The PHILCO Phorum
37-84 No RF after overhaul - Printable Version

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37-84 No RF after overhaul - spierce328 - 12-30-2018

Hello all. 

I'm trying to bring a customer's 37-84 back to life. (schematic attached) Originally it had the typical hum and would tune no stations. After a complete recap the hum is much reduced, but still no RF what so ever. The radio itself is in very good condition with no work performed on the chassis. I have: replaced all caps, out of spec resistors, checked all tubes, extensively checked continuities with no luck.  All voltages are dead on. Resistance and voltage across the FC is perfect.  I have tried applying the antenna to 77 tube grids with no effect. I'm stuck !!!!

There is one section that does seem odd to me. There is a parallel capactor and resistor (6, and 7) I would assume there to create bias for the 77 tube. However, there is no voltage of any kind across them. Nothing , nada, zip.  This doesn't seem normal. 

Unfortunately, my signal generator shot craps so I am awaiting a new one. Any suggestion where I might look next in the mean time.  I do have replacement 77 and 42 tubes on the way and I'll give that a try. 

Thanks Steve

RE: 37-84 No RF after overhaul - Radioroslyn - 12-30-2018

Check the resistance of the primary of the ant coil. This can easily be accomplished by measuring from the ant and gnd post on the chassis w/the volume control turned up.
Next check the resistance of the feedback winding on the osc coil, one end is grounded and the other connects to a 6K resistor.
Both of these winding should show a low resistance like <20 ohms. If higher you'll need to rewind them, not difficult abt 20-25 turns.
OBTW there's two different versions of the 37-84 a code 121 (older using standard base tubes) and a code 122 using octals.
Original service info is here: http://www.audiophool.com/Philco.html  Look up 1934 and click on 84.

Terry

RE: 37-84 No RF after overhaul - spierce328 - 12-31-2018

I checked both and they are 3 ohms and 6 ohms respectively. Does it make any sense to you that there is no voltage across that 6k resistor. Note that it is in parallel with a capacitor. ( items 6 and 7). 



Thanks

RE: 37-84 No RF after overhaul - Radioroslyn - 12-31-2018

No I would think that there should be some voltage at the cathode. Possibilities are bad tube, bad socket or connection at socket, shorted cap.

GL.

RE: 37-84 No RF after overhaul - spierce328 - 12-31-2018

Upon further review the coil is open at the 6K resistor. i get about 7 megs and it keeps bouncing around. I went as far as taking the resistor connection loose to double check. That would be the coil between pins 4 and 1 on the schematic. 

Yes, this is the 121 chassis. 

Having never rewound a coil before can you point me to a link or advise me how it's done. 

Thanks for all you help. Steve

RE: 37-84 No RF after overhaul - Radioroslyn - 12-31-2018

http://www.philcorepairbench.com/rewinding-philco-rf-coils/

This is for earlier sets but it's all pretty much the same. The important thing is that you need to wind the new winding in the same direction as the original so take note when before you remove it. On the ant and rf coil it doesn't matter but on the oscillator the phasing is important. Wire ga isn't real critical something between 32-38ga is fine. A telltale sign of a bad coil is if you look closely at  the winding thru the wax coating you probably is green dots. This is where the copper wire is corroded. If you try to unwind it it falls apart. It's not very many turns it's like 20-25.
OBTW   http://philcoradio.com/phorum/showthread.php?tid=14779   Your customer might like it.

Have fun it's an easy fix!

RE: 37-84 No RF after overhaul - klondike98 - 12-31-2018

+1 on what Terry said.  The only coil I've rewound was on my 60MB.