[R-390] RF subchassis separation

[Mark Richards]

Roy,  Lots of good ideas, and a few comments..

On 03/14/2014 15:16, Roy Morgan wrote:
> Sounds like you did well about the crystals. At $20 to$30 each, you 
> might better find some ole time test things at a hamfest.  Yes, a 
> scope does a fine job of testing crystals to see if they are working.
I can only assume that if they pass the scope test, they are fine in 
circuit.  Is this reasonable?  Might I safely assume that crystals 
either work or they don't?  Never heard of a weak crystal.

Gosh, I remember as a novice grinding these to change frequency! Like 
cutting wood: you can take off, but can't put back on.


>
> If all you have is digial meters, there is nothing like a moving 
> needle on a meter to give you the clear peak on the circuit you are 
> tuning up.  RCA Senior Voltohmist, Triplet 980(?) are all fine.  The 
> military ones include the TS-505 and many others.
Poor timing on my part to work on R-390A in advance of finishing rebuild 
work on a Simpson 260 whose internal batteries were left to rot the 
battery contacts and unfortunately also munging one section of the 
rotary switch.  But I have little Radio Shack handy meter that does OK.


>
> I expect to put together a test fixture for working on R-390's (and 
> other receivers).  It will have a 0-20 volt meter for the diode load, 
> a well insulated connection (teflon) for the AVC line, an audio meter 
> and loads for the audio output (or at least connections so I can use 
> the military or GR audio output meters I have.) and may be one or two 
> other things, maybe line voltage meter.  There will be an umbilical 
> like bundle of wires to fasten to the back of the radio and be 
> connected to all the test points.

Wow!  Won't that be handy.  Would it be feasible (and of course, useful) 
if this included input for signal gen with perhaps switch to route it to 
IF or antenna in as needed for test?  Maybe a step attenuator on the 
signal generator line and balanced antenna input load resistors, too?

Would your test fixture comport with extension cables for working on 
modules apart from main chassis?  I built these form my R-390a restore, 
and they have already been very handy.


>
> Just because the crystal is acting like a crystal does not mean the 
> oscillator is working.  Here is another thing to do:
>
> (Every boatanchor test bench should have a general coverage receiver 
> handy, like a second R-390A or an R-388.)  Get some other receiver 
> nearby and feed the antenna jack with a little sniffer - this can be 
> two turns of wire, or just the center conductor of a piece of coax - 
> put the sniffer around or near the oscillator tube and see if you get 
> the signal in the other receiver.  It will be loud and clear if the 
> oscillator is working.
>
> Also, the VTVM with a one meg resistor in the probe can be used at the 
> oscillator grids or mixer input test points to see if the dc volatge 
> is what it should be - the R-390 has test points at some or all of the 
> oscillator grids or mixer input points - DMM's with their plain test 
> leads will or can swamp out the oscillation and lead to a false negative.

I am about to do this, as the calibrator still doesn't work.  I replaced 
v207 (5654) with a known good one and got a stronger signal from cal but 
it still has a 60HZ (or maybe 30HZ) rasp on it, and it drifts wildly.

Noted that when peaking T207 that this part of the circuit is acting 
unstable.  I'd peak it and get tremendous improvement and then it will 
suddenly drop to a lower level.  Re-peaking, there seemed to be a point 
where tuning past the peak would cause it to drop off completely.  
Strange.  Seems dynamic enough to be tube-related.  But during tuning 
the 500KC and 1MHz bands went completely dark...  So might have to pull 
the RF deck (after I change out some more tubes)... as it seems there 
may be flaky caps in the general area that failed as things got warmer.


>
> The URM-25 is a very versatile generator.  Roger on the list tells how 
> hundreds of technicians used them for decades to align the R-390 
> receivers and that's all they needed, plus a VTVM and audio output 
> meter.  They produce 400 and 1000 cycle output signals and have a 1 mc 
> calibrator
I love mine.  I have two with one for parts as the other works better 
and on all bands.


>
> About the audio output meters:
> All these meters share common characteristics:
> - Will measure from milliwatts up to 20 or 100 watts
> - Have a range of internal audio load resistances from an ohm or two 
> up to maybe 20,000  Set usually by a rotary knob and decimal 
> multiplier switch
> - Frequency response across the audio spectrum
> - Use metal oxide rectifier type meter system - which can probably be 
> repaired if not working.
>
>
> - the military one is TS-585 (?).  It is quite clunky, somewhat 
> unimpressive looking,  and is often missing its cover (and maybe 
> strap) if you find one at a fest.  (likely once installed at a test 
> bench, the covers got heaved out since they were in the way).  I have 
> one that did not compare well with my GR one, but a bit of fiddling 
> with the calibration pot(s) and shunt resistors brought it back to 
> accurate.  Totally adequate for all receiver and even small hi fi amp 
> testing.  Will last forever.
>
> - The GR modern one is the 1040A.  These are offered on eBay for up 
> toward $400.  That is too high.  At a fest you may get one for a 
> quarter of that or less.  They are sought after and often pricey, but 
> worth a fair amount if not bashed or fitted with odd knobs (as is one 
> on ebay just now).  Will last forever.  If you see one at a fest for 
> $100 or less, buy it. If you see two, buy them both and put one on 
> ebay and come out ahead!  This is one example of GR excellence - for a 
> small (heavy!) box with three controls, one set of terminals, and a 
> meter, it gives astounding versatility, accuracy and functionality.  
> Measures 2 milliwatts to 20 watts full scale, 0.6 ohms to 32 kilohms.

It kills me to think that I owned one of these years ago.. and a bunch 
of other test gear (I was in broadcast engineering).  Time, and life, 
and all my wonderful tools departed.  Maybe will meet up with some at 
ham fests someday before I am planted.

/Mark Richards
K1MGY