SB-301/SB-401 repair - one more thing.

[Kenneth G. Gordon]

On 2 Oct 2010 at 13:07, Glen Zook wrote:

> One thing:  The cables for the heterodyne oscillator, BFO, and LMO
> connections definitely should be RG62/U.  However, the cable for the
> antenna connection should be RG58/U.

Yes, thank you. I did not make that clear in my last post. Good 
catch, Glenn.

One other matter that might be considered here:

In the SB-401, I believe the final amp circuit is very close to the 
same one that Heathkit used in all of their SB/HW 5-band tube-
type transceivers which use a pair of 6146s. 

That circuit includes a bunch (6?) 1 ohm resistors in parallel in the 
cathode to meter the plate current. In several of those rigs I have 
worked on, those resistors have drifted very high, throwing the 
plate current indication, and thus the bias level, way off.

If you cannot achieve normal full-load plate current (~250 mA) 
with known good finals, check those resistors.

Be aware that if they ARE off, adjusting the bias to show 50 mA 
will instead really result in much higher plate current, and the 
stage will no longer be operating at class AB1, but instead much 
closer to class A.

Although this might be somewhat more linear, it is hard on the 
finals as they have to dissipate more heat.

Ken Gordon W7EKB

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