformer since it has a tap off the secondary to the filter cap prior to the choke. Maybe theres something going on there?
Eddy you really put it all into perspective with your calculations. Once the short is found I will be surprised if the choke is usable.
By the way, the schematic link that I uploaded was taken from BAMA. The circuit changes (6AG7 and parts that he has crossed out) do not apply to mine. My sx25 appears to be a stock version.


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 Post subject: Re: Hallicrafters Power Supply Problem Help Sought
PostPosted: Dec Sat 02, 2017 5:02 pm 
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Joined: Mar Wed 16, 2011 10:44 pm
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Location: Peekskill, NY
If you have not done so, disconnect both choke wires and measure from either of them to ground (body of choke) and
be sure that there's a high resistance to ground there. More then 10K ohms or so.

If this test is good then my approach is to start disconnecting every connection to the various B+ busses
and verify the short has cleared. Then start re-connecting and see which one is the problem. Doing so
with a reduced line input, in one way or another, is a good idea.

Once the B+ connections are opened, it would be a good idea as mentioned to ohm out each stage, as the
shunt might be apparent with the small voltage applied by the ohmmeter - before doing the powered up test.


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 Post subject: Re: Hallicrafters Power Supply Problem Help Sought
PostPosted: Dec Sat 02, 2017 5:15 pm 
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Todd,

The audio output transformer won't be affecting that and the tap is actually the center tap on the primary side of the output transformer which feeds the high plate current to the output tubes. Because of the signal level involved the output tube B+ doesn't require as much "ripple smoothing" as the rest of the receiver and because this is by far the highest current draw portion of the receiver it allows a much lighter duty filter choke to be used since this current doesn't flow through the choke.

To narrow things down disconnect R38 in each receiver where it connects to the B+ line and then measure the resistance from the choke output to ground to see if it is the same (or within 10%) in both receivers. If it is different you have narrowed down the problem area, if not disconnected the B+ lead in each receiver at the junction of R38/R39 in the bleeder/voltage divider string and compare the resistance from this lead to ground.

These two tests won't catch a tube fault that is dynamic (based upon operating conditions) but it will catch a fault that is due to a static tube fault (short) or a capacitor or other failure to ground and help you to further narrow down the problem area. Once you start to zero in on the suspected area measure and compare between the two receivers and I suspect you are going to find a shorted cap or faulty insulation involved.

Rodger WQ9E


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 Post subject: Re: Hallicrafters Power Supply Problem Help Sought
PostPosted: Dec Sat 02, 2017 5:53 pm 
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[quote="Todd Electron"]. When it is in circuit it draws 650 miliamps. So then I measured the good choke in the other (good and working) sx25 and it measured 8.8 henries, 295 ohms and draws only 120 miliamps. [quote]

How and where did you measure the current.?
How many ohms do you read from the choke output to gnd (with the set unpowered)

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 Post subject: Re: Hallicrafters Power Supply Problem Help Sought
PostPosted: Dec Sat 02, 2017 7:24 pm 
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Location: 649 Lacy Wilkerson Rd Roxboro, NC 27574
One of the new electrolytic wasn't installed backwards was it?

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Joel Walker
Fix something.........it will make you feel better!


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 Post subject: Re: Hallicrafters Power Supply Problem Help Sought
PostPosted: Dec Sun 03, 2017 5:06 am 
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Joined: Jan Fri 08, 2016 1:19 am
Posts: 27
Location: mapleton sd
Christmas came early tonight in my shop.
Buried deep beneath the oscillator compartment is the tube socket for the 6k8 mixer. As access is
difficult I had my VOM alligator-clipped to a long skinny screwdriver. When I probed pin number 6 there the VOM showed a short to ground. I then moved over to the working sx25 and at its 6k8/pin 6 there was no short. Checking the schematic I saw Pin 6 is connected to the plate of the 6k8. Could I be so lucky? I jabbed at it with my high-powered super duper x-ray goggles and spread everything out as best that I could with a small flat blade. Then I decided I was either going to blow it up real good or else know the reason why. Variac power-up and suddenly the receiver came to life What a beautiful sound. So I don't know if I cleared a solder blob, snipped-off wire debris from my re-capping project or what but something was bridging over to pin 6 (I think to the pin 7 heater pin). Tomorrow when I am fresher I will remove the band 2 oscillator coil to gain access to the 6k8 socket and neaten it up in there. The choke no longer even warms up and the most amazing thing is that the choke is still functioning after all those bubbling overheats. I've checked the choke current draw and it was only 70 miliamps. With the good sx25 drawing only 90-100 miliamps I am guessing that 70 to 100 ma is the typical range. The choke in the good sx25 is different in appearance so maybe there is a slight parameter difference too. Appreciate all you guys and your helpful suggestions. Special thanks to Rodger. You nailed it.


Last edited by Todd Electron on Dec Sun 03, 2017 6:06 pm, edited 1 time in total.

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 Post subject: Re: Hallicrafters Power Supply Problem Help Sought
PostPosted: Dec Sun 03, 2017 4:23 pm 
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Todd,

Congratulations on finding and solving the problem!

A solder blob in the wrong place wouldn't surprise me because over the life of the set plenty of stray material (wire clippings, solder debris, flaked metal from solder lug, etc.) is created. Even though I try to be very careful during repairs I always give the chassis a good shot of air from the compressor/air gun and then shake it out when possible; holding and shaking something like a Hallicrafters HT-33 chassis isn't very practical :)

Rodger WQ9E


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