[R-390] IF alignment questions and noise issue.

Mon Jan 19 15:42:32 EST 2015

Dennis,

Its a mixing thing.

A stage is making noise.
It depends on what it has to mix with to determine if it comes out in the audio.

Stop guessing.
Start a proper systematic trouble shooting sequence.

Roger AI4NI

-----Original Message-----
From: Dennis Wade <sacramento.cyclist at gmail.com>
To: Roger Ruszkowski <flowertime01 at wmconnect.com>
Cc: R-390 HF Receiver List <r-390 at mailman.qth.net>
Sent: Sun, Jan 18, 2015 11:33 pm
Subject: Re: [R-390] IF alignment questions and noise issue.

Thank you Roger.  I'll work on those tests this week.

A little more information after playing with it more last night.  Its definitely present on all bands.   

Disconnecting the RF deck at J208 stops the bacon frying.

Pulling V201 stops the noise.  Re installing V201, and then pulling the 1st mixer, etc on down the chain I can still hear the frying, but at a reduced level. I noted that the only silver mica in the signal path on all bands is the 33pf grid coupling cap. The others are ceramic or other types.  

Another list member was puzzled by being still able to hear the noise with the mixers pulled and wondered if the AGC line was involved.  At his suggestion I looked at the AGC bus with a scope and could see some noise in tandem with what I was hearing.  I also grounded the AGC bus which silenced the noise, and indeed the noise is completely gone in MGC mode.  By the way, its clearly a heat related problem, I turned on the receiver this afternoon about 2:15.  The noise didn't show up until after 4.  I almost declared is a ghost. 

And finally, prior to discovering the noise, I had been working with the IF deck checking alignment and setting gain.  No noise whatsoever after several hours powered on.  It really seems to be narrowed to the RF deck, possibly a component on the AGC line?  I'm headed back to the schematic and see what's there.  

And thanks to all for the DC block responses.  

               Dennis

On Sun, Jan 18, 2015 at 3:49 PM, Roger Ruszkowski <flowertime01 at wmconnect.com> wrote:

Dennis,

Does the receiver fry bacon on all frequencies.
If you are  frying bacon under 7.999 as well as over  then the first mixer and 17 Mhz osc are OK.

Inject 455 Mhz at 150 micro volts into the If deck at J518.
Set the band width to 2Khz
set the BFO off
Adjust the IF gain for -7 volts on the DC load.
Set the generator modulation on at 30 %
Observe about 1/2 watt on the local output across a 600 ohm load.

Listen for your frying bacon and be certain it is not in the IF deck.
Set the generator to CW and observe a 28 - 30 DB drop in signal level at the local audio output
on the back panel.

If your IF deck and audio do not pass this test start fixing problems in the IF and Audio deck.

pull V203 the second mixer.
Set the receiver to some number over 8 Mhz.
Inject 455 Mhz into E 211.
reduce the generator level until the DC load level is - 7 volts.

Repeat the noise test. you may only get 20 : 1 or better but not 28 - 30 DB difference.
Listen for your frying bacon and be certain it is not in the third mixer and VFO.

Pull the RF tube V201
Do the math and inject a frequency into E210
Listen for your frying bacon and be certain it is not in the second  mixer and crystal oscillator deck

If you fry bacon only under 7.997 you have a first mixer and 17 Mhz osc problem.

If you got this far you have an RF Amp V201 problem.

Go looking for a sliver mica cap as suggested.

Open the ground lug bolt and retighten it on any cap to ground you suspect before you
just replace it.

You can turn the receiver up on end and work on the RF deck was it hangs out of the receiver.

You just can not go dialing the VFO all over.
But set the VFO to 500 KHz and you can work up and down the Mhz at the 500 KHZ point.

Its enough to trouble shoot the problem.

Roger AI4NI

-----Original Message-----
From: Dennis Wade <sacramento.cyclist at gmail.com>
To: R-390 HF Receiver List <r-390 at mailman.qth.net>
Sent: Sat, Jan 17, 2015 10:05 pm
Subject: [R-390] IF alignment questions and noise issue.

Good evening,

       Setting out to check alignment of the variable IF stages, and the
Y2K manual states to couple the generator through "Test Lead CX-1363/U" to
test points E209 and E210.

       I have an HP8657B, and my guess is the test lead mentioned above
isolates the generator from the receiver, probably through a DC block.  How
to you all couple a modern generator to those test points in practice?

        Noise issue:  The RF deck (that some may remember had shorted B+
cap) now seems to develop a "bacon frying" sound several minutes after a
cold start.  I've isolated the noise to the RF deck by unplugging J208 when
the noise is present and it completely goes away.  I've also swapped all
the tubes in the signal path with no change.

           Any ideas on how I might isolate the noise to a stage in the RF
deck?

 Thanks,

 Dennis

--
"If they trust you, it is an extraordinary privilege, and you simply can't
abuse it."
      - A. Alan Post  1914-2011.  California Legislative Analyst 1949-1977.

-------------------------
Dennis L. Wade
KG6ZI
Carmichael, CA

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Please help support this email list: http://www.qsl.net/donate.html

-- 

"If they trust you, it is an extraordinary privilege, and you simply can't abuse it."   
      - A. Alan Post  1914-2011.  California Legislative Analyst 1949-1977.

-------------------------
Dennis L. Wade
KG6ZI
Carmichael, CA