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37-650 schematic discrepancy??? - Printable Version

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37-650 schematic discrepancy??? - raydog - 03-29-2014

My latest toy, a 37-650, worked great when I got it and now about 3/4 done with the resistors and caps and still plays great. Tag shows it was rebuilt in 1963, looks like a number of caps were replaced and a little wiring.

The problem is that I have a difference between the actual radio and the schematic at the volume pot. Looking at the schematic, part #40, a 75pf cap, is missing. What I have is resistor #41, 40k resistor is tied to cap #43 and the top of the pot, not to the center tap. Center tap is tied to cap #33. I did install a 75pf cap between top and center taps, but I cannot say it made any difference. The tone sounds great except the first setting almost sound like a pocket radio... normal for this setting??? The volume pot looks to be original, it is a 3 tap 2Meg.

Also in my output section, I have a resistor not on the diagram and looks like it was from the factory, it is between the junctions of parts #45, 46 and parts #47 and 53. I think it is a 330k ohm, any ideas why this is here? I left it in as the radio plays fine.

Thanks, Don

http://www.theoldradiofixerupperguy.com/pdf/Philco/37-650.pdf

RE: 37-650 schematic discrepancy??? - TA Forbes - 03-30-2014

I checked my Philco "Changes in Models" data but didn't see anything under model 37-650 that addressed these specific changes.

Philco was notorious for making circuitry changes to their models.

RE: 37-650 schematic discrepancy??? - Tom Hamilton - 04-02-2014

Hi Don,

You're not alone with undocumented Philco changes.

I don't want to hijack this thread, but I've got a question about a factory-wired bakelite block part number not agreeing with the schematic's part number in a 37-670. I'll start a new topic.

Tom

RE: 37-650 schematic discrepancy??? - raydog - 04-02-2014

I hear ya. I think I will just eliminate part #40, the mica cap since it is not clear to me on it's purpose. Radio plays without it anyways, just have to track down why the volume drops out intermittently. Going to check and clean the tube pins and sockets first and see if that helps. Then an alignment.

RE: 37-650 schematic discrepancy??? - TA Forbes - 04-03-2014

With respect to the intermittent volume drop.....

Check the AVC circuitry vic the 6J5. Also check the 6J5 for a cathode to filament leak.

RE: 37-650 schematic discrepancy??? - codefox1 - 04-05-2014

Often parts were used in production and one or more "legs" of candohms were just not used. If there was not an original connection that you can tell, that is what happened. Agree swap out another 6J5. Easy to come by so beg or buy one from classifiedsa here, don't have a spare myself, or would contribute ir.

RE: 37-650 schematic discrepancy??? - raydog - 04-05-2014

Thanks guys, I did order a 6J5G last night so I won't know until next week if it solves the problem. I turned the radio on the other night and have now lost all volume except for a faint whisper. What should I being seeing for voltage on the 6J5? The schematic isn't showing any numbers for that tube.

RE: 37-650 schematic discrepancy??? - Joeztech - 04-06-2014

It looks like that 75pFd capacitor might be there to roll off the high frequency response. This may have been done to help protect the output transformer from extreme high frequency spikes. The output transformer has .003mFd capacitors on either side of the center tap on the primary windings for snubbing the spikes too. Make sure that those parts have about a 1KV or better rating.

As far as any effect on audible response, it probably would not be noticed as the frequencies affected would be in the ultrasonic range.

Joe

RE: 37-650 schematic discrepancy??? - raydog - 04-16-2014

Finally an update on my lost volume. Aside from waiting a week and a half for my 6J5 tube to arrive. No fault of the seller, usps sent it half way around the country when it was only 90 miles away from me to begin with. I plugged in the replacement tube and all is well again. Just coincidence that it crapped out at the time of the recap I guess. I also rewired the volume control per schematic and installed a 75pfd cap where noted. Plays great, now time for a quick alignment and wrap things up with this one.
Thanks for the help,
Don