SB-301/SB-401 Functional Repair Guidelines

[Robert Groh]

I have actually tackled my SB-300/400 pair so I speak only from experience on 
other radios and from prior comments on this and other lists so here goes:

#1: As you noted, all the electrolytics are suspect. Some or all may be ok - now 
- but they will probably fail at some time.  So best to tackle them now. I did 
have the caps in an old HP scope (circa 1959 vintage) which came back to life 
(gradual power up on a variac over 4 or 5 hours) with no problem.  But then that 
is HP!  Replacing radial lead caps is easy.  Replacing the can types is really a 
bit of a hassle. The whole key is leakage current - if low, you can leave alone. 
Not sure how to check that easily.  If they run hot - that is sure indicator of 
'badness' - usually will be a self-revealing thought.

#2: Resistors are almost sure to have drifted off in value - sometimes drifted a 
lot (usually up in value by 100's of percent)!  So I would plan on checking many 
of them with an ohm-meter particularly those in circuits where the value is 
somewhat critical.

#3:  DeOxit or equivalent on all the contacts, a bit of good thin machine oil in 
the bushings and a shot of pot cleaner in the variable R's. All of them.

#4:  Other caps can fail - the ceramics seem to be pretty reliable, the other 
types not so much.  I would leave finding those to the point in time where you 
are trying to find problems (i.e. don't replace unless they are really a 
problem).

#5:  Grounds seem to come up as a problem frequently so maybe loosen up mounting 
screws on boards and then re-tighten just as precaution.

#6:  As you noted, changing to modern 3-wire plug is (in my mind but not 
everyone agrees) an essential.

So let us know how you make out.  And, by the way, a SawzAll is not an 
electronic tool of choice !  (Private joke).

73
Bob, WA2CKY
 




________________________________
From: John H. Klingelhoeffer <wb4lnm at AOL.COM>
To: HEATH at LISTS.TEMPE.GOV
Sent: Sat, October 2, 2010 12:35:24 AM
Subject: [HEATH-TEMPE] SB-301/SB-401 Functional Repair Guidelines

A local club has asked me to look at returning an SB-301 / SB-401 pair to 
operational condition.  The internal condition looks good and I don't see any 
mods.