[Hallicrafters] R-274 Sensitivity Testing

[Thomas Hoyer]

I have my 274 up and running on all bands now. A couple tube swaps (good for good but one works better the the other....) and a couple capacitor changes.


In checking the sensitivity I find that while it seems to receive well, the sensitivity is somewhat lacking. A quick check based on the RF stage gain chart in the manual confirmed my suspicions. For example, I need 12uV input to get 10 (-10v actually) across R107 on band 1. This is after alignment per the manual. Walking through the other section confirms that each stage needs more input then shown to get the -10v specified.


So I decided to back up and check the other sections per the manual. I found just the opposite when checking the 2nd and first 455 IF stage gaines in that I needed less, much less, input to get the specified output. 


Normal Broad, V10-1, calls for 30,000uV to get -10V. I only need to input 12,500uV to get -10v
V9-1 calls for  3200uV and I input 1100uV to get -10v
V3-7 calls for 700uV and I only need 260uV.


These values seem to be way off..... Too much stage gain?




Going through the second mixer starts to show a possible issue:


T26-7 calls for 1500uV, but 500uV will get me to -10v
V6-1 calls for 170uV, but 120uV will get me to -10v


V3-7 calls for 120uV but I need 220uV to get to -10v. So it looks like this stage has some issue. I went over all of the resistance and voltage checks around V3 and all are well within spec. Most all of the resistors have been replaced through the entire radio as have the capacitors except for the ceramic ones and a couple of the micas that seem to measure fine.


There are a couple things I'm wondering about (as I bounce around subject areas here), I did replace a couple of the 33pf coupling capacitors between stages. I used mica caps vs the ceramic called for in the parts list. Should I go back to a ceramic? Maybe the mica is messing with the coupling between circuits?


I did verify that the output of my HP signal generator is correct at the various frequencies and voltages levels specified using my spectrum analyzer.


Looking for some clues.......


Tom
W3TA