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 Post subject: Upgrading Heathkit FM-3 Tuner to FM-3A?
PostPosted: Feb Sat 03, 2018 7:00 am 
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I just found a Heathkit FM-3 FM tuner (https://www.radiomuseum.org/r/heath_fm_ ... _fm3a.html), which I plan to use with my W-6M amp and WA-P2 preamp.

According to page 4 of the following article, there were two versions of this tuner. The later (FM-3A) version added AGC and a few other improvements, and Heathkit produced a kit (C-FM-3) to upgrade the FM-3 to FM-3A:

http://www.w6ze.org/Heathkit/Heathkit_045_FM4.pdf

The only schematics that I've found are for the later (3A) version. Has anyone seen a schematic for the early (3) version? By comparing schematics, I could upgrade my FM-3 to 3A. Even better would be a manual for the C-FM-3 upgrade kit, but that may be a real long shot.

Thanks!

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


Last edited by philsoldradios on Feb Sun 25, 2018 11:23 pm, edited 1 time in total.

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 Post subject: Re: Upgrading Heathkit FM-3 Tuner to FM-3A?
PostPosted: Feb Sat 03, 2018 8:46 pm 
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I went through my huge Heathkit library and found no trace of a FM-3 manual or schematic. There is a schematic in the 1956 Heathkit general catalog but the print is so small it's illegible.
BUT, I found a complete description (including a legible schematic) of the FM-3 in the March 1956 issue of Radio & Television News (p.100) and the complete magazine can be downloaded here:

http://americanradiohistory.com/Archive ... 6-03-R.pdf


And there's another article in the January 1957 issue of Radio Electronics (p.123) about improving the FM-3 by adding AFC , a much welcome upgrade as local osc. stability was never a strong feature in the FM-3/FM-3A original design. Here's the download link for the magazine:

http://americanradiohistory.com/Archive ... 957-01.pdf

That's all I was able to find so far, not the requested original Heathkit infos but still better than nothing. (I'll keep searching)

Please note that the only way to tell if your tuner is actually a FM-3 or FM-3A is by looking inside and comparing the circuits because from the outside both have an identical appearance.


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 Post subject: Re: Upgrading Heathkit FM-3 Tuner to FM-3A?
PostPosted: Feb Sat 03, 2018 9:15 pm 
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Thanks, that's very useful! I will definitely be comparing my circuits to what is shown in the schematics.

Edit: I just noticed that my tuner says FM-3 (not 3A) on the dial, so there's a clue.

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: Feb Sun 04, 2018 12:40 am 
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Here's an article about installing the Heathkit upgrade kit: http://americanradiohistory.com/Archive-Poptronics/50s/57/Pop-1957-12.pdf page 89.

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 Post subject: Re: Upgrading Heathkit FM-3 Tuner to FM-3A?
PostPosted: Feb Sun 04, 2018 1:05 am 
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The FM3 schematic is posted over in Electrical/Mechanical Repair and Restoration

viewtopic.php?f=6&t=328171&start=440


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 Post subject: Re: Upgrading Heathkit FM-3 Tuner to FM-3A?
PostPosted: Feb Sun 04, 2018 4:44 am 
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A little better shot of the FM3 schematic.
Attachment:
20180203_192407.jpg
20180203_192407.jpg [ 132.78 KiB | Viewed 1797 times ]

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 Post subject: Re: Upgrading Heathkit FM-3 Tuner to FM-3A?
PostPosted: Feb Tue 06, 2018 5:39 am 
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After comparing my tuner to the FM-3A schematic and assembly manual, it looks like I have a model FM-3. Here are schematics for the FM-3 and FM-3A:

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

https://antiqueradio.org/art/Heathkit_F ... ematic.pdf

The FM-3A adds the components circled in green -- better power-supply filtering:

Image

Those components also appear in the FM-3A assembly diagram:

Image

But the circled components are absent in my (FM-3) tuner:

Image

Strangely, pin 7 (cathode) of the 6X4 rectifier (tube A) in my tuner is connected to nothing, although both schematics show pin 7 connecting to the power supply filters. Note the green arrow in the previous photo. Wiring mistake?

Anyhow, it shouldn't be hard to add those parts to my tuner and improve the power-supply filtering. I haven't finished comparing the rest of the schematics for other changes.

Regards,

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


Last edited by philsoldradios on Feb Sun 25, 2018 11:30 pm, edited 1 time in total.

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 Post subject: Re: Upgrading Heathkit FM-3 Tuner to FM-3A?
PostPosted: Feb Tue 06, 2018 6:58 am 
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Good work Phil. Don't see how the unit could have worked with no pin7 connection. The power supply mod looks simple. Your unit looks in about the same shape as mine. Could be yours has never worked. I re-caped the PS and replaced a few, very bad wires. Would be a good idea to get yours back in operation before attempting retrofitting. Mine works real nice, except the constant drifting.
Pin7 looks like something may have been soldered there. Look for a loose wire in that area. It runs over to the filters.

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Last edited by john8750 on Feb Tue 06, 2018 7:10 am, edited 1 time in total.

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 Post subject: Re: Upgrading Heathkit FM-3 Tuner to FM-3A?
PostPosted: Feb Tue 06, 2018 7:08 am 
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here is that area in the builders manual Phil.
Attachment:
20180205_220222.jpg
20180205_220222.jpg [ 127.83 KiB | Viewed 1713 times ]

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 Post subject: Re: Upgrading Heathkit FM-3 Tuner to FM-3A?
PostPosted: Feb Tue 06, 2018 8:13 am 
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Thanks, John.

Don't ask me how the tuner works at all with pin 7 disconnected, but it actually played fairly well in brief tests.

That build diagram is helpful. I can see at a glance that somebody -- either the builder or a later tinkerer -- also eliminated the 10K resistor between the two 20mfd filter electrolytics. And the 10K resistor between the 6X4 rectifier (tube A) and 6BQ7A RF amp (tube B) is connected to pin 6 of the 6X4 instead of pin 5 as specified. Maybe the builder mis-counted pins and got stuck in "off by one" mode when making some of these connections on the rectifier.

I'd gladly pay "retail" for an FM-3 manual, but I only see FM-3A stuff on eBay. Is there any way that you could bring your manual to a local Kinko's (or FedEx Office, etc.) and have them make a copy of the whole thing? It doesn't have to be bound, just legible copies on white paper. I'll happily pay for the cost of copying + postage + something extra for your effort.

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 Tue 06, 2018 11:48 pm 
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I have FM-3 manual; see PM
If 6X4 was really used as the rectifier, then the cathode, pin 7, has to be connected. Maybe somewhere within the circuitry the tube rectifier was replaced with solid-state devices.

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 Post subject: Re: Upgrading Heathkit FM-3 Tuner to FM-3A?
PostPosted: Feb Wed 07, 2018 12:36 am 
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philsoldradios wrote:
Thanks, John.

Don't ask me how the tuner works at all with pin 7 disconnected, but it actually played fairly well in brief tests.

That build diagram is helpful. I can see at a glance that somebody -- either the builder or a later tinkerer -- also eliminated the 10K resistor between the two 20mfd filter electrolytics. And the 10K resistor between the 6X4 rectifier (tube A) and 6BQ7A RF amp (tube B) is connected to pin 6 of the 6X4 instead of pin 5 as specified. Maybe the builder mis-counted pins and got stuck in "off by one" mode when making some of these connections on the rectifier.

I'd gladly pay "retail" for an FM-3 manual, but I only see FM-3A stuff on eBay. Is there any way that you could bring your manual to a local Kinko's (or FedEx Office, etc.) and have them make a copy of the whole thing? It doesn't have to be bound, just legible copies on white paper. I'll happily pay for the cost of copying + postage + something extra for your effort.

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



All ready done Phil. Will go out tomorrow, no charge.

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 Post subject: Re: Upgrading Heathkit FM-3 Tuner to FM-3A?
PostPosted: Feb Wed 07, 2018 3:08 am 
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Hey, thanks. I will put it to good use!

Phil Nelson


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 Post subject: Re: Upgrading Heathkit FM-3 Tuner to FM-3A?
PostPosted: Feb Wed 07, 2018 7:02 am 
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philsoldradios wrote:
Hey, thanks. I will put it to good use!

Phil Nelson




Your welcome Phil.
I have the same plan for my FM3. Works great, but has some drift. After a good warm-up its not so bad.
Please post all your work :wink:

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 Post subject: Re: Upgrading Heathkit FM-3 Tuner to FM-3A?
PostPosted: Feb Fri 09, 2018 1:40 am 
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john8750 wrote:
philsoldradios wrote:
Hey, thanks. I will put it to good use!

Phil Nelson



I have the same plan for my FM3. Works great, but has some drift. After a good warm-up its not so bad.
Please post all your work :wink:


All FM3/FM3-A's have some thermal drift, it's a (free) built-in feature of the design ! To correct this issue you'll need to add AFC which is an easy mod if you read and follow the article in the link I posted above (see my post #2).

Here's it again:

http://americanradiohistory.com/Archive ... 957-01.pdf


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 Post subject: Re: Upgrading Heathkit FM-3 Tuner to FM-3A?
PostPosted: Feb Fri 09, 2018 3:27 am 
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Tubologic wrote:
john8750 wrote:
philsoldradios wrote:
Hey, thanks. I will put it to good use!

Phil Nelson



I have the same plan for my FM3. Works great, but has some drift. After a good warm-up its not so bad.
Please post all your work :wink:


All FM3/FM3-A's have some thermal drift, it's a (free) built-in feature of the design ! To correct this issue you'll need to add AFC which is an easy mod if you read and follow the article in the link I posted above (see my post #2).

Here's it again:

http://americanradiohistory.com/Archive ... 957-01.pdf



Thanks for that, I am very interested and will check it out. I do want to thank Heathkit for that 'free' feature. Right now I have the receiver just above my work bench so I can reach up and give it a little twist. It connects to an old, tube of course, Philco, then to a Zenith console. I replaced the speakers, sounds awesome, even though just monaural from the FM3.

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 Post subject: Re: Upgrading Heathkit FM-3 Tuner to FM-3A?
PostPosted: Feb Fri 09, 2018 3:47 am 
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Very cool. That mod is about the FM3a difference, plus a little filter upgrade.
Check that out Phil. Its just what we are wanting.
Thanks Tubologic...

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 Post subject: Re: Upgrading Heathkit FM-3 Tuner to FM-3A?
PostPosted: Feb Fri 09, 2018 1:21 pm 
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Could the drift be caused by ceramic disc caps with the wrong tempco?

If so perhaps replacing them with the right tempco would fix the drift.

AFC is indeed a good thing to add though.

I added AFC to my Magnavox CR-192 FM tuner and it was fairly easy to do and works good.


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 Post subject: Re: Upgrading Heathkit FM-3 Tuner to FM-3A?
PostPosted: Feb Fri 09, 2018 5:37 pm 
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Tube Radio wrote:
Could the drift be caused by ceramic disc caps with the wrong tempco?

If so perhaps replacing them with the right tempco would fix the drift.

AFC is indeed a good thing to add though.

I added AFC to my Magnavox CR-192 FM tuner and it was fairly easy to do and works good.



I am looking at getting an FM4. I believe its an improvement over the FM3/FM3a :?:

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 Post subject: Re: Upgrading Heathkit FM-3 Tuner to FM-3A?
PostPosted: Feb Fri 09, 2018 10:20 pm 
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I did win the auction for an FM4.
https://www.ebay.com/itm/Tube-Receiver- ... 1438.l2649

Another FM4 is coming up
https://www.ebay.com/itm/Vintage-Heathk ... 1438.l2649

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