SB-301/SB-401 Functional Repair Guidelines

[Kenneth G. Gordon]

On 2 Oct 2010 at 1:35, John H. Klingelhoeffer wrote:

> 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.

That's a good start: no mods.

> From an initial power on, it's quite clear by sound that the receiver
> needs new electrolytic capacitors.  They appear to be the originals. 
> That will be my first step with the receiver and the transmitter, too,
> including several cathode bypass caps.

Tom N0JMY at http://www.hayseedhamfest.com/ has NEW direct 
replacements in stock. He also sells complete kits for both the 301 and the 
401, among others, the last time I communicated with him. He is 
unquestionably honest and a very good fellow.

> Beyond that, checking the tubes,

and making certain the correct tubes are in their correct sockets.

> and a tuneup by the manual, what else
> should I look to do as far as cleaning and lubrication?

As has been mentioned by at least two of us, make certain that the small 
nuts-and-bolts that mount the various circuit boards to the chassis are tight. 
These loosen over time, and often some corrosion will form between the 
boards and the chassis causing some really weird symptoms. In some 
cases, I have had to loosen each bolt (of those I could easily reach) and dab 
some DeOxit between the board and chassis, then retighten.

>  Bandswitch
> contact cleaning?

DeOxit 5 on a "Q" tip.

>  LMO maintenance?

These can become a problem in that the contact between the various 
movable bits of the main tuning cap can become corroded and cause 
intermittants, causing the frequency to jump while tuning. This occurs 
especially if they have not been used for a while.

Often, especially with the tube-based LMOs, running the tuning from one end 
of the scale to the other multiple times will clear this up. Very rarely, you will 
have to, somehow, open the LMO and carefully clean and DeOxit the 
bearings, but only do this as a last resort. The LMOs are for the most part 
unusually reliable.

>  Solder joint reheating?

Not generally, IFF the kit was wired properly. Sometimes this does help with 
a particularly stubborn problem.

>  What are
> problem areas with this pair? 

Other than resistors that have gone high by now, none, really. There are a 
few circuit changes that can make them a bit more handy to operate, but the 
transmitter seldom needs anything done to it to improve matters. The 
receiver can use some help, but I would leave them both alone at this point. 
Just get them working and let them be used for a while.

BTW, the problem with the resistors is VERY common with old equipment. 
Carbon-composition resistors were NEVER very reliable. Even sitting on a 
shelf, never used, their resistance drifts high. However, in my experience 
with the Heathkit gear, the "tolerances" are so loose that resistor values can 
change quite a bit without any symptoms showing up. I would not do a "shot-
gun" replacement of them, but only attack those which have a bad effect on 
some particular circuit.

The SB-301/401 are a really good pair.

Don't forget that the cables between the two are NOT RG-58. They are 
supposed to be RG-62.

> The 'widowmaker' power cords will certainly be changed with more
> modern 3 wire types.

I mount one of those standard (now) computer power connector sockets on 
the back panel and use old computer power cords.

>  Thanks in advance and 73.  This pair is a bit
> before my time at Heathkit.  

Yes. They are pushing better than 40 years old. I really like mine.

Ken Gordon W7EKB

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