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Harbach Keying Mod Analysis
Michael S. Higgins (K6AER)
on
January 16, 2010
View comments about this article!
Many of the hams have had to build and install the Harbach low voltage keying mod on SB-200, 201's and SB-220,221's must have noticed the instructions do not contain a schematic. The circuit board has is missing a trace and the driver transistor collector must be hardwired to the gate of the IRF610. If you do not have design experience this can lead to a bit of confusion. The basic operation of the keying mod simple and limits the keying voltage to the transceiver key output.
I drew the schematic of the board and it is presented below.
The amplifier keying circuit functions in the following way. The power input from the 110 VDC power on the heath amplifier is grounded via the T/R relay and bias zener to change the bias on the Cathodes and engages the T/R relay. This key voltage is typically over 125 VDC in some amplifiers. This circuit has been around for ages and is very common on glass tube grounded grid amplifier. Older transceivers can handle these key voltages but new transceivers made after about 1995 need an interface either between the amplifier and the radio or installed in the amplifier itself.
The keying mod PCB is connected to the amplifier bias power supply and the key side of the T/R relay. The power supply biases Q1 on via R1 and the voltage is held by the voltage drop through the base of Q1 and D1. This holds the voltage at 1.7 VDC. The key current is about 1 mA. When the mod key line is grounded, Q1 is unbiased and the voltage from the voltage divider, R3 and R2 in no longer grounded and the gate of Q2 is biased on and the MOSFET conducts pulling the key voltage from the T/R relay to ground. C1 and C2 are for RF bypassing.
That is it. This keying mod can be used for any number of older amplifiers such as the Heathkits, Kenwood TL-922, L4,L7 and others from the 50's-80's. I have had quite a few hams in the area ask me about the keying mod and my guess others out there might be confused as well. I hope this add some clarity for I see postings about amplifier keying every month.
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Harbach Keying Mod Analysis
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by OK1RR on January 16, 2010
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No schematic at all. Tried both Opera and Firefox, my browser is not the culprit...
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Harbach Keying Mod Analysis
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by OK1RR on January 16, 2010
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It is a bad format! The .emf file is definitely not suitable, use .jpg, .png or .gif! Extracted the .emf and converted to .png, now I see what you talking about.
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RE: Harbach Keying Mod Analysis
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by K9FV on January 16, 2010
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As previous posted says - no image in Firefox. Doesn't even show up as an image, but I was able to do a "save" then open and view.
A very well drawn schematic and thanks for posting it - it is info sorely needed by many.
73 de Ken H>
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Harbach Keying Mod Analysis
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by K0FF on January 16, 2010
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This kind of information is very useful, thanks for going to the trouble. The schematic shows up fine here by the way. Geo>K0FF
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Harbach Keying Mod Analysis
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by KG4RUL on January 16, 2010
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Here is a PDF of the eHam screen with the schematic:
http://www.kg4rul.info/KeyerSchematic.pdf
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Harbach Keying Mod Analysis
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by K5END on January 16, 2010
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Image good in Internet Explorer. (Not visible in Firefox, for both Ubuntu 8 and Windows XP.)
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Harbach Keying Mod Analysis
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by KJ6BSO on January 16, 2010
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I extracted the file using Adobe Illustrator and saved it as a JPEG file. E-mail me at <mycallsign>@arrl.net and I'll send you a copy.
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Harbach Keying Mod Analysis
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by K7LJ on January 16, 2010
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Has something changed? I have two - SB-220s, and an SB-200, all with the Harbach soft-key circuit board installed. I've also installed the Kenwood TL-922 version, and have one to yet install in a Drake L-4B.
I've installed these Harbach Soft Key kits in many friends' SB-200/220/221 amplifiers, too. I looked through my files, and the instruction sheets I kept for them all have a schematic and detailed parts list on the back of the one page sheet.
Has the circuit board changed, and the instructions no longer have a schematic on them? That seems strange...
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RE: Harbach Keying Mod Analysis
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by QRZDXR2 on January 16, 2010
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While harbach makes a good keying mod it contains too many parts. We have gone over to a much more stout keying module from jackson harbor press called the Key All.
One small board does it all. has optic isolation as well as better transit handling.
Only requires a small button battery (3volt) and your in business for years to come. Easy to tie into the existing relay with just two wires as you would a ground switch. (we plug the output of the keyall into the regular keying jack on the amp. The radio then plugs into the Keyall's small box... and your done.
We made small boxes and mounted them on the back of the amps we have. now all we do is hook up the radio with a patch cord like normal. I doubt that we will ever go back to tubes or TR relays so the mod is fixed as part of the amp now.
Simple and less compex than the harbach mod so we found. Cheaper too.
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Harbach Keying Mod Analysis
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by AD3G on January 16, 2010
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I can see the schematic fine in Firefox...no problems here
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RE: Harbach Keying Mod Analysis
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by K0UA on January 16, 2010
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I can see the schematic fine in Firefox...no problems here either
K0UA
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Harbach Keying Mod Analysis
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by SWMAN on January 16, 2010
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Great info Mike. It came in perfect on my little Blackberry Curve. No problem. 73. Jim
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RE: Harbach Keying Mod Analysis
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by KG4TKC on January 16, 2010
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Thanks K6AER,nice article,well done for sure.
Strange that some cannot see the schematic,it shows up great on a double boot xp/Ubuntu box. Right clicking properties shows it is a gif file. Firefox renders it perfectly in Ubuntu Karmic Koala,or 9.10.
Perhaps it is a personal settings issue?
73,es GL,KG4TKC
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RE: Harbach Keying Mod Analysis
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by WA1RNE on January 16, 2010
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Just a note of caution:
The circuit configuration shown in the schematic will operate only with the SB-220 positive bias configuration.
- It will not operate correctly with an SB-200/201.
The SB-200 uses a negative bias supply, approx. -120 vdc for use with 572B's.
About 20 years ago, I designed a keying circuit around a MOSFET for my SB-200 and know this to be the case, so Harbach must be providing 2 configurations of the circuit depending on the amplifier. A quick check of the download section of their web site confirms this.
So if you have an SB-200/201 you must reference the schematic for a negative 120 volt bias supply configuration.
...WA1RNE
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RE: Harbach Keying Mod Analysis
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by W9AC on January 17, 2010
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> "While harbach makes a good keying mod it contains too many parts. We have gone over to a much more stout keying module from jackson harbor press called the Key All."
First, another excellent write-up by K6AER...
The Jackson Harbor Press "Keyall" is a favorite of mine as well. It contains just four parts, offers 100% optical isolation, a MOSFET pair is polarity independent, the output requires no ground reference, and it's rated at 500V AC or DC at 2.5A. Total build time on the bench is under five minutes.
As good as the Keyall is, it comes with no specific instructions for many of the popular amplifiers. Showing how to connect to say...the most common Heathkits, Drake, Dentron, and Collins amps would probably help Chuck sell more of his kits.
Many ops are caught in that area between no soldering/electronics knowledge and at the other end, fully understanding how a circuit works and have no issues adapting a circuit to fit their needs. For those in between, Harbach's instructions make it easy to just follow along without having to actually think about how the circuit is working.
Paul, W9AC
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RE: Harbach Keying Mod Analysis
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by K6AER on January 18, 2010
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WA1RNE,
Thanks for the notice about the SB-200 series amplifier and their negative keying. This is true.
There is a correction to the Schematic.
R3 100K should connect between the Gate of Q2 and the Cathode of D3. In the schematic I have it going to the anode side of D3 which is incorrect.
I should not do Schematics late at night.
73, Mike
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RE: Harbach Keying Mod Analysis
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by KC7OTG on January 26, 2010
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Seems like you could replace D2 with a wire and remove D3 after you hook R3 to the same node where D3's cat currently is... But I'd still want to opt for an optically isolated version if I were to make something...
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RE: Harbach Keying Mod Analysis
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by K4TLJ on January 31, 2010
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Here is the schematic of my modified SB-200. Note the use of a FOD852 optoisolator for keying. To use the isolator for keying amps with positive 110 volts just reverse leads three and four. Cheap and simple.
http://home.comcast.net/~suptjud/Heathkitsb_200wdenhanced.jpg
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