[NostalgiaRadioTime]

Problem: Open Primary on the OSC coil.

I've searched through the information here and on ARF regarding this common problem, but have a couple of questions for someone who has perhaps rewound one of these recently.

I measure 3.1 ohms on the secondary. What should the primary measure?

I've read anything from 14 turns and 25 turns of 36 to 40 ga wire. Before I start unwinding...and lose count...does anyone know which it should be?

I had an extra OSC coil from a model 89 that I rewound years ago and I tried it on this model 84. I was able to get the local (strong) station in at full volume faintly. It does get a bit louder when I touch the antenna wire I have connected (but not as loud as it should). That coil measures 6 ohms on the primary and 5 ohms on the secondary.

I then tried one of those AES replacement OSC coils and couldn't get a single station to come through with it. The model 89 coil worked better.

Could the low volume be caused by too much resistance on the model 89 coil? The secondary on the model 89 coil has a lot more windings than on the model 84 coil.

I measure 180 ohms from the antenna clip to GND with the volume control all the way open (and the set off). I *think* the ANT coil primary is good based on that?

Here's the schematic:

   

[Radioroslyn]

Hi Greg,
>I measure 3.1 ohms on the secondary. What should the primary measure?

Don't know and it's not particularly critical as long as it's not 100's or 1000's or ohms.

>I've read anything from 14 turns and 25 turns of 36 to 40 ga wire. Before I start unwinding...and lose count...does anyone know which it should be?

Good question. I think I've read that the wire ga is 38 but again it's not particularly critical. The direction IS. As for the # of turns if you don't have enough the oscillator won't oscillator at the low end of the band. If you have too many it's tends to distort the output of the oscillator. There is a couple of ways to measure the original. You can measure the width of the coil and wind it's replacement to match it's width using 38ga wire. You can clean off some of the wax and use a pin dragging it over the turn and count to bumps/turns. Or you can unwind it although  it can be a little tricky as the turns sometimes just fall apart from rot.

>I had an extra OSC coil from a model 89 that I rewound years ago and I tried it on this model 84. I was able to get the local (strong) station in at full volume faintly. It does get a bit louder when I touch the antenna wire I have connected (but not as loud as it should).

It could be a tracking issue as the osc (460kc higher than the incoming signal) and ant circuit (at the incoming signal's frequency) must be tuned to the proper frequencies for the set to have good sensitivity. It must do this across the whole band. By touching the ant wire it's telling me your ant is too short.

>That coil measures 6 ohms on the primary and 5 ohms on the secondary.

Doesn't mean a thing other than the coils aren't open. Resistance varies w/the length  and ga of the wire. It's all about the inductance. Generally in an oscillator circuit the feedback winding is going the be about 20% of the # of turns that are on the tuned circuit. So if the tuned circuit (secondary in your case) is 80 turns your feedback winding (primary) is going to be about 16 turns.

>I then tried one of those AES replacement OSC coils and couldn't get a single station to come through with it.

Well there are a lot of factors in play. Mostly was it oscillating? And if it was was it anywhere near the frequency it needed to be (460kc above the incoming signal). The good thing about it is that the AES coil does have a slug adjustment so you can set it to track properly. I've never bother to try using one in a Philco set as it's easier and quicker just to rewind the old one.

>Could the low volume be caused by too much resistance on the model 89 coil? The secondary on the model 89 coil has a lot more windings than on the model 84 coil.

No the resistance doesn't matter. I would suspect that it wasn't oscillating and that your local station is strong enough to bypass the mixer and get to the detector stage.

>I measure 180 ohms from the antenna clip to GND with the volume control all the way open (and the set off). I *think* the ANT coil primary is good based on that?

Hmmm Seems a little suspect. Would think that it should be lower in resistance. If you do the math, we know the resistance of the 38G wire is .66 ohms per foot. This = about 273' of wire @ 180 ohms around a 1'' dia form. I think this works out to about 1043 turns although my math skills are not very good. I'd pull it out and look for green spots but I'll bet it's no good or there's a volume control issue.
Just as a quick example 38G wire is .66 ohms per ft. Just two sizes smaller (40G) is 1.0 ohms per foot, almost twice as much. I rewound a shadow meter coil w/40g, the total resistance was a little over1500 ohms. That 1500' of wire on a 1" x 3/8" form!

[NostalgiaRadioTime]

Thanks so much, Terry!

I rewound the primary of the OSC coil. 16 turns of 38 ga wire. Same direction (clockwise) as the original. When I used a bit of low heat from the heat gun to melt the wax over the original winding to remove it, one turn of wire fell off. Guess I found the break right away! Took off the old clear ring under the winding and replaced it with a strip of thin clear mylar. Now I will be the first to admit I am not very good at winding this tiny wire. There are gaps between the turns. It ain't pretty. Not sure if that matters much if the turns are touching or have a gap between?

When I gator clipped it back in, I get very faint reception when I clip my antenna wire to the ANT clip on the chassis. Pretty much what I was getting when I gator wired in the OSC coil from the model 89 I was trying.

With my newly rewound OSC coil from this set, if I move the antenna wire to the center tap of the volume control, it made no difference. This is I essence Pin 2 or the primary of the ANT coil.

Now if I put the antenna wire on Pin 3 of the ANT coil, or the secondary, I had steady wide open volume and received many stations up and down the dial. So, despite the gaps in the windings, I think my rewound OSC coil is working.

I can measure resistance with the ohm meter on both the primary & secondary of the ANT coil, so either the coil is shorting between the windings, or one of them is partially open. Or the volume control could be a problem...

The schematic shows it should have a 20K ohm volume pot, but this set has one (stamped Philco and looks factory correct) that measures 600K ohms. So I tried one I had on hand that measured 100K so see if it made a change. It didn't.

So I'm thinking I should just let the OSC coil as is, as it seems to be working, reinstall it and concentrate on the ANT coil? I'm guessing, as you thought, the outer winding on that is bad too.

[Radioroslyn]

>It ain't pretty. Not sure if that matters much if the turns are touching or have a gap between?

What I do when rewinding is wind three or four turns fairly neatly and then use my thumbnail to close the small gaps between the turns. It's better not to have big gaps between the turns but it is what it is.

> I think my rewound OSC coil is working. 

Well the acid test is to use a counter or an AM/SW radio to listen for the oscillator signal. W/the dial set to 550KC the osc signal should be be @ 1010KC and w/the dial set @1500KC the osc signal should be @ 1960KC. Ah yes that was when hurricane Donna hit. But I digress.

> So I tried one I had on hand that measured 100K so see if it made a change. It didn't.

The value of that pot needs to be fairly low  < 50K else wise all of the attenuation will be in the first few deg of movement.

>So I'm thinking I should just let the OSC coil as is, as it seems to be working, reinstall it and concentrate on the ANT coil? 

I would make sure the it's oscillating across the whole range of frequencies. Then go on to the ant coil. Typically it is the primary that is the issue. Also I find that the resistors can drift badly especially the higher value ones around the detector stage and the two 2 w voltage dividers.
With a good alignment using a signal generator (not this by ear crap) they are fairly sensitive, no 116X but not bad.

GL & say hi to ur dad 4 me.

[NostalgiaRadioTime]

Thanks once again, Terry. Here's the progress report:

I did the OSC test with a second radio. I had this 37-84 set to 620 AM on the dial, and the second radio set to around 1100 and could hear the OSC coming through the test radio. The station didn't come through but I knew I found the OSC, as when I turned the 38-84 off, I got normal static on the second radio. Repeated the test with another restored tube radio and got the same results. Now the rewound primary on the OSC coil still has gaps between the windings, but the right number of turns and wound in the right direction. I'm getting a lot of stations (with the test ANT coil...see below), just not the strong local one at 1470 but I suspect I may need to do an alignment to find it. So I'm kinda on the fence if I should try rewinding the coil to get the windings closer or leave it as is?

I unhooked the existing ANT coil wires (which measured 2.6 ohms on the primary and 5.7 ohms on the secondary) and gator wired in the antenna coil I have from a parted out model 89. That primary measures 16.5 ohms and the secondary 9 ohms.

With the model 89 ANT coil, the radio has good volume and control of volume. And received lots of stations. Then I disconnected the model 89 ANT coil and  gator wired the ANT coil from the radio back in, not touching the tuner or volume control. With the ANT coil from this radio I have very low volume. Yet neither winding reads open. Possibly it's green spots cutting down on the volume? Or could the higher resistance on the model 89 coil be boosting the volume? I suspect rewinding the primary of the ANT coil is next. I'm still using the volume control that was in the radio, and measures around 500K. I'm trying to find one that measures 20K as the schematic calls for. Not sure how much it will affect the performance, though.

Oh, and Dad says hi

[462ron]

Hi, I am just a little curious as to how sensitive and good of a performer this set is? I've never seen a set like this where the signal goes right from the 1st det/oscillator into the second detector with no IF amp in between. I do see where cap 17 looks like gives some kind of feedback to the tapped IF transformer so maybe that helps. No RF or IF amp has me wondering! Thanks

Ron

[Radioroslyn]

> I do see where cap 17 looks like gives some kind of feedback to the tapped IF transformer

Yes this circuit is called a regenerative detector and when adjusted properly has quite a bit of gain, much better that a simple diode detector. It uses feedback to amplify the rf signal and convert it to audio. It also can be quite selective depending on how much feedback is used. Not to be confused with a super regenerative detector which is very different.

If someone was interested in radio I would recommend  building a simple regen set. It will get you a good sense of how oscillators work and how good a simple set can really pull in many signals.  Something like this :  http://onetuberadio.com/2017/04/11/1947-...ast-regen/  could be assembled in  long afternoon after all the parts are sourced.  Can also double as a signal generator and a dip meter in the right hands.

[462ron]

Thanks Terry. I thought it was something like that. I've heard of regen sets just never saw one before. That is an interesting little set in that article.

Ron

[NostalgiaRadioTime]

With thanks once again to Terry, I finished up this radio tonight. I was waiting on a new 20K volume control / power switch, which, happily, arrived today. It is amazingly powerful...it picks up the local station and strongest Milwaukee station without ANY antenna lead connected! I learned a few things in the past week restoring this set: the OSC coil worked fine even with the gaps between the windings of the primary I rewound. Most important are the number of turns and the direction they were wound. I also was happy to find nothing was wrong with the ANT coil...the low volume and poor reception was all due to the 500K volume control someone decades ago replaced the original 20K one with. I got lucky with the photo finish on the front of the cabinet....it cleaned up well and some careful touch up of the chips with a black magic marker and a quick thumb wiping the excess off so only the chipped areas got colored really made it pop again!