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Re: Halicrafters SX17 transformer

 

Joe,
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Yes it was. The primary on the original had failed open. A Stancor A-4723 was also suggested by Jim Whartenby, but I found that difficult to source in the UK. It was possible to obtain from the US, but with the addition of substantial shipping costs + tax. A UK source suggested the Hammond 124 range. I initially thought to use the 124D (as per Franz Hottinger's solution) because of the similar secondary coil resistance, but got further advice elsewhere that the 124A would suffice and to just go for the cheapest option. All options in the 124 series had the required 1:3 impedance ratio in any case. The 124A was some 30% cheaper than the 124D but is is also physically smaller than the 124D and significantly smaller than the original. I did have to make an adjustment in order to fit it. This would have been avoided had I opted for the 124D as it has a larger footprint and matching hole spacing. I also had to add a tag strip as this transformer has no terminals, just floating wires. There wasn't much difference between the cost of the brand new 124A and something used on eBay, so it made sense to buy the brand new part. Neither option would have been particularly cheap.
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I used the same solution as Franz, that is, feeding the anode of the 6R7 directly via a series resistor and having a capacitor from anode across to the transformer primary, which was then connected to ground at the other end. I didn't know what had caused the original to fail, so it seemed sensible to avoid putting the anode DC feed through the transformer primary.
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Jim,
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Thank for the information about speakers. Much appreciated. I am not unhappy with the result and the main thing is that it does work and produces a very reasonable volume.
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On Wed, Aug 7, 2024 at 01:16 PM, Joe Connor wrote:

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I want to make sure I understand this. Was the Hammond 124A used to replace the interstage transformer??
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On Tuesday, August 6, 2024 at 03:03:10 PM EDT, Jim Whartenby via groups.io <old_radio@...> wrote:
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John
I figured that Richard would chime in about your audio power comment.??
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18 watts sounds to the normal ear as twice as loud as that from a 1.8 watt amplifier.? The SX-17 manual mentions that the audio amp puts out 13 watts of "undistorted" audio, whatever that meant before 'hi-fi" became popular.? In any event, if you are just going to listen to speech you may not notice much difference but music or singing might be a different story.??
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The reserve power might make the listening experience a bit more life-like especially if your speaker response is pretty flat across the audio spectrum.? Again, the smallest change the normal ear can perceive is 3dB or twice the power.? The difference in performance between a 3 ohm speaker and a 4 ohm speaker is not worth the worry.? The more efficient the speaker is, the louder it will sound.??
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I use an old Zenith wood cased bookshelf speaker from a thrift shop with an added 70 volt line transformer using the 10 watt tap.? This is enough to convert the headphone 600 ohm output of a military type radio to the 8 ohms needed by the speaker.? I believe the audio power of the amplifier inside the radio is under half a watt maximum but it sounds just fine for speech!
Regards,
Jim
Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy
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On Tuesday, August 6, 2024 at 12:16:17 PM CDT, John via groups.io <siloam@...> wrote:
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In the end purchasing a new transformer proved to be necessary. The Hammond 124A appears to be working just fine although I doubt that the set is currently putting out anywhere near 18W. After fixing problems in the decoder and IF section (mostly caused by leaky paper wax capacitors that had to be replaced, the set sprang into life and started receiving stations. I implemented an arrangement similar to Franz (HB9HPZ) - so thank you for posting that. I am using a temporary speaker setup which is nowhere near ideal, and perhaps not correctly matched but good enough to test with and I can get good and clear volume from the set, but it does have to be turned up over halfway, maybe two thirds of the way. In any case, it is working so thanks for the comments with advice regarding the transformer.
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