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 Post subject: Re: Upgrading Heathkit FM-3 Tuner to FM-3A?
PostPosted: Feb Sat 17, 2018 2:48 am 
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Location: Woodinville, WA USA
Yes, the add-on unit does connect near the 6AL5, so your guess that it's an AFC circuit might be right. This schematic shows the connections, as near as I can make them out:

https://antiqueradio.org/art/Heathkit%2 ... dified.jpg

I can't draw a schematic of the add-on without removing it from the chassis. Most of the components are wired to the hidden side of the added terminal strip, and it's hard even to count them, much less trace their interconnections. The strip contains a dozen components. Very tidy (and cramped!) construction, whoever did it.

Phil Nelson
Phil's Old Radios
https://antiqueradio.org/index.html


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 Post subject: Re: Upgrading Heathkit FM-3 Tuner to FM-3A?
PostPosted: Feb Sat 17, 2018 4:53 am 
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philsoldradios wrote:
Yes, the add-on unit does connect near the 6AL5, so your guess that it's an AFC circuit might be right. This schematic shows the connections, as near as I can make them out:

https://antiqueradio.org/art/Heathkit%2 ... dified.jpg

I can't draw a schematic of the add-on without removing it from the chassis. Most of the components are wired to the hidden side of the added terminal strip, and it's hard even to count them, much less trace their interconnections. The strip contains a dozen components. Very tidy (and cramped!) construction, whoever did it.

Phil Nelson
Phil's Old Radios
https://antiqueradio.org/index.html



Phil
I have the FM3, beautifully built, and can take photos of it 'as built' if you need. Let me know.....

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John Smith, over and out.
If I did something right, I made a mistake.


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 Post subject: Re: Upgrading Heathkit FM-3 Tuner to FM-3A?
PostPosted: Feb Sun 18, 2018 12:59 am 
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Thanks, the FM-3 assembly manual shows clearly how an FM-3 is built. I just need to clear space on my workbench and start with the basic recapping.

Phil Nelson


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 Post subject: Re: Upgrading Heathkit FM-3 Tuner to FM-3A?
PostPosted: Feb Mon 26, 2018 3:22 am 
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I got a copy of the C-FM-3 conversion manual, and it's more minimal than I expected. It does make a simple (move one wire) mod to add AGC to the first IF stage. It also adds an RF filter (1 resistor, 1 cap) to the output of the rectifier, and it includes an optional mod (1 fixed ceramic cap, 1 trimmer) to improve stability at the oscillator. The rest of the manual describes how to install a new drive gizmo on the tuning cap and install a new dial glass and an extra pilot light. Here's the part of the manual describing electronic changes:

https://antiqueradio.org/art/Heathkit%2 ... ersion.pdf

However, the C-FM-3 conversion kit did NOT incorporate a couple of other changes that appear when you compare the FM-3 and FM-3A schematics.

In the FM-3, a 2mfd/150V electrolytic is connected through a 47K resistor to pin 5 of the 6C4 output tube. In the FM-3A, that cap is changed from 2mfd to 20mfd; another 47K resistor is eliminated; an 8.2K resistor is added between the 20mfd electrolytic and the second 20mfd filter electrolytic; and a line is rerouted from the junction of the 8.2K resistor/20mfd electrolytic to a 6.8K resistor on pin 1 of the 6U8 tube.

This schematic is annotated to show the schematic differences between FM-3 and FM-3A:

https://antiqueradio.org/art/Heathkit%2 ... hanges.pdf

Notice that the FM-3A does not show any trimmer cap where one would be added via the C-FM-3 kit.

Meanwhile, I replaced the two power-supply electrolytics and the two .05 caps on the output jacks. The tuner sounds very nice, although it drifts a LOT during the first few minutes. After that, it's stable.

Phil Nelson
Phil's Old Radios
https://antiqueradio.org/index.html


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 Post subject: Re: Upgrading Heathkit FM-3 Tuner to FM-3A?
PostPosted: Feb Mon 26, 2018 4:19 am 
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Thanks Phil.
My FM3 works very well, after warm-up. Does not drift after that.
I have the FM4. Will re-cap soon, and post results.

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If I did something right, I made a mistake.


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 Post subject: Re: Upgrading Heathkit FM-3 Tuner to FM-3A?
PostPosted: Mar Sat 10, 2018 1:27 am 
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After finishing my FM-3 restoration, I got a chance to try out the tuner with my W-6M amp and WA-P2 preamp. They sound great together!

This article has details and more documentation, for anyone interested:

https://antiqueradio.org/HeathkitFM-3Tuner.htm

In a nutshell, I made a couple of the changes mentioned in the C-FM-3 manual, but skipped the others.

Regards,

Phil Nelson
Phil's Old Radios
https://antiqueradio.org/index.html

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 Post subject: Re: Upgrading Heathkit FM-3 Tuner to FM-3A?
PostPosted: Mar Sat 10, 2018 3:20 am 
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What a pleasure Phil. Read the whole article with great interest. I wont up-grade my FM3 either. It has a few minute warm-up, then stays in tune. I will work on the FM4 soon and post results.

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 Post subject: Re: Upgrading Heathkit FM-3 Tuner to FM-3A?
PostPosted: Mar Sat 10, 2018 9:10 am 
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If you restore an FM-4, you may want to create a new thread. It's easier for people to find things if topics aren't all mixed up :)

Regards,

Phil Nelson


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 Post subject: Re: Upgrading Heathkit FM-3 Tuner to FM-3A?
PostPosted: Mar Sun 11, 2018 10:43 pm 
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philsoldradios wrote:
If you restore an FM-4, you may want to create a new thread. It's easier for people to find things if topics aren't all mixed up :)

Regards,

Phil Nelson



Will do Phil :wink:

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If I did something right, I made a mistake.


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 Post subject: Re: Upgrading Heathkit FM-3 Tuner to FM-3A?
PostPosted: Mar Mon 19, 2018 9:52 pm 
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OK. Here is the new thread about the FM4.
viewtopic.php?f=13&t=337644

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If I did something right, I made a mistake.


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