[EdHolland]

Hi Arran,

The speaker wire is cloth covered, but is very fragile on this example. It has frayed to reveal bare wire where it passes through the chassis, and through the opening in the cabinet shelf. RCA didn't go to the trouble of using a grommet or eyelet, so the sharp edges of the sheet metal did their worst. That and time, as the cloth covering has become rotten and brittle. The antenna wires, and those for the dial bulbs are a similar story.

I do wish that cloth wire was available which has patterned covering as well as the solid colour.

Cheers

Ed

[mikethedruid]

I agree about wishing that cloth covered wire was available with pattern covering. Green and white 20 gauge would be very useful for restoring the speaker harness for Philcos. I take plain green, put little 1/2 inch pieces of masking tape all along it spaced about 1/8 inch apart, then spray it with white paint. Gives me the "green with white trace" I need, but does not look like the original.

[EdHolland]

I didn't get to anything yesterday, save a check that a bundle of my new wire will pass through the hole in the chassis. It is thicker than the original, with four lines needed to the speaker plug.

Also, I have a mystery cap on the band switch, thanks to previous repairs. It looks like a 0.1uf Orange Drop, and if I am reading the value correctly, I cannot sees how it corresponds at all to the schematic. It wouldn't be the first anomaly...

More soon,

Ed

[EdHolland]

I think the circuit location of the mystery cap is traced. If my electrical sleuthing is correct, it is in place of a 5600 mmF (pF) cap (C25). This is switched into the oscillator circuit on some wavebands. One end permanently on the hot end of the main variable capacitor (C26), which is in circuit all the time, except when the bandswitch is at the preset position, and the other end at another wafer switch terminal.

Quite a "bold" move to replace 5600 pf with 0.1 uf! Given that I'm not sure the exact task of this cap just yet (but it is probably bypass duty) I wonder the effect this has on the circuit.

Do my esteemed forum members have suggestions for a suitable cap to use in this location? The one very grainy under-chassis picture I can find HERE suggests a paper cap in what appears to be the same location, but it's hard to see exactly... Bandswitch is near the top of the image, and the cap is tucked in to the right of the rear wafer, below the mounting posts.

Thanks

Ed

[Arran]

Ed;
  You could try Tom's Small Engine Barn if you want some repro cloth covered wire with a tracer rather then just a solid colour for the speaker cable, however 18 AWG is the smallest gauge they have.
Regards
Arran

[EdHolland]

Thanks Arran,

I think I found them in a search some while back, or it was another supplier aimed at vintage vehicles and engines.. As you suggest 18 AWG is a bit hefty for radio use.

Concentrating now on the best replacement for the 0.1 uF cap that shouldn't be there. Tracing the band-switches is a good brain teaser.

[David]

Ed What a big difference between .1uf(100000pf) and 5600pf guess somebody was thinking they could improve the circuit or change the range on one or more bands. I have seen the same thing in CB radios by changing one of the crystals so they would be talking out of the allowed band, only works if someone else has made the same mod. Changing certain resistors in the output stage increasing the power out so as to drive a liner amp. One last idea the Sch that I posted was from 1941 this may have been done to prevent the use of certain bands during the war very common on West Coast radios during the war years. David

[EdHolland]

David,

Your 1941 schematic matches the one I have, at least in the area of interest around the band-switch. That is an interesting thought about disabling one of the ranges. The cap in there at the moment is certainly post-war, but may have been added to replace one that was of a value different from spec.

There has been a lot of work beneath the chassis on this set, apparently on several occasions in it's history, and with varied levels of attention to detail and accuracy. I reasoned that the best option was to clean it up throughout, and get things back to trustworthy values - this is not an untouched piece where originality can be preserved, but it should be a nice performer.

For the 5600 pf, I'm leaning towards finding a silver mica cap (can't really go wrong) but could try a film cap until parts arrive.

[David]

Have a Halli SX-130 from 1967 every cap under the hood is a ceramic disc, except the Ecaps. Silver mica would be a good choice. David

[EdHolland]

I got some time at the radio bench today - very welcome, as outside temperatures reached 100 F, and the basement work area is nice and cool. Following in the footsteps of Ron Ramirez's postings, I played "Let's make a speaker cable" to replace the original (no wire nuts) but fatally frayed wires on this chassis.

The socket in this case is a one-piece Bakelite part, and I was able to disengage the snapped-in contacts by squeezing them carefully to pull them out. New wires were attached and the contacts clipped back in to place. Although my wire had thicker insulation than the factory stuff, all four leads passed neatly though the hole in the chassis. All looks good and the cable reaches the speaker.

After that, I replaced the dial bulb wiring to both bulbs on the left side. Two more of those to go, plus some antenna circuit wiring.

Getting close now

Ed

[EdHolland]

OK picture time. Please excuse the untidy bench, and the hastily tied flying leads. Next steps will be to replace that last 5600 pF cap - A trip to my favourite surplus place is in order . Then on to dim-bulb testing and alignment.  But I couldn't resist a quick snap with all the new wiring and valves polished up and back in place.

   

[mikethedruid]

I applaud your re-stuffing of the multi section electrolytic filter condenser. For the RCA 18T I am soon going to restore I was very lucky. Antique Electronic Supply had a PERFECT replacement, same capacities, same voltage, same dimensions, even the same tab mounting. (  https://www.tubesandmore.com/products/ca...20101010-f   ) Needless to say I have ordered one to replace the one in the set.

[EdHolland]

Thanks Mike!

I looked at AES and other options for a while before deciding to restuff - there was nothing quite close enough to what I needed. I'd certainly do this again, and indeed, the forthcoming Zenith project has a multi-section can that needs attention.

Cheers

Ed

[EdHolland]

Progress today. I replaced the capacitor on the bandswitch for a film type 5600 pF part. I found elsewhere that the originals were paper, so this seems a fair swap.

With one or two last thinker items, it was time to pop all the valves back out, and set up the dim bulbs tester. The usual routine - check voltages, then everything but the rectifier. This all came together nicely, and power was applied without drama, until I saw that the 6SQ7 wan't lighting up. There was then a diversion as I wiggled it, saw the dial lamps and dim bulb flicker, and realised that the octal base was loose. I disassembled the valve from the base, and carefully tinned then resoldered all the pins. This may still be a problem valve, but I have a spare.

From there I added the rectifier, and monitored HT. It came up nicely, and then I moved the clip to the return from the speaker field coil. 0.28 Volts. The cause was found to be at the socket on the speaker - one of the connectors on the cable had not seated properly and pushed out of the back. Easy fix - connection and voltages appropriate, but drooped noticably as the emission came up on the output valves - expected with the dim (now brighter) bulb in circuit.

At this point, with everything looking just fine I cut the bulb out and applied direct mains power. Still no drama, and a feint buzzy hum from the speaker that responded to the volume and tone controls. An intermittent noise was traced to a dry solder joint. Alas, no stations on AM or shortwave. Switching to the phono input and adding the customary finger to the input terminal showed that the audio stage is working nicely - good and strong with plenty of volume.

At this point, it was getting a bit late, so diagnostics were somewhat cursory. I grabbed a portable AM/FM/SW radio and twiddled the RCA listening for signs of oscillator Success on the SW bands, and at seemingly appropriate frequencies, but nothing on the AM broadcast band (Medium wave to we of UK origin).

So, we have a solid audio stage, an oscillator on some bands, but no stations, and no oscillator (I think) on AM. So on to diagnostics! It's possible the repaired 6SQ7 is not working, amongst other things unknown.

Cheers

Ed

[Arran]

Ed;
  If the oscillator isn't working I doubt whether the 6SQ7 has much to do with it, I would focus on the 6SA7 and the circuitry around it on the AM broadcast band, particularly the tank circuit for the oscillator. Quite often there is a 47K ohm resistor in that area that can go up in value causing the oscillator not to work below a certain point, if that has already been replaced then I would look at wiring errors, and finally the B.C oscillator coil and the mica caps. The Mica caps don't often go bad but they sometimes do anyhow.
Regards
Arran