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 Post subject: R-390A/URR first mixer
PostPosted: Feb Wed 21, 2018 4:25 pm 
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Joined: Sep Sun 08, 2013 11:53 pm
Posts: 42
Thanks to the suggestions in another thread , i made tests and yes , all that's above 9.000 is working , so further i went.
Got a signal generator at 1.5 mhz at the antenna and followed the trail. Found it at the rf amp , went all the way , step by step to the grid of V202 ( really .. if you want to follow this discussion , break out a schematic ) pin 6 and the signal reaches that point. My second test was to look for 17mhz at pin 7 coming from the first osc V207 . All fine there . But i can't make heads or tails of the output. If i measure at pin 3 of Z213-1 d.c voltage is 208 volts , at V202 pin 1 201 volts . pin 6 -0.16 on MGC and + 0.2 on AGC pin 7 is 7.52 VDC MGC and 8.0 on AGC
So grid to cathode measures in AGC mode is -7.2 V . ( Round up last digit )

The output of that stage is 17.000 mhz measured with the counter at Z213-3 pin 3
The manual indicates that the output for a 1.5 meg input signal should be at 18.5 MHz.
Like the input signal is missing from the mixer .. ideas ?

I did swap the tube for a known good one, no change.

I'm out of ideas how to debug this further .. If you got suggestions ill certainly give them a go and report.


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 Post subject: Re: R-390A/URR first mixer
PostPosted: Feb Wed 21, 2018 6:11 pm 
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Joined: Sep Wed 10, 2014 2:01 am
Posts: 1821
Location: Costa Mesa, California
I found that following the alignment procedure in the manual item by item, line by line, was pretty critical to finding the mechanical problems early. When my top bands all worked--8 MHz and up--and the lower bands did not, it was because the bandswitch was out of alignment. Go to page 110 paragraph d. It tells how you determine if the switch is correctly positioned. Do that before anything else so you don't waste time trying to find a problem that is strictly a result of the bandswitch being out of position. Also the page before describes how to tell if the cams are aligned.

Norm

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 Post subject: Re: R-390A/URR first mixer
PostPosted: Feb Wed 21, 2018 6:44 pm 
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Joined: May Sat 12, 2012 1:33 pm
Posts: 1628
Location: Rochester, NY.
On mine, I had a cam hub clamp that had cracked and was not turning the bandswitch shaft on certain bands. I didn't see the problem until I removed the front panel.


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 Post subject: Re: R-390A/URR first mixer
PostPosted: Feb Wed 21, 2018 8:51 pm 
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Joined: Sep Sun 08, 2013 11:53 pm
Posts: 42
Thanks the bandswitch is right on . Field change 7 not being applied i read 55k instead of 200k but pin 6 of the 17mhz osc is going infinite switching to 8 , band switch clamp is fine . Mechanical alignment is fine so are all the came at 7+000

Mechanics are ok .

Thanks for the suggestions .


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 Post subject: Re: R-390A/URR first mixer
PostPosted: Feb Wed 21, 2018 9:21 pm 
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Joined: Sep Wed 10, 2014 2:01 am
Posts: 1821
Location: Costa Mesa, California
What happens when you attempt the IF alignment protocol on pages 114- 116? Where does the process break down?

Norm

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 Post subject: Re: R-390A/URR first mixer
PostPosted: Feb Wed 21, 2018 9:28 pm 
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Joined: Jun Mon 24, 2013 3:00 pm
Posts: 1182
Location: Champaign IL 61822
One would expect a counter to measure 17 MHz at the plate of a mixer whose
oscillator is 17 MHz. If working properly with 1.5 going in to be mixed 18.5 and
15.5 will be present but won't be strong enough to swamp the 17. Even with
a tuned circuit at 18.5 unless the 1.5 is really really strong it won't swamp the
17.

To analyze this, you need either a spectrum analyzer or another known working radio
with good image rejection very loosely couple to that plate, then you should find all three
frequencies.


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 Post subject: Re: R-390A/URR first mixer
PostPosted: Feb Wed 21, 2018 11:18 pm 
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Joined: Sep Thu 01, 2016 3:56 am
Posts: 238
Location: San Jose, Ca.
Ric, Sounds like the problem is related to the 1st Var IF. Check that the slugs bottom out at 0.500 mc and are at the top at 7.+000. Try feeding 18.5 mc into the 1st mixer grid (with the dial at 1.5 mc) and see if you get that at the grid of the 2nd mixer. To check the top alignment, feed 24.5 mc in with the dial set at 7.5 mc.

Regards, Larry


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 Post subject: Re: R-390A/URR first mixer
PostPosted: Feb Thu 22, 2018 1:30 pm 
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Joined: Sep Sun 08, 2013 11:53 pm
Posts: 42
Larry i believe i see what you're driving at , for that injection test should i defeat the 17mhz osc to have just the injected 18.5 ?

Thanks for your patience , my first real repair job on a receiver this complex , AA5's are for juniors compared to this .. :D
When ill be done , ill have learned so much . Guess this is a rite of passage , after this , everything is simple.


Ric


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 Post subject: Re: R-390A/URR first mixer
PostPosted: Feb Thu 22, 2018 5:22 pm 
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Joined: Jun Mon 24, 2013 3:00 pm
Posts: 1182
Location: Champaign IL 61822
[quote="FuzzyTheBear"... , after this , everything is simple.
Ric[/quote]

Not quite! Not even for radios, let alone TVs.

Next, try a Racal RA-17 It uses extreme electrical complexity instead of mechanical.
But the result is probably second only to the R390A.

Then move up to to the Rohde and Schwarz EK-07 which can be used as anchor for
a 7000 passenger mega cruise ship.


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 Post subject: Re: R-390A/URR first mixer
PostPosted: Feb Thu 22, 2018 10:46 pm 
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Joined: Sep Thu 01, 2016 3:56 am
Posts: 238
Location: San Jose, Ca.
Ric, Very good question. I think it would definitely be easier if the 17 mh osc were disabled, but not necessary. The signal levels will be fairly low, so checking for the signal at the 2nd mixer might be easier with a good rx tuned at the correct frequency.

Since this is the problem area, a common problem is the contacts on the cans are not making good contact. Try using some deoxit sparingly on the pins and sockets of the 3 1st var IF tuning assemblies. While you have them out, you might want to do continuity checks for open coils and shorted caps. The silver micas in there do go bad and change value.

Have fun. Regards, Larry


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 Post subject: Re: R-390A/URR first mixer
PostPosted: Apr Mon 09, 2018 5:14 pm 
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Joined: Sep Sun 08, 2013 11:53 pm
Posts: 42
Well , turning the bandswitch all the way left while powered up started serious smoke signals this morning .. so .. more work da****
more work to be done , obviousy something is WAY wrong and the rf deck needs an overhaul now .. patience is growing thin :D
Im thinking shopping for a working RF deck and have one to repair at leisure. Patience patience patience


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 Post subject: Re: R-390A/URR first mixer
PostPosted: Apr Mon 09, 2018 9:54 pm 
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Joined: Sep Thu 01, 2016 3:56 am
Posts: 238
Location: San Jose, Ca.
Hi Ric, I understand the frustration. Since you're thinking about getting a replacement deck, you might as well remove this one and look at it. Can you identify the location where the smoke is at? The problem could be a an overloaded resistor, or one that just went bad (probably not). There is might be something wrong with the band switch - check all the segment centers to be sure they are reasonably tight on the shaft and turn correctly. And, sometimes it's a tube that went bad putting too much load on a resistor. Or one of the coupling caps shorted and causing high current in a plate coil and/or resistor. I'd start with measuring all the coupling caps.

There's a rainbow at the end.

Regards, Larry


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