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Heathkit SB401 AM conversion




 
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Author Topic: Heathkit SB401 AM conversion  (Read 513 times)
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K8DI
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« on: January 10, 2019, 11:57:00 AM »

So, besides trying my hand at building an AM transmitter, I have in my shack a Heath SB301/401 pair. I bought them off eBay around 1993; shortly after I gave them a full alignment, ten years ago I replaced a couple capacitors in the receiver. They have spent a LOT of time just sitting, as time went on and I had no place to put an antenna and other things taking my time... 

Fast forward to now:  I haven't powered the transmitter up in five or more years, but I use the receiver nearly daily. It provides a good audio monitor of what my main radio is doing, with a 18" length of 12g wire stuck in the antenna socket as a probe/antenna.  I know at this point I have to approach the 401 as an unknown; check capacitors, alignment, etc. An internet search shows that a very few people have modded the 401 to be an AM transmitter, but details were lacking. Has anyone here done this or have any documentation on doing this? If it's something reasonably simple/doesn't cut up the radio too much, I'm tempted to try it out just to see how it works. And because I like tubes. For those familiar, the 401 has the full crystal set/can operate standalone.

Ed/KB8TWH


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Ed, K8DI (fmr. KB8TWH)
N1BCG
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« Reply #1 on: January 10, 2019, 12:57:19 PM »

I restored a 301/401 combination and they're a great pair for SSB and CW, although for AM, you can expect a carrier level of about 25 watts based on its 100 watt PEP SSB rating. I have not heard of an AM mod for this rig but it will be interesting to hear how this project progresses!

Do you plan to use them coupled?
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w8khk
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This ham got his ticket the old fashioned way.


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« Reply #2 on: January 10, 2019, 01:07:15 PM »

There is an article about using the HW-101 and SB-102 on the AM Window here:

http://www.amwindow.org/tech/htm/hw100plusmods.htm

I have done this mod with the HW-100, and it works well.  

The architecture of the HW and SB rigs here is so similar, I would be very surprised if this mod did not work for you.

If you follow just the three steps in the first paragraph, you will be on the air, no mods that are not easily reversible.  The other mods give you audio improvements.

Your 301/401 pair look very nice.  I would crank up the 401; it probably does not need any major TLC....


Give it a try, let us know how it works for you.  You will probably find it makes a great driver for a linear amplifier as well.
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Rick / W8KHK  ex WB2HKX, WB4GNR
w3jn
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« Reply #3 on: January 12, 2019, 09:05:10 PM »

I converted one.  Basically you bypass the crystal filter, feed the audio directly to the balanced modulator, and unbalance the modulator for about 25 watts output.  By doing so you need to do outboard audio processing since the audio bandwidth can be REALLY wide.

I wired it such that it retained the SSB capabilties, and used the CW position to bypass the filter.  I might have done a writeup here at some point on what I did, it was so long ago I can't remember the details.

The downside of this was that the balanced modulator kept drifting and the RF output kept creeping up/down.  Were I to do it again I'd replace all of the resistors in the balanced modulator with 1%ers and replace the pot with something much better, and remote a fine adjust pot on the front panel since chasing the drift was a real PITA.
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WD5JKO
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WD5JKO


« Reply #4 on: January 13, 2019, 01:31:57 PM »

The downside of this was that the balanced modulator kept drifting and the RF output kept creeping up/down.  Were I to do it again I'd replace all of the resistors in the balanced modulator with 1%ers and replace the pot with something much better, and remote a fine adjust pot on the front panel since chasing the drift was a real PITA.

   I have seen this same drift running AM with my central electronics 20A. I have wondered for a long time if a closed loop feedback loop could be added to maintain a constant level of carrier. The loop bandwidth would have to be limited to something sub audible like a few hertz. One thing I did do that helped was to change the balanced modulator diodes to a matched quad of 1N5711's that are glued together for thermal stability. That helped a lot. Would any of this be applicable to the SB401?

Jim
Wd5JKO
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