[Heathkit] SB-102 low drive problem

[Edward B Richards]

Hi Bob;

I had a similar problem with a HW-101. I realigned it 3 times but still
low power out. I finally found the problem by taking RF level
measurements through out the transmitter. The transformer on the VFO was
out of alignment, causing low signal into the second mixer (I think).
This adjustment is not called out in the alignment procedure, so I missed
it each time I did an alignment. Good luck.

73, Ed Richards K6UUZ


On Sun, 27 Feb 2005 23:30:45 -0600 Robert Nickels <w9ran at oneradio.net>
writes:
> My SB-102 has low output on every band except 75 meters.  40 is about 
> 
> half, and 20 and above will output produce a few watts.  The tubes 
> in 
> this radio all test very good and it receives fine.  The main 
> problem 
> was is was VERY dirty, but cleaned up well.
> 
> I've aligned the receiver per the instructions using the crystal 
> calibrator as a source.   Every coil peaked, no broken slugs and 
> uisng a 
> good signal generator I can hear a 1uv signal on all bands 80-10.  
> I've 
> checked signal levels all the way up to the 2nd Transmit Mixer V6, 
> the 
> 6CB6 just before the driver.   The driver grid coils are in the 
> mixer 
> plate circuit and  the RF level at the grid of the driver drops just 
> 
> like the output, even though the level of both of the mixer inputs 
> are 
> the same on all bands.  It's pretty obvious that only the 80 meter 
> coil 
> is being tuned right, yet they are peaked.  . In all the SB-100 
> series 
> the 80 meter coil is in the circuit all the time, and a the 3F wafer 
> on 
> the bandswitch switches the coils for the other bands in parallel as 
> you 
> change bands.   I suspected dirty contacts - and they were - and 
> I've 
> cleaned them with DeOxit twice.  Checked and replaced a couple of 
> resistors but no improvement.
> 
> Here's what's really got me stumped:   The same coils and capacitors 
> are 
> used on Receive as well as Transmit so how can there be anything 
> wrong 
> with them?   That's why the alignment is done using a received 
> signal, 
> once they're peaked they are supposed to be good to go on both Rx 
> and 
> Tx.     My coils are peaked, as proven by the receiver sensitivity.  
> Yet 
> the drive level out of this stage falls like a rock as you move up 
> the 
> bands! 
> 
> I would really appreciate any ideas where to look as I know this 
> same 
> scheme was used in thousand of HW and SB radios.  I'd normally 
> suspect 
> weak finals but like the rest of the tubes, they test high on the 
> good 
> scale, and even weak finals should put out more than this.
> 
> Thanks!
> 73 Bob W9RAN
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