[criageek]

Ok, it's time for me to get back to this.  First I need to apologize for jumping back and forth between the Philco schematic and the Sams schematic.  In general I prefer working with the Philco schematic because I find it easier to follow.  But the copy I have of the Sams schematic is clearer and easier to read.  From here on I'll try to use just the Sams schematic when posting here.

I haven't done anything on the 47-1230 for the last 2 weeks or so, but I have done some research, studying, and thinking.  I've watched a few videos and done some reading on tube theory and feel like I have a better understanding, but that hasn't helped me determine if I have a problem with too much current being drawn through the 15k, 2W resistor (R127).  My current thinking is that the circuit is working as designed.  Part of my reasoning is logic (albeit somewhat flawed logic), and part of it is wishful thinking.  The wishful thinking is because I really want to call this radio done and surprise my Mom with it.

Here is my logic.  As designed, R127 dissipates about 0.96W in AM mode, which is already higher than I would think they'd want to run a 2W resistor.  But this is based on the voltages listed on the Philco schematic...310v on one side and 190v on the other, so 120v across the resistor.  Simple math tells us that it's dissipating 0.96W.  So now, if we assume that more current will be drawn through R127 in FM mode, then we will be running this resistor even hotter.  Now here is where the 'flawed' logic may come in.  My assumption is based entirely on the fact that my radio DOES draw more current through R127 in FM mode.  I'm not smart enough to be able to tell from the schematic if it should draw more current in FM mode.  Here is a schematic with the circuit in question highlighted in blue.  The yellow section is switched into the circuit in FM mode.  Basically, it connects a 4.7k resistor across the 27k resistor connected to the plate of the oscillator section of the 7F8, resulting in 4.004k.

   

I didn't find this current draw through R127 as a result of some in depth troubleshooting.  Basically I just stumbled onto it.  I noticed the voltage drop across it and did some math.  I tried touching it the last time I worked on it after running in FM for a while and I couldn't hold my finger on it for more than a second.

Unless someone can convince me that there really is a problem here, I plan to replace the 15k, 2W resistor with a 15k, 5W resistor and button it up.  But, if you are going to tell me there is a problem, please give me some guidance for determining what is causing the problem and how to fix it.  What would you do?

Thanks,
Rich

[jcassity]

try something new...............

condition- unplugged and the loctal tube removed.
check to see if there is **ANY** resistance from any pin at the top side to chassis. If your in the same boat i was in,, my loctal was suffering from a minor carbon trace. or carbon strike,, whatever its called. i had no socket replacement so i proceeded against the advice i was given and pulled the socket out. i took it apart to reveal the two wafers . I laid the wafer with the obvious gully of a carbon strike down on different sand papers until i sanded way past the carbon trace. i put it back together and that solved that problem.
although its not typical ,,, i actually desoldered all terminations to this socket so i was sure my meter would read just the pins to chassis.

Check to see if the theory on my set holds true to yours.... see if you have an AC and DC voltage drop across that resistor. I could not actually detect it on the meter but,, it should be there invisible to your meter or not. A oscope would surely see it. Because of where it is actually living in the circuit, the thought on my set for this same problem resistor was that it not only has dc but also some ac on it,,,
I was able to gain control of how i calibrated it by observing my voltage at my 6k6 to get it below 300v or close to what the schematic says.

[criageek]

Thanks again jcassity - I REALLY hope your suggestion (possible carbon trace) is not true, especially for the 7F8 (mixer/oscillator) or 6BA6 (1st IF amp).  They are both buried under the function switch on the tuner sub-chassis and would be impossible to service without removing it, especially the 7F8.  I did do tube resistance measurements in the past and the only unusual readings I got were on the plates of the tubes that are fed by the B+ line(s).  In each case the expected reading is in the 180k - 300k range and I read infinity (with a digital meter).  I would think that I would read much less rather than much more if there were a carbon trace problem.

I'm sure I do have an AC component across R127, but if I remember right it's only about 200mV.  At this point I feel like I may be chasing my tail and I don't want to do that.  I did install the 5W resistor, but I'm going to go a step further.  I ordered a chassis mount resistor and will install that.  There may still be a problem in the circuit, I don't know.  But at least R127 will not be a failure point, and the heat it generates will not cause a failure.  So if there is no problem in the circuit it will be extra protection.

I have a possible lead on a 47-1230...I may pick that up so I have a chassis to compare to.

Thanks!

Rich

[Greggshere]

Rich if you do find another 47-1230 to compare to, I would love to buy it from you when you are finished with it. My existing one is pretty much a lost cause, since it was my first victim and I didn't know crap. Now I have a few restoration s under my belt and if I have one in fairly original shape I'm confident that between the two I can make one good one. I have a beautiful cabinet, a restored record changer and no working chassis at this point.

[criageek]

Sorry Gregg - I messed up...again.  This is the second time in about 2 weeks that I've mistaken a 46-1209 for a 47-1230.  I need to look closer at the dial next time    

Rich

[Greggshere]

Just keep me in mind if you do find one. It would be greatly appreciated.

[jcassity]

so.............. current status check...
set fully restored for radio purposes......

symptom: No FM?? or intermittent weak FM?

[jcassity]

another thing...
you may have to find the tables associated to your set that show expected voltages along the entire schematic.

usually these tables denote the band switch orientation as the "given".

you have a fully restored radio, now its time to plot your voltage readings against the oem documentation.

I dont think i have seen this from you yet but,, i may be wrong. I have seen tables you have built but dont recall seeing the actual oem tables.
if you dot have this literature, it can be purchased here easily, plus its going to add value later when a future failure happens.

[criageek]

The chassis mount 15k resistor should arrive today.  Once I have that installed I plan to do the entire alignment procedure (yet again!) and then I'm prepared to put everything into the cabinet and call this project complete.  AM works well.  FM works well.  Phonograph works well.  The only performance problem I've seen is that there are a couple of FM stations at the low end of the band that I also pick up at the high end of the band. I don't know what to do about that so I'm prepared to live with it.  Otherwise, it works really well.

Rich

[jcassity]

hey,
scroll up..
you posted at the same time as me...

its all good now??????????????????????

[criageek]

Yes jcassity, that was in reply to your 'current status check'.  Other than the current draw through R127 in FM, and the low end FM stations showing up at the high end, I am happy with the way it works.

I have taken voltage readings at each tube pin and compared to those listed in Sams, but those readings are in AM mode.  I have taken all the same readings in FM but don't know if they are valid or not, since I don't have reference voltages for FM.  The Philco schematic does show voltages at various points in the circuit for both AM and FM, but not for each tube pin.

Rich

[Jake Blake]

What is the bandwidth of the FM band on your 47-1230 ?  
Is it anything like what we have today?   I notice older models have a much lower bandwidth for FM, ( 42-50mc),  but the info given in the Gallery doesn't say the bandwidth for FM for this model.

I wonder when that change occurred.

[Radioroslyn]

I think you mean frequency coverage. Bandwidth typically refers to narrow or wide the IF stage(s) will pass.
For instants the typical bandwidth for FM is 300kc and AM is 10kc.
The frequency coverage or tuning range for FM is 88-108mc (Modern) and Pre war is 42-50mc but the signal width is the same.

>I wonder when that change occurred.
It was right after WWII. There were a few sets that have both FM bands (referred to as the FM 50 and the FM 100 bands) These where built in '46 or so. I don't think Philco made one w/both Zenith did. The demise FM 50 band was TV it was use for channel 1or2 I forget.

[EdHolland]

The Zenith 12H090 I just completed has both FM bands, and that set was from 1947 (earlier variants go back to 1946).

There is quite a story behind FM, and the competition as it was introduced, rejected by RCA, and E.H. Armstrong attempting to strike out with his own radio network. If you ever get the chance to watch the Ken Burns documentary "Empire of the Air", it tells the story rather well and is well worth viewing for lots of other radio history.

[criageek]

Today was a big day for the 47-1230.  First a quick update.  I installed the 15k chassis mount resistor on the top side of the chassis and used some heat sink compound to help the heat transfer.  That thing is absolutely cool after running in FM mode for a while, so I am happy with that decision.

Now, about that big day.  I stuffed everything into the cabinet...actually I did it many times, trying to get everything just right.  Finally, everything fits nicely and looks great.  Well, the pushbuttons are badly off-center, but I couldn't see a way to correct that without some pretty major surgery, so we'll have to live with that.  So, time for the first full test in the cabinet, and it's a huge success.  Since the dial is mounted to the inside of the cabinet and not to the radio chassis, this is the first time I've been able to see exactly where the pointer is on the dial.  AM works very well and I was able to align it so that it tracks quite well on the dial.  It picks up a ton of stations and sounds great.  FM also works very well.  It picks up many stations, some I would not expect it to pick up.  And I even picked up some shortwave!  And all this is in my basement, using the built-in FM dipole and AM loop antenna.  I've very, very happy with the performance.  The one thing that is disappointing is the tracking on FM.  If I adjust it so it picks up a strong station at the proper spot on the dial at the high end, the low end is quite a ways off.  For instance, if I set it to get 104.5 at 104.5, then 93.9 is way low, somewhere around 92.5.  I'll probably set it so it's accurate near the middle, and just live with it being high at the high end, and low at the low end.

Other than that, I'm very pleased.  I'll post some pictures and video once I get it into some decent light.  I can't wait to surprise my Mom with it  .

Rich