Model 70 Low Freq Sensitivity - Printable Version
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Model 70 Low Freq Sensitivity - Guest - 06-12-2006
This is an "early series" Model 70....and this one has baffled me for quite a while.
Sensitivity drops off a lot below about 750 Khz, but is fine above that.
Besides a total chassis overhaul (bath tub caps, electrolytics, bad wiring, etc), I also replaced the resistor and cap across the 600 Khz trimmer with a silver mica type and a good 5% carbon resistor...and that helped a bit. I also replaced the dial, and made sure it fit "on the mark" properly with the old one.
Also made sure the IF was properly aligned, and that the freq tracks across the dial...and it does within 10Khz of dial markings. Followed the rf/osc/ant alignment procedures to a "T".
Also removed the tuning condensers and re-mounted them with good grommets...I mean, I really went through this thing.
All coils check out fine. Resistances are just about where they should be.
All tubes check fine on a Hickock 539B.
Still, it's not up to par.
I'm grabbing at straws, now...
Antenna is a 20' run of wire across the ceiling, set is grounded to a cold water pipe.
Any suggestions greatly appreciated.
- Steve Chambers - 06-12-2006
You might try a longer antenna.
Steve Chambers
- Chuck Schwark - 06-13-2006
Longer antenna than 20 ft. will be much better. The early non-AVC version may not be as sensitive as the later Model 70 with AVC.
Clean the mica wafers in the tuning gang and conpensation trimmers. Grundge and crud can impair them.
Weak tube or tubes. Check tubes with a good transconductance tube tester.
There may be moisture absorption (fairly common) in one or more RF or osc coil form that needs to be gently "baked" out if it possible.
Check for poor electrical contacts (very common) in all parts of the tuning gang assy., especially the grounding and rotor contacts.
- Guest - 06-13-2006
Longer antenna than 20 ft. will be much better. The early non-AVC version may not be as sensitive as the later Model 70 with AVC.
Interesting. This seems very variable from one early brand of non-AVC superhet to another. Some in my collection don't suffer as much from this as others, though they're from the same superhet era. It may, indeed, be a design factor with the Philco 70 early series (below s/n 22,000).
I _did_ hook it up to a homebrew BCB tuned loop, and that sure helped, of course...problem is, the only space for the 70 in the house is a very tiny room...looks like I'll be building some more tuned loops!
Clean the mica wafers in the tuning gang and conpensation trimmers. Grundge and crud can impair them.
Done when I overhauled the tuning capacitors; completely diassembled and (very gently!) cleaned them. I may have missed the low freq oscillator trimmer...I'll go back and check.
Weak tube or tubes. Check tubes with a good transconductance tube tester.
As per previous post, tested all on a Hickok 539B.
There may be moisture absorption (fairly common) in one or more RF or osc coil form that needs to be gently "baked" out if it possible.
This may prove a bit difficult, as I live near the ocean... conditions that regularly run 75-90% humidity. But I'll give it a go.
Check for poor electrical contacts (very common) in all parts of the tuning gang assy., especially the grounding and rotor contacts.
Cleaned all that up when I overhauled the tuning capapcitor assy', but will look again... basic assumptions should often be tested!
All in all, I think the idea that this is a design and longer/better antenna issue is probably correct. But I will go back and see if any more bugs need swatting.
Thanks to all who have responded for your kind input! It's deeply appreciated. This is a great group of people.
73 DE Lin/KJ6EF
- Chuck Schwark - 06-13-2006
Regarding the coils.
Melt off as much of the sealing wax as possible, then keep them "warm" for a while at say a low, low oven temp; no more than say 150 deg. to bake the moisture out (and maybe some leftover wax).
Then spray coat the windings with clear lacquer to seal them (INSIDE the coil forms too!!) so there is a moisture barrier for your high-humidity area.