RCA 6T75 Restoration (16GP4)

[dobulee]

Picked up the Set in South-Eastern Ohio on Friday, and have just gotten it back home to South-Eastern PA.

Other than some dry rotted insulation, and a heap of dust, the set looks pretty decent for 59 years old.

'As is' Pictures attached. (The modern cartridge fuse holder is my own temporary addition). The cabinet is in reasonably good shape, save the one tear in the veneer. It looks better in person, the water spots are on my lens, not the cabinet!

The front and rear access panes for the HV cage are completely missing, so I'll have to machine some new ones-- but that's for another time...
More to follow!

Alex

[Bill Cahill]

That's one of the harder models to come by. Congrats are in order.That cabinet style according to RCA was "Moderne".
I may have a rusty door for your cage.It could be cleaned, and, re painted the original flat balack. If this set has the un painted cage, that will work, too. Pm me if interested. Cagee door free. Shipping charges, and, pro. packing by ship, and, pack is all I want. I'll make nothing on this.I just wish to help.
Bill Cahill

[John Folsom]

Alex, welcome to the forum and to the wonderful world of vintage TV restoration. Please keep us all posted on you restoration progress. For you electrolytic twist-loc cans, you may want to consider the brand new manufacture cans offered by Antique Electronic Supply. They are not cheap, but a good alternative to restuffing.

I would like to see anything you have on early color TV, as that is my primary interest these days.

And don't hesitate to ask questions and post comments, that what the forum is for. We ARE interested. Have fun!

[bandersen]

If you're curious what it might take to restuff those cans, check this thread out: http://www.audiokarma.org/forums/sho...=231914&page=2

Looks like a fun set to work on. Good luck!

[dobulee]

That was a great thread, thanks. Got the chassis about half cleaned up last night, I'll post some more pictures once I start recapping.

[dobulee]

Got the chassis relatively clean, top side. And dove into 'the works'.

I had previously heat shrunk some tubing over the leads of the main transformer, but the cotton an rubber was still causing issues, flaking off at the 'mouth' of the transformer and doing it's best to short out. I decided to pull open the transformer, remove the original leads, and solder new leads directly on the windings. Where I couldn't get adequate access to the windings, I cut the original lead as short as possible, and heat shrunk the new connection. It turned out okay! I'll post pictures when I get it back together.

I also took out the first twist lock cap, and began the restuffing process. Boy does it take a lot of heat to wick up solder off the chassis connections! I'm used to working with 0402 SMT parts under a magnifying glass -- so this is quite a change.

When I finally got the cap out, I went the dremel route, and created tabs to pull back. However the 'Pull out the terminals and fiber disc and save it,' part didn't go so hot. Even with a lot of heat, the terminals and disc refused to let go from the innards. Eventually I broke the disk by pulling on it, so I'll have to make a new one out of phenolic. Any other tips for better success??

With the disc out, I finally was able to stab some pliers into the can, reheat, and pull the works out. Ive got the new capacitor assemblies ready to go, as soon as I make a new insulator disc. The process is a ton of work, I can understand why people would just put the new caps under the chassis.

Other than that, I just started replacing some caps and resistors -- almost all that I have checked, even the lower valued ones, are out of tolerance.

Once the power supply and other electrolytics are replaced, and new tubes are in, do you generally find that it is relatively safe to attempt to fire up the set? Or is it always advisable to replace all the paper caps first? I have read that people have success with the 'replace a few and test' method.

Any help appreciated as always,
Alex

[Eric H]

Alex, I usually just cut the can off right above the step, this leaves the Phenolic intact, then I drill some very tiny holes near each leg for the + leg of the new cap to go through, tie them all together on top and drill one more hole for the ground wire to pass through, glue the can back on with JB weld.

If the can is Hacksaw accessible you can do this without even removing it from the chassis, you do have to be careful not to bust the Phenolic mounting wafers when you do it that way.

Sometimes you can't get all 3 or 4 new caps inside the can, in that case I just put the remaining one under the chassis connected to the lugs.

[bandersen]

Hi Alex,

Seems like a good night for stuffing caps I've been using a slight variation of the technique you used. I open up the can and removing the steel ring like you've done. Then, I heat up the can with the wafer and pins in place. A slow, steady twist & pull with pliers on the pins and viola the whole slug comes out. Finally, I unrolled the aluminum and goop to free up the pins. This way the entire base is in great condition.

I like Eric's technique too. A pipe cutter can make quick work of the aluminum can. It's especially nice when the cap is in a cardboard sleeve.

Good luck,
Bob

[Bill Cahill]

No, not safe to try now. Especially with those black beuties in there, and, in hv cage.
Not advisable.
Where origninals are 1,00 volt, use at least that high voltage, as they have quite a pulse on them. Cap values on damper, and, yoke circuits are fairly crittical, and, disc caps will not work properly. Especially not on vertical circuit.
Clean hv thoroughly, as there is tendency to arc.
Yokes, and, flybacks are prone to failure in these sets, are as vertical oscilator transformers.
I do with out line caps, and, line resistors as without them set chassis is electrically safer to touch.
They stopped noise such as car generators, and, diathermy machines, which we no longer use.
Bill Cahill

[dobulee]

Thanks for the input,

I'll take a stab at the heat and twist method on the next twist lock.

I have access to a wide array of HV capacitors, so getting 1kV 5% Films in just about any value isn't a big problem. I'll ohm out, and hipot test the Flyback and vertical oscillator trans. before firing the set up for the first time.

As for the line caps and line resistor, my work in the power electronics industry tells me that the line caps are there as much for RFI suppression from the set out to the world, as for the world into the set. Without them I imagine you could get some local oscillator noise out onto the line which would interfere with other sets. However, I doubt the single order filter really works any miracles -- modern RFI filters are quite an artform in themselves! I'll put in new Y2 safety grade ceramics. I have an isolation transformer at my disposal as well.

[dobulee]

I was replacing caps around the vertical oscillator, and something didn't look quite right --My first schematic change!

Sam's shows the vertical oscillator as a 6J5, where as the RCA chart in the cabinet shows the 6SN7 Dual Version. 1/2 of the 6SN7 is connected exactly as the 6J5 would be, with the other 1/2 pulling duty as the synch amp, where sams shows 1/2 12AU7. The 6SN7 is biased differently than what is shown on the 12AU7, but I guess that is to be expected.

Things get even weirder upstream of that at the DC-Restore/Synch Sep tube, where my chassis bears little resemblance to the Sams schematic. There appear to be both halves of a 12AU7 interconnected up there, where sams shows only 1/2.

I haven't fully traced out that schematic yet, but if I get time, i'll do my best to plot it out.

For what its worth, my Sam's guide for the 6T75 shows chassis KCS47, A; where the actual unit is stamped KCS47AT. Does anybody have this particular schematic? Right now i'm pretty much operating on the 'Blindly replace what is there' technique with no schematic of some sections.

Other than that, a few pics of the re-leaded transformer. Luckily wire is available in all colors under the sun!

Alex

[John Folsom]

Send me an email address and I will send you the schematic.

[Eric H]

Great job on rewiring the transformer!!

[dobulee]

Well, here it is a week later, and I really went to town on cap replacement. And, with many thanks to John, I now have a (more or less) accurate schematic of the AT chassis. Incidentally, I have come across several 'missing' components in the IF section. There has been at least one omitted capacitor that should be in parallel with an inductance to form one of the traps -- my guess is that this was not a design change, but an error in the original construction. So far I haven't put it in -- we'll see what falls out.

I'm about 90% done with all the paper caps, I am leaving all the mica's in place at the moment. And, I have one of the 3 electrolytic twist locks bodged back in. For now, I've givin up on restuffing them -- I'll worry about authenticity later -- right now I'm still in the 'new project euphoria' stage, and want to get the set up and running.

Current state of the chassis is shown -- Mylar is my capacitor brand of choice -- but there's some Panasonic, Vishay, and a little SBE in there, too.

Incidentally, how in the world do you guys clean the underside of the chassis? When I get a group of caps removed, I clean what I can of the underside of the metal, but even the wiring is completely coated in 'goo.' I dare not use alcohol to clean it, as I fear it will brittle the aging plastics. This set was picked up in major coal country, -- maybe it just lived a dirtly life.

As the crow flies, this was picked up about 80 miles from the Columbus Oh TV market, and also about 80 miles from Charleston, WV. I'm suprised there was anything on the air in 1950...

If I can keep it up a little while longer, I think I'll be ready to bring it up on a variac next weekend. And then, either great exhilaration or mild depression will result

Alex

[John Folsom]

Alex, which inductor has the missing capacitor? As for cleaning, other than a good dusting with an air compressor, I don't try to clean the underside of chassis. The underside of your chassis looks pretty clean to me.