When will I ever learn? (SB-101)

[Bob Burns]

Hi Kenth
 
It passes the smoke test so not all bad. You can probably get a circuit  
from ebaman or mods.dk and you can get a full manual in the USA for a  price. 
It can only take one faulty component in a shared circuit to stop it all  
working.
 
Can you hear any noise in the speaker of headphones with the AF Gain fully  
up?
 
Does the RF gain control have any effect on the S-Meter? It should.
 
As it does not smoke and assuming that all valve heaters are working I  
would start by measuring the supply voltages at the incoming 11 way  socket and 
then the circuit voltages - the manual does give a table of  expected 
values.
 
With the SB101 switched off, cold and disconnected from the PSU, measure  
all of the fixed resistors and see how many are significantly outside their  
tolerance range. This is a well known problem with 30+ year old equipment 
that  used solid carbon resistors as they do go high in value over time. 
Replace any  that are out of tolerance with a carbon film type of the correct 
value and  rating.
 
With it powered up:
 
Is the regulator tube lit? Located near the audio output valve towards the  
back right when looking from the front panel.
Can you measure any output from the internal LMO with a scope or counter?  
Set the Frequency Control to "Locked Normal".
There is a test point for the band oscillator which is a tuned anode tuned  
grid type. You should see a non zero reading on all bands - some minor  
adjustment may be required to the oscillator tuned circuits but take care with  
any very stiff cores as they break easily.
 
Just a few items to look at first but keep in mind that these are  
repairable with some patience and I reckon that they are much easier to work on  
compared to the modern black boxes.
 
73

Bob

Bob F Burns
Amateur  Radio Callsign: G3OOU
G-QRP Member No: 6907; QRA: IO91WH; WAB: TQ25
Joint  web site with Crystal Palace Radio & Electronics Club: 
_www.g3oou.co.uk_ (http://www.g3oou.co.uk/) 
Technical web site: _www.qsl.net/g3oou _ (http://www.qsl.net/g3oou) 

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