[heathkit] Heathkit SB400 low output

["Robin McNeill" <robin@venturesouthland.co.nz>]

Bob,

Many thanks for that. I have already replaced all the resistors for precisely the reasons you state and resoldered the joints. I had considered replacing the bypass caps, but that would have affected all bands and the caps seldom fail (I think). That just leaves the coils. I don't have a Q-meter: I read somewhere that the old coil slugs lose permiability over time and maybe this is true? I am not sure how the the slugs could deteriorate. The circuit is so simple that there is simply nothing else to replace. 

Upon further reflection, I think that the problem may be that the SB400 loads the oscillator too much. I added a double RCA adaptor to read the HFO frequency with a (digital) frequency meter and it pulled the HFO frequency quite a bit (but still dead with, or without the meter on 40m). I notice that the later SB301 instructions for the interconnect lead specifies "must be RG-62/U cable 24" long  supplied with SB-301 Receiver". So perhaps that is critical? (RG-62/U 93 Ohm coax seems impossible to find in NZ). Maybe this is a design fault (the reason I checked the SB301 instructions was to see if there had been a mod)? What circuit did you use for a buffer, please?- that may be the best way to go. Alternatively, do I just add some capacitance between grid and plate?

I have to say that I get good sig reports with the rig and (other than 40m) it is much nicer to use than my fancy silicon rig with DSP etc., etc.,

73
Robin ZL4IG



-----Original Message-----
From: G3OOU@aol.com [mailto:G3OOU@aol.com]
Sent: Thu 26/03/2009 21:55
To: heathkit@cyberventure.com; Robin McNeill
Subject: Re: [heathkit] Heathkit SB400 low output
 
Hi Robin
 
I would check that the components in the oscillator circuit are still  within 
tolerance, particularly the resistors. Carbon composition resistors go  high 
over a long period of time and I threw out my "new old stock" of carbon  
resistors as they were 30+% high. Check the oscillator coils for dry joints etc  
and if you have a Q bridge measure the coils on that. Also make sure that the  
valveholder sockets are not oxidised.
 
If you disconnect the HFO lead to the SB400 does the SB300 oscillator  work 
correctly? I found it useful to monitor the HF oscillator grid current  with a 
test meter when I did my rebuild, re-alignment and subsequent tests with  an 
SB401.
 
You need to keep the interconnecting lead as short as possible to minimise  
loading the SB300 oscillator.
 
It was designed to work in this mode so it should be possible to make it  
work correctly. However, in my particular case, I installed a buffer in my SB301  
as I needed to use longer interconnect leads in my station console.
 
73

Bob 

G3OOU
G-QRP  6907
_www.g3oou.co.uk_ (http://www.g3oou.co.uk/) 

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