[Ron Ramirez]

Two years on...

Same radio. And it's been mostly sitting on a shelf collecting dust. I just missed out on an opportunity to sell it, because it now has an odd problem.

Every time I turn it on, instead of the sound gradually coming up as it should, it remains silent until it suddenly "pops" into playing mode.

No, there is no "pop" - what I mean is, that it suddenly comes on instead of gradually coming up.

I think the sensitivity on the upper end of the AM band may be a bit lacking. I did go over the set's alignment today and it helped on the highest SW band, but there was no real improvement in the other bands.

I've switched every tube in case the sudden volume was a tube cathode - no dice, it still does it.

I suppose it is time to get out the signal tracer and see if I can find where the sound stops at turn on before the tubes are fully warmed up.

Any thoughts, guys?

[morzh]

Ron

What if your oscillator is starting late? That would do it.

[Ron Ramirez]

Two different 6A8s...same results. I suppose I could try a third.

[morzh]

No, no, I saw your note that you checked the tubes.
I mean, for whatever reason the oscillator kicks in late. Coils wet, gain low....dunno, it's a shot in the dark. But it only kicks in when the tube is fully hot.

[Ron Ramirez]

Interesting...I should try it on the other bands and see if it does the same thing.

[Mondial]

You can also confirm if it is the oscillator by injecting a 470 kHz IF signal as you turn on the radio. If you hear it immediately, then probably the osc not starting on power up.

If there is a delay in hearing the 470 kHz signal also, then the problem is elsewhere.

[Ron Ramirez]

You guys are good. Injecting a 470 kc signal into the 6A8 oscillator grid was easy enough, and revealed that - yes indeed, the 470 kc signal comes up gradually followed by the sudden "popping in" of the radio signal.

And it is doing this on all five bands.

So...that means it must be something in common with all five bands.

Let's see...other than the 6A8 tube which has been replaced once already, there is capacitor (31) and resistors (32) and (33). I think cap (30) can be eliminated as a potential problem as it appears to only be used on the upper three bands.

That's my thoughts...unless all five oscillator coils are suffering from the same moisture absorption (possible, I suppose).

Regardless, unless youse guys have any other ideas, it looks like the RF unit is coming out again...ugh...

I hear you already. "Post the schematic!" This will get you started:
http://www.nostalgiaair.org/pagesbymodel...013222.pdf

[Mondial]

Ron, you might also check resistors 78 and 79. These form a voltage divider and filtering network (along with C37A) for the osc B+ supply. If they have changed in value it could reduce the voltage available at turn on, delaying start of the osc.

[Ron Ramirez]

(78) and (79) were already replaced with new resistors.

[Ron Ramirez]

Grab a copy of the Service Bulletin for the 37-670 HERE.

Yup, after ten months, I'm finally going back into this set's RF unit. Wish me luck, I shall need it.

[Brad Winder]

Good luck Ron! Knock on wood, my 37-670 works fine...

[morzh]

Good luck, Ron!

[Kenradio]

Best of luck Ron on your 37-670. I'll be following your troubleshooting of the RF unit--I have a 38-2670 Code 325 that works very well and has a simplified RF unit with a 6 wafer bandswitch .   Ken

[Ron Ramirez]

Here's the underside of the patient. Again.

[Image: http://www.philcoradio.com/images/phorum...70_001.jpg]

In case you have forgotten or did not read page 1 of this thread, the paper capacitors have already been restuffed with new caps.

I don't know if I have ever mentioned this or not, but over the years I have developed my own system for unsoldering the wires of a 1937/38 Philco RF unit, and then soldering them back up again. It has worked well for me over the past few decades, so I see no reason to change now.

I make a drawing of where each of the wires go that come from the RF unit, making careful note of just where they connect under the chassis. I then number every wire. I think those wire labels would be dandy, but I've never had any of those, so I just make dots on each wire corresponding with the proper number. One dot = 1, two dots = 2, etc. Once I get to 11, I make one dot, leave a space, and make another dot. 12 = one dot, space, two dots. And so forth and so on.

I have a notebook in which I keep all of these drawings. Now, I had to tear this one out so I could scan it, so I will probably draw it again and keep it in the notebook.

[Image: http://www.philcoradio.com/images/phorum...70_002.jpg]

I make no claims to be an artist. It's a bit crude, but it works for me. This will help me reconnect each of the wires correctly.

Yes, there are two wires labeled 7. They both connect to the same point under the chassis, so I assigned the same number to both.

The drawing also shows the speaker wiring harness. I've made a new harness for this radio, I just need to install it. This drawing will help me do that.

I have now unsoldered each of the wires and removed the volume control shaft as it needs to be out of the way before the RF unit is removed. Tomorrow is a holiday, so I will disassemble the dial assembly and remove the RF unit in the morning, and we'll see what happens from there.

[Ron Ramirez]

So it was time to remove the RF unit. Again.

First, we remove the dial scale. Use some needle nose pliers and put the jaws through the two holes of the metal disk which holds the dial scale in place. Twist counter-clockwise, then remove the disk and the dial scale.

[Image: http://www.philcoradio.com/images/phorum...70_003.jpg]

It is easier to remove the dial scale if you first remove the shadow mask arm from the band switch shaft. Photo to follow.

Next up is the metal and phenolic piece behind the dial scale, with the glass rod in the middle. This is what produces the fine line in the dial scale which is the indicator.

[Image: http://www.philcoradio.com/images/phorum...70_004.jpg]

Remove one bolt on either side of this piece, then remove the piece.

[Image: http://www.philcoradio.com/images/phorum...70_005.jpg]

Now let's remove the shadow mask arm from the band switch shaft. One set screw holds it in place.

[Image: http://www.philcoradio.com/images/phorum...70_006.jpg]

[Image: http://www.philcoradio.com/images/phorum...70_007.jpg]

The metal piece which holds the dial scale in place must also be removed. This is also held on with one set screw.

[Image: http://www.philcoradio.com/images/phorum...70_008.jpg]

Also remove the felt washer behind it. Now, remove the shadow mask. You are left with this:

[Image: http://www.philcoradio.com/images/phorum...70_009.jpg]

Remove two bolts and remove this piece.

[Image: http://www.philcoradio.com/images/phorum...70_010.jpg]

more...