[Ron Ramirez]

Here is how the RF unit looks now - quite bare.

[Image: http://www.philcoradio.com/images/phorum...70_011.jpg]

Unsolder all 13 wires under the chassis as shown in the rough drawing I made and showed you in post #29, page 2, this thread.

Remove the bolt from the back of the RF unit.

[Image: http://www.philcoradio.com/images/phorum...70_012.jpg]

Make absolutely sure that every wire coming from the RF unit is unsoldered and loose now.

Having made absolutely sure of that, tilt the RF unit forward and then work it out by pulling it forward.

[Image: http://www.philcoradio.com/images/phorum...70_013.jpg]

Eventually, you will have it out and on your workbench.

[Image: http://www.philcoradio.com/images/phorum...70_014.jpg]

more...

[Ron Ramirez]

Now, I needed to remove the oscillator section from the RF unit. This involved unsoldering no less than five wires, removing two bolts, and then removing it.

[Image: http://www.philcoradio.com/images/phorum...70_015.jpg]

Look - I had already replaced resistor (33)! You can see the new 10K resistor on the band switch.

[Image: http://www.philcoradio.com/images/phorum...70_016.jpg]

Resistor (32), 32K, still measured almost exactly 32K.

I decided to go ahead and replace (32) and (33) as well as mica capacitor (31), 250 pF. I replaced (33) with a 9.1K resistor this time, in case that was causing the oscillator to start late. I did not have any 250 pF caps so I replaced (31) with a 270 pF mica cap. And (32) was replaced with a new 33K (I had no 32Ks, that is no longer a standard value).

[Image: http://www.philcoradio.com/images/phorum...70_017.jpg]

Now it was time to start reassembling the RF unit.

[Image: http://www.philcoradio.com/images/phorum...70_018.jpg]

Once I had reassembled the RF unit, I put it back in the chassis and resoldered all 13 wires to the proper places.

While I had it apart, I went ahead and replaced the speaker cable.

more...

[Ron Ramirez]

So I hooked up a test antenna and the set's speaker, plugged it in, and turned it on.

Guess what?

It still "popped" on after a warmup of several seconds, instead of coming on gradually as it should.



Hmmm...

[Ron Ramirez]

I decided that I would not pull the RF unit a third time. So I decided to try a desperation move - and that was to replace some tubes and see what happened.

I tried two other 6A8G tubes. No change.

I then replaced the 6K7G RF amp tube with another 6K7G and...

SUCCESS!!!

Yup, the 6K7G RF amp was faulty. Apparently it would not start conducting until one of its internal elements reached a certain temperature and then the radio would suddenly "pop" on.

Now it comes on gradually as it should.

Whew! That turned out to be an easy fix! And I probably did not even need to pull the RF unit a second time.

So that's it. I will now put the chassis and speaker back in the cabinet and call this one finished.

[NostalgiaRadioTime]

Excellent work! I've chased many tubes trying to find which one was the one causing just the symptom you described...that sudden "pop" on instead of a gradual volume increase as the set comes to life. It's frustrating when tubes test good, but yet one of the internal elements have a "hidden" fault that isn't known until the set is tried out. I've got a tube in my big round gold dial Silvertone that leaves that soft "whooshing" sound in the background noticeable when the volume is turned down. Still haven't found which one is the culprit on that.

[mikethedruid]

If your tube tester has a headphone jack, it is for checking just these sorts of problems. With the headphones in, and the controls set properly, you should hear virtually nothing. A whooshing sound, or hiss means a gassy tube. If you tap the tube and it rings like a bell for a bit after the tap it is microphonic. If you tap it and it pops and crackles you probably have an internal short that the shorts test didn't catch.

[NostalgiaRadioTime]

I'm guessing it's probably a bit gassy. I've swapped them all but I suppose it's possible my spare has the same malady. Unfortunately my 1951 tube tester has no headphone jack.

[Ron Ramirez]

I actually replaced the 6K7G twice. The first time, the set came up gradually but the reception was very weak. I turned the radio off and attempted to remove the tube.

Next thing I knew, I was standing there holding the glass envelope of the 6K7G and the bakelite base was still in the RF unit.

That was fun to get out of there. Not.

So I tried another 6K7G and all was well.

The 6K7G I had in the 37-670 to begin with went into the round file along with the one that came apart on me.

p.s. my tube tester (Heathkit TT-1) has no headphone jack either...

[WyTex]

Nice tip on using dots to mark wires. Helped me replace speaker wires this week. Thanks!

Do, or should, RF units be rebuilt as part of a general restoration?

Thanks,
Paul

[Ron Ramirez]

They absolutely should.

They have paper capacitors hidden deep within their insides which can be extremely difficult at times to reach.

Not to mention old carbon composition resistors which are probably drifting way out of tolerance.

And in addition, mica caps in them are now 81-82 years old and these are beginning to fail on us also.

But if you don't replace the paper caps at the very least, the radio likely will not work properly if at all.

Curse David Grimes and his pain-in-the-behind RF units! Now, every time I see a Philco RF unit, I want to take a sledgehammer and destroy it!

But I don't.

[GarySP]

Come on, Ron.  Didn't you know that was a chapter of "Grimes Anger Management Therapy"!  If you didn't bust up one of his components working on it, you were ready to deal with anything else life could throw at you!  Take care, Gary

[Nathan Slingerland]

Oh come on - it's a huge improvement.  

Just have a read of the Sept. 1936 issue of 'Philco Serviceman' where there's a long article enumerating the reasons and benefits:

https://drive.google.com/file/d/1RuuLkCv...sp=sharing

   

[Ron Ramirez]

Yuuge improvement, my behind. What a load of crap. Who did Grimes pay off to write up that load of bull.... ?

and yes, I have previously read the Philco propaganda about the "ease" of service of the Unit Construction sets. I still say BULL!!!

[Nathan Slingerland]

Heh - I was sure you had seen it but I noticed it recently when scanning in issues Serviceman. Its pretty amusing that Philco is clearly in sell-mode to the technicians as much as to the end customers!

Anyway, a strong link between physical layout and schematic layout of the stages just doesn't hold water.  Probably it just made it cheaper to build in the first place.

[Arran]

If they had made the sets truly modular, as in using plugs and sockets, or screw terminals to connect the modules together, then what they said would have been true. Arguably the featur that you can dismantle the chassis by undoing the front and rear plates is kind of convenient, but only to get around the inconveniences of having components hidden in various nooks and crannies around the chassis.
Regards
Arran