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Philco 71 E IF adjustment instability - Printable Version

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Philco 71 E IF adjustment instability - sandbar40 - 07-27-2012

Have just joined altho' been restoring radios inside and out for many years.
current project, 71 e chassis C 34468.
Completed circuit restoration including restuffing bakelite cap. cases with 630 v axials, electro cans with 10 uf 450v, and replacing any resistors too far out of tolerance.
Coils all checked ok and set worked across bc band.
Started to align by peaking if's to 260 khz and all went well until the last trimmer, no 4 lh side back - 1st if primary - whitch caused set to oscillate noisily before a peak was reached.
Backing off this trimmer resulted in the set operating ok, but i wonder if this is normal behaviour, any suggestions very welcome.

RE: Philco 71 E IF adjustment instability - codefox1 - 07-27-2012

Well, always could be usual crap or dust amongst the leaves. Why not loosten it up, blast it out with Deoxit, leave it dry over night and try again. Usual stuff, count how many turns accurately to tighten, write down, ad repeat backwards the nest day. Yo'ull be really close.

Hopes this helps.

RE: Philco 71 E IF adjustment instability - sandbar40 - 08-01-2012

thanks codefox, but trimmer tests out ok. will dig deeper.

RE: Philco 71 E IF adjustment instability - Chuck Schwark - 08-01-2012

Might I suggest checking for marginal tubes, and/or an open or shorted IF transformer winding.

Also, if you flushed a mica trimmer with a contact cleaner, like DeOxit, there could be an oily residue that's throwing the capacitance of the trimmer out of spec. Many contact cleaner formulas leave a non-oxidizing film behind when they evaporate. I use DeOxIt at work for contacts only because of this.

Try flushing the mica with denatured alcohol to clear off any oily residue, that's good for metalic contacts in a switch, but bad for a mica trimmer cap.

Been there.

Chuck

RE: Philco 71 E IF adjustment instability - Radioroslyn - 08-01-2012

Good thoughts Chuck. It maybe just the electrical differences in the new parts vs the old. The caps may have the same value of capacitance but the inductance may be a bit different along with wiring layout. Agree with Chuck check all the other stuff to be sure it's working ok and I poses you could beef up the bypass and decoupling parts values.
GL
Terry

RE: Philco 71 E IF adjustment instability - sandbar40 - 08-03-2012

Thanks for the suggestions guys,
I ruled out the trimmer by opening it to min. capacitance and bypassing it with a modern trimmer which gave the same instability.
I removed the IF coils, tested them, and used a hot air gun to melt out old wax ( and hopefully moisture ) then melted in some new wax.
Will look at bypassing next.
Tried another ' 36' and '44' - much the same.

RE: Philco 71 E IF adjustment instability - Chuck Schwark - 08-03-2012

Also make sure lead dress on the grid and plate wires are not near each other. Push the wire(s) going to the grid close as possible to the chassis and be sure the plate lead(s) are as far away as possible from the grid leads and socket terminal.

Chuck

RE: Philco 71 E IF adjustment instability - sandbar40 - 08-05-2012

Checked lead dress - ok - most of wiring is still original.
Re-did alignment starting with 2nd IF feeding sig. gen. at 260 khz via 0.1 cap to grid cap of '44' which peaked ok, then moved gen. to grid cap of '36'. This time adj. of either trimmer resulted in osc. while approaching peak.
This behaviour reminded me of the old regenerative sets where max. sensitivity was just before osc.
With a stable setting of these two trimmers set goes quite well, but i'm still puzzled by this behaviour.
Also i wonder why there are blanking plates covering the two IF primary trimmer access holes on the rear of the chassis, - were these settings more critical and not normally touched ?

RE: Philco 71 E IF adjustment instability - Steve Davis - 08-05-2012

Some of the IF trimmers that have B+ on them, the covers are there to prevent someone from getting an accidental shock. When adjusting these trimmers you should wrap your screwdriver with tape so to not short the B+ to the chassis.

Steve

RE: Philco 71 E IF adjustment instability - codefox1 - 08-05-2012

Agree, use a nylon diddle stick, and if you don't have one, beg a dry chopsick from your kitchen and file it to fit slot. If you are using an ancient signal generator like most of us, a really cheap modern frequency counter should join you bench. Check without modulation, and not hhooked up to set, rehook it or use 20 turns of anything on a TP roll near the set, you probably know the drill. Turn on modulation and let us know whaty happens. Yes, hypercritical adjustments need to be very slightly be detuned, espedially with over performing tubes, which are becoming somewhare rare nowadays as are we.

RE: Philco 71 E IF adjustment instability - sandbar40 - 08-05-2012

Yes, the protection from HT makes sense. The trimmer adj. are by way of hex nuts not screw slots, and the tool i use for these is a length of stiff plastic tube that has been softened at the end with a heat gun and molded over the nut, so insulation from HT and effect of metal eliminatsd.
For slotted screws i use strip of fiberglass PCB filed to shape, with copper removed of course.
Signal generators are, ' Windsor mod. 66 a ' restored, and ' AVO MK 3'. Frequency counter is a' Leader' digital.
Amazingly, some of the tubes in this radio still have stickers on them stating they were tested ( by a radio shop ?) in 1938 and found good. They still seem to be good now!




pcb

RE: Philco 71 E IF adjustment instability - codefox1 - 08-05-2012

Good, so just follow the volts.