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TRF volume TOO HIGH!!

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Post by datsun280 on Fri Aug 17, 2012 1:39 pm

I hoped that would not be a problem. It was the first thing I removed from the set and had a mishap that broke one of the cap to coil leads. I carefully de-tarred the components, soldered the connection, tested the ohms and it seemed to be ok. Installed it and had the high volume reception. Then used the diagram on the schematic and made the alternate, installed it and still had the high volume reception.

I had sent a finder out on Antique Radio Forum to locate a replacement vc as well back when I thought it was a defective vc only. Had since received some feedback to identify and correct the problem.

So far:
Moved the vc to the RF circuit, with no change, still high volume reception.

Moved the vc to the screen circuit and have the original vc finally going to low volume. It increases normally to about 1/3 of the way to full volume. Then at 1/3rd of the way to full volume, it jumps to high volume. Then the remainder of the way to full volume, it increases slowly to a higher, more distorted volume.

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Post by Bill Cahill on Fri Aug 17, 2012 1:53 pm

These are jury rigs. They won't do. It must be fixed properly. Who the heck told you to do these things? No. Never mind. I dont' want to know.
Bill Cahill

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Post by datsun280 on Fri Aug 17, 2012 2:01 pm

I didn't mean to get you frustrated Bill. Sorry.

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Post by Bill Cahill on Fri Aug 17, 2012 2:03 pm

I'm not frustrated. Just trying to help. No problem here.
Don't worry..........
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Post by datsun280 on Fri Aug 17, 2012 2:15 pm

Ok. And I want you to know that I really appreciate you taking the time to help me Bill.

I feel kind of uncomfortable having ended up working with two different forums on this set.

The last reply on ARF helped me to understand that the vc is now matching the current curve of a tetrode (which I had no idea of).

It seems that the vc is operating correctly (I mean not defective) then since it is matching the curve of the tetrode? If so, then this is as good as the volume control will be (which isn't very pleasant)?




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Post by Bill Cahill on Fri Aug 17, 2012 5:19 pm

That's up to you. But, it isn't correct. If it works o.k., you can leave it that way. Might be simplest that way.
Current curve? It's varying the voltage. Current? I don't know. A number of the guys at ARF are very intelligent. But, a number of them need to learn more. Heck. I know I do. Haven't learned enough yet.
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Post by datsun280 on Mon Aug 20, 2012 6:21 am

I was hoping to get a response this weekend from the folks that were helping on the project, but wasn't able to get one yet. Kind of behind the eight ball to have this done for today. Would you be able to help me with the answer for this?

First is the direction to smooth out the volume control and then is my question.

Received direction to smooth out the volume control that is currently matching the curve of the tetrode.. as follows:
Yes, you can somewhat smooth it out by figuring out what range of rotation, from full off, gives a smooth increase. Then turn off the radio and measure the resistance from CW to wiper. Figure out what fraction that is of the total pot resistance. Then put a resistor equal to that fraction times the pot resistance between the CW terminal and the source of screen voltage.

For instance, if you get a smooth increase up to one-third of a turn of a 10K pot (linear), then you will measure 6.6K from the CW terminal to the wiper. That means you need 10K times ( 6.6 / 3.3 ) or about 20K between the CW terminal and the wire that used to go to it.

If it smoothed out to only 1/4 rotation, you'd measure about 7.5K, so you need 10K times ( 7.5 / 2.5) or 30K


Hoping for clarification of this:
The current configuration of the vc connections is connecting the vc inside the screen circuit of the 1st and 2nd RF with connections with wiper and one tap, and the last tap to ground.

The antenna is connected to the 1st IF coil only and then to ground but no longer connected to vc.

I understand the calculations, but, am I testing from antenna lead (going to 1st IF coil) and wiper (in series with screen circuit) and then installing the resistor between same?

Thank you for any help you can offer.
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Post by Bill Cahill on Mon Aug 20, 2012 10:40 am

????????
I'm sorry, but, I only repair the right way. Current limiting on screen?
Sounds more like you are shrting out the B+ to that tube. How does that limit current?
If you create more of a load that isn't limiting current. That's limiting voltage, but, causing that circuit to draw more current.
I'm sorry, but, I've never tried to repair a radio this way.
I just don't understand how a jury rig like this improves the radio.
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Post by datsun280 on Mon Aug 20, 2012 11:32 am

I understand.
I can't answer any of the questions.
Thank you.
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Post by Bill Cahill on Mon Aug 20, 2012 12:43 pm

I'm not berating you. I just don't understand why it's being done this way. Sorry.
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Post by Bill Cahill on Mon Aug 20, 2012 12:44 pm

I'll try to think of some ideas to help.

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Post by datsun280 on Mon Aug 20, 2012 1:04 pm

I am not offended Bill.

I received a reply for clarification but am awaiting another reply as I did not understand it completely.

I do know that the reason the project has gone this direction is that all other methods to control the volume failed. We had not addressed the filter, most likely, because the alternate filter shown on the schematic was used.

I have the orig coil with the original caps. It showed continuity. But, it was replaced with the alternative as one step in trying to control the volume.

I don't know how to test the coil/cap assy to know anything further to evaluate its actual condition.

I would much prefer to have the set original but I need to have the set operating today unfortunately. ARGH!

If I knew how to test the coil/cap assy to be certain of its condition, I would do that.


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Post by Bill Cahill on Mon Aug 20, 2012 10:19 pm

If the coil still has continuity, just replace the capacitors, and, use the original coil, and, wire it back the way schematic shows. Try it. Leave the jury rig out. You might be pleasantly surprised.
The coil isn't there for resistance. It's there to keep rf signals out of the rest of the set. Take a hint from this. It may just work.......
Houmor me. I could be wrong. But, that coil was put there for a reason.
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Post by Resistance is Futile on Tue Aug 21, 2012 3:18 am

You may have the wires reversed in the control. it is a control that is wired from the antenna to ground, and the center tap is the gain control adjustment. If you connect an ohmmeter (with the radio unplugged) to the center tap and the other end to ground, then increase the volume control and read less resistance you have the control wired backward. As you increase the control clockwise the resistance should go up in resistance, this has the effect of removing more of a short to ground. If the volume control is midway point you should have 5000 ohms from center-tap to ground, as you increase the control clockwise you should be getting higher resistance. Up to 10,000 ohms at highest volume setting.
At the halfway setting half the signal is going to ground and the other half is going through the RF antenna coil. As you increase the volume less of the signal is going to ground (chassis), and more through the Coil.
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Post by datsun280 on Tue Aug 21, 2012 5:23 am

Thank you guys for your continued help.

To confirm that the coil assy has been reassembled correctly:
The two leads coming from the coil are connecting to one end of each of the mica caps. At each of these two connections, a wire is also connected which would lead to Det plate and to the 250k resistor.

The other end of the mica caps are connected to each other, and, this end connects to the coil assy metal box which grounds to chassis.

Testing the coil assy out of circuit from wire lead to wire lead, the resistance is 90ohms.

Does that sound like it is the proper resistance value?

The mica caps do not have any color code or printed value on them. When I had made the alternate filter, I used .0005 mfd caps. The schematic is difficult to make out the value with certainty so I had researched other similar sets by Kennedy and most had the .0005mfd.

If the 90ohms is not the desired value, I should get your opinion as to what value to use as replacement for the two mica caps. The coil wire is so small that I would like to be able to replace the caps correctly the first time. The coil wire from the center of the coil has only 1/2 inch of wire to work with.

If the 90 ohms is not the desired value, would you want the coil resistance value without the caps connected to test the coil itself?

If the 90 ohms is the desired value, then I will install the coil assy and rewire back to original schematic.

Thank you.
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Post by Bill Cahill on Tue Aug 21, 2012 10:27 am

Nope. O.K. NOw, I found out. That coil is actually a 10,000 ohm wire wound resistor.
You have bad caps, and, it's wired wrong.
The resistor is in series with the plate of det. and,the 50,000 ohm B+ resistor.
At that point where two resistors connect the .06 coupling cap to grid of 226 goes.

Now, the two caps.
The first one looks like a 200 mmf mica going to ground from plate of det. At that ground connection the other mica cap, I think it's a .005 mf goes to other side of resistor at the junction of the 10,000 ohm, and, 50,000 ohm resistor.
That cap is for tone.
The other cap is part of the det. circuit itself.
The coil is just a wire wound resistor, which I didn't realize myself.
You should be getting a resistance of 10,000 ohms from the plate of the det. to the end that goes to B+.
The values are not clear on inside caps, but, It looks like the values I've given you.
Hope this helps.
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Post by datsun280 on Tue Aug 21, 2012 11:16 am

Currently, the alternate filter is comprised of a 10k resistor with one end sharing connection to the Det plate with a .0005 mfd cap.

The other side of the 10k resistor is sharing connection to one side of the .06 coupling cap and the 250k resistor.

The other sides of the two .0005 mfd caps are sharing ground to chassis.

The resistance from Det plate to the junction of the .06 cap and 250k resistor measures exactly 10k.

My schem shows the one side connecting to the .06 cap and 250k resistor. The 50k resistor is actually on the other side of the .06 coupling cap.

I am using Kennedy Coronet 42 schem http://www.nostalgiaair.org/PagesByModel/554/M0039554.pdf

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Post by Bill Cahill on Tue Aug 21, 2012 12:24 pm

Those connections are wrong.
Need to re do them the right way.
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Post by Bill Cahill on Tue Aug 21, 2012 12:26 pm

No it isn't.
The schematic shows that resistor going to B+.
The other side of .06 coupling cap on grid of amplifier has a resistor going to chassis.
That's bias.
I looked at the schematic, but, I'll go look again.

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Post by Bill Cahill on Tue Aug 21, 2012 12:30 pm

I just looked. It's the same schematic. You are wiring it wrong.
Sorry. Look at the line running from other end of 50K resistor. It's going to the second filter in the power supply.
That frsistor feeds B+ to the plate of det.
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Post by Bill Cahill on Tue Aug 21, 2012 12:33 pm

You are shorting out the det., and, indirectly putting rf right into the amplifier. How can the radio work right when the det. is being disabled?
The det. is there to help regulate the signal along with turning it fromrf to audio.
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Post by datsun280 on Tue Aug 21, 2012 12:36 pm

That is the 1st AF tube. The prior tube is the Det 224 tube that has the output filter outlined box above the plate.

The other end of the 50k resistor between Det and 1st AF goes to ground.

Am I reading this wrong?

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Post by Bill Cahill on Tue Aug 21, 2012 12:40 pm

I know that.
You aren't wirring it right.
I see the lines indicating the parts are in a can.
If you are describing the way you have it wired correctly, it's not. It's wired wrong.
Sorry. Look carefully at schematic, and, compare your wiring.

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Post by Bill Cahill on Tue Aug 21, 2012 12:42 pm

Don't get upset. Just go at it very carefully to compare what you have with what they show.
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Post by datsun280 on Tue Aug 21, 2012 12:50 pm

It is currently connected the way that I have interpreted the schematic Bill. The box containing the alternative detector output filter has the 10k in series with the det plate and the junction of the .06 cap/250k resistor.

I see that the otherside of the 250k resistor ties to the 1st AF plate 50k resistor and then down to the 16 mfd electrolytic.

But I can't see on my schem where the non-det plate side of the 10k output filter resistor could connect to anything other than the .06 cap and 250k resistor.

And that is how the set is connected.

What am I missing?

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