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Ok, I'm still having issues with the 37-650. I just finished the extensive teardown of the RF subchassis- anything that wasn't addressed before, was this time. However- I still have no audio output. If you all recall, last we heard, the radio had gone from low volume to no volume. However, I am better equipped to check things now- I have my oscilliscope, my RF generator and my Precision Model 654 Tube Tester and High Sensitivity Set Checker...
So I am making power checks. With the black probe aggie clipped to the case of the chassis, and touching the indicated spots on the underside of the tube sockets, with tubes in place, I am getting some spurious readings: What should be 6-6.5vac is between 6 and 7. This is reasonable. However- all my 100 vac points are running around 200vac-225vac, my 250vacs are running around 510-520vac! I've switched the radio off for the night, and until I get some ideas...
The power transformer is the stock 32-7606N, for 110vac50/60hz. Power here is about 120vac, 55hz. I have a spare transformer I pulled out of the 38-3 I tore down last week, but the number on it is 32-7853N. Is this a suitable substitute?
All my tubes check out fine on the Precision tube tester(this is the type that powers them up, and checks them- not just a filiment checker) I am going to test them again, to be sure they were not damaged by the higher than standard voltage.
I replaced all the condensors, resistors, bakelite and micas as well, and all the electrolytics. I also replaced the hard wiring with new- the insulation was decayed and flaking off, so it only made sense. I used 20ga. stranded for most of it, except where smaller was called for. The two large wire-wrapped resistors were replaced with similar spec. metal oxide ones- could this be my problem?
So now I am looking at two issues: too high a power on the high AC circuits, and no volume. I am suspecting the volume control, and will pull it later today to check it out. It looks like the 38-3 uses the same one,(I checked the numbers) so I will probably give it a swap-out, and see the result- once the power issue is addressed.
Help? Please?
Thanks!
Scott
Old Cars, Old Radios, Old Pipes and Young Women... What more is there?
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Scott:
Where are you measuring these AC voltages? Shouldn't be AC on anything except the heater pins and the rectifier. Maybe your meter is set wrong? If you measure overly high DC voltage on the screens and plates then tubes aren't drawing current. I would suspect a wiring error since you said you replaced all of it. You're going to have to trace it all out very carefully, I think.  Hang in there, it's a great radio, have one on my "bench" now.
Steve Chambers
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Hi- Thanks for the reply. The data I'm using is the Model 37-650 Service Bulliten, No. 254, from Philco. On the front page it gives voltages in Figure 1, with each socket referenced as to pin location and voltages. Most are just given as "100V" or "250V" and only the 6.3V positions are marked as AC. Is it possible I'm doing the wrong test? Should the higher readings be DC? and if so, would that account for the grossly higher readings? I have been using this chart as guide to checking voltages. I have the negative probe clipped directly to the chassis, near the power supply, and touch the tip of the red to the point indicated.
The paragraph under the test point chart states: "The voltages indicated by arrows were measured with a Philco 025 Circuit Tester which contains a voltmeter having a resistance of 1000 ohms per volt. Volume control at minimum, range switch in broadcast position, line voltage 115 A.C." My meter, in AC has 1000 ohms per volt sensitivity, and in DC has 20,000 ohms per volt sensitivity. (I have a 0-6000V ability)
As to the wiring, I replaced each wire one at a time- and referred greatly to the schematic- I have one of the large ones from Chuck S. laminated and clipped up in front of me.(I must say- sometimes takes a bit of figuring to reconcile reality with the schematic sometimes)
Let me know if I'm testing the power in the wrong way, or need a different probe- Right now I'm using a simple straight probe.
(Once I figure out these fancy toys- the ossy, the rf gen, I'll be set... lol)
Thanks!
Scott
Old Cars, Old Radios, Old Pipes and Young Women... What more is there?
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Scott:
Only the filaments have AC, on the other pins you need to test for DC. Most meters require that you switch between AC and DC (mine does anyway). You may still get high readings, but at least they will be accurate this way. I'm still guessing there's an error somewhere and you will need to trace the B+ from the source, make sure the cathodes of the tubes are connected properly and so on. You are testing the right places, and the probe makes no difference. There are other things that can cause a tube not to conduct and so raise the voltage.
Steve Chambers
Posts: 147
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Cool- I'll recheck the numbers this afternoon in DC, I'll post the results. Thanks!
Scott
Old Cars, Old Radios, Old Pipes and Young Women... What more is there?
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Ok- I rechecked with the multimeter in DC... This time things come out a little closer to spec: everything is +-5VDC of the specified voltage, and given the higher input voltage, (line here is 120 or so) seems about right.
I am now going to remove the volume control, and check it out. I have an unknown quality spare to test with...
Scott
Old Cars, Old Radios, Old Pipes and Young Women... What more is there?
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Scott:
Does the speaker make any sound at all? I see in your previous posts that it did at one time. If you are getting good voltage measurements then the field coil should be ok. You might want to check the output transformer. Try injecting an audio signal through a small capacitor to the top or middle of the volume control. Ground the other side of audio source wire to the chassis.
Steve Chambers
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Steve- I had been, the first couple days, getting reasonable sound, but not great. It "blinked out" after a few hours. (This was before I tested voltages last time) I replaced one 6F6GT tube which has a short, and hoped that would take care of the issue, but has not. (And checked all my Ant. RF and Osc. coils- clleaning all the wafer switches at the same time. I also checked the RF cans...)
The only signal generator I have is RF- I don't have an audio generator- will the RF work for this purpose?(I still don't have manuals for it, so I don't know what it will do!)
Scott
Old Cars, Old Radios, Old Pipes and Young Women... What more is there?
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Scott:
You can use any audio source for this. Rf will not do for this test. Most signal generators have an audio only output though. I sometimes use an old CD player. Be careful and use the coupling cap to keep any high voltage away from source in case you slip  or something is miss wired.
Steve Chambers
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Well, seems I do have an audio generator... I have a cable toner- used for identifying a cable outlet. It produces a single volume warbling tone, and can be injected into the area desired. I tested the spare vol. control, and seems to come out the same way the one in place does.
What next?
Scott
Old Cars, Old Radios, Old Pipes and Young Women... What more is there?
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When I injected the signal into the volume control, I heard it through the speaker, and the control did vary the sound. There are three terminals on top- one goes to ground, at the chassis, then on to the #3 position for the antenna connections. I tested on the other two. I did not test on the one below...
What should I look for next?
Scott
Old Cars, Old Radios, Old Pipes and Young Women... What more is there?
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Scott:
Now I'm confused. The antenna should not be connected to the volume control. The volume control has a ground, (which may be shared with other components that need to be grounded) a connection that takes the signal from the detector, and another (the wiper) that feeds a portion of the signal to the first audio amplifier tube. If you get a sound, then the audio part of the radio is OK. How loud was the sound? It should be possible to make it pretty loud.
Steve Chambers
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It has never gotten read loud- except on one or two very close(and strong) radio stations, and even those would not be as loud as normal.
I just went back and checked my pre-op photos of the volume control, and it very definitely connects between the RF subchassis ground point on the Antenna section, and the antenna #3 or GND terminal, with the same wire also grounding the no.1 terminal on bakelite cap no. 32.
I just went and rechecked all my tubes, and one 6K7G(RF amp) has a short indicated on the 8 & 9 pins. Intermittant, but there none the less, so I swapped in a spare. I now have a burr on AM band, but no reception as such. Sounds like electric razor interference... The tone changes with frequency change on the am band- higher freq, faster noise, lower, slower... If I touch the top terminal of the 1st audio tube(6K5GT) it also changes dramatically. becomes a more pronounced wuhh-wuhh-wuhh. Touching any other top terminal just dampens it a little. No action on the sw bands, at all... Burr was there before I swapped tubes, just to clarify.
Scott
Old Cars, Old Radios, Old Pipes and Young Women... What more is there?
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I've got a great Cessna airplane noise simulator at the moment...
I just tried swapping out the tone switch, and apparently something else has joined the fray- I started popping fuses, so now I need to find that problem, as well... The tone switch made no difference. Tomorrow evening, I will try swapping the volume control- or at least the mica cap on it, which was replaced with new. I will try the original- I swapped it for general purposes, but maybe it will help...
Definitely a frustration day. Well, off to bed- 05 comes early, and I get to play KC-10A Crew Chief for the weekend... I'll probably take a circuit digram to study... Maybe if I sketch through it, I'll come up with some ideas.
Any suggestions welcome!
Thanks!
Scott
Old Cars, Old Radios, Old Pipes and Young Women... What more is there?
Posts: 147
Threads: 28
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Ok, here's the latest... Pulled the sub chassis again, tore it down enough to confirm all my connections are right. They are. Replaced the volume control with another, no discernible difference. I did eliminate the buzzing by disconnecting the shadow meter- not sure that affects anything otherwise, however, but since the shadow never appeared, and I am pretty sure the piece which is supposed to make the shadow in the first place is missing, it really doesn't bother me at the moment. (I am of the opinion the wiring to the shadow meter is the buzzing cause- I shrink tubed it due to many broken areas in the wiring: no open spots, but precautionary, and I think it is creating a field of its own.
So, Now all I get is a hum, but no radio activity at all... All the coils have checked good for continuity. What next?
Chuck- Any ideas? Anybody?
Scot
Old Cars, Old Radios, Old Pipes and Young Women... What more is there?
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