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Ron's RCA T7-5 Electronic Rescue - Printable Version

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Ron's RCA T7-5 Electronic Rescue - Ron Ramirez - 06-03-2018

I found this poor little tombstone at Kutztown last month.

[Image: http://www.philcoradio.com/images/phorum/T7-5/T7-5_001.jpg]

The radio has seven tubes and covers three bands (540-1625 kc; 1.625-5.7 mc; 5.7-18 mc).

[Image: http://www.philcoradio.com/images/phorum/T7-5/T7-5_002.jpg]

The cabinet is not in the best of shape. It has water/moisture damage and missing veneer - two strips on the bottom of the right side and an entire section on the left side:

[Image: http://www.philcoradio.com/images/phorum/T7-5/T7-5_003.jpg]

Below, you will see more of the water/moisture damage - the inner substrate is loose and wavy, and the left side of the cabinet is pulling loose from the bottom. Also the top is slightly warped at the back and has pulled away slightly from both sides at the back.

[Image: http://www.philcoradio.com/images/phorum/T7-5/T7-5_004.jpg]

Also above, other issues such as the following are obvious:

This thread will cover only the electronics. I will attempt to do something with the cabinet later in a separate thread.

RE: Ron's RCA T7-5 Electronic Rescue - Ron Ramirez - 06-03-2018

Preliminary

To remove the chassis from the cabinet, one must remove the knobs.

They simply pull off, right?

Well...they should.

I had to resort to the old string trick to get the three lower knobs off. For the double tuning knob, a combination of string and brute force finally broke them free.

All of the knobs were pretty well rusted onto their respective control shafts.

Having finally removed those stubborn knobs, I unplugged the speaker cable and pulled the chassis.

[Image: http://www.philcoradio.com/images/phorum/T7-5/T7-5_005.jpg]

As you can see, the dial scale has faded to oblivion. Fortunately, Radio Daze sells replacements. I will not invest in one until I know the radio is going to work, however.

Top view of chassis:

[Image: http://www.philcoradio.com/images/phorum/T7-5/T7-5_006.jpg]

and bottom view:

[Image: http://www.philcoradio.com/images/phorum/T7-5/T7-5_007.jpg]

Finally, for those of you who wish to follow the bouncing capacitor symbol, here is the complete service manual (warning - large file - 22.2 MB).

I thought I would have to invest in some .047 uF capacitors before I began, but this radio does not use any! It uses .005, .017 (!), .01, 0.1, 0.25, a couple 10 uF electrolytics and one 18 uF electrolytic. Easy. Icon_thumbup

On the other hand...did you take a good look under the chassis? Rubber-covered wiring! Icon_mad Icon_evil Icon_thumbdown Icon_wtf Icon_crazy

RE: Ron's RCA T7-5 Electronic Rescue - NostalgiaRadioTime - 06-03-2018

In spite of the rubber wire, this should be a breeze for you compared to the last project. The nice thing about these RCA chassis' is you can remove the two cross braces where the chassis bolts go. It makes for a much more "open" chassis to work on. I'm betting that candohm will have to go for sure. It just doesn't look healthy.

If I'm not mistaken, RCA (and GE) were using all metal tubes at this point. Nice for not having to deal with tube shields, but kinda hard to tell if the tubes are lit! Icon_lol

RE: Ron's RCA T7-5 Electronic Rescue - Ron Ramirez - 06-03-2018

Yes, and I will definitely remove those braces in order to have as much access to the parts under the chassis as possible. Of course, the braces will be put back when chassis work is done.

You're right, Greg...this T7-5 was among the first RCA models to use metal tubes. Since this radio was designed for them, I don't mind using metal tubes in this set and, assuming the radio works after recapping and replacing rubber-covered wires, I will replace the few GT tubes this set has with metal.

I forgot to mention earlier...it was just as difficult to remove the tube shield from the 80 tube as it was to remove the knobs. The shield was almost completely rusted to the shield base. Another casualty of moisture. I hope the coils are OK.

RE: Ron's RCA T7-5 Electronic Rescue - Ron Ramirez - 06-03-2018

I've ordered Solen Fast capacitors to replace the electrolytics, and ordered some mica caps as well. With any luck they might be here by next weekend. I think I will use Solens in all of my better radios from now on...that is, as long as they will fit in the capacitor cans in which they will be used. They cost quite a bit more than new electrolytics but they will likely not require replacement for decades. And they fit very nicely in the large prewar aluminum cans.

RE: Ron's RCA T7-5 Electronic Rescue - Steve Davis - 06-04-2018

Should be a piece of cake after the 38-690.

Steve

RE: Ron's RCA T7-5 Electronic Rescue - Arran - 06-05-2018

This brings back memories of the Canadian RCA 5T5 and CGE E-81 sets I have owned and worked on, those were somewhat newer then this set though as they were 1936-37 models, but with those the bottom was made up of three pieces, two L shaped steel end plates one which the chassis mounts were riveted, and a thinner sheet metal cover for the bottom center. To get at much of anything the end plates had to come off. The bottom of this T7-5 chassis reminds me more of the Victor 118 I used to have and the CGE M-61 I currently have, which were 1934-35 models, I can kind of see the evolution of RCA/G.E chassis design here.
Regards
Arran

RE: Ron's RCA T7-5 Electronic Rescue - Electrothaumaturgist - 06-05-2018

It is so incredible to be amongst such knowledgeable technicians of ancient technology in this day and age that there is no way to adequately express my gratitude for allowing me to be a part of this "living thing" but to say THANK YOU ALL!!!  Icon_biggrin

RE: Ron's RCA T7-5 Electronic Rescue - Ron Ramirez - 06-06-2018

Glad to have you here in the audience, Electro. Icon_thumbup

My Solen caps and mica caps arrived today from Arizona. That was really quick...I ordered them Sunday night and three days later, here they are. And I went the cheap USPS route on the shipping, also.

No time to even open the package tonight due to other commitments. I probably won't actually start working on this T7-5 until this weekend.

Fortunately I still have some plastic insulated wire left over from that Philco 41-616 project from a few years ago, which I will use to replace the rotten rubber-covered wire.

RE: Ron's RCA T7-5 Electronic Rescue - Ron Ramirez - 06-06-2018

(06-05-2018, 05:20 AM)Arran Wrote:  This brings back memories of the Canadian RCA 5T5 and CGE E-81 sets I have owned and worked on...

Funny you should mention that...for me, this brings back memories of the 1970s and an RCA 5T1 which was my first complete antique radio. That was so many years ago (more than 40 years Icon_eek ) that I don't even remember whether or not it had rubber-covered wiring. If it did, it must have still been good for the most part at that time. The 5T1 would have been around 40 or so years old at that point, really not that old at the time. Now decades later, this T7-5 is 82 years old and the rubber-covered wires have long since become dry-rotten.

RE: Ron's RCA T7-5 Electronic Rescue - Ron Ramirez - 06-08-2018

...and so it begins.

[Image: http://www.philcoradio.com/images/phorum/T7-5/T7-5_009.jpg]

Replacing rubber-covered wire is fun...not. I only worked on this for a couple of hours tonight and you can see what I have accomplished so far.

When it comes to sets like this with rubber-covered wiring, I simply pick a place to jump in, so to speak, and go from there. I'm working around the 1st audio tube (6F5).

A closer look:

[Image: http://www.philcoradio.com/images/phorum/T7-5/T7-5_010.jpg]

I can hear some of you now...Oooohhh, eh eh, you didn't restuuuffff the old caaaaaappsss!!! Well, no, I didn't. There is a very simple reason why...the old paper cap tubes are unraveling and falling apart! So, yes...I'm taking the easy way out.

The RCA solder terminals are quite unusual compared to Philco. Each terminal...including the terminals on the tube sockets...is a set of three "fingers" if you will, through which the wires and lead ends are threaded. Then these "fingers" were squeezed together and the whole thing covered in solder. Even though it's been something like 42-43 years, I remember these same type of terminals on that 5T1 I had so many years ago. And, like the 5T1, I found that these can be fragile; one of the three "fingers" has already broken on one solder terminal so I am trying to be as careful as I can.

Oh, and this radio had extra electrolytics added under the chassis. That "repairman" did not bother to disconnect the originals, merely adding the then-new ones in parallel with the originals. I have now removed all of the extra added electrolytics, leaving the aluminum can electrolytics in place. I will deal with those later on in this restoration.

RE: Ron's RCA T7-5 Electronic Rescue - Electrothaumaturgist - 06-09-2018

Wow! That baby has seen some serious moisture to have the waxed(?) caps disintegrate like that  Icon_wtf

RE: Ron's RCA T7-5 Electronic Rescue - Ron Ramirez - 06-09-2018

It seems that way, Electro. Although, the chassis isn't rusty, which I find amazing considering the condition of the cabinet.

So, what did I do today?

Not a lot on the T7-5. Grocery shopping, mowing, replacing the handle on a storm door, more mowing, and then...finally, after supper, a few hours with the RCA.

[Image: http://www.philcoradio.com/images/phorum/T7-5/T7-5_011.jpg]

[Image: http://www.philcoradio.com/images/phorum/T7-5/T7-5_012.jpg]

What took the most time was the little .01 uF cap in a metal case. I managed to get the unit apart (no easy feat) and restuffed it with a yellow film cap. I filled the unit with hot glue for good measure. That took up most of tonight's time with the radio.

I also replaced a couple wires and one resistor. I also removed the second IF can cover, cleaned it, and put it back in place.

In addition, I checked the two Candohm resistors. One is good (R16-R17-R18), but the other has one bad section (R15 and R20; R20, 15K, is the bad section). Greg, you called it; one bad Candohm section. Only, it was the Candohm that was not visible before I removed the cross-braces and the extra added electrolytics. Time for a Mouser order. By my calculations, 5 watt resistors should be sufficient for both R15 and R20 but I will likely use 7 watters instead.

While I am at it I might just go ahead and replace the three section Candohm (R16-R17-R18) as well.

RE: Ron's RCA T7-5 Electronic Rescue - NostalgiaRadioTime - 06-09-2018

You're right, Ron,  I couldn't see that candohm along the rear side of the chassis either with the brace in place. I was referring to the 3 section one running vertically from front to back. Still, as you said, you may as well change that one too. It may test good now but who knows how reliable it will be.  As with the crumbling wire replacement, now is the time to do it for peace of mind. Icon_thumbup

I remember those flimsy 3 finger terminals on the tube sockets from the late 30's RCA's I've worked on. After almost breaking one off, I copped out and did the hook and loop dance... leaving the original lead attached to the terminal and cutting out the old caps and resistors close to their body. It may not be the most ideal way, but it saved me some unneeded headaches.

RE: Ron's RCA T7-5 Electronic Rescue - Arran - 06-10-2018

If you want to know how to work with those forked tube socket terminals it is actually not that hard. The secret is to use a solder sucker, which I do anyhow even in sets without the forked terminals, heat up the terminal, place the solder sucker on it once it's molten, then let go of the bulb or trigger. Sometimes you have to use it more then once, and that takes care of 90% of the old solder on the terminal. Then what you do is use a sharp, flat bladed, jeweler's screwdriver, a small one, to spread the fingers apart, sometimes heating up the joint again whilst you are at it, and the old component leads and wires will lift right out, replace in reverse order. I've worked on a lot of Canadian RCA and G.E sets from the late 1930s and early 40s, and they used the same forked terminals on the tube sockets as Ron's T7-5 uses, the American RCAs from that era did not according to what Ed Holland found in his RCA 110K2, so I had to work out a technique, no cutting and J hooking involved.
Regards
Arran