[R-390] test equipment question

[2002tii]

Steve wrote:

>Some questions for the veteran radio techs - I'm organizing to start 
>work on a couple of R-390A's I just acquired.  What test equipment 
>do I need (as opposed to "would like to have") to get these babies running??

I'm no fan of the military signal generator, but if it's working 
properly it will get you through (particularly if you can trust your 
counter).  I third (or fourth) the recommendations of the HP 
8640B.  You can get brass replacements for the nylon gear that 
shrinks.  In any event, I rarely use a sig gen for *troubleshooting* 
-- rather, for calibrating and optimizing (in particular, the RF gain).

I happen to like using a 'scope for most of my measurements, but 
YMMV.  A Tek 465B is way overkill, but nice to have.  I prefer the 
comparable HP scopes personally (1740 series), but I like Teks just fine.

I see no need for an audio oscillator as long as the sig gen has an 
internal modulation oscillator.  If you feel the need for an RF 
voltmeter, get (better yet, build!) an RF probe to use with the Heath 
VTVM.  I don't have one, and have no idea what I'd do with one (but 
then, I use a 'scope for that).

The last thing I'd like to address is the philosophy of radio 
service.  Many of the comments posted so far seem to advocate a 
"shotgun" approach.  I don't.  There are certain components in most 
electronic products that are either failure-prone or that have 
devastating consequences if they fail (the infamous C553 in 390As is 
a prime example of the latter; all of the tubular capacitors in 
SP-600s are good examples of the former).  For the 390 and 390A, 
these components are are well-documented.  Beyond these, I see no 
reason to replace anything that hasn't given you a specific reason 
(except, perhaps, the electrolytic power supply caps, which might 
justify replacement just on principle).  Tubes, in particular (with a 
precious few exceptions), don't need to be changed "for the heck of 
it."  In most cases, they don't even need to be replaced when a tube 
tester says they are "weak."  With the exception of the first RF 
tube, all the tubes in a 390A can be dead flat -- 25% or less on a 
good tube tester -- and the radio will easily meet its 
specifications.  (I know -- I once filled a 390A with dead flat tubes 
on purpose and measured it.)  Time was when one might replace all of 
the tubes just so you wouldn't have to think about them again for a 
decade or three.  Nowadays, however, many of the new tubes available 
-- even lots of NOS tubes -- are not reliable (the new tubes because 
they are not made to a high standard, the NOS tubes because they have 
been leaking ever so slightly for 30-60 years and now have less than 
perfect vacuum).  So, often, the tubes in the radio are as good as 
you are going to do in terms of future reliability.

The real bugaboo in 390s and 390As are the crystals, which have aged 
all out of spec by now.  If I wanted to refurbish a 390 or 390A, I'd 
start there (if you can find anybody who can supply quality crystals 
today; I've heard mostly horror stories the last ten years or so).

Best regards,

Don

-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://mailman.qth.net/pipermail/r-390/attachments/20090213/9eff190e/attachment.html