[R-390] R-390 (non-A) - Low RF Gain or bad Antenna relay?

Wed Nov 6 15:25:45 EST 2024

Jerry,  Since you will be using a tube extender, you can check the
oscillators by scoping the mixer cathodes on the extenders.  You may need
to decouple your scope using a 1 M res.

Regards, Larry

On Wed, Nov 6, 2024 at 12:11 PM Larry Haney <larry41gm2 at gmail.com> wrote:

> Jerry,  Yes, you can do  it with a tube extender.  Use a small coupling
> capacitor between sig gen and grid of the mixer.  If a mixer oscillator is
> not functioning, you can simulate it with your sig gen and feed it into the
> cathode.  It will require a little larger cap for this, maybe around 500
> pf.  Have fun.
>
> Regards, Larry
>
> On Wed, Nov 6, 2024 at 12:00 PM <k1jos at att.net> wrote:
>
>> Hi Larry,
>>
>>
>>
>> Thanks.  Do I need a set of extension cables to inject into the 3rd
>> mixer or would a tube extender work?  I recently repaired my KWS-1 and
>> thought that was tight, but I didn’t have to disassemble module to test
>> each stage !!
>>
>>
>>
>> Any practical advice would be appreciated.
>>
>>
>>
>> Jerry
>>
>>
>>
>> *From:* Larry Haney <larry41gm2 at gmail.com>
>> *Sent:* Wednesday, November 6, 2024 2:00 PM
>> *To:* k1jos at att.net
>> *Cc:* r-390 at mailman.qth.net
>> *Subject:* Re: [R-390] R-390 (non-A) - Low RF Gain or bad Antenna relay?
>>
>>
>>
>> Hi Jerry,  For setting the IF gain, you can try a couple things, remember
>> where the gain pot is at right now and then turn it all the way up and see
>> if that improves your problem.  If not, put it back where it was at (it's
>> not critical). Are all the bands about the same deafness?
>>
>>
>>
>> The input to the 390 & As IF decks is high impedance (about 1 k ohm I
>> think), so the 390A suggests a 50 ohm shunt for use with the 25D sig gen.
>> If you can adjust your sig gen for 1 k ohm, that should work close enough
>> for you to tell if the IF deck is the problem or not.
>>
>>
>>
>> It sounds like it's the RF deck.  You could work backwards from the 3rd
>> mixer injecting a signal and see if the required signal input keeps getting
>> considerably less and less the closer to the antenna you get.  I'd check
>> the 3rd var IF alignment real closely, you can manually lift the inductor
>> bar up and see if the signal strength goes down right away or not.  It
>> should.  One of the small SM coupling caps may have gone bad
>>
>>
>>
>> Regards, Larry
>>
>>
>>
>> On Wed, Nov 6, 2024 at 10:18 AM <k1jos at att.net> wrote:
>>
>> I have checked all tubes and found two weak 6JB6 in the AF module which I
>> replaced.  The primary issue seems to be inadequate RF gain.
>>
>>
>>
>> I did a side-by-side comparison of strong and weak stations above and
>> below 8MHz with my HF-380 and R-390.  I found the R-390 was deaf to signals
>> with less than an S-6 reading on the HF-380.  I also compared signal
>> strength to a Flexradio 6700 and the Flex and HF-380 gave similar
>> readings.
>>
>>
>>
>> Would adjusting the overall Gain (TM-11-856, para. 71 a and b) be a
>> reasonable next step?  If my R-390’s Gain is indeed low, the adjustment
>> requires a signal generator with a Network, Impedance Matching device
>> (CU-406/URM-25F, part of the AN/ URM-25”.  I have a Rigol signal generator
>> where the load impedance can be set to provide correct voltage readout.
>>  Is there any recommended method when not using the AN/URM-25 and impedance
>> matching device?
>>
>>
>>
>> Jerry
>>
>>
>>
>>
>>
>> From: k1jos at att.net <k1jos at att.net>
>> Sent: Monday, November 4, 2024 8:14 PM
>> To: 'r-390 at mailman.qth.net' <r-390 at mailman.qth.net>
>> Subject: R-390 (non-A) - Low RF Gain or bad Antenna relay?
>>
>>
>>
>> Several years ago, Don W5OR (SK) asked me to share administrator
>> responsibilities with him to manage the r-390 mail list server.  While I
>> have several R-390/R-390A, I never had the need to repair any.  So, this is
>> my first post for help…
>>
>>
>>
>> My R-390 has gone partially deaf.  It sits in a fan-ventilated rack which
>> rarely gets rolled around.  Its last maintenance was in 2002 when Don
>> Heywood did an incredibly thorough job.  It worked great until a few weeks
>> ago when it became completely deaf.  My initial inspection found something
>> bizarre, the RF cable had popped loose (see attached picture)  After
>> reconnecting, it was only partially deaf.  I could tune in moderate and
>> strong signals across all bands but I need to set Local Gain to 7 or higher
>> just about where hum comes into the speakers.  The best description is what
>> you might expect to hear if RF Gain is not at maximum. Also, another ‘clue’
>> is I noticed that switching to CAL does not remove or attenuate a CW or
>> phone signal.. sounds same as if in AGC.  I checked that the antenna relay
>> is working:
>>
>>
>>
>> *       On powering to STBY – the relay clicks and pulls in visually and
>> checking both twin-ax connector pins, I confirmed both 2 pins are
>> grounded.
>> *       On switching to AGC or MGC – the relay clicks off and neither of
>> the twin-ax pins are grounded.
>> *       On switching to CAL – the relay clicks on again and the twinax
>> pins are grounded. The manual says in CAL, unlike in STBY, +180 volts de is
>> applied to the rf. if., af, and calibration oscillator stages and in both
>> STDBY and CAL, terminal 2 of CR801 is grounded to apply 6 volts dc to relay
>> K101.
>>
>>
>>
>> I also checked Break-In and that works in AGC giving complete silence,
>> but the manual says that’s achieved by grounding the AF output in addition
>> to grounding the antenna inputs.  How does that prevent overloading the RF
>> section with a local transmitter?
>>
>>
>>
>> My 1st question is, why is there no signal attenuation when switching to
>> CAL?  Is there anything in the electromechanics that could explain this?
>> Is this coincidental to the R-390 partial deafness or part of the same
>> problem?
>>
>>
>>
>> Pending some replies, my next step is to check the tubes in the RF and
>> audio stages but I’m hoping that my description may sound familiar to the
>> many experienced members of the list.
>>
>>
>>
>> TIA Jerry
>>
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