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 Post subject: Re: R-390A alignment, some questions
PostPosted: Sep Mon 27, 2021 8:37 pm 
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Joined: Apr Wed 01, 2020 7:22 pm
Posts: 334
Gianni...

Even measuring the crystal oscillator voltages will load down the circuit and cause very inaccurate voltage readings, and even kill the oscillations. The higher the frequency of the crystal, the more inaccurate the reading will be. That's why I believe that the ultimate test is the sensitivity of each band rather than the output voltage of the oscillator.

73...Jordan VE6ZT


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 Post subject: Re: R-390A alignment, some questions
PostPosted: Sep Mon 27, 2021 8:50 pm 
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Joined: Jul Tue 06, 2021 6:12 pm
Posts: 71
I removed the RF deck. It was easier than the first time. The receiver is in good state and all the resistors I checked have retained their value.

C327 appears "less good" than the others (that seem almost new!). C327 has a little dirty dark "patina". Obviously I hope it is a sign of illness...

I can not wait that the new capacitors arrive!


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 Post subject: Re: R-390A alignment, some questions
PostPosted: Sep Tue 28, 2021 1:30 pm 
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Joined: Sep Thu 01, 2016 3:56 am
Posts: 450
Location: San Jose, Ca.
I have used both methods of measuring the oscillators outputs as described in the 1956 tech ref with success. I think measuring the dc on the mixer grid in standby is easier, so that is the one I use most of the time. I have not had an issue with the accuracy of the readings. I've found and fixed many problems using this method. If you prefer to measure the mixer cathode signal level voltage, make sure your scope can go up to the measured frequency and the probe will not load the circuit down.

Regards, Larry


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 Post subject: Re: R-390A alignment, some questions
PostPosted: Sep Tue 28, 2021 5:35 pm 
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Joined: Jul Tue 06, 2021 6:12 pm
Posts: 71
Hi Larry,
your document was very helpful. Without it I would not dare to afford this repair! :D
I found what I hoped: the six ineffective trimmer had the rotor and the stator stuck together! But now I have another doubt. I explain better with the photos. When I remount the trimmer, I must to be sure that the (A) contact touches the contact (E) on the stator. But ...they can rotate and even remounting could be imprecise. Have I to glue the parts in some way?

I found some trimmers that had the stator so sticked to the rotor that I had to use a blade to separate them.

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 Post subject: Re: R-390A alignment, some questions
PostPosted: Sep Tue 28, 2021 7:37 pm 
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Joined: Jan Fri 20, 2012 10:25 pm
Posts: 345
Location: Warminster, PA
Ahhh yes, the good old stuck trimmers............ There is a good article on how to repair them here:

http://www.collinsradio.org/wp-content/ ... ywrite.pdf

Be careful when trying to separate the discs they do break easily - don't ask how I know.

Been following this thread with interest, you are in good hands with those responding.

Tom
W3TA


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 Post subject: Re: R-390A alignment, some questions
PostPosted: Sep Tue 28, 2021 9:29 pm 
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Joined: Jul Tue 06, 2021 6:12 pm
Posts: 71
Aaaaaaah..... I saw your post ten minutes too late!

I BROKE one stator! They are really incredibly fragile. It is broken in 3 pieces and is not repairable!

I am trying to think some way to replace it, Perhaps a 0,5mm plastic or fiberglass with a small area in conductive paint?

Finding a spare capacitor is very difficult here in Italy and from USA it takes almost 20-30 days.


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 Post subject: Re: R-390A alignment, some questions
PostPosted: Sep Wed 29, 2021 12:55 am 
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Joined: Apr Wed 01, 2020 7:22 pm
Posts: 334
I've had excellent success freeing up "stuck" or stiff trimmers on the crystal oscillator board by spraying them thoroughly with De-Oxit D5 and letting them soak for a few hours or overnight. Once I get them to move more freely, I flush the D5 residue out with Electrosolve contact cleaner.

In fact, that's about the only thing I use D5 for as I find it far too messy to work with.

Cleaning the trimmers in the RF and IF transformers is simple enough, but you have to be careful not to break the coil leads when you move the contact clip on the underside to free the rotor disk. There's very little slack in the coil wires... but that's a subject for a different day...!

As always, YMMV...! 73...Jordan VE6ZT


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 Post subject: Re: R-390A alignment, some questions
PostPosted: Sep Wed 29, 2021 1:31 pm 
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Joined: Jul Tue 06, 2021 6:12 pm
Posts: 71
I am not sure I did the right thing to rebuild the broken ceramic disk. I used a 0,55mm thick PVC card, applying on it a thin metallic foil. This is the result:
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I believe that the area should be important for the capacity. It is not perfect (it is very difficult to handle that small foil), but it could work.
What do you think?
By the way... note on the original disk the signs that witness that has rotated (it should be fixed).


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 Post subject: Re: R-390A alignment, some questions
PostPosted: Sep Wed 29, 2021 4:03 pm 
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Joined: Jul Tue 06, 2021 6:12 pm
Posts: 71
A question: can the crystal deck be tested without the RF deck? I.e. there is some problems switching on the RX without the RF deck?


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 Post subject: Re: R-390A alignment, some questions
PostPosted: Sep Wed 29, 2021 5:56 pm 
Member

Joined: Jan Fri 20, 2012 10:25 pm
Posts: 345
Location: Warminster, PA
Tubetron wrote:
I am not sure I did the right thing to rebuild the broken ceramic disk. I used a 0,55mm thick PVC card, applying on it a thin metallic foil. This is the result:
Attachment:
IMG_0451.jpeg

I believe that the area should be important for the capacity. It is not perfect (it is very difficult to handle that small foil), but it could work.
What do you think?
By the way... note on the original disk the signs that witness that has rotated (it should be fixed).



Nice craftsmanship, but I think you'll find that the difference in the dielectric constant of the two materials will not give you the capacitance you are looking for.

I could be wrong.......

Tom
W3TA


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 Post subject: Re: R-390A alignment, some questions
PostPosted: Sep Wed 29, 2021 6:10 pm 
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Joined: Jul Tue 06, 2021 6:12 pm
Posts: 71
The best way to know is to try... :D

Maybe you know the answer to the above question? (can the crystal deck be tested without the RF deck?)


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 Post subject: Re: R-390A alignment, some questions
PostPosted: Sep Thu 30, 2021 12:39 am 
Member

Joined: Sep Thu 01, 2016 3:56 am
Posts: 450
Location: San Jose, Ca.
Hi Gianni, Very nice work. Really creative.

Yes, the 2nd osc will work without the rf deck being in. The trick is the capacitance load on the output - it needs to be the same as if the rf deck were installed. You need to put about a 1.5 foot coax on the output mb connector because the capacitance is reflected back to the osc output tuning circuit.

Have fun and good luck.

Regards, Larry


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 Post subject: Re: R-390A alignment, some questions
PostPosted: Sep Thu 30, 2021 10:51 am 
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Joined: Jul Tue 06, 2021 6:12 pm
Posts: 71
Signal received, Larry, thanks!


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 Post subject: Re: R-390A alignment, some questions
PostPosted: Oct Fri 01, 2021 7:57 pm 
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Joined: Jul Tue 06, 2021 6:12 pm
Posts: 71
NEWS!
The capacitors arrived. I have done what follows:
- The RF deck is dismounted from the mainframe. I just connected P108, without remounting the RF deck on the mainframe and without the OSC2 module
- I tested again E209 in standby, to be sure: -2.97V
- I replaced C327 with the new one
- I tested again: -4V. The recapping had worked!

I was happy BUT... adjusting the nucleus of T207, the voltage at E209 can reach an incredible -15V.
What am I doing wrong? How must I tune T207 at best? I found no indication in the manuals.


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 Post subject: Re: R-390A alignment, some questions
PostPosted: Oct Fri 01, 2021 8:45 pm 
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Joined: Apr Wed 01, 2020 7:22 pm
Posts: 334
You can't properly align T207 until the RF deck is re-installed and active... I "usually" peak T207 for maximum receive sensitivity on the 7MC band using background noise and the Line Level meter with no antenna connected. Others are sure to have their own method...

You'll find a world of information about maintenance of the RF Deck here:

https://www.r-390a.net/Pearls/RF_deck_electrical.pdf

It's well worth reading before you re-install the deck...

73...Jordan VE6ZT


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 Post subject: Re: R-390A alignment, some questions
PostPosted: Oct Fri 01, 2021 8:59 pm 
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Joined: Jul Tue 06, 2021 6:12 pm
Posts: 71
Thanks, I will read it.

So have I not to worry about this high value?


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 Post subject: Re: R-390A alignment, some questions
PostPosted: Oct Sat 02, 2021 12:24 am 
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Joined: Sep Thu 01, 2016 3:56 am
Posts: 450
Location: San Jose, Ca.
Hi Gianni, Great work.

Jordan is right in that you will not be able to correctly set T207 until the RF deck is reinstalled or with the 2nd osc connected to it. The reason - that reflected capacitance. No way to get operating resonance until it's al connected together.

For setting T207, there's a lot of discussion about it in the Pearls (as noted). The conclusion is that it should be set at the point where all trimmers will adjust for a peak and they are not at their end of travel. You will need to make an exception for the one you repaired as it's capacitance may not be in the correct range. If it is not, you can add or subtract capacitance in parallel with it to get it in range.

You will probably need to adjust T207 a few times a little at a time until you find the 'sweet' spot.

And no, you don't need too worry about too much output.

Regards, Larry


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 Post subject: Re: R-390A alignment, some questions
PostPosted: Oct Wed 06, 2021 11:37 am 
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Joined: Jul Tue 06, 2021 6:12 pm
Posts: 71
Hi Friends,
GOOD NEWS! The 0-7 MHz bands are no longer deaf.
Now I have AM sensitivity of 1.5...2.0 uV on almost all the bands. LONG LIFE THE RECAPPING, you were right!

Sensitivity has been measured as follows:
· using LINE METER as indicator
· -10dB (modulation OFF) vs. 0dB (modulation ON)
· SELECTIVITY= 4KHz
· generator Marconi 2022E with DA-121/U-like adapter, modulation 1KHz 30%, BALANCED ANTENNA input

Perhaps it is not the best setup but it is what I used and it is enought repetible and quick to measure.
I recapped C256, C275, C276, C277 C278, C279, C286, C309, C322, C323, C327. Note that I did not change C321, because it is not 12pF as expected but 18pF and I had not such capacitor. I’ll do it next time. (I found no note about this difference, do you know anything?).

But now to the PROBLEMS: after the overhaul, the levels from OSC2 (E210 in stand-by) are all 1-2V lower than before and neither adjusting T401 I can return to previous values. The only thing I did that could be related to this, is the replacement of C287 with a new 5000pF 1000WVDC good quality mica capacitor, but it should be better or equal, isn't it? The trimmers rotate smoothly and peak very well in not-extreme positions. Even the by-me-rebuilt trimmer seems to work (you were right about the dielectric constant... I had to add a capacitor in parallel!). But the DC values are lower (I mean: less negative). Worst values are 4V.
What could I did wrong?

@Larry: has T207 some effect on OSC2? Is it a mistake for T402?


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 Post subject: Re: R-390A alignment, some questions
PostPosted: Oct Wed 06, 2021 12:03 pm 
Member

Joined: Sep Thu 01, 2016 3:56 am
Posts: 450
Location: San Jose, Ca.
Hi Gianni, Very good work. Sorry for my brain check on T207, I meant T401. C287 replacement should be fine and not the problem. Did you replace the caps in the 2nd osc that are in parallel with the trimmers - they can cause low voltage output. But, if the low bands are 4 volts, what are the normal ones? The crystals can also cause low output. How's the sensitivity on the bands with low output? And, the band switch in the 2nd osc can also cause problems - contamination on the switch contact insulator.

Regards, Larry


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 Post subject: Re: R-390A alignment, some questions
PostPosted: Oct Wed 06, 2021 2:30 pm 
Member

Joined: Apr Wed 01, 2020 7:22 pm
Posts: 334
Tubetron wrote:
Hi Friends,
GOOD NEWS! The 0-7 MHz bands are no longer deaf.
Now I have AM sensitivity of 1.5...2.0 uV on almost all the bands. LONG LIFE THE RECAPPING, you were right!

Sensitivity has been measured as follows:
· using LINE METER as indicator
· -10dB (modulation OFF) vs. 0dB (modulation ON)
· SELECTIVITY= 4KHz
· generator Marconi 2022E with DA-121/U-like adapter, modulation 1KHz 30%, BALANCED ANTENNA input

Perhaps it is not the best setup but it is what I used and it is enought repetible and quick to measure.
I recapped C256, C275, C276, C277 C278, C279, C286, C309, C322, C323, C327. Note that I did not change C321, because it is not 12pF as expected but 18pF and I had not such capacitor. I’ll do it next time. (I found no note about this difference, do you know anything?).

But now to the PROBLEMS: after the overhaul, the levels from OSC2 (E210 in stand-by) are all 1-2V lower than before and neither adjusting T401 I can return to previous values. The only thing I did that could be related to this, is the replacement of C287 with a new 5000pF 1000WVDC good quality mica capacitor, but it should be better or equal, isn't it? The trimmers rotate smoothly and peak very well in not-extreme positions. Even the by-me-rebuilt trimmer seems to work (you were right about the dielectric constant... I had to add a capacitor in parallel!). But the DC values are lower (I mean: less negative). Worst values are 4V.
What could I did wrong?

@Larry: has T207 some effect on OSC2? Is it a mistake for T402?


Gianni...A few comments:

The voltage charts in the manual are not hard and fast "specifications", and only serve to indicate if a circuit is working nominally. Those numbers in the manual were determined using a USM-116 VTVM which has a DC input impedance of 100 Megohms which keeps it from loading down the circuit as much as possible. Even then, the numbers are only an acceptable range of DC voltages at the listed test point.

You said that you checked all the resistors in the RF deck and they were all OK. I've never had a 390 RF deck which didn't require at least a half dozen resistors replaced because they were more than 5-10% out of spec or they appeared to have been overheated.

For example, if a 2200 ohm resistor measures 2350 ohms, it gets replaced, if a 470k ohm resistors measures 500K it gets replaced, even though they measure within 10% tolerance. I do this because if a resistor is already drifting upwards in value, it's likely to keep drifting upwards. I really dislike having to pull the RF deck out, especially if I have to check for problems that I could have avoided by checking components more thoroughly the first time.

C321 should be a 12pf cap, not an 18pf cap so it may have been installed in error, or replaced later with an available cap. I'd replace it with the proper 12pf SM cap or a 10pf SM cap if you have one on hand.

C287 should be a ceramic disc bypass capacitor, not a mica cap, perhaps you mis-stated the type of cap you used..?

Good work on reworking the broken trimmer capacitor...!

Keep at it and I'm sure your efforts will be rewarded...!

73..Jordan VE6ZT


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