[Hallicrafters] SX-28 help please

Wed May 5 22:56:21 EDT 2010

I think this is good advice but I will have to admit to using a variac (with precautions) all the time with a lot of my old stuff. Some of the old radio equipment we drag home hasn't been fired up in years so we expect multiple problems and faults (emphasis on more than one) and the question is where are they and how serious. Your first job is to get a good understanding of what the physical condition of your SX-28 is. Pull all the tubes and do an initial cleaning, blow the dust out, etc., and perform a thorough visual inspection making a note of anything that does not look right. Inspect for any missing, burned or broken parts as well as any previous repairs or modifications. After determining your receiver is reasonably safe to proceed, clean and test the tubes and reinstall except for the 5Z3 rectifier. You will want to perform a brief operation test to establish if you have working order before you replace components. This will help you locate a problem later if it stops working at some point during capacitor and resistor replacement, etc. Also, a brief test will help you decide how to proceed with your restoration and to help gauge the effort and cost that will be involved. 

A word about capacitors: The power supply electrolytics are the only capacitors in your receiver that you stand a chance of reforming and if they are going to reform they will usually reform in an hour or less. It is also generally true that if an electrolytic needs to be reformed they usually won't stay reformed for long. You would have to repeat the reforming process again every time you attempted to use the receiver. I play these games with some of my 1930's stuff because I am an originality nut but I do it with test equipment hooked up to monitor leakage current and B+ while ramping up and closely monitor it the entire time it's on. This is also like playing Russian Roulette with an antique. If the plate and screen bypass capacitors test bad, and they will, they will be bad from now on and they have the potential to take out power, audio and IF transformers. The coupling capacitors will not necessarily be dangerous to run as-is but they could hurt sensitivity, introduce distortion or worse case cause IF amplifier tubes to run hot, etc. 

If you do not have experience with the point at which tube rectifiers begin to conduct you should get a solid state rectifier to substitute for the 5Z3 during this brief phase of the testing. As Bob suggested you should also have the receiver connected through a light bulb to the AC. The bulb protects an unknown receiver while you are in the initial stages of testing. If there is a serious short, worse case is you light the bulb with no further damage to the receiver. Connect a voltmeter to the B+ buss (one of the high voltage terminals on the receiver's filter capacitor) and advance the variac to about 50 volts and observe behavior of B+ reading. Is it responsive and did it come up quickly to around 100 volts or more or was it lazy and stayed down low? If low watch to see if the B+ eventually comes up. If so the cap is reforming. Touch the filter cap (carefully) to be sure it is staying cool as you advance the variac in steps on up to 110 volts. You should see the B+ rise quickly with each advance of the variac until you reach full rated B+ if the electrolytic cap reformed and there are no seriously leaky plate and screen bypass caps. If the B+ does not want to increase stop until you find out why. One other piece of equipment that will help you stay out of trouble is a DC milliameter. As you advance the variac you will want to keep leakage current below 10 mills. With a milliamp meter you would advance the variac until you get a reading of 10 mills. As the cap reforms the leakage current will drop. Keep advancing the variac in steps while maintaining leakage current at no more than 10 mills until you arrive at 110VAC. B+ should have risen to full rated value and leakage current will have dropped to less than 2 mills. Generally speaking a serviceable cap will have less than 2 mills leakage at WVDC and a good cap will be less than one, more or less, depending on the original rating. Modern electrolytics are reliable and have negligible leakage current which is why they are preferred by most. Remember, this was a bench test to establish working order. Your receiver still needs to be restored electrically to achieve reliability and level of performance it was originally engineered for.

73, Greg

-----Original Message-----
From: Robert Nickels <ranickel at comcast.net>
To: hallicrafters at mailman.qth.net
Sent: Wed, May 5, 2010 11:09 am
Subject: Re: [Hallicrafters] SX-28 help please

On 5/5/2010 1:32 AM, hallicraftersx28 at aol.com wrote:
 90 V over 5 days !! Were  you  slowly trying to ROAST the rcvr ?

 hate to pile on, but just to amplify what Carl, Nick, and others have 
aid, everyone should just forget about this "bring it up slowly on a 
ariac" myth.  It is almost surely not going to properly reform the 
lectrolytics and is likely to have unintended consequences.   That's 
he problem with the internet - beginners have trouble separating the 
rban legends from sound technical facts and procedures.
A variac is a useful tool when used as Carl described.  (I like to add a 
ight bulb in series just for added protection), and with care, proper 
easurements, and solid-state rectifiers - can be used for in-situ 
eforming.    But the only way to properly restore a piece of vintage 
lectronics is to understand first how it works, and then go through a 
ystematic troubleshooting and restoration process like has been 
escribed.   Not only will you end up with a properly-functioning piece 
f gear but you'll understand how it works!
73, Bob W9RAN
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