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41-250 oscillator problems - Printable Version

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41-250 oscillator problems - electroubleshooter - 02-05-2008

Hi

I am working on a Philco 41-250. You can see the schematic here: http://www.nostalgiaair.org/Resources/407/M0013407.htm

I have replaced all of the rubber wiring. I was not able to test the radio before replacing the wiring due to the many exposed bare wires.

I now have multiple oscillator problems.
First, the oscillator does not oscillate on the AM band, either using the pushbuttons or tuning cap. On the shortwave bands, it would only oscillate for a portion of the range of the tuning cap. After checking the tuning cap for shorts and not finding any, I found that terminal 6 on the oscillator coil was open. It appears that someone had worked on that terminal before as the wire was not rubber coated. I got that fixed.

Now, the oscillator will operate on both shortwave bands for the full range of the tuning cap, but still no AM.
The frequency range for the oscillator on the SW bands is wrong:
SW1: 2.5-5.6 MHz
SW2: 7.5- 8.8 MHz

I found that cap 25 (from the grid of the oscillator tube to the oscillator coil) had been previously replaced). The cap was color coded green, orange, green, which makes no sense to me.(How do you read this color code?) It does however measure 288 pf. I replaced it anyway with a new 270pF for good measure since I didn't have a 250.

Now, the same symptom: no oscillations on AM either from pushbuttons or tuning cap.

I then did some more checking of the wiring. All resistors are in tolerance. The other windings on the oscillator coil are good.
I found that there was a wire running from the top of the variable capacitor which connects to terminal C9 on the wafer switch. This makes no sense at all as the schematic shows terminal C9 going to cap 9 (which in turn goes to the antenna). Also from researching this on the Internet, it appears that the wire going to the top of the tuning cap is suppose to be a "gimmick" cap, a very short piece of wire that is suppose to be fed thru the hole on the other half of the tuning cap and then bent back. On this radio that wire was fed through the hole but was then fed through a hole in the chassis and ran to the wafer switch. BTW, this wire was not rubber coated and falling apart, so obviously it was newer.

The question is? Did someone modify this radio? Did it ever work after being modded? I removed the long wire that went from the tuning cap to terminal C9 on the wafer switch and just put a short one on. I ran it through the other tuning caps lug and then bent it in a "U" shape. This made no difference in my symptom.

Does anyone have a pix of what the gimmick is supposed to look like? Did they all have this gimmick or was their different versions?

Any other suggestions on this radio?

BTW, I swapped the XXL tubes, no change. When oscillating the oscillator tube has -2.2V on the grid and the plate is 33V. When not oscillating the grid is OV.


Thanks

- Terry Judkins - 02-05-2008

The dual cap marked 21 and 21A should be replaced. It looks like a mica cap but in fact is paper. It has been bad in most of the radios of this design that I have refurbed.

Does the oscillator work with the pushbuttons?
That might give a clue.

- Guest - 02-05-2008

I just got done with a 42-350 that had osc problems. I had to clean all the tube pins first. It uses a similar osc with the XXL(7A4) tube. I replaced the grid resistor and the 100pf cap going to the grid and now it works. I guess you just can't depend on mica caps like the old days. It sounds like you need some good pics of a working set to find out the placement of those wires. The osc needs some feedback from that gimmick to get it working right.

- electroubleshooter - 02-05-2008

Terry Judkins Wrote:The dual cap marked 21 and 21A should be replaced. It looks like a mica cap but in fact is paper. It has been bad in most of the radios of this design that I have refurbed.

Does the oscillator work with the pushbuttons?
That might give a clue.

no it does not work with the pushbuttons either.

I will check the cap. Is that the square white one mounted on the bandswitch?

Thanks

- electroubleshooter - 02-05-2008

Lou deGonzague Wrote:I just got done with a 42-350 that had osc problems. I had to clean all the tube pins first. It uses a similar osc with the XXL(7A4) tube. I replaced the grid resistor and the 100pf cap going to the grid and now it works. I guess you just can't depend on mica caps like the old days. It sounds like you need some good pics of a working set to find out the placement of those wires. The osc needs some feedback from that gimmick to get it working right.

Maybe someone will have a pix.

Thanks

- Terry Judkins - 02-05-2008

electroubleshooter Wrote:
Terry Judkins Wrote:The dual cap marked 21 and 21A should be replaced. It looks like a mica cap but in fact is paper. It has been bad in most of the radios of this design that I have refurbed.

Does the oscillator work with the pushbuttons?
That might give a clue.

no it does not work with the pushbuttons either.

I will check the cap. Is that the square white one mounted on the bandswitch?

Thanks

Yes it is on the band switch.
I just checked a 41-250 here that I am listening to and the 'gimmik' is totally missing. It works fine on all bands.

- electroubleshooter - 02-05-2008

Terry Judkins Wrote:
electroubleshooter Wrote:
Terry Judkins Wrote:The dual cap marked 21 and 21A should be replaced. It looks like a mica cap but in fact is paper. It has been bad in most of the radios of this design that I have refurbed.

Does the oscillator work with the pushbuttons?
That might give a clue.

no it does not work with the pushbuttons either.

I will check the cap. Is that the square white one mounted on the bandswitch?

Thanks

Yes it is on the band switch.
I just checked a 41-250 here that I am listening to and the 'gimmik' is totally missing. It works fine on all bands.

Thanks,

Not having to worry about the gimmick will help.......

- justlrnin - 02-06-2008

Hello. Newbie here, and I'm also restoring a 41-250 that I just bought over the weekend. (first non-car radio I've restored, only the second tube radio I've touched, but I'm fascinated (obsessed?) by these old radios.)
I have an Eico 666 tube tester with various tube charts, none of which list the 7A4 (XXL) and the 41 tubes, so I can't test them. Anyone happen to know the settings to use to test these tubes on the Eico?

Also, what are the value(s) of the 21 and 21A capacitor? I was just getting ready to order all the caps when I found this forum today and didn't realize that capaitor was paper. I've been reading posts here for the past couple of hours. What a great source of info and expertise!

- Terry Judkins - 02-06-2008

justlrnin Wrote:Hello. Newbie here, and I'm also restoring a 41-250 that I just bought over the weekend. (first non-car radio I've restored, only the second tube radio I've touched, but I'm fascinated (obsessed?) by these old radios.)
I have an Eico 666 tube tester with various tube charts, none of which list the 7A4 (XXL) and the 41 tubes, so I can't test them. Anyone happen to know the settings to use to test these tubes on the Eico?

Also, what are the value(s) of the 21 and 21A capacitor? I was just getting ready to order all the caps when I found this forum today and didn't realize that capaitor was paper. I've been reading posts here for the past couple of hours. What a great source of info and expertise!

21 and 21A are listed as 370 uuF (pF). Not a standard value.
Replace with two 360 or 390 pF silver micas. These caps affect the pushbutton tuning.

Can't help with the tube tester settings. I have had weak 7A4 (XXL) tubes though. Watch for shorted 84 rectifier.