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Philco 90 - Extra Paper Caps? - Printable Version

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Philco 90 - Extra Paper Caps? - etech - 04-06-2008

Hi all,

When I was restoring my Philco 90 (single 47 output tube) chassis, I found three paper capacitors which are not shown on the schematic or in the Philco drawings. See the circled caps in the photo below.

[Image: http://i135.photobucket.com/albums/q136/etech_photos/PhilcoChassisHighLight.jpg]

Two of them are Philco brand and the third is some off brand. All are .05uf. From what I can tell, they seem to be in parallel with caps that are in the bakelite blocks, drawings #18, 26, and 46. Do you think they were added by a repairman? Or is it possible that they were part of a product design change?

Oh, the paper block capacitor on the chassis right side wall is definitely an added unit. It was installed 10-10-36. I have removed that unit since this photo was taken.

Regards,

Ed

- planigan - 04-07-2008

Quite possible added by repairman rather than pull bakelite block to replace open cap. PL

- BDM - 04-07-2008

I agree. I've run across many Philco chassis this was dome to. Usually the leads coming from inside the block at cut at the terminal ends.

- Ron Ramirez - 04-07-2008

Hi Ed

Ditto what Planigan and Brian said. Back in the old days, repairmen really couldn't rebuild bakelite blocks the way we do today because the old paper capacitors were physically larger than they are now. So it was often common practice to bridge a new paper cap across the old bakelite block. I've seen this done many times, especially where one terminal of the block has been cut off to get the old block out of the circuit! Icon_eek

- etech - 04-07-2008

Thanks for the info guys. I just wanted to be sure that there was not some product design change causing these caps to be added for performance reasons. I am comfortable now that they are not. After having rebuilt the bakelite blocks and the metal filter blocks, the radio plays very well.

On a side note, I still need to rewind the primary of the RF transformer, which is open. The radio plays beautifully with the open primary bridged by a 20K resistor. In fact, I am wondering if there is anything to be gained by rewinding the primary of this transformer? I see that the later Philco 70's use an open primary with the B+ feed to the RF tube plate (shunt fed) via what appears to be an RF choke.

Ed

- etech - 04-08-2008

I found the answer to my own question, lol! I spent some time on that RF coil today. I found the bottom turn (B+ side) was open. I was able to repair it by soldering it back together. Icon_biggrin

Now, as far as the performance of the radio is concerned, I noticed no difference in sensitivity or performance with the coil operating properly. I was still able to receive 740 AM Toronto in the day time on my outside wire antenna. It was the same relative signal that I had before I repaired the coil. In this case, since the open was near the bottom end of the primary, there were lots of turns available to provide sufficient coupling for the signal to transfer to the secondary. Others may not be so lucky, depending on where the break is. However, I would try that shunt feed idea again.

Regards,

Ed