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Old 14th Oct 2020, 5:30 pm   #16
usradcoll1
Heptode
 
Join Date: Mar 2017
Location: Cedar Grove, Wisconsin, USA.
Posts: 823
Default Re: Hallicrafters SX-25 Rebuild: New Mains Transformer Questions.

Quote:
Originally Posted by mrrstrat View Post
Update on radio: I repaired the poor work and bad modifications. One curious "mod" made by the previous owner was the soldering large wire wound resistor off of the A/C plug to bleed off the excessive voltage from the 700VCT transformer he had installed on it. And it looked to be a 10-15 watt Ohmite of a value I could not read as it was obviously smoked at some point.

Then I did the smoke and fire test and the radio came right up!

The alignment is near perfect (amazingly) and the crystal is sharp and functional at all of the settings. BFO is strong and works as expected.

I am getting about 285VDC off of the rectifier (for the B+) - I would have thought this would have been higher but the set seems to be doing great - I played it for a couple hours yesterday and will be using it to power music in the office today (I do an extended smoke and fire test and extended endurance test for my own radios).

I am amazed at the rich sound this set has. The P/P audio is definitely a difference. I SX-43 has great sound but the SX-25 is in a different league altogether.

I enclosed the replacement cap the previous owner put in - I have been told it is a "universal" replacement can. I never heard of these and always used the correct replacement. Anyway it is a 20uF to 40Uf replacement can and exceeds the 30-10-10-10-10 I need, but since its a filter I expected no ill effects. But my experience with amplifiers told me the overall bass response of the audio would be affected. I do see that effect as it has crisp highs and thundering lows and can be overloaded when too loud - I will replace the can at some point but I am still proofing the circuit.

Thanks all for the help in getting this going and it looks like I dodged a major bullet with the grounded CT on the 5.0VDC line. The set appears to have had no ill effects whatsoever. The time it was on and burned up the last type 80 was literally 2-3 seconds at the most. I have burned power valves up in seconds in the past so I was ready to switch it off during this test - I would have thought the 80 valve was more robust than that, but apparently they are not invincible.
Do you have different service information on the SX25. The Riders pages I have don't show the voltages expected on the various valves or the H/T or B+.
Dave, USradcoll1 About 150 miles from the O/P in Wisconsin.
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