SB-102 - more troubles

[Bob Burns]

In a message dated 14/07/2009 20:58:49 GMT Standard Time,  
kgordon2006 at verizon.net writes:

However,  MY SB-102 LMO has the same problem, so I intend to 
drill the spot-welds  out on mine (carefully) and will reassemble it 
with self-tapping screws  like the earlier ones have been done.





Hi Ken
 
I have a well worn and much used SB301 which I acquired second hand many  
years ago. The self tapping screws in the LMO enclosure are showing signs of 
not  staying tight so I made an experimental enclosure to see what could be  
achieved using 16swg flat aluminium cut sheet for the six  sides held 
together using 1/4 inch (or 6mm) square aluminium bar  tapped 6BA for the 
fixings. The result is a rock solid case that just does not  give in any direction 
and is relatively simple to make. There is a circuit and  photo in the 
Vackar Oscillator section of my technical web site. The square bar  takes up less 
space than a right angle fold. There is no reason why this  technique could 
not be applied to the shape used in the Heath LMO  enclosure.
 
On a parallel theme I have been investigating the design of a valve  VHF 
transverter to go with my SB101 and I discovered that some of the classic  
1950/60 valve overtone oscillators like the Squier circuit have poor  
stability, overheat modern small quartz crystals and are extremely sensitive to  
loading by the following stage. Heathkit frequently used the tuned anode tuned  
grid circuit which is more stable than the Squier but still suffers from  
(modern) crystal overheating which shows up as drift immediately  following 
switch on and lasting up to a minute. Lowering the supply to the  oscillator 
does improve matters. One of the best results to date in respect of  
stability uses the crystal in a feedback loop from the anode tuned circuit to  the 
cathode with the grid being grounded - switch on drift was almost zero and  
no evidence of frequency wandering. All of the oscillators used the same 
HC18/U  crystal for comparison and one half of a 12AT7 as the active device. I 
will not  be using 6146 valves in the PA for 144MHz as Heath did.
 
73

Bob 

G3OOU
G-QRP  6907
Joint site with Crystal Palace Radio & Electronics Club: _www.g3oou.co.uk_ 
(http://www.g3oou.co.uk/) 
My technical site: _http://www.qsl.net/g3oou/_ (http://www.qsl.net/g3oou/) 

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