[R-390] R-390 Poor Sensitivity

[hackmohr at myfairpoint.net]

Greetings All,

Looking for some help trying to figure out the cause of overall poor sensitivity in a Collins R390. I've been rebuilding this one over the past few years and finally brought it back to life in April. (Details here: http://home.myfairpoint.net/~hackmohr/r390/) After going through one round of alignments, subjective reception comparisons with an R390A and a 51J-4 show it performs rather poorly compared to these two. I've begun to check stage gain readings and I'm now finding discrepancies which are starting to point me in the right direction. I think. I'm using an AN/URM-25 Signal Generator that I'm assuming is in the ballpark when it comes to output levels. They may not be exact but I beleive they are close enough to detect the discrepancies I'm seeing which are on an order-of-magnitude level. 

It appears that the IF Unit is functioning normally. I can more or less duplicate the stage gain measurements shown in the manual all the way through the 6 stages. A ~50uV 455kHz signal at J526, the input to the module gives the -7VDC reading at the Diode Load terminal. Moving to the RF Deck I find the first problem. At the input to the 3rd Mixer at E210 a 50uV 455kHz signal sometimes passes through OK, other times it goes into some type of oscillation. A 2.4mHz signal at the same level always goes into oscillation. At E209 on the 2nd Mixer I get no oscillations, but it consistantly takes around 4000uV to get the proper Diode Load reading at 2.4mHz. I've tested and swapped out both the 2nd and 3rd mixer tubes. All voltage and resistance measurements for the 2 tubes are normal. Z222 has also been swapped out with an assumed good one but to no avail.

Going back through the RF stages with the 2.4mHz signal I find: E208 10uV,[??] E207 11000uV, E206 2800uV, Bal Ant: 400uV. Anyone with experience in stage gain mesurements see any patterns here? Something I'm missing? Maybe something in my test setup is causing the oscillations? Any and all help will be appreciated. Thanks.

Mark
WA2FXM