[rck46]

I am getting low volume on this radio with the volume control turned all the way up when tuned into a very strong local station.  I have gone through this radio and replaced all the electrolytics, paper caps, some of the micas, and out of spec resistors.  All the antenna, RF, oscillator, and IF coils show continuity.  In the process of aligning this radio I noticed that I could not get the secondary of the 1st IF to peak no matter how much I turn the adjusting trimmer screw.   I took the cover off of the IF to see if there was anything obvious and noticed that there is a black wire that makes one wrap around the secondary coil between the third and forth windings.  Here is a picture of the IF.  The top coil with four windings is the secondary coil and you can see the black wire with some of the insulation flaking off.
 
One end of the wire goes to chassis ground and the other end attaches to pin 5 of the 6K7 I.F. amp tube.  I see nothing on the circuit diagram that indicates this.  Circuit diagram is at http://oldtech.net/Philco/37-630p1.gif.  

Any ideas as to the purpose of this wire and if it relates to my problem of not being able to peak the secondary of the 1st IF and low volume?

Other things I have tried or checked.

1.  Voltages check out reasonably against those listed on Riders, page 7-59.

2.  Subbed tubes - no change.

3.  Get loud hum when touching grid cap of 6Q7 tube.

4.  My shadow meter is open so I subbed a 1500 ohm, 1/2 watt resistor to replace it.

5.  I noticed I get somewhat louder reception when a gound is connected to the gound terminal on the back.

Two more questions.  The circuit diagram shows the aerial and ground terminals.  Is the antenna to be connected to the red or black terminal or does it make any difference.  Also there is a link attached to the ground terminal that can be swiveled between the black terminal and terminal 4.  What is the correct position for this link?

I also posted this over at the antique radios site.


Thanks, Bob

[Mondial]

The black wires are a negative feedback winding which was added in later production. It applies an out of phase IF signal to the suppressor grid of the 6K7 tube and serves to stabilize the IF amp. I guess Philco had problems with the IF stage oscillating and came up with the extra winding as a fix. Don't think it has anything to do with your problem unless it is obviously shorted to the secondary winding itself.

It seems that the secondary of the IF trans is not resonating with the associated trimmer and that will definitely cause low sensitivity. Have you measured the actual resistance of the secondary and how does it compare to the value noted on the schematic?

There is not much there to the tuned circuit, so there is a problem with either the coil, or the mica compression trimmer which tunes it to resonance. You can disconnect the coil winding from the trimmer and test each individually.

[PAradiogeek]

Hi Bob,

Yes, the red terminal is where you should connect the antenna.  If you attach a 5-10 foot wire, that should be sufficient to receive a strong local station.

Another thing to check would be the AVC resistor (R44).  This should be a 1meg resistor.  Did you replace this resistor?  If it was open or if it's value is significantly higher, then the gain in the first stages will be reduced.

Jon

[45philcodon]

I'm surprised that you noticed an increase in volume by adding a ground connection. Normally, the presence of capacitive coupling from the chassis ground to the power line provides the effective ground connection. Capacitors "67" should provide an effective ground.

Don

[rck46]

Thanks for the comments and the purpose of the black wires on the 1st IF.  I replaced all the insulation on this wire.  There was no short between it and the secondary coil.

I isolated the secondary coil.  Resistance is 15 ohms.  Circuit diagram indicates 11 ohms.  No change after connecting to the trimmer.  Primary measures 13 ohms and circuit diagram indicates 7 ohms.  The trimmer and mica piece looked good.

R44 was replaced with a 1 meg resistor.  I verified that I replaced it with the correct resistor.

Capacitors 67 were replaced and they are grounded.  Still get more volume with a ground.  

I went through another complete IF and RF  alignment of the short wave and BC bands following the Riders proceedure. The link on the aerial/ground terminal was between the black and ground screws.   With the exception of the secondary of the 1st IF I was able to peak all the trimmers.  I screwed the trimmer on the 1st IF secondary trimmer all the way down and then backed it off until it peaked.  Further backing of the screw produced no change.  I would say volume on the BC band is slightly better but not the best. Still have to have volume control turned most to all the way up.   Volume at the lower end of the dial was very low until I detuned the 600 trimmer slightly from the setting that was made during the alignment process.

I get no reception on the other two bands. 

I also have a post on this radio over at the antique radio site.  See http://www.antiqueradios.com/forums/view...6&t=320133
One comment asked about the effect of touching the grid caps of the RF  6A8 and 6K7 tubes.  Touching the 6A8 silenced the radio and touching the 6K7 reduced the volume.

Bob

[Mondial]

My guess is there is a problem with the mica trimmer. If you loosen the screw and it still does not peak, it seems to have too much capacitance to tune the coil to resonance.

 You can try connecting a mica or ceramic cap in the range of 220 pF to 470 pF to the grid cap terminal of the trimmer (remove coil wire and grid cap wire)  and connect the coil winding and grid cap wire to the other lead of the added cap. This will effectively place the added cap in series with the trimmer, reducing its capacitance, and you can see if the secondary coil will now peak.

[rck46]

Mondial: 

In response to your comment I pulled the IF apart to add a 390 pf capacitor in series with the IF trimmer.   Before I did this, out of curiosity, I checked the capacitance of the trimmer with my Heathkit capacitance checker.  I checked it with both coil leads disconnected.   With the trimmer screw tightened completely the capacitance of the trimmer is about 60pf.  This number seems too low rather than too high.   As I loosened the trimmer screw the capacitance dropped.

To confirm this I checked a trimmer out of a Grunow 580 that was in my junk pile because of open coils.  Capacitance of this trimmer was 150 pf with the screw tightened and as the screw was loosened the capacitnce dropped.

Based on this comparison it seems like I should be increasing capacitance by adding a capacitor parallel wih the trimmer.

Am I thinking correctly?

Does anyone have an idea what the capacitance range of this Philco trimmer should be?

Thanks, Bob

[Mondial]

It does seem the capacitance of the Philco trimmer is quite low, but if you notice the secondary coil has a lot more turns than the primary and as such will need a smaller resonating cap. 

My feeling that you may need lower capacitance is based on your observation that you had the greatest output from the secondary when the trimmer screw was backed out all the way, corresponding to minimum capacitance. Since you never achieved a definite peak, it is possible that even lower than the minimum trimmer capacitance is necessary to bring the secondary into resonance.

My guess is that the trimmer cap plates could have lost some of their springiness and therefore are unable to increase the spacing between them enough when the screw is fully loosened.

[rck46]

For further discussion on this problem see http://www.antiqueradios.com/forums/view...6&t=320133.  Still looking for a solution.

Bob