RCA 110K2 Unexpected project - Printable Version

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RE: RCA 110K2 Unexpected project - David - 07-01-2018


Ed was looking at the location of the 47K resistor (R-21) and the sch I posted is not easy to read the pin out checked Nos Air and the same thing also looking at C-13 100mmf if it was my set an I had to replace R-21 would also replace C-13 with my skill set C-13 would be in real danger.

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RE: RCA 110K2 Unexpected project - EdHolland - 07-01-2018

Thanks Arran & David.

I missed your posts (busy day) but have made a lot of progress.

One thing early on I noticed was that the feed to the tone switch, strapped across an unused pin on the 6F6 socket changed hum when "poked about". I had to extend the wires of a capacitor to make this connection, and the wire extension had broken, hidden by the sleeving I had used in the interest of neatness and safety.
Restoring this (Really... and after all the time I spent cursing the bad repairs of others), I found that there was some activity on bands B and C . Now there's progress.

Noise, but nothing sensible on the 31 m spread band and still nothing from the broadcast band.

Then I set about trying to set about tuning in the IF. Alas, my sig gen is very noisy and unmodulated - making it extremely hard to determine the fundamental from the hash. Instead, I found a station, and using an accurate radio, found the oscillator frequency. Then I adjusted the oscillator to the correct 455 kHz offset, and brought the IF back in to line. I do wonder that there is something amiss in the second IF transformer (mica caps?). When I was trying to do this with the signal generator, it was quite easy to get a nice peak at 460 kc, but very hard to tune it down. Perhaps something to revisit?

Anyway, still troubled by the lack of basics, such as the main 550 - 1400 kHz broadcast band, I started some diagnostics, and found that the oscillator coils for this band are open. A little investigation found a broken wire at the earthy end of the coil that is likely the culprit, but at this point, other duties beckoned me away. I will join the ends and see what this accomplishes, and then decide if a re-wind is in order.

No clue on the 31 m band or the preset tuning yet, but I suspect all will fall in to place with some more investigation. Oh, and the 6-18 MHz bans drops out around 7 MHz - for some reason the oscillator dies, but is active either side of the dead spot. As one tunes, the set loses sensitivity gradually before dropping out, then fading back in.. Strange!

So, I am happy to say the RCA is stumbling back to life, and there is more to do.

Cheers

Ed

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RE: RCA 110K2 Unexpected project - EdHolland - 07-02-2018

Some good clues! Nostalgia air has an entry for this radio including the Riders "Simplified Schematics" This has proved to be a big benefit in understanding which coils are in circuit. for each band.

..I now know the presets won't work until L28 for the AM band is repaired. Other stuff should fall in to place - I read elsewhere that the mica caps in these sets can be problematic also. All good stuff to dig into.

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RE: RCA 110K2 Unexpected project - Arran - 07-04-2018

Ed;
  Two other thoughts occurred to me the other day after I had posted last. On these sets the shortwave bands each have their own antenna coils located around the band switch, but the antenna coil for the AM broadcast band uses the loop antenna, so if the loop antenna is not connected, or incorrectly connected the broadcast band will not work. The puishbutton bank could also cause this problem, if it is connected incorrectly the radio will also not work, however if it is connected correctly this should be a good way of checking the oscillator as each coil and capacitor assembly on the P.B bank switches into place of the B.C oscillator coil near the bandswitch.
  The IF cans may or may not be worth investigating, they are slug tuned (permeable tuned) but the fixed caps are either molded micas, or NPO ceramic caps, not cheap silver mica sandwiches like a 1950s kitchen radio would use. 
Regards
Arran

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RE: RCA 110K2 Unexpected project - EdHolland - 07-04-2018

Thanks Arran,

I had been running the set with the loop connected.

Today - Big progress. I desoldered the oscillator coil former and looked at the broken "A band" winding, which was open at the ground end common to all 3 coils. I could have tacked it together, but since it was all the way out of the set, and I had 34 AWG wire, I took careful notes (52 turns, and 4.5 turns after the tap) and rewound. The wax was scraped off beforehand and carefully saved, to be melted back on with a heat gun.

With the rewound coil back in the chassis, I found everything working better. AM reception was returned and the presets too. The latter share the A band coil, with their own oscillator preset tuning caps.

Further, the dead spot on the C band is now gone - I think this was caused by the open A band coil resonating while open circuit (when not in use, both ends are grounded), and the absorption killing the oscillator. Of course it could have been the baking I gave the coils with the heat gun.

Either way, I was able to perform a rudimentary alignment on the radio, and establish reception on all bands, so good progress.

I am still concerned that the IF's don't tune well, and have ordered mica caps to replace the ones internal to the transformers.

Anyway - very pleased to keep this one moving in the right direction.

Cheers

Ed

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RE: RCA 110K2 Unexpected project - EdHolland - 07-07-2018

More today, mostly fiddling with alignment, but one other important step forward.

Following the datasheet calls for set-up of the 2nd, then the first IF. When I try injecting a signal as specified, this kills the stage completely. Instead, I found a "gimmick" insulated wire wrapped loosely around the same point, I was able to inject and hear 455 kHz from my generator. This wasn't working quite as expected, as the adjustment of the second IF still showed rather little response - but there was enough pickup from the first stage that it did respond, and I could tune the first IF.

Jiggling things around a bit, I find it is possible to get first and second IFs to peak up, although I do feel this could be improved upon. There are also periodic crackles from the radio after switch on from cold, so I may investigate for "silver Mica disease".

Aligning the front end is very rudimentary on this radio, and seems to fall in just fine, although there is one anomaly. The 31 m "Spread Band" is derived from the 6-18 megahertz band using padding and trimming capacitors. However, the oscillator trimmer has to be adjusted to maximum to meet the set point. It is in parallel with a tubular ceramic (82 pf) and I wonder if this has drifted in value over the years.

All this fiddling around had, until today, ignored one thing - terrible audio from the speaker. For one thing, the surround had detached from the frame, making for a big "slap" on heavy bass accents. But also, the voice coil was rubbing. Looking closer, there were two screws adjacent to the field coil. These secured clamps that held a plate to which the spider is attached. A little adjustment restored clean movement to the voice coil, and lovely audio.

Cheers

Ed

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RE: RCA 110K2 Unexpected project - EdHolland - 07-11-2018

Oh dear. I do get obsessive compulsive with these things.

Looking again at the schematics, I realise another detail overlooked, and blind-sided by the previous fixer(s). The RF bypass caps after the 2nd IF should be 120 pF on the "earthy end of the IF secondary, and 100 pF on the 6SQ7 anode circuit. These had been replaced with film caps, larger in value. The one picture found on-line, and a quick shot in a repair video clearly shows that originally these were the lozenge shaped micas used elsewhere in the set. Fortunately I have suitable values among the parts just received.

Further up-stream, I am working out why the spread band is hard to align (air trimmer all the way at max). Since this is 2-18 pF, it won't take much change elsewhere to confuse matters. I've done a few calculations already to help understand which component might have the most significant effect.

A shiny new capacitance meter (Actually capable of Capacitance, Inductance and resistance) is already helping out, and with care, I can make sense of the first few pF value caps I have been able to test out of circuit. I might put up a review of this instrument elsewhere on the forum. For around $30 from a well known online source, I think it is a handy bit of kit.

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RE: RCA 110K2 Unexpected project - EdHolland - 07-16-2018

Saturday offered a chance to work on the hassis again.

The film caps were replaced with micas, and I found another bypass cap that was absent... 100 pF on the 6SQ7 anode circuit which I thought was there!

No time to test, hopefully I can play with it one or two evenings this week.

Ed

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RE: RCA 110K2 Unexpected project - Arran - 07-17-2018

Ed;
  The 100 mmf (pF) in the anode circuit of the 6SQ7, if it on the triode or is on one of the diodes? If it's on the triode it was probably for tone compensation, so it wasn't really needed to make it work. On the other two micas, if they were involved in the IF circuits then that could throw the alignment off, if they were in series with the fixed micas inside the cans. The black coffin shaped micas in RCA sets (as well as Canadian G.Es) aren't really that bad in spite of the neurosis about them on the barking dog forum, unless one is falling apart or there is a weird fault I leave them alone, in fact shotgun replacement is more likely to cause a problem then to head off a potential one. That cap checker you have is a reasonable way of weeding out the bad ones, I'm not sure whether it will do it in circuit or not, some will and some won't.
Regards
Arran

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RE: RCA 110K2 Unexpected project - EdHolland - 07-17-2018

Thanks Arran.

One was in the triode anode circuit... probably tone as you say but it was missing and not replaced by previous restorer..

The others were more closely associated with the 2nd IF, and I indeed was expecting an effect on alignment. The shotgun had hit this set at point blank range long before I came along with the surgical tools and bandages..

I played with the cap tester on some of the oscillator section caps. It is easy enough to disconnect one side for reliable readings.

Cheers,

Ed

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RE: RCA 110K2 Unexpected project - EdHolland - 07-22-2018

Improvements!

First of all, with the changes back to mica caps in the IF circuit it was possible to get the IF tuned up with much more ease. However, I was still faced with an inability to align the 31 m spread band, so started looking at the caps in the oscillator circuit, as there is a lot of padding and trimming done to achieve the narrow oscillator tuning range (with repect to frequency). Eventually I found a 120 mmF mica that was suspect in value, so substituted a new cap. Success! I was able to adjust the osc. trimmer and get the 9.5 Mc/s signal rocked in at the proper dial setting. Prior to this, I could only get close, with the trimmer at full capacity.

The capacitance meter helped here, I could determine the original 120 mmf cap was low, and was able to select a new cap from stock to be close in the required value. Now the circuit aligns with the trimmer roughly midway along it's travel.

I'm pleased with the improvements.

Edit: I forgot to mention that I also found time to look at the speaker. The outer edge of the cone had become detached from the frame, the rubber grommets accompanying the fasteners were hard and split, and there was no sign of a gasket. I re-glued the cone, and had some felt left over from working on the Philco 16X. When I had removed the speaker a while back, I noticed it was very poorly fastened in place - something else the original "restorer" had hacked together.

It is no wonder the friend who gave us the radio was disappointed. Aside from the casual electrical restoration and quick cabinet wipe over with shellac, the speaker was:

1) Coming unglued at the cone surround, rasping and slapping the speaker frame
2) Voice coil rubbing the pole pieces
3) Barely secured in the cabinet with no gasket to to the sound board

I'm aware of the sum that changed hands for this radio, which was high even for a top condition item. Some people have no scruples whatsoever.


Cheers

Ed

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RE: RCA 110K2 Unexpected project - EdHolland - 07-29-2018

More investigation yesterday. I was curious as to the state of the capacitors within the I.F. transformers. (Pics to follow). Removing the easiest candidate -The first I.F.- took but a few minutes, and the can was easy to open, revealing two tall windings on a waxed paper tube. The capacitors were miniature versions of the clamped type mica style. I removed each in turn, and measured the capacitance. Expecting 100pF from the schematic, I found these were each accurately 110 pF. Too much of a coincidence for me to consider making a change to what was found, so everything was returned to place as found.

I am still not convinced that the two I.F.s peak up quite as they ought to, but am reluctant to risk causing damage.

However, the most fun was had listening to the set. With a fully operational speaker, and connected to the outside long-wire, it was performing nicely. AM reception is good to very good on the main A band on long-wire or loop antenna. Moving to the "Special service" B band, 1.4 - 4 Megs, reception seems a tad weak where it overlaps with AM. The "International" 6-18 Megs band is working well and I was able to pull in many Shortwave broadcasts, especially in the 41 m to 22 m range. This included a very nice signal from Radio New Zealand International, a not uncommon visitor to the West Coast. On to the 31 m Spread-band and this was curiously quiet, although on the wide range SW band I thought there were plenty of 31 m stations. Perhaps there is still an alignment issue, I'll check

Pretty happy so far with the set's performance, and I am just about ready to declare the chassis "done".

Ed

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RE: RCA 110K2 Unexpected project - EdHolland - 07-30-2018

Here is a picture of the IF transformer I checked:

   

The view is from the primary side, with the small cap soldered between the outermost pins in the phenolic base. In this picture, it is behind the center, unused, pin. An identical arrangement is made on the other side for the secondary winding. Replacing the capacitors would be easy - they desolder without much trouble, and there is room for replacements, but I doubt that this will be required for a long time to come, and the value marked on my schematic differs from what was measured.

Regards,

Ed

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RE: RCA 110K2 Unexpected project - EdHolland - 10-06-2018

After a busy morning of "Honey - do" items, I got back to this RCA, its time to look again at the cabinet. First up was to make an impression on the shellac "washover" that was applied by my arch enemy "restorer". That was taken of quite easily, first by wiping over an area with denatured alcohol, then going over it with a plastic kitchen "scourer". This removed the stuff pretty well, and was safe to the underlying finish and "photo finish" cross banding details.

Next, to deal with the problem area on one side, where the original finish has flaked away. More working with alcohol on a cloth helped to rework this, and blend it in. I don't want to strip the whole thing, thanks to the photo-finish. I think this will smooth out and rework/rebuild OK with time and care.

To get into all the nooks and crannies around the bars in front of the grillecloth, I removed the speaker mounting board. This is definitely a cost cut construction. The speaker mounts to a board ~ half the height of the grillecloth, with the cloth itself applied to a piece of cardboard. This is obviously original, as it is coated with the silver paint applied everywhere on the inside of the cabinet.

Anyway, things are starting to look better

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RE: RCA 110K2 Unexpected project - EdHolland - 10-07-2018

I looked at the cabinet again today, and the right side, with the worst of the flaking lacquer, really demanded more effort in preparation and rework. Reluctantly, I broke out the 150 grit paper and rubbed back carefully to wood in these areas, being careful to avoid passing over the photo finish "crossbanding". This was quite successful, and was followed with 200 grit. A first wipe over, sparingly, with tung oil brought out the colour, and I will continue to build up finish in this way. I'm rather pleased with the results, and this radio is going to look sharp again, and play as well as it ever did.

Pics to follow.

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