Help with an RCA 65X1

3/1/2008 6:32:02 PMTonyJ

Started working on an RCA 65X1 I recently received and the set wasn't receiving any stations. Proceeded to recap the set and attempted an alignment after. Found the adjustments way off on the 2nd IF and finished the alignment thinking that was the problem. Afterwards it still didn't appear to receive stations. Verified the voltages on the 12SA7 and subbed in a known good tube but no dice. Put a frequency counter on the 12SA7 oscillator grid and that appears to be working and tracking 455K above the station dial as well. For kicks I tuned to a strong local station and tweaked the secondary of the 2nd IF and could get that station to come in very well. Tweaked the primary and it got a little better. Tried to change stations and this is the only station the set receives. When it's tuned to the correct spot on the dial it comes in great - other places along the band it's there quietly amid noise. I also set the tuning dial as low as it would go and put the frequency counter on the plate of the 12SA7 expecting to see 455K. What I actually saw was 915K, approximately double. Also found that changing the tuning also changed the frequency. Might have been harmonics the counter was picking up?

One thing I noticed is that when adjusting the 2nd IF, there are adjustment points that result in a loud AC type hum. The IF can has the number that I've read about some mica problem (119 I think). Not sure if that would cause a problem like this. Took the 1st IF can apart for kicks and the resistance checks match the print and the variable caps dont seem to have damaged mica. Needed a break and haven't looked at the 2nd IF can yet.

Any ideas?? Here is the schematic for the set:
http://www.nostalgiaair.org/Resources/065/M0015065.htm
So much for what I thought would be a simple recap. Thanks in advance!

3/1/2008 6:55:55 PMPeter G. Balazsy

When you align the IFs, use a 455khz modulated signal and inject it to the grid of the first mixer via a .01 cap

(Turn your tuning condenser to max open.)

Keep the signal generator signal low but not too low... just easily audible.

Then tweak the 2nd IF first.

Then tweak the 1st IF next.

At this pointy you can check to see if the radio is tuning properly... but then you can go to the calibration step..

Then tune to 1600khz and feed in a 1600khz modulated signal via a small wire loop near the antenna.

Tweak the cap on the small section (osc) of the tuning condenser for max signal at 1600kc.

Then change signal generator to 1400kc and find or tune in the tuner knob to 1400kc for max signal then tweak the cap on the large section of the tuning condenser for strongest signal.

Then change everything to 600kc and check to see if the tuner is tracking properly.

As for "bad mica" in the IF cans... that refers only to slug tuned IF cans with a fixed integrated capacitor.
If your IF cans have adjustable trim caps you will not experience the "dreaded silver mica disease"

3/1/2008 7:09:13 PMTonyJ

Yep - the Conar generator I have has a setting for modulated output. That's what puzzles me - followed the alignment per the schematic and even the alignment using 1300 and 600 through the antenna seemed to work.

Incidentally the 2nd does show two 120pF caps in it. What symptoms does this 'silver mica disease' usually exhibit?

:When you align the IFs, use a 455khz modulated signal and inject it to the grid of the first mixer via a .01 cap
:
:(Turn your tuning condenser to max open.)
:
:Keep the signal generator signal low but not too low... just easily audible.
:
:Then tweak the 2nd IF first.
:
:Then tweak the 1st IF next.
:
:At this pointy you can check to see if the radio is tuning properly... but then you can go to the calibration step..
:
:Then tune to 1600khz and feed in a 1600khz modulated signal via a small wire loop near the antenna.
:
:Tweak the cap on the small section (osc) of the tuning condenser for max signal at 1600kc.
:
:Then change signal generator to 1400kc and find or tune in the tuner knob to 1400kc for max signal then tweak the cap on the large section of the tuning condenser for strongest signal.
:
:Then change everything to 600kc and check to see if the tuner is tracking properly.
:
:As for "bad mica" in the IF cans... that refers only to slug tuned IF cans with a fixed integrated capacitor.
:If your IF cans have adjustable trim caps you will not experience the "dreaded silver mica disease"
:

3/2/2008 2:19:54 AMTonyJ

I think it's time to set this one aside for now and come back another time with a clean head ;) Went through the alignment again and still only receive one station normally at it's frequency (910KHz) and still receive it faintly across the band. Tuning
approximately 100K below 910 I get bad motorboating. Resistors checked in tolerance, all paper and electrolytic caps were replaced, verified IF transformer coils were good, tried subbing in new tubes, checked and rechecked wiring, shield connections on tubes was correct.

Something else weird - I found that when I rotate the radio that the adjactent station came in (without touching the tuning). What's odd is that if I use my signal generate and connect it to a loop of wire near the set, I can tune to and pick it up, so I have to believe the oscillator is working. At a loss and hopefully someone comes up with an idea or something simple I just plain overlooked. Thanks.

:Yep - the Conar generator I have has a setting for modulated output. That's what puzzles me - followed the alignment per the schematic and even the alignment using 1300 and 600 through the antenna seemed to work.
:
:Incidentally the 2nd does show two 120pF caps in it. What symptoms does this 'silver mica disease' usually exhibit?
:
::When you align the IFs, use a 455khz modulated signal and inject it to the grid of the first mixer via a .01 cap
::
::(Turn your tuning condenser to max open.)
::
::Keep the signal generator signal low but not too low... just easily audible.
::
::Then tweak the 2nd IF first.
::
::Then tweak the 1st IF next.
::
::At this pointy you can check to see if the radio is tuning properly... but then you can go to the calibration step..
::
::Then tune to 1600khz and feed in a 1600khz modulated signal via a small wire loop near the antenna.
::
::Tweak the cap on the small section (osc) of the tuning condenser for max signal at 1600kc.
::
::Then change signal generator to 1400kc and find or tune in the tuner knob to 1400kc for max signal then tweak the cap on the large section of the tuning condenser for strongest signal.
::
::Then change everything to 600kc and check to see if the tuner is tracking properly.
::
::As for "bad mica" in the IF cans... that refers only to slug tuned IF cans with a fixed integrated capacitor.
::If your IF cans have adjustable trim caps you will not experience the "dreaded silver mica disease"
::

3/2/2008 3:38:01 AMPeter G. Balazsy

Tony:
You may just not really have the IFs properly tweaked somehow.
...because... it just seems to me that if, in fact, each IF transformer is tuning/peaking nice and strong at 455kc .... AND then...you DO also see the osc tracking nicely at 455kc above the tuned freq everywhere along the dial... Then it has to work... y'know?

"Silver mica disease" comes from failure of the integrated mica caps in the IF can and the silver oxidizing over the years and then the silver migrates across from one part of the cap to the other cap.
That allows B+ to leak over to the grid of the next stage creating great loud crashing static sounds.

3/2/2008 8:44:11 AMTonyJ

Hi Peter:

Thanks for the explanation on the silver mica problem. Now I know what you mean as both of my IF cans have trimmer capacitors in them. One thing I saw in looking at my 2nd cap was the lack of the 120pF caps shown on the print which I assume is to filter out any remains of the 455KHz. I saw the brown and black wire terminated on posts in the ceramic top piece but no caps. Are those caps usually internal to the ceramic piece somehow?

I have another weird thought - since the radio will tune in and pickup a signal other places on the band from a loop of wire on my signal generator except that point around 800K or so where I get the terrible motorboating, is it possible something in the chassis is essentially radiating RF that overpowers the normal antenna signal? Example, if I set the dial to 1300 on the radio, the station broadcasting at 910 is still heard. Put a coil of wire on my signal generator, set it a few feet from the radio and the modulated signal generator RF comes through fine. That coupled with the fact of the motorboating spot on the dial makes me wonder if it is picking 'itself' up somehow.

Thanks again for the advice!

:Tony:
:You may just not really have the IFs properly tweaked somehow.
:...because... it just seems to me that if, in fact, each IF transformer is tuning/peaking nice and strong at 455kc .... AND then...you DO also see the osc tracking nicely at 455kc above the tuned freq everywhere along the dial... Then it has to work... y'know?
:
:"Silver mica disease" comes from failure of the integrated mica caps in the IF can and the silver oxidizing over the years and then the silver migrates across from one part of the cap to the other cap.
:That allows B+ to leak over to the grid of the next stage creating great loud crashing static sounds.
:

3/2/2008 10:48:38 AMTonyJ

Finally got it!

You were spot on with the alignment Peter. I should have realized it sooner but whoever had the radio prior to me had the IF trimmers backed out quite a bit. When I aligned the set (twice mind you lol) I think that since I started with them so far off that I was tuning to an image perhaps. Before throwing this one against the wall I decided to give it one more alignment but instead first tighten down all the trimmers in the IF cans. Lo and behold it after doing this it lives. General question about aligning then - when you receive a set that is really way off or someone has been in it, is this a safe bet to start a fresh alignment with all the trimmers tightened down?

Thanks again for your help!

3/2/2008 7:46:17 PMPeter G. Balazsy

:Finally got it!
:
:You were spot on with the alignment Peter. I should have realized it sooner but whoever had the radio prior to me had the IF trimmers backed out quite a bit. When I aligned the set (twice mind you lol) I think that since I started with them so far off that I was tuning to an image perhaps. Before throwing this one against the wall I decided to give it one more alignment but instead first tighten down all the trimmers in the IF cans. Lo and behold it after doing this it lives. General question about aligning then - when you receive a set that is really way off or someone has been in it, is this a safe bet to start a fresh alignment with all the trimmers tightened down?
:
:Thanks again for your help!
:
::Tony:

Well Great Tony... glad I could help.
I would say that you should re-start an alignment on a mis-aligned set by turning down the trimmers snug ...but then turn them back about 1-1/2 turns as a good starting point. NoP trimmers should ever be designed to work at the extreme of their range but instead somewhere closer to the midrange of their travel.