[francois8890]

I finished restoring 2 Philco 16B Tombstones this Summer. A code 121 and a code 125 and did a complete restoration on both of them including rebuilding the bakelite blocks, replacing all the other caps and out of tolerance resistors, stuffing the electrolytics and filter cans as well as doing alignment by the book and now both sets play extremely well with the exception of the shadowmeters on both radios.

Version 1 code 121 meter coil reads 949 ohms and AVC voltage seems to be tracking along with tuning. Also have checked magnet and the "vane" in the assembly returns promptly when flipped with a toothpick. Of course on this version there is no shunt resistor so that isn't the issue either.

Version 125 meter coil measures just over 1100 ohms and shunt resistor is 5,050 ohms. I had read that the resistor was subject to a Philco revision that swapped a 2,500 ohm resistor here so I clipped another 5,000 in parallel and there was no difference in the operation of the meter. I didn't take the code 125 meter assembly apart yet as I'm a little reluctant to take this one apart again since it took me so long to get it working as well as it is now and don't want to take a chance of screwing something up. As I said, both of these are exceptional radios with the exception of the shadow meters, especially the code 125 , getting stations from literally everywhere. I just would like have them truly finished before I put them back in their cabinets. Thanks for any ideas,

Frank
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[PAradiogeek]

Frank,

My 37-630 schematics in Rider's includes instructions for adjusting the shadow meter. Do you have anything like that for your 16B? Basically there are two adjustments that can be made. The coil can slide forward or back on the housing. Also, if I read it correctly, you also can twist the coil in one direction or another on the housing in order to get good operation of the meter. I don't really understand how the latter could affect this, but that is what the instructions said.

You might want to refer to the schematics in Rider's for another radio if yours doesn't have this informaiton. The scematics for the 37-630 are not on NA, but here is where information on the 37-640 can be found.
http://www.nostalgiaair.org/Resources/216/M0013216.htm
Consult the second page.

I hope this helps. Good luck.
Jon

[francois8890]

Frank,

My 37-630 schematics in Rider's includes instructions for adjusting the shadow meter.  Do you have anything like that for your 16B?  Basically there are two adjustments that can be made.  The coil can slide forward or back on the housing.  Also, if I read it correctly, you also can twist the coil in one direction or another on the housing in order to get good operation of the meter.  I don't really understand how the latter could affect this, but that is what the instructions said.

You might want to refer to the schematics in Rider's for another radio if yours doesn't have this informaiton.  The scematics for the 37-630 are not on NA, but here is where information on the 37-640 can be found.
http://www.nostalgiaair.org/Resources/216/M0013216.htm
Consult the second page.

I hope this helps. Good luck.
Jon

Jon, thanks for your response. I do have those instructions or some that are similar that came from the Philco Repair Bench. My problem is that there is no movement on the screen as I am tuning. I am thinking this may be related to AVC or something in the operation of the radio itself that is not providing proper voltage to track the tuning for the meter. I am going to go back over all the procedures in the Philco service hints that affect the sharpness and width of the shadow and see if that helps. Thanks for your input,

Frank

[Mondial]

Frank, how much AVC voltage are you developing on a strong signal? You should have -10 to -15 volts of AVC, which will make the shadow meter deflect and narrow appreciably. Anything which reduces the overall gain of the receiver will affect the amount of AVC generated, which in turn will affect the shadow meter operation. 

The meter really indicates the plate current of the AVC controlled tubes, which is reduced as the AVC voltage becomes more negative.

[PAradiogeek]

You had mentioned that the "...AVC voltage seems to be tracking along with tuning" so I assumed the problem was with the adjustment of the meter itself. What is the voltage difference on the AVC line between a strong station and no station?

My next guess would a weak tube somewhere between the RF and 2nd IF tubes. Have the tubes been tested? Even if you have and they test OK, it may be worth swapping out a few tubes in there to see if you get any better response out of the meter.

[francois8890]

Frank, how much AVC voltage are you developing on a strong signal? You should have -10 to -15 volts of AVC, which will make the shadow meter deflect and narrow appreciably. Anything which reduces the overall gain of the receiver will affect the amount of AVC generated, which in turn will affect the shadow meter operation. 

The meter really indicates the plate current of the AVC controlled tubes, which is reduced as the AVC voltage becomes more negative.

I will need to measure it again but I don't believe it is that much. More along the order of -0.5 to -5.0  Where is the best place to measure? I think I put it on the grid of AVC tube. Is it better to open the circuit and measure current?

I will measure again and post back.
Thanks,

Frank

[francois8890]

You had mentioned that the "...AVC voltage seems to be tracking along with tuning" so I assumed the problem was with the adjustment of the meter itself.  What is the voltage difference on the AVC line between a strong station and no station?

My next guess would a weak tube somewhere between the RF and 2nd IF tubes.  Have the tubes been tested?  Even if you have and they test OK, it may be worth swapping out a few tubes in there to see if you get any better response out of the meter.

What I actually meant was that I could watch the AVC voltage go up and down as I tuned on and off a strong station. I am going to go back out to the shop now and measure again. I addressed what I thought the voltage might be in my response to Mondial in the previous post. It sounds like after reading his post that the amount I was reading would not be sufficient to move the meter.

Thanks,  Frank

[francois8890]

OK went out and measured AVC voltage  again and measured at grid of 78 QAVC tube. -1.75 tuned in and - 0.387 off tune so it looks like not enough AVC to move the meter.  Now to figure out what is causing the lack of voltage there .
I didn't mention that I live in a very rural area and get virtually no am stations here so I am talking about my AMT 3000 transmitter as I mention a "strong" station. I do get Quite a few SW stations at various times though so the radio does seem to perform pretty well. As far as the code 125 version, I am getting excellent performance with stations all over the SW dial. But right now I am working on the early code 121 and trying to solve these one at a time.

I also swapped out all of the front end tubes with no change in performance at all just to be thorough.

Thanks,

Frank

[francois8890]

I don't seem tp be getting anywhere here. I have done another alignment to see if anything changed and it hasn't. I replaced another resistor,2.0 meg that measured 2.8 meg and again , no change in AVC voltage. Also as I mentioned before, swapped out all of the front end tubes with no difference. I must be missing something. I am puzzled as to why the radio performs so well with the low AVC voltage. Any thoughts out there? Thanks,
Frank

[francois8890]

Well, although not 100% perfect, I do have this shadow meter working now. I replaced 6 resistors in and around the IFs and Detector  circuits which seems to have increased the AVC voltage some. I am now getting -0.5volts off tune and up to -4.0 volts when tuned to a station. I also took the meter assembly back apart and worked with the magnet some (seems kind of weak) and put back together and adjusted the light some as well. So now getting movement as station is tuned in and out. I also have found that putting a small magnet near the shadow meter increases the movement some.
I am now ready to quit while I'm ahead with this one and move on to the Code 125 version and see what I can find there.
Thanks to all here who helped,

Frank

[Warren]

Sounds like the magnet might need remagnetized. There are directions here on the forum , and other places on the net showing how to do this.

[francois8890]

Sounds like the magnet might need remagnetized. There are directions here on the forum , and other places on the net showing how to do this.

Thanks Warren, I tried those instructions and there was a slight improvement but not enough. What I was able to do though was place 3 small magnets on the outside of the meter housing which has greatly enhanced the action and definition of the meter and I am going with this for now. I will put the chassis back in the cabinet and let it play for a couple of weeks to see if the affect lasts.

Thanks,
Frank