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Just started on a Philco 38-15 a couple weeks ago. Got all the caps replaced along with all of the out of tolerance resistors. It comes up and plays but there is a bad 60hz hum coming coming out of the speaker. I used my signal tracer and traced back through the circuit and I don't pick it up in the 6A7, but I do pick it up on the 78 tube. I haven't been able to track down where that hum is getting into the audio from.
If anyone had any ideas let me know.
I have double checked that the E-caps are connected correctly.
Schematic here:
http://www.nostalgiaair.org/PagesByModel...013239.pdf
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Heater to cathode short or leakage was my first thought, but the cathode is grounded. Make sure circuit grounded points are not relying on just a rivet. They loosen and corrode/rust.
Make other tube element leaskages? Try another 78?
Be sure of ALL electrolytic polarities on power supply and B+ lines.
Tube socket grundge a/o leakage?
Phase of the Moon or height of the tide?
:-)
Chuck
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I believe the bias cap across 33 and 34 should have + side to chassis, - side to transformer center tap so as to provide a source more negative than "ground." All to many times the symbols are drawn incorrecltly but looks to be correct on this schematic. Don't know if set would play at all if this one get put in backward. Easy enough to check.
I'd check the voltages on 78, if voltages are OK, pull it out and see if you still have hum. I am assuming the hum varies with volume control, and that control is known good. Then there is the possibility of a component touching the filament wire. I'd have a good look for that, maybe poke around with a chopstick.
Let us know.
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Looing at the schematic the gird bias cap is drawn correctly - positive to ground. The symbol is small thick bar = POS, thin cup shape = NEG.
Chuck
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I bet I got that cap in wrong. Actually I'm positive I got that cap wrong as I grounded all the E-caps to the chassis so my bad I didnt pay close enough attention to the schematic and just assumed..
I will check it out this weekend..
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Found the cap I installed backwards and I removed it from the circuit and jumped in another correctly this time and the dang thing still hums. So next I'll start looking at the tubes and see if there is anything funky there.
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I finally got time to get back on the project. I had the the new caps to replace #31 hooked up all wrong. Once I studied the schematic I was able to figure out how they needed to be hooked up.
I have the radio playing and it works very well pulls in stations good, and the shortwave is very sensitive. Also cleaned up the chassis, painted the power transformer bells, and shined up the IF can and old E-Cap shells.
The power transformer gets a little hot so I'm working on figuring out what value/wattage dropper resistor to put in line with the power cord. I tried a 50ohm/50W which seems to do good drops the input voltage from 125 to ~110.
Here is a couple pics.
[Image: http://i1129.photobucket.com/albums/m501...3486C9.jpg] [Image: http://i1129.photobucket.com/albums/m501...D0517B.jpg] [Image: http://i1129.photobucket.com/albums/m501...F35EBD.jpg]
I added an input for an MP3 player or phone to the back of the set.
Now to start on that cabinet.
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The chassis looks great! 
What did you use to clean that chassis? Although, from looking at some of your other Photobucket pics, it appears the chassis wasn't too dirty to begin with.
Two nit-picky things, if I may:
- You do realize those are not the correct knobs, yes?
- Philco stopped using cloth-covered power cord during the 1937 season and switched to brown rubber.
Good job with the MP3 jack, it is not too noticeable. Is it one of those Radio Shack jacks that breaks the circuit from the second detector when a plug is inserted? I installed a similar jack in wife's little Sony boom box for use with external speakers. When a plug is inserted, it cuts off the internal speakers and applies the audio to the plug, which carries the sound to the external speakers mounted in the ceiling of our patio out back.
--
Ron Ramirez
Ferdinand IN
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Nice job on the chassis, Keith!!
Color me impressed!
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Quote:Two nit-picky things, if I may:
- You do realize those are not the correct knobs, yes?
- Philco stopped using cloth-covered power cord during the 1937 season and switched to brown rubber.
Good job with the MP3 jack, it is not too noticeable. Is it one of those Radio Shack jacks that breaks the circuit from the second detector when a plug is inserted?
You nit-picky Ron? Never.. 8)
The knobs are from a 610 that I am working on they were on the bench so that's what I used. I do have set of the correct knobs.
Power cord - I ran out of the brown rubber cords, but I just happened to have a spool of the cloth covered so I figured it was the next best thing.
The MP3 jack is the radio shack piece that breaks the detector circuit. Works quite well, I also put one in my 71B.
Cleaned the chassis with navel jelly. The chassis wasn't the super dirty but it was pretty grungy a lot of that yellow oxide from the plating.
Thanks for the comments guys..
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Messing up the negative connection on the filter caps is a very common mistake that people make if they are used to working on sets newer then 1939, I know because I did it myself before. Most sets built before 1939 used discrete filter capacitors for the input and output filters so they would isolate the input cap from the chassis with a cardboard insulator and bolt or clamp the output filter cap directly to the chassis, so if you connect the negative of the input cap to the chassis it behaves like it's open circuit.
It was very common practice during the 1930s to use fixed bias on the tubes by installing a resistor network in series with the B- or center tap of the power transformer. It's kind of a cleaver way of achieving a negative bias for the tubes by making the common negative less negative then the center tap of the power transformer. The newer method, in the case of the power output tube, is to make the cathode more positive with respect to the grid by connecting a resistor in series between the cathode and common negative rather then making the grid more negative. I hope I didn't get mixed up here on a few points but that is what they did even if my grasp of the theory is wrong.
Regards
Arran
P.S If the power transformer is running warm make sure that someone didn't substitute a #42 output tube for a #41, a #42 draws a lot more filament current then a #41.
(This post was last modified: 09-30-2012, 08:44 PM by Arran.)
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Thanks for the explanation Arran.
It has all the correct tubes including a 41 output tube. I have a power resistor of 50W at 50ohm in line with the hot side of the plug after the fuse. It drops the incoming voltage to around 108 but the power trans still gets a bit warm around 125 degrees with it just sitting on the bench not in its cabinet. The resistor gets very warm/hot at around 160 degrees I have it heat sinking to the chassis with some thermal paste which keeps it a little cooler.
I am starting to wonder if there isn't something pulling more current then it should although I do have a 1amp fuse in line and it doesn't blow open even on start up.
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Check for carbon tracks in/on tube sockets maybe.
Poor rivet ground(s) in the power circuit?
Possible partial winding short in transformer maybe?
 Let's hope this is not the case.
Rememeber that the power transformer was rated at lower AC mains voltage. How's
it do on the variac at 110-115 VAC from cold start for an hour or so?
Any old end-wrapped dogbone resistors still in there?
S'bout it for the moment. Back to work...
Off Top of Head
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Quote:Check for carbon tracks in/on tube sockets maybe.
Poor rivet ground(s) in the power circuit?
Possible partial winding short in transformer maybe?
Icon_eek Let's hope this is not the case.
Rememeber that the power transformer was rated at lower AC mains voltage. How's
it do on the variac at 110-115 VAC from cold start for an hour or so?
Any old end-wrapped dogbone resistors still in there?
All the resistors have been changed. There was one resistor (13) that called for a 10k @ 1W. I didn't have any so I used a 13k at 3W and it gets pretty D**n hot also hot enough you can't keep your finger on it. not sure if this is causing the PT to run hot or not.
I will put it back on my Variac @ 110 tonight and see how it acts, and check the sockets for carbon and make sure the rivets are tight.
Thanks Chuck.
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Just looked at resistor 13 and it only feeds B+ to the mixer and IF tubes.
BUT, B+ goes thru the band switches and some coil windings.
Funky switch contacts or carbon tracks in the switch wafer(s) or coil shorts could be something also to look at. Maybe a dangling switch terminal wire, some kinda short to switch frame from a solder lug on a wafer......wracking my brain....crumbled insulation on any wires in that neighborhood.....pri to sec short/leakage in one of the osc coils...still a bad ('new') 2uF cap hanging on that B+ line, below the IF tube in dwg...
It is Full Moon right now. All else fails, use the tried and true
Six-Foot Navy Drop Test Procedure.
Chuck
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