[Heathkit] sb-303 problem

[yash at aol.com]

Just finished with about 8 hours bench time with a sb-303. It worked poorly,but did basically work. Biggest problem, poor selectivity, poor sensitivity some hum and bad,tinny harsh audio.
  Did the following
1.   Pulled apart and cleaned all pots,switch's,connectors and a good general board and chassis cleaning,  looks  great.
2.   replaced all the filter caps in the power supply and replaced all the electrolytic's and coupling caps in the audio section.
3.   Did a total alignment,per the 303 manual with a AC vtvm and sig. generator,every thing peaked up,  as per the manual right on the numbers.
4. did some spot checks of voltages ,everything is well within spec.
     Put the 303 back in line, added the 47 ohm shorting plug for the LMO per the manual and turned it on
  SAME problems I started with,maybe  marginally better ,but just so ,all the original problems still exist, no more hum,same bad audio.and all the other problems.
       I don't know where to go from here. This is not a complicated receiver,but I just can't seem to get a handle on the problem. I had a 303 a few years back and it would stay right there with my drake r4b,so I know the 303 can perform very well. 
  Any suggestions at this point would be very welcome.
thanks
dale wt4t