Model 37-84 Help Needed - Printable Version
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Model 37-84 Help Needed - dseverson - 04-01-2015
Hello, everyone.
I recently purchased a Model 37-84 and did the usual recap and resistor replacement as well as replacing the 4 meg with a 2 meg in the I.F. can as noted on the Philco Repair Bench website. However, I have run into a problem that I hope someone can help me with. The radio turns on but I do not get any stations unless I touch the grid cap on the 6J7G detector-oscillator. A couple local stations come in loud and clear. Touching the grid cap on the 2nd detector 6J7G gives a loud hum. Also, the volume control doesn't do anything. In other words, the volume is constant and doesn't get louder or softer. I changed to a 50k ohms volume control (the schematic calls for a 20k) with the same result.
Here is a link to the schematic: http://www.nostalgiaair.org/PagesByModel/225/M0013225.pdf
I look forward to getting any advice you can share.
Dave
RE: Model 37-84 Help Needed - morzh - 04-01-2015
This means your antenna transformer (position number 2 in the sch) is open. Either side could be. Measure it with an ohmmeter.
RE: Model 37-84 Help Needed - dseverson - 04-01-2015
I was afraid of that.
RE: Model 37-84 Help Needed - morzh - 04-01-2015
Don't be afraid, just measure and see if it is the case.
Those are easily rewound coils usually if one-layer type.
RE: Model 37-84 Help Needed - Radioroslyn - 04-01-2015
Easy way to test ant coil primary is Unplug set ,Turn volume full up, Measure resistance from ANT post the GND post. Should read an ohm or two.
Terry
RE: Model 37-84 Help Needed - dseverson - 04-01-2015
Thanks, all, for the replies.
From the ground post (#1 on the schematic), I got 3.1 ohms on post #2 and 6.3 ohms on post #3. Does that sound like the antenna transformer is open and in need of a rewind?
Dave
RE: Model 37-84 Help Needed - Radioroslyn - 04-01-2015
It's hard to say. The volume control is connect in parallel with the primary of the coil. If you try to measure it without having the control all way up (loudest) you are measuring the control rather than the coil. That's why in my previous post I gave instructions how to measure it.
Terry
ps The reason the vc doesn't do much when the set is work properly is the set needs to be grounded.
RE: Model 37-84 Help Needed - dseverson - 04-02-2015
The radio was unplugged and the volume was all the way up when I measured.
RE: Model 37-84 Help Needed - Radioroslyn - 04-02-2015
Ok looks like coil is GOOD! Next think to check is the feedback winding on the osc coil. Measure resistance between the chassis and Where the 6K/1400mmfd connect to the oscillator coil (pins 1 and 4) If good should show a few ohms.
Terry
RE: Model 37-84 Help Needed - morzh - 04-02-2015
Dave
actually Terry asked you to measure from Antenna post to Gnd, and not from pin 2. While it is a useful measurement and might seem the same, it is not.
Now, just to close this step, could you measure from Ant to Gnd
1. with volume fully up
2. with volume fully down.
RE: Model 37-84 Help Needed - dseverson - 04-02-2015
So, if I am understanding correctly, there are two posts on top of the chassis that wires can be clipped to. One is labeled ground and the other is labeled antenna. I clipped the negative lead from my meter to the ground and the positive to antenna. With the volume turned to full, I have a reading of 42.5. With it turned all the way down, I get 0. Hopefully, this is what is needed.
RE: Model 37-84 Help Needed - dseverson - 04-02-2015
Also, the radio was unplugged when I took the readings.
RE: Model 37-84 Help Needed - morzh - 04-02-2015
By 0 do you mean...zero? or infinity? (open)
Can't be zero (shorted).
If open, your volume pot is open end to end and this what may be causing your problems.
It should read low ohms (single to maybe 10 or so) when volume is fully UP, and 20Kohm when fully DOWN.
RE: Model 37-84 Help Needed - dseverson - 04-02-2015
I'm sure you can tell I'm a total amateur, just recently picking up this hobby. This morning I was in a rush when I took the readings but I'm almost certain I got zero, not infinity, at no volume. When I get home, I'll take another reading and get back to you. When you say the pot might be open end to end, does that mean that mean the pot is bad or is something wrong with the way it's wired up?
In any event, I truly appreciate the assistance you all are providing.
Dave
RE: Model 37-84 Help Needed - morzh - 04-02-2015
Well,
your pot is connected between Antenna and GND with its end pins.
If you disconnect the pot's middle pin from antenna transformer primary coil, the resistance between these end pins will be the pot's listed value, 20K, or whatever it is today after wear, maybe 30K. If you have a crack in the pir's resistive element, then you will measure infinite, or very high (in Megaohms) resistance.
Pot's wiper slides along the resistive element, and is connected to the middle pin, and so if you measure between it and any end pin while rotating the shaft (this moving the wiper) you will see the resistance changed from very low (Ohms, maybe tens of Ohms) to the full scale (20 kOhm or, again, maybe 30kOhm if it is worn).
If it is cracked, then you will see the resistance change and then, when you pass the crack, jump up to that very high value.
Now what can be happening in your case, IF you indeed see the infinity on your meter (seeing zero in the Low position is wrong) and 40 ohm or so in Full position this may mean your pot is cracked, otherwise you's see that 40 ohm going gradually to 20 kOhm.
Now. Since at the Full position you do get 40 or so, you have contact there.
So, have you tried the antenna reception at Full position?
Depending where the crack is (if it is) you might lose reception at the very top, or close to the bottom.
So.
1. Connect your DMM between A and GND, and note the resistance at FULL (clockwise all the way).
2. Slowly rotate the volume CCW, see if it jumps to Zero or Infinity (very high) and notice the place where it happens.
3. If it indeedgoes to zero......your pot ends are swapped and your CW position is Min Vol, and CCW is Max Vol.