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Philco model 048 set tester - Printable Version

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Philco model 048 set tester - Blaine - 01-31-2010

I am at a standstill. Got some battery packs and a new #30 tube. Few resistors had to be replaced. and restufed the capacitors.Problem with the frequency generator. I get excellent readings on my frequency counter.That's if a pull the top corner of the variable condensor towards the front of the instrument. Let it go and I loosethe signal.I cleaned the variable and resolderd all the joints and still the same problem. Any one had any experiance with a similar tunning problem.
I also replace the attenuator control .This was a 500ohm wirewound pot. I subbed with a AB 500ohm pot.Would this bother the attenuation . Not sure if the original is linear or audio I really not sure of the difference.
Thnks..

Re: Philco model 048 set tester - codefox1 - 01-31-2010

Get an analog ohm meter and see where the variable capacitor is shorting out. Ususlly it's a bent leaf on one of the ends. If you can slip a piece of paper in, you should be able to find it. Otherwise there may be crud in there. And if there are trimmers, take 'em apart and clean the contacts and micas (very carefully, they are real fragile.) I usually blast away with deoxit, put a paper towel underneath, and wait a day.

If there has been trauma to the capacitor, as in a bashed in knob or some rough handling, mre severe actions might be necessary. There are some heroic examples on this forum, including mine, which is taking the D**n thing off, and boiling it for a while and then regreasing the bearing when it dries out.

Finally, look for something really simple like a rubber coated wire that has become bare and is shorting.

Good luck.

Re: Philco model 048 set tester - Blaine - 01-31-2010

Thanks I will try your ideas. This instrument has not been handled roughly . By looking at it its been well kept. On one side of the tuning cap there is an adjustment of sorts .Not sure if this is for padding .This term may be wrong . But it has an insulating material between it that is loose, but in tack. I didn't want to tight it in case it sets it out of alignment.
Well I will do some more trouble shooting in a few days..
Thanks

Re: Philco model 048 set tester - Blaine - 02-06-2010

found that the500ohm wirewound potentiometer is bad. Not sure if its termed a wirewound but beleive so.You can plainley see inside the pot andwatch the arm rotatearound the wire. What should I use to replace this pot? I can,t seem to locate this pot on the schematic.It is the attenuation control. Are there still some of theses style of pots around? Thanks

Re: Philco model 048 set tester - Radioroslyn - 02-06-2010

Do you think it was added by a previous user? Where in circuit is it connected? If it is in the output a WW pot is not a good choice as it will look like an inductor and may cause some resonance at various frequencies.
Terry

Re: Philco model 048 set tester - codefox1 - 02-06-2010

I agree. Sometimes wirewounds are used instead of carbon pots because they can dissipate more power. I don't have the schematic in front of me, but typical carbon pots can dissipate a half watt, and most wirewounds can dissipate 4 watts or more. The wirewounds last longer, but as has been pointed out can really screw things up at higher frequencies.

Knowing nothing further, and assuming that this was a substitute part, I would just check for anything in the vicinity that may have caused the original control to fail, (other than plain old age.)

I've blown up parts on perfectly good test equipment when connecting it wrongly to a set I was working on a couple of times. So that may have been what happened originally.

As for the trimmer caps, yeah, you gotta take them apart, clean out the crud with deoxit or whatever your favorite solvent is, let everything dry out, and reassemble. The mica chips are really fragile, so take your time. If you ruin one, think unobtanium or scrounge-ola! Good luck.