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 Post subject: Re: Please help me understand schematic....
PostPosted: Feb Mon 12, 2018 4:22 am 
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It seems we never resolved the terminals on the padder.....

There are only TWO functional connection points on the padder. Because they are on opposite sides of a capacitance, they would have infinitely high resistance between them. A reading of 0.2 ohms probably means you are measuring between two mechanical features that are in fact the same finctional point (AKA "node")... All this was discussed earlier.

So....find the TWO functional connections to the padder....disconnect it from the circuit if you have to.

Next, check the wiring of the coil as follows:
Pick a terminal on the coil and note where it connects. Eg point B on Curtis's diagram goes to C3. Now find the opposite end of that winding**, labelled point E. That should go to ground. Physically, that might happen at the padder.
Next find point A, verify it goes to 6a7 pin 5, and to the tuner. Now, find the other end of that winding and verify it goes where it is supposed to. also find the tap, and verify it is connected to the right place.

**the schematic shows 2 windings between points B and E. Since their junction does not connect to anything else, I'm now calling them one winding.

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 Post subject: Re: Please help me understand schematic....
PostPosted: Feb Tue 13, 2018 3:04 am 
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Location: Cottage Grove Oregon; 97424
Hope this works new pictures of the chassis....as you can see the padder and the oscillator coil and the 6A7 tube socket.


Attachments:
fairbanks morse 56.JPG
fairbanks morse 56.JPG [ 123.8 KiB | Viewed 435 times ]
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 Post subject: Re: Please help me understand schematic....
PostPosted: Feb Tue 13, 2018 8:11 pm 
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Mike;

The first thing that jumps out at me is that the wiring to the padder is in fact wrong.

Remember the discussion about 3 terminals, but only 2 functional nodes. Look at the padder and you see 3 connection points--in a line. the 2 outer ones are the same node (the one that goes to ground). You can verify this with your ohmmeter.

Hopefully, you can now see that R5 goes to the wrong terminal. Looking at your picture, I'm also not sure you have the right value in there.

I will post more after you confirm what i am seeing.

Here is an enhanced snip from your picture:
Attachment:
FM_56_front-end.jpg
FM_56_front-end.jpg [ 99.94 KiB | Viewed 422 times ]

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 Post subject: Re: Please help me understand schematic....
PostPosted: Feb Tue 13, 2018 9:30 pm 
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You are right that the outer two points are connected and are not related or connected to the center wire. R-5 is 10K and is in fact 10K.


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 Post subject: Re: Please help me understand schematic....
PostPosted: Feb Tue 13, 2018 9:45 pm 
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Are we talking about the same part?? I'm looking at the one with red and orange bands....hard go get 10K from that.....

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 Post subject: Re: Please help me understand schematic....
PostPosted: Feb Tue 13, 2018 9:59 pm 
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I used the larger picture to attempt to identify and attach "Letters" to the terminals of the oscillator coil to match the schematic where terminal letters were added. It appears that point E has no connection. It also appears that R5 may be connected incorrectly at the padder.
Attachment:
FM_56_front-end[1][1].jpg
FM_56_front-end[1][1].jpg [ 127.94 KiB | Viewed 410 times ]
I could be wrong about my identifications so don't do anything until Mark leads you through this.

Curtis Eickerman

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 Post subject: Re: Please help me understand schematic....
PostPosted: Feb Wed 14, 2018 1:06 am 
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Mike, if you go look at the first photo you posted you can see the 10k resistor, looks like it is attached to the center pin of the padder. Can't tell from your last photo the colors (brn, blk, org?), but for sure is attached to the wrong place.

Jeff


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 Post subject: Re: Please help me understand schematic....
PostPosted: Feb Wed 14, 2018 8:27 am 
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It is brown, blk, and orange with a red % stripe that's 10K and does go to the outer side of the padder, which is wrong.


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 Post subject: Re: Please help me understand schematic....
PostPosted: Feb Fri 16, 2018 3:02 pm 
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So--is it fixed?

You have quite a few other issues with lead dress and solder joints. In particular, the wiring around pins 4 and 5 of the tube needs to be fixed. Note that pins 4 and 5 are NOT supposed to be connected.

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 Post subject: Re: Please help me understand schematic....
PostPosted: Feb Sat 17, 2018 12:26 am 
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Wooo Hooo!! It's Alive!! Just like that scene in the movie Wrath of Khan, when Spock went into the engineering section and did something in the Anti matter chamber and the Enterprise was able to go to warp and get away? Yeah, just like that; I hooked up that ground wire from the coil and moved R-5 the the center leg of the padder and the radio came right to life!

I CAN NOT thank you good folks enough! Now she still has an issue or two, the tone control makes no difference, and that horrible sequel from the grid cap of the 75 tube is still there when you touch the volume control and try and increase the volume.

Every one of the adjustment caps on the various IF's were way out as well, but now I get decent volume and sound quality. (as long as I don't touch the volume control)

So I guess the next thing to do is an alignment?

God bless you folks and thanks again!! may God bless you for your patience!


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 Post subject: Re: Please help me understand schematic....
PostPosted: Feb Sat 17, 2018 12:16 pm 
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Quote:
So I guess the next thing to do is an alignment?

No....

The steps I would take:
1. Check ALL of the wiring against the schematic. As part of this, review what happened so far and try to ID what proceedures you need to study up on. The example that sticks out is the padder. The schematic shows this as a two-terminal device, and yet you found 3 connection points. This thread includes hints about how to resolve such an issue, but they apparently got lost.
As needed, post questions wherever you still have issues with how to read schematics.

2. Clean up any workmanship issues...eg the problem identified around pins 4 and 5 of the 6A7. Make sure all solder joints are good, sleeving on component leads where needed, etc.

3. Fix the problem with the audio "squealing". This may include elements of steps 1 and 2.

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 Post subject: Re: Please help me understand schematic....
PostPosted: Feb Sat 17, 2018 12:49 pm 
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You may get a better grip on wires & components like resistors when soldering, if you use a hemostat as pliers. These allow a grip closer to the action without burning fingers & can actually keep a bit of heat away from the component. Also very useful in awkward places & component removal. The faster you can solder a joint the better: Too much heat and prolonged heating is not good.

In some cases I have used one in each hand to get parts into & out of places they should never have been put.

Marc


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 Post subject: Re: Please help me understand schematic....
PostPosted: Feb Sat 17, 2018 4:59 pm 
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mjohansson2 wrote:

I CAN NOT thank you good folks enough! Now she still has an issue or two, the tone control makes no difference, and that horrible sequel from the grid cap of the 75 tube is still there when you touch the volume control and try and increase the volume.

Every one of the adjustment caps on the various IF's were way out as well, but now I get decent volume and sound quality. (as long as I don't touch the volume control)


I suggest that you be more precise in your descriptions. Here you say the grid cap of the 75 squeals; is it the cap squealing or is the sound coming from the speaker? Does it squeal when you touch the metal shaft of the volume control? Or touch the case of the volume control? Or, begins to squeal when you turn the volume control?

On the IFs, were they physically unscrewed way out? Or, were they way out of adjustment?

Good to hear that you are making progress.

Jeff


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 Post subject: Re: Please help me understand schematic....
PostPosted: Feb Sat 17, 2018 11:46 pm 
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What Tone control? If I am looking at the right circuit there is none on it. All there should be is C13 a 0.006mfd (0.0056) cap broadside across the speaker transfomer primary (not to ground).

Another issue is that if it had paper filter caps and these were replaced by electrolytic; It may need a 0.1mfd cap B+ to ground.

+1 Translate the volume control issue.

Marc


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 Post subject: Re: Please help me understand schematic....
PostPosted: Feb Sun 18, 2018 1:32 am 
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Ok more clarification: When the radio is turned on and the volume is at 3/4 or higher there is a squeal from hell. I h ave to turn the volume down. Now if it turn the volume up it squeals. If I touch the grid cap of the 75 it will either squeal or make some other "funny" sounds. On the volume control center tap if I touch that with a pencil or what ever I get the squeal. There is a cap there and that is the correct value.

the tone switch is a cheesy two position switch next to the band switch and it's either on of off no intermediate position. And it's ruff as a cop I will try some lube on it to smooth it out.

As for the IF cans, and the other screw in adjustable caps, all were screwed way out.

I may try and different 75 tube and see if that helps.


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 Post subject: Re: Please help me understand schematic....
PostPosted: Feb Sun 18, 2018 2:53 am 
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Re-iterating what I wrote: I have looked at all of the schematics posted and on none of them is there a "Tone switch", or a tone anything. So I conclude it should not be there.

The squeal is likely feedback if the tube is good: Leave the finger out of it. You replaced what were probably "Outside foil caps"; that can have ramifications as the modern ones are not like that. The biggest PIB is the area around the second detector, or leaving shields off of tubes like 6D6 and sometimes 75.

Tatty wiring (aka lead dress) bad solder joints (bonding) and the new caps all can add up to radiation,induction & instability, especially in that area. Wires running parallel can induct (antennas); Wires running through the air can be radiators. It is important to keep wireds as close to the chassis as is practical. Wires running parallel can induct into modern caps. It is never unusual to have to replace long wire runs to volume controls etc. with shielded wire.

You could go eat Chinese & bring home, the chopsticks, wind up the volume to where it starts oscillating & use the chopstick to prod & probe to see if you can change the frequency of the oscillation? That should pin it down to a wire, or an area.

Marc


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 Post subject: Re: Please help me understand schematic....
PostPosted: Feb Sun 18, 2018 5:28 am 
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mjohansson2 wrote:
Now if it turn the volume up it squeals.
It might be worth double checking that C11 (0.1 µF) and C12 (.25 µF) are installed, the correct value, and really go to ground as they are supposed to. I mention this because missing grounds seems to have been a theme with this radio (missing filament wire ground, missing LO coil ground).

Curtis Eickerman

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 Post subject: Re: Please help me understand schematic....
PostPosted: Feb Sun 18, 2018 7:10 am 
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+1 Ground & decoupling, one related in part to the other. causing instability.

Marc


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 Post subject: Re: Please help me understand schematic....
PostPosted: Feb Sun 18, 2018 9:03 am 
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I took out the volume control as I did not remember if I had taken it apart and cleaned the insides with deoxit. It was filthy so I guess I had forgotten to do so. I ohm ed it out and it read a steady 680 ohms, the schematic calls for 500K I found a replacement on/off volume switch which ohm ed out at 498 and installed that. I also found pader B in the schematic located by the 6A7 tube shown in the chassis picture in the Riders manual. When I turn that screw in or out, the sequel either gets louder or goes away. So....long story short, I now have better volume control and more volume, and the sequel is greatly reduced, I'll bet the problem lies in the pader B.

Now I do not know how they originally had the grid cap of the 75 tube routed originally, but it was mickey moused when I got it, you pulled on the wire and it broke off of one of the resistors.
So I replaced the wire, anchored it to a terminal strip, attached the correct resistor and cap and it's good and secure now.


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 Post subject: Re: Please help me understand schematic....
PostPosted: Feb Sun 18, 2018 12:54 pm 
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The screw in the padder changes frequency: Normally that is set to the dial at around 600KHz and affects the band spread. It should not be responsible for the oscillation. One of the problems I see is the tatty wiring.

As has been pointed out wires & caps should be run as close to the chassis as is possible. The wires on 3 are a classic example of what should not be. Wires carrying RF /AC have a tendency of inducting one into the other. The light wire going across the Mica cap neeads to be on the other side of the Ant coil flat on the chassis. the one going 3 to padder is also asking for trouble. I do not like the two yellow caps between the ant coil & the padder. With the one with spaghetti, get it low down, next the chassis, or parallel with the coil, in 2D it looks like a short waiting to happen.

The Chopstick trick works there as well; move a wire & change the squeal frequency that's a problem.

Marc


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