[462ron]

Ok, just about ready to button up this radio, replacing grille cloth soon. Replaced  all the caps, checked resistors, checked the multi section wire wound resistor, tested tubes and did alignment. The only thing that was bad was the primary for the first detector transformer #6 on the schematic was open, plate circuit in the RF amp. I was able to rewind it no problem. The set isn’t particularly sensitive, better at night as expected. Tried a new set of 24s with no difference. All voltages check out. I’m wondering how sensitive of a receiver the model 70 is? One thing I noticed in checking all the coils in this set is the antenna transformer #2 on the schematic, is constructed differently from most coils. The primary is wound on a separate bobbin mounted at the top of the coil form whereas most are wound directly over the top of the secondary. I was kicking around the idea of winding a new primary directly over the secondary like most are, to see if it improves any. Thoughts!

Ron

[Paul Philco322]

l love those paper tube testing labels you occasionally see, how they took the heat and survived, who knows.

Nice job and some sets even if not super performers are cool, they made all kinds for all customers.. Enjoy.

Paul

[Ron Ramirez]

But there isn't a good replacement for the 70/90 grille cloth.

Ah, but there is...

Michael Katz is back in the grille cloth business, and the first cloth on his list of current offerings is Philco 70/90 cloth. Please see this post:
https://www.philcoradio.com/phorum/showt...?tid=22498

[462ron]

Thanks Ron, I already got the whiskey rayon cloth from Richmond Designs. Just need to install it. What do you think of me wanting to wind the primary of the antenna coil on top of the secondary instead of using the primary that is wound on the bobbin and mounted to the top of the coil form? I’m thinking it would get he more signal to the secondary and maybe making the set a little more sensitive!

Ron

[Ron Ramirez]

Other Ron - I would rewind the coil as Philco did from the factory; after all, there is a reason they designed it in the manner in which they did. But Terry could probably answer this question better.

[BrendaAnnD]

Ron Ramirez is right. You don't want that primary wound on top of the secondary. Reason is that the coil is designed for nominal coupling across the band. If you were to wind it on top, you may get better coupling in the lower part of the band, but you would lose total Q in the circuit and also change how it tracks across the band...

[462ron]

Thanks Ron and Brenda for your input. I will leave it be then. It is working, better at night. Just wonder how sensitive this model is or am I expecting too much from 90 year old technology?

Ron

[Radioroslyn]

+1 Brenda.

Yup would over couple the signal thus broadening the width of the rf amplifier stage. Not a great idea.

Think of it like this. You have a very nice high end hifi set and between the tuner and amp there's an equalizer. You set the eq to a level frequency across not boosting or cutting the audio range. This is like your ant coil stock from the factory. Rewinding the primary over the secondary or very close to it is like readjusting the knobs on the eq. It's going to make some frequencies more predominant than others. No it's not going to cause it to pickup stations in the wrong places on the dial. Just the rf amp circuit don't be working @ it's full efficiency.

Rewinding the bobbin is not particularly difficult. Remove the ant coil from the chassis. Nip off one or both of the tops of the slots on the coil. Remove the two wires. The original winding is 49 ohms so if you were to use 40ga wire it would take 49'. You can sort it for 38ga it will be a little longer. I don't remember if you can remove the winding without grinding off the rivet in the center. If it's brass use a small brass screw to replace it. Brass changes the inductance slightly vs steel. You can scramble wind it doesn't have to be a honeycomb or super neat. The phasing doesn't matter either (direction or which wire connect to which terminal). You can remount the two coils back together again with a couple of dabs of hot glue @ to top of the slots.

Remember by 1932 or so Philco was in the musical instrument business.

Have Fun!

[Radioroslyn]

To answer your last post you pose an interesting question. Sensitivity. I guess it depends on how you judge it. Techs would say hook it to a SG with a calibrated attenuated output and do the limbo with it. Some may have a 5' piece of wire in their basement that they call an ant. All of their set that don't have a loop ant have poor sensitivity. Others have a hundred feet+ of wire up in the air and a lightning arrester. A separate earth ground rod. All their sets work dandy. A marginal set with a good ant can perform pretty good. Same with a good set and a lousy ant. But neither is the best situation.

I'm not sure if I've ever had a set that had poor sensitivity that was do to it age or design. It would end up being a part failure or a failed repair. Heck you can go back to the late teens and 1920's when regenerative sets were king. One or two tube sets that had all kinds of sensitivity. You might want to look into building one for fun. They are simple and will give you a good working knowledge of how an oscillator works, hand capacitance, and inductive coupling to name a few.

I find that the early70 is a little beastly because of it's lack of avc. Don't have a working 70B but do have a 370. It's playing as I type. It has a 15' wire that runs behind the couch. Picks up all the local stations with good fidelity. Don't like the knob setup, hard to grip, it's very different from the 70B.

The 90 yr old technology works ok if you use the sort of antenna that was used back then. This is if all is in good working order. With that being said we have a lot of "new problems" to deal with that were not around back then.

One thing I forgot to mention the open ant coil will also effect the volume control. With a good ant and the volume all the way down you'll still hear stations. The 5000 ohm section of the vc is to shunt all of the signal to ground when the control is turned all the way to the left.

Now just for test purposes wink wink you could connect a small cap like 5 or 10mmfd from the ant post to grid cap of the 24A rf amp. Voltage rating whatever lightning is.

Sorry went into the ramble mode...

[462ron]

        Thanks Terry. I guess I wasn’t very clear when talking about the antenna coil. Everything ohms out good on mine, no opens. Primary comes in at 50 ohms. I was just surprised to see the primary so far from the secondary, not wound on top of it. I thought it could be a little more sensitive compared to some others I have. My antenna is little over 15’, I know, not stellar! I have finished it up doing the new grille cloth last and I left the antenna coil alone. I will call it finished. Thanks to all for your input and to Rob (rfeenstra) for graciously printing new labels for me!

Ron