[R-390] R-390 Digest, Vol 174, Issue 11

Sun Oct 14 10:58:17 EDT 2018

Hi Dave,Sounds like you have to isolate the problem.  I think I would start by putting a 455 KHz signal with some modulation into the IF unit at about 75 uv, monitoring the diode load voltage and listening to the audio out at startup.  If you're good there then start putting a signal into the various test points in the rf deck.  Put signals in starting at E208 then E209, E210, E 211 and T208 output.   Do this in MGC only.

I've had similar problems and have found that mica caps with B+ on them are the most failure prone components in the radio and the toughest to pinpoint.  That's what I would suspect and probably in the rf deck.  Good luck! 

In a message dated 10/14/2018 10:07:05 AM Eastern Standard Time, r-390-request at mailman.qth.net writes:

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Today's Topics:
  1. Re: Gain drift (dog)  2. Re: Gain drift (Larry H)  3. Re: Gain drift (dog)  4. Re: Gain drift (Larry H)  5. Re: Gain drift (dog)

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Message: 1Date: Sat, 13 Oct 2018 13:32:31 -0400From: dog <agfa at hughes.net>To: r-390 at mailman.qth.netSubject: Re: [R-390] Gain driftMessage-ID: <c2c0a87c-3908-2bfe-f063-1e662f6a28e2 at hughes.net>Content-Type: text/plain; charset=utf-8; format=flowed
I'll try some of these things, especially measuring the AGC when it happens. It takes a while because it's not always happening. I'm still not sure if it happens below 9MHz but I'm thinking it does. It's not just on one band and the BW on the IF appears irrelivant except for more or less noise out which is normal. This is one of those buggars to find, it takes so long to happen. I thought about cleaning the bandswitch but haven't gotten it out yet. I see the 2 green screws on the front of the 2nd Osc module to the RF module are missing. I'll replace them next time the front panel is off.

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Message: 2Date: Sat, 13 Oct 2018 17:28:28 -0700From: Larry H <larry41gm at gmail.com>To: dog <agfa at hughes.net>Cc: R-390 Forum <r-390 at mailman.qth.net>Subject: Re: [R-390] Gain driftMessage-ID:    <CAOSJ_nuZvRM6PRSMc=AwNs-tJOdStEjmOe46v6UPRu8yRE9FWA at mail.gmail.com>Content-Type: text/plain; charset="UTF-8"
Hi Dave, This sounds like a tough one.  It could be most any contact: tube,switch, or wiring socket.  RF deck switch would be my 1st choice.  Do youhave any subchassis you can switch out?  You might need to narrow it down.And yes, it could certainly be one of those SILVER MICAs acting up.  Youmight try some cold spray on them one at a time.  Since it happens on morethan one band, you can eliminate the 'band' section of the RF deck (but notthe switch or tube contacts), but it could be the 2 variable IFs.  If ithappens above 8mh, then it should not be the 1st var IF.
Good luck Dave.
Regards, Larry
On Sat, Oct 13, 2018 at 10:32 AM dog <agfa at hughes.net> wrote:
> I'll try some of these things, especially measuring the AGC when it> happens. It takes a while because it's not always happening. I'm still> not sure if it happens below 9MHz but I'm thinking it does. It's not> just on one band and the BW on the IF appears irrelivant except for more> or less noise out which is normal. This is one of those buggars to find,> it takes so long to happen. I thought about cleaning the bandswitch but> haven't gotten it out yet. I see the 2 green screws on the front of the> 2nd Osc module to the RF module are missing. I'll replace them next time> the front panel is off.>>> ______________________________________________________________> R-390 mailing list> Home: http://mailman.qth.net/mailman/listinfo/r-390> Help: http://mailman.qth.net/mmfaq.htm> Post: mailto:R-390 at mailman.qth.net>> This list hosted by: http://www.qsl.net> Please help support this email list: http://www.qsl.net/donate.html>

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Message: 3Date: Sat, 13 Oct 2018 21:13:29 -0400From: dog <agfa at hughes.net>To: r-390 at mailman.qth.netSubject: Re: [R-390] Gain driftMessage-ID: <ac78846a-555c-5a6f-cd55-cf3c970a6d3f at hughes.net>Content-Type: text/plain; charset=utf-8; format=flowed
OK, new data. I socked 50uV into the antenna balanced input right at cold start up. After a few minutes and things settle down, the agc is at -5VDC. Carrier meter is 48. Let it run for a while and soon the Carrier drops to 25-30 and the AGC goes to -2.4VDC. I've got AM mod going into it and the AC output of the diode load is holding steady at about .5VAC no matter which carrier reading. Then after a while the Carrier meter goes back to 48 with the corresponding AGC of -5VDC. Does this several times and I get tired of watching it so one time while it's on the 25 carrier, I switch it to STBY and back to AGC. Goes right back to Carrier of 48 and corresponding AGC. Something is drifting but it's hard to hear on the audio, the AGC seems to be working but something is changing the gain during warm up.
After letting it sit for over an hour it seems to stabilize on Carrier level of 48 and AGC of 4.3. I have a list of uV vs. AGC voltage, I'll check all the agc vs uV voltages out after it warms up good.
I'm thinking with no antenna the AGC is down (.6VDC) where it's not working so a small change in gain produces a large output on the diode load audio output. That's why I'm hearing the difference with no antenna. Not so much output difference when there's a fairly large signal present and the AGC is working. I'm really not sure how to find this other than a shotgun approach. It's very difficult to get to the components to shoot freeze spray on them while in operation.
I do have another RF and IF section, but the other RF section doesn't appear to work, I tried it once, but I may try it again. The other IF section,? the 2KHz mechanical filter has a shorted output and I've got it apart for repair so it's disabled at the moment. I may have to fix it first before I can proceed.

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Message: 4Date: Sat, 13 Oct 2018 20:55:12 -0700From: Larry H <larry41gm at gmail.com>To: dog <agfa at hughes.net>Cc: R-390 Forum <r-390 at mailman.qth.net>Subject: Re: [R-390] Gain driftMessage-ID:    <CAOSJ_nswVUfmTPSX+A8fcg4yfEyjE55sXv2S8LyyNmNTYJjeEA at mail.gmail.com>Content-Type: text/plain; charset="UTF-8"
Dave, When the gain drops and goes back to normal during the 1 hr warm up,are the transitions from good to bad gradual or quick?  About how long doesit take?  About how long does it stay in each state?  Is there any unusualnoise in either state or during the transition?  Do you need  to let the390A cool down for some length before it will happen again?  About howlong?  Are you using a 3tf7 osc current regulator?  Have you checked allthe mini bnc connectors?
Regards, Larry
On Sat, Oct 13, 2018 at 6:13 PM dog <agfa at hughes.net> wrote:
> OK, new data. I socked 50uV into the antenna balanced input right at> cold start up. After a few minutes and things settle down, the agc is at> -5VDC. Carrier meter is 48. Let it run for a while and soon the Carrier> drops to 25-30 and the AGC goes to -2.4VDC. I've got AM mod going into> it and the AC output of the diode load is holding steady at about .5VAC> no matter which carrier reading. Then after a while the Carrier meter> goes back to 48 with the corresponding AGC of -5VDC. Does this several> times and I get tired of watching it so one time while it's on the 25> carrier, I switch it to STBY and back to AGC. Goes right back to Carrier> of 48 and corresponding AGC. Something is drifting but it's hard to hear> on the audio, the AGC seems to be working but something is changing the> gain during warm up.>> After letting it sit for over an hour it seems to stabilize on Carrier> level of 48 and AGC of 4.3. I have a list of uV vs. AGC voltage, I'll> check all the agc vs uV voltages out after it warms up good.>> I'm thinking with no antenna the AGC is down (.6VDC) where it's not> working so a small change in gain produces a large output on the diode> load audio output. That's why I'm hearing the difference with no> antenna. Not so much output difference when there's a fairly large> signal present and the AGC is working. I'm really not sure how to find> this other than a shotgun approach. It's very difficult to get to the> components to shoot freeze spray on them while in operation.>> I do have another RF and IF section, but the other RF section doesn't> appear to work, I tried it once, but I may try it again. The other IF> section,  the 2KHz mechanical filter has a shorted output and I've got> it apart for repair so it's disabled at the moment. I may have to fix it> first before I can proceed.>>> ______________________________________________________________> R-390 mailing list> Home: http://mailman.qth.net/mailman/listinfo/r-390> Help: http://mailman.qth.net/mmfaq.htm> Post: mailto:R-390 at mailman.qth.net>> This list hosted by: http://www.qsl.net> Please help support this email list: http://www.qsl.net/donate.html

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Message: 5Date: Sun, 14 Oct 2018 10:06:07 -0400From: dog <agfa at hughes.net>To: Larry H <larry41gm at gmail.com>Cc: R-390 Forum <r-390 at mailman.qth.net>Subject: Re: [R-390] Gain driftMessage-ID: <989b3739-8928-a5c0-565e-b097346d331e at hughes.net>Content-Type: text/plain; charset=utf-8; format=flowed
Larry,
I have to let the rx cool off before it acts like this again. Upon startup, it has what I would call normal gain. Then after about 5 mins it drops, not all of a sudden but a gradual, but over maybe 5 seconds to the low gain point. Then after about another 5 minutes it drifts back to normal the same way. It seems to go through this cycle until after about an hour or so it stops and the gain is very near what it was at startup. I have seen the gain drop all of a sudden, but that was just once. When the rx is interrupted if it's on low gain cycle, the gain jumps back to normal, usually but not always. Yes, using the 3tf7. I have played with the mini bncs when the gain is low to no avail.
I was thinking about pumping in a signal to the balanced input from my transceiver, it puts out a +7dBm port that I can pad down if necessary, and watching various spots through the RF/IF signal path with the spectrum analyzer. But first I need to get some baseline measurements. Was thinking about doing this also in MGC mode to stop the AGC effect. Maybe I could isolate at least a stage that's causing it that way instead of having to shotgun it?
I also have a Fluke 189 recording voltmeter that I can use if necessary but I'm thinking the SA will be the easiest method.
I should check it below 9MHz first.
Dave

On 10/13/2018 23:55, Larry H wrote:> Dave, When the gain drops and goes back to normal during the 1 hr warm > up, are the transitions from good to bad gradual or quick?? About how > long does it take?? About how long does it stay in each state?? Is > there any unusual noise in either state or during the transition?? Do > you need? to let the 390A cool down for some length before it will > happen again? About how long?? Are you using a 3tf7 osc current > regulator? Have you checked all the mini bnc connectors?>> Regards, Larry>

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End of R-390 Digest, Vol 174, Issue 11**************************************