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Philco 90 Chassis Restoration - Printable Version

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+--- Thread: Philco 90 Chassis Restoration (/showthread.php?tid=1031)


Philco 90 Chassis Restoration - etech - 03-19-2008

Hi all,

Some of you probably know that I am in the process of of restoring this model 90 chassis.

[Image: http://i135.photobucket.com/albums/q136/etech_photos/Philco90Top.jpg]

After cleaning it, I decided to do a quick check to see if I could get reception. After all, there is no sense in investing in the cabinet and other parts needed if the chassis is not restorable. With that in mind, I tacked in two 10uf caps in place of the Mershon caps and connected a field coil speaker. Since I did not have the Philco speaker, I connected a 10 inch Zenith speaker I use for a bench test unit. It has a 500 ohm field coil, so I added a 2,500 ohm power resistor in series with the field to simulate the proper Philco speaker. This speaker had a push-pull transformer, so I used the center tap and one end.

I connected an antenna and slowly powered up the chassis with the variac, watching the current draw. At 80 volts the radio started coming to life, although it was oscillating, even up to 100 volts. I figured there were some bad decoupling caps, so I tacked in a .01uf cap from the RF/IF screen supply rail to ground. The oscillation stopped and the radio was picking up many stations across the dial. Frequency calibration was not too bad either. The only problem was that the RF sensitivity seemed weak. I checked voltages in the set and all were normal except for the plate of the RF tube (#24), no voltage. It turns out the primary of the RF coil is open. Icon_sad I temporarily tacked a 20K resistor across the primary to get some voltage to the plate of the RF tube. Lo and behold, the radio plays great. It has lots of volume and very good sensitivity. Icon_smile

So, I guess I am going to have to do a complete recap and repair that bad RF coil. It looks to me like the primary is the bottom winding? Anybody know for sure? Maybe I can repair the winding without have to rewind it?

Anyway, having had reasonably good success with the preliminary checks on this chassis, I guess I'm going to have to stay on the lookout for a cabinet and speaker, lol!

Regards,

Ed

- Ron Ramirez - 03-19-2008

Hi Ed

The primary of that coil is wound over the secondary, and is much smaller than the secondary winding. It can be easily rewound; you will need some 38 gauge enamelled wire for this, along with something suitable to use as insulation between it and the secondary winding.

Get rid of the old insulator between primary and secondary - it is made of celluloid, and deteriorates over time, causing the windings to corrode. Let's hope your secondary winding does not have green spots - if so, then it, too, has been attacked by a combination of moisture and the celluloid itself.

But I'm getting ahead of myself. You will need to know how many turns you need to rewind, and in which direction (critical in most coils such as this). There are ways you can find this out. A good primer can be found at Chuck's site:
http://www.philcorepairbench.com/tips/svctip36.htm

- etech - 03-19-2008

Ron,

Thanks for the info. You are correct. It is the primary that is open. I will edit my original post to reflect that it is the primary. I will take a look at Chuck's site for some more info. At this point, the secondary is still good, so I hope I can just rewind the primary. Oh well, I guess that is not so bad. At least the radio works well. It should be a keeper.

Ed

- Texasrocker - 03-20-2008

Great restoration Ed!! That chassis looks Great!! You are doing it *justice* indeed!! Another vintage Philco saved!! Icon_wink Randal

- etech - 03-20-2008

Thanks Randal. I appreciate the kudos. Icon_smile I enjoy the electronic restorations. I am learning to do cabinet work also, but have a way to go yet.

Ed

- Texasrocker - 03-20-2008

Yea Ed!! I understand!! I also enjoy the electronic restoration better than restoring vintage radio cabinets!! You did a GREAT job cleaning & bringing that mod 90 chassis back to life!! I would suggest doing a *full-recap* of all the chassis as well. Just my .02! Those other bakelight blocks are trouble "squeels", bypass caps waiting to happen sometimes!?? I ran across a mod 90 cathedral cab a couple yrs ago in E Texas at a antique shop (barn find). The cab was in very-poor condition, (been wet many yrs deterioration),.. missing much veneer & rear arch support,busted & missing wood spkr scroll-work,missing all orig knobs, the chassis was rusted, broken tube sockets, missing all tubes,missing spkr, etc!!! It was a real *trainwreck* Maximus!! The owner of the antique shop was asking $800. (firm)!!!.. because she had read in a vintage radio collectors guide for antique shops, that it was valued at that amt! She overlooked the part of "in good condition"!! I offered her $25 for it and she laughed at me!! Then I "counter-offered"!! I said I will take it off your hands if you pay me $25. just to "try" to salvage the rusty chassis!! She was offended!! (hee hee)! The cabinet was truly, absolutely, unrestorable dumpster materials only! Most of the veneers of orig plywood & cab trim was missing also!!
Just wanted to share a true Philco mod 90 "find" !! There out there!! Enjoy your Philco!! Icon_wink Randal

- etech - 03-20-2008

Hi Randal,

Yes, I am planning to do a full recap on this 90 chassis. Chuck's site has nice tutorials on how to restore/restuff the Mershon caps and bakelite blocks.

How about the tone switch? What is the best way to replace the caps without destroying it? It looks to be bakelite also. Would I use the same methods as the bakelite blocks? Heat with the heat gun to remove caps and tar block?

Also, what about the volume control? This one is not scratchy, but is a bit stiff. If it is safe, I will lube it with WD-40. I use that on most of my 30's and 40's sets with good success.

I think my biggest problem will be to find a useable speaker. If I could find one in need of repair I would try that. I think the model 20 speakers have a bolt on field coil. I have a homebrew coil winding machine and have rewound field coils before.

Coil Winder
[Image: http://i135.photobucket.com/albums/q136/etech_photos/CoilWinder1.jpg]

I think I can do the recone job myself also. I can get the knobs and escutcheon from Larry Bordonero and the cabinet from Dick Oliver. So I think it would be possible to piece it together, if I do not find a junker.

Regards,

Ed

- Chuck Schwark - 03-20-2008

Use the same method on the potted tone controls. Thought I mention that in the bakelite block pages? Maybe not.

Same thing just somewhat easier.

Chuck

Model 14, 91 oscillator coil rewinding - theblurrrr - 05-30-2008

For Ron and the rest of the Phorum:

Is the 38 gauge magnet wire required for this application? I have to rewind the primary oscillator coil on this 14LZX; the closest wire size I can readily get is 34 gauge and the original size appears to be slightly smaller, maybe a 36 gauge?

Thanks in advance.

- Ron Ramirez - 05-30-2008

36 gauge *might* work.

Most of these coils use 37 or 38 gauge for the primary (small outer) winding.

Wire size and winding - AI2V - 05-31-2008

For years I've been using #34 wire and never had a problem in any Philco. I've also found that the number of turns isn't super critical in the plate circuits and one or two turns more or less won't matter. A perfectly wound coil looks professional but if turns overlap that's OK too. The most critical thing in the coil winding is the phase (direction of winding); if it's wound backwards it's not a problem either- just reverse the leads of the coil where they solder.

A bit of sticky beeswax melted on the surface that the coil will be wound upon helps greatly. When finished winding, a pinch of beeswax melted into the coil with a soldering iron will hold it all in place nicely.

- Steve Davis - 05-31-2008

Wall warts are a good source of wire for winding these coils.

Steve