[heathkit] Heathkit SB400 low output

["larry allen" <larya@rogers.com>]

Hi Robin...
It may also be that you may have to simply resolder old solder 
connections... That radio was not built yesterday... If I got an old ham 
radio, I would simply start by resoldering the connections.. In a sence 
pretend you just got the kit and build it... again...
Larry ve3fxq

----- Original Message ----- 
From: "Robin McNeill" <robin@venturesouthland.co.nz>
To: "heathkit List Member" <larya@rogers.com>
Sent: Friday, March 27, 2009 5:41 PM
Subject: [heathkit] Heathkit SB400 low output


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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