[Heathkit] need help on a sb-301

[Glen Zook]

First of all, go back and replace ALL of the electrolytics!  They don't have to "look" bad to be bad!  Electrolytics in a unit as old as the SB-301 are definitely suspect and really need to be replaced.  Those capacitors are inexpensive and, when they "go", they can take some fairly expensive parts with them.

Replace D3 and D4 and go ahead and replace C113 and C114 while you are in the AGC circuit.  Also, check R112.

C228 and C234 are also suspect as well as R212, R214 and especially R206.  Any of the bypass filters in the AGC circuit could be leaky and that will cause all sorts of problems.

Have you carefully looked at both sides of all the circuit boards in the unit for dirt, corrosion, etc.?
 
Glen, K9STH


Website:  http://k9sth.com


________________________________
 From: "yash at aol.com" <yash at aol.com>
To: heathkit at mailman.qth.net 
Sent: Thursday, February 28, 2013 4:12 PM
Subject: [Heathkit] need help on a sb-301
 
                           Started earlier this week with a really rough 301,did a full cleaning on the chassis, touched up all the switch contacts with dexoit,went thru all the tubes and replaced a 6au6, all the rest looked fine. Replaced all the electrolyics that looked bad saw nothing else burnt or out of place. Fired the rig up and every thing worked as it should ,but very low sensitivity. I did a complete alignment on the rig and she really came back to life. My only problem is that when tuning in a station,usb or lsb,the rig acts like you had the agc off. the signal is loud and distorted,if you VERY carefully tune can get things somewhat clear but not good. backing off the rf gain changes nothing, subbed any of the audio tubes ,no change. The agc circuit seems fine . Have not done any voltage readings as of yet. T have a second 301 in a set of twins that works fantastic. Can any one give me a starting place to check on the 301  to
 maybe narrow the problem
thanks for any help.