Oak Hills Research OHR-100A QRP Radio Review

Front panel of the Oak Hills Research 100A QRP radio kit after assembly. The controls are RF and AF gain, RIT, Bandwidth, and main tuning.

Front panel of the Oak Hills Research 100A QRP radio kit after assembly. The controls are RF and AF gain, RIT, Bandwidth, and main tuning.

My Christmas present in 2014 was an Oak Hills Research OHR 100A 20-meter QRP radio kit. I’ve played around with QRP in the past and found it a fun way to operate. QRP is the Q-signal for “low power,” by convention meaning less than five watts output, as opposed to QRO, generally taken to mean higher power of at least 100 watts. QRP generally means Morse code (CW) because CW gets through where voice modes, such as single side-band, do not. Some people think that QRP is the cat’s meow, and other people prefer a lion’s roar! For me, five watts on 20 meters is enough to have solid and satisfying CW QSOs.

The OHR 100A comes as a kit and is available from www.ohr.com, a subsidiary of Milestone Technologies. As of this writing the kit is $179.95. A ten-turn potentiometer that acts as a bandspread is available for $17.95, and a built-in keyer is available for $29.95. Although I ordered both the ten turn potentiometer and the keyer, right now I’m operating the rig “stock.” I will install both of these options in due time; in the meantime I’m getting reacquainted with my straight key.

Rear panel of the Oak Hills Research 100A. The keyer option hasn't been installed yet, so those holes are vacant. Note the unusual key connector: an RCA jack! The radio has separate connections for headphones and a speaker; attaching headphones will mute the speaker.

Rear panel of the Oak Hills Research 100A. The keyer option hasn’t been installed yet, so those holes are vacant. Note the unusual key connector: an RCA jack! The radio has separate connections for headphones and a speaker; attaching headphones will mute the speaker.

The radio’s design is about fifteen or twenty years old. It was reviewed briefly in the December, 2000, issue of QST, and was available for several years before that. QRP radio kits really haven’t changed much in that time, and the “maturity” of the kit means that the bugs have been worked out.

The radio comes with a nice case. I must say I admire the way the chassis is constructed—it has stand-offs already mounted to accept the circuit board. The front panel letters and numbers are easy to read.

Building the kit. The kit comes with a printed and drilled circuit board that’s ready to accept parts. No parts are pre-mounted. There are a few integrated circuits; each of these goes into a socket that’s soldered to the circuit board. There are quite a number of toroid coils that need to be wound by hand. Apparently some people must object to this, because (for a fee) you can get the coils pre-wound. Personally, I have no problem winding toroids, and the instructions for doing so are clear and easy to follow.

A closeup of the circuit board for the OHR 100A. D107 is a small 1N34 diode, which I mismounted the first time around. My repair wasn't terribly neat, but it works. Note that the ICs are all in sockets; there are no surface-mount components.

A closeup of the circuit board for the OHR 100A. D107 is a small 1N34 diode, which I mismounted the first time around. My repair wasn’t terribly neat, but it works. Note that the ICs are all in sockets; there are no surface-mount components.

The instruction manual is not as detailed as a Heathkit manual. For example, one step is to solder in all the resistors. All of them in one step! I would say that the kit is at an intermediate level and not a good starter kit for a beginner. You will need a magnifying glass to identify the different components. I started out by mounting each component on a small card, noting the component value and the circuit diagram number (for example, C114). I had trouble sorting some of the part and ended up doing some detective work to separate them out. Attaching each part to a card with Scotch tape turned out to be a great idea and hugely facilitated finding the right parts to stuff onto the board. I did mount one diode backwards. Fortunately I caught this while inspecting the board carefully. I’m not very good at desoldering, but was finally able to get it out and turn it the right way. Moral of the story: be careful, take your time, and check your work frequently. I found myself wishing the manual were a little more explicit about where to put things. For example, it would say “install C125 and 126” without showing a board diagram that indicated where they went. That meant I spent lots of time hunting for the right spot on the board to put the parts.

Fortunately, when it was finally time to apply power, the radio actually worked! No smoke, no overheating, no fuse-blowing. I attribute this to being exceptionally careful while building it. I also attribute it to sheer dumb luck.

The circuit board of the OHR 100A, showing two of the transmitter toroids, one of which is bifilar (two turns on the same core).  C116 (yellow) is one of the trimmer capacitors that must be adjusted.

The circuit board of the OHR 100A, showing two of the transmitter toroids, one of which is bifilar (two turns on the same core). C116 (yellow) is one of the trimmer capacitors that must be adjusted.

Aligning the radio. One aspect of building a radio is that you have to align it. Aligning means getting all the inductor, capacitor, and resistor values just right so that the receiver works with maximum efficiency and the transmitter is set up to send out the right signal without distortion. The instructions for alignment turned out to be easy to follow. I used my MFJ-259B antenna tuner as a frequency counter. I won’t say that the MFJ-259B is that accurate a frequency counter, given that counting frequency is not the MFJ-259B’s primary purpose, for but alignment it worked just fine. During the alignment, there are three test points where you need to measure a voltage. What the manual does not say is that you will be measuring DC voltage at these points. I assumed this and did fine. I point out that the manual specifies using a digital voltmeter (DVM). This is because a good DVM can measure voltages without loading down the circuit. This is NOT true of a cheap VOM. You can tell the difference: a DVM has a digital readout, and the VOM has an analog meter. (Please note this is true of today’s equipment. If you’re using test equipment that was new decades ago, it’s a different story. Back then the DVM equivalent was called a vacuum-tube volt meter, or VTVM.) In any event, the rig aligned up well.

OHR 100A circuit board, showing two more trimmer capacitors that must be tweaked for best receiver performance. I had best results using a ceramic adjustment tool rather than a metal screwdriver. Note the toroids. The toroids were not hard to wind.

OHR 100A circuit board, showing two more trimmer capacitors that must be tweaked for best receiver performance. I had best results using a ceramic adjustment tool rather than a metal screwdriver. Note the toroids. The toroids were not hard to wind.

I do want to mention that the instructions say to use a small screwdriver (such as a jeweler’s screwdriver) to do the alignment for the trimmer resistors and capacitors. That’s fine for the resistors, but even putting a metal screwdriver near a trimmer capacitor will affect it. So you have to tweak, pull out the screwdriver and listen to the result, and repeat this several times. I did this as best I could and the radio works just fine. But what you really need for this job is a non-metallic screwdriver so you can tweak without the screwdriver’s metal affecting the results. I contacted Marshall Emm, N1FN, at Milestone Technologies, which owns OHR; he suggested their part # PK034D ceramic screwdriver. I purchased one and then set about to re-align the radio. I found that with the ceramic screwdriver I was able to align it more precisely. I would recommend that you use such a tool when doing the alignment.

The first step in aligning is to spread or bunch up the windings on a toroid to get the oscillator output precisely on frequency. The setting was very sensitive to the slightest movement. You’re then instructed to put some fingernail polish or some such on the windings to hold them in place. The manual suggests letting whatever you put on the windings dry for 24 hours. I did, though I’ll admit it took some willpower because I wanted to get on with it!

The transceiver uses a sidetone generator so that you can hear what you’re transmitting. The frequency of the sidetone is independent of what frequency you might be transmitting on. I set the sidetone to 700 Hz. I then followed the instructions to align the OHR 100A’s transmit and receive frequency so that if I match the tone of an incoming signal to the 700 Hz sidetone, I would zero-beat the other signal; that is, the ham on the other end hears you transmitting exactly on his frequency. You will need your “main” transceiver to do this; in my case I used my TenTec Jupiter. Both the sidetone frequency and loudness are adjustable.

The VFO frequency is determined by a potentiometer rather than a variable capacitor. I plan to replace this with the OHR-provided ten-turn potentiometer.

The VFO frequency is determined by a potentiometer rather than a variable capacitor. I plan to replace this with the OHR-provided ten-turn potentiometer.

On the Air. The unit exhibits some warmup drift of a few hundred hertz for the first ten minutes or so. I saw this complaint in other online reviews. And Marshall Emm said that most of the OHR 100s exhibited this drift. So the point is that you turn it on first thing, then arrange the rest of your paraphernalia (key, logbook, notebook, etc.) to give it time to settle down. Once warmed up, the VFO is rock-solid.

I really wish there was a way to “spot” other stations. That means using the sidetone frequency as a reference so I can tune other stations to exactly match. Unfortunately, the only way to do this is to transmit briefly (one dit) so you have a reference in mind while tuning the other station. Remember, if you are not transmitting on the other station’s frequency, the other station will hear you as a tone that’s quite different from what it’s transmitting. You don’t want to be too far off.

The rig uses an RCA jack to plug in the key. An RCA jack! I’ve never, ever seen a radio that uses an RCA jack for the key! At first my key line went through several adapter cables to be able to plug in properly. Yesterday I modified my key to use a simple jumper cable from the key to the radio. I do worry though—plugging and unplugging the key really tugs the connector and I’m worried that it will eventually cause the circuit board to crack.

Marshall Emm suggested I run the RF gain control about one o’clock and the AF gain control about ten o’clock. I tried this and it works fine. The radio has no automatic gain control (AGC), so if you have the RF gain wide open, a loud signal will have you yanking off your headphones in a hurry!

The variable bandwidth control is sometimes helpful, but the bandpass skirts are anything but steep. So a loud station, even though a couple kilohertz away from your station, will still come through even with the bandwidth tuned to its narrowest point. I just put it midway and leave it there. I’m used to the “brick wall” bandpass filter on my TenTec, so this rather loose bandpass filter takes some getting used to. It reminds me of using my old Heathkit HW-16 when I was a novice. Back then any bandpass filtering was done in your head, not in the radio!

The RIT (receiver incremental tuning) control allows you to tune the receiver without changing your transmitting frequency. It’s effective and allows tuning a couple kilohertz away on either side. Be sure it’s centered (there’s a center detent) when you start a QSO, then use it to follow another station’s drift without changing your transmitting frequency. Otherwise you and the other station will chase each other all over the band!

Unlike many other QRP kits, the VFO frequency is changed using a potentiometer instead of a variable capacitor. My MFJ-9040 QRP rig uses a variable capacitor with a reduction drive, so a little indicator tells me where I am on the band. However, the OHR 100A changes from 14.000 MHz to 14.080 MHz in about three-quarters of a revolution of the knob. That means barely breathing on the knob changes the frequency by a few hundred hertz. And the pot exhibits some stiction, so fine tuning is hard. I know that this will be fixed when I install the 10:1 potentiometer, but the potentiometer reduction drive has no indicator. So, since the radio tunes across 80 kHz, I will have to remember that each turn is 8 kHz and will have to count revolutions from a band edge.

The rig gives a pretty loud click when it’s turned on, so you may want to plug in your headphones after you switch it on. There is a separate speaker jack, and the audio output is sufficient to drive a 4-inch speaker quite loudly.

There’s a bit of a thump when keying, but not objectionable. The on-the-air reports I’ve received so far indicate solid keying with no chirp or clicks.

The rig is specified to run at 13.8 VDC. My station runs on a solar-charged battery, so in the evening the voltage is down closer to 12 VDC—the rig seems to tolerate this with no problem. My straight key is a Japanese JJ-38, which I acquired back when I was a Novice, and the antenna is a Butternut HF-9V vertical.

An aside about straight keys. While operating I found many hams using slow code and straight keys. I’d always thought that a straight key made a great shelf ornament, and real CW was done with an electronic keyer and iambic paddles, but apparently the straight key is making a comeback. Key to this is an organization called the Straight Key Century Club (SKCC), and those who belong exchange their SKCC numbers. I quickly joined—my SKCC number is 13205. The SKCC folks are found on 20 meters in the neighborhood of 14.050 MHz, and I’ve found they’re quite willing to QRS (send more slowly) for me. (I do note that SKCC considers “bugs” to be straight keys, but I think pretty much everyone I’ve worked uses a straight key. Straight keys have a certain “sound” that’s unique to every operator, something called the operator’s “fist.”)

Other wishes. I wish that the power connector was an Anderson PowerPole connector. But these became popular long after the kit was designed. I created a short pigtail connector that connects to the radio and has a PowerPole connector on the other end. I also wish very much that the rig had a pilot light just so I can tell when it’s turned on! I am ambivalent about the lack of an AGC. On the one hand, you really get a good feel for the band without it, but on the other hand when really loud signals come in, my hand goes for the RF gain control. Oh, and the biggest wish of all? That my rusty CW, once a solid 18 to 20 words per minute, will recover!

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14 Responses to Oak Hills Research OHR-100A QRP Radio Review

  1. Thanks for the tip Dave, there is! a bag of parts labelled 30mtr band pack ,i’ve heard that in some instances if you want to change a rigs freq it’s just a case of crystals for the band you want and a couple component changes

  2. Dave says:

    Gary, if you haven’t started yet, note that there is a bag of parts labeled 30m bandkit. Perhaps you can talk with OHR and exchange this for the 20m bandkit. 73, Dave

  3. I got the DD-1 and the 100A in an equipment swap ,so far i’ve built the DD1 but haven’t started the 100a the kits are very well built nice enclosures, my 100A is for 30mtrs i wonder how much work it would take besides crystals to convert it to 20mtrs?

  4. Dave says:

    Steve, I’m sorry to hear you’re having trouble. Frustrating! I suggest contacting http://www.ohr.com. Give them a call. I had some trouble with my kit after I wrote the review, and they were very responsive. Good luck! 73, Dave

  5. Steve says:

    I built this rig and the drift is much worse than what you describe. It doesn’t just happen in the first 10 minutes, but continues for 45 minutes or so during which time it’s basically impossible to have a QSO with anyone without constantly touching up the tuning.

  6. Dave says:

    Hi Richard, I got the keyer option but never built it. I’m still very much in a mode of rebuilding my CW skills, and all the slow code is related to the Straight Key Century Club, so I’ve been using my straight key. Even with my new Yaesu, I’m using my straight key. My current comfortable code speed is about 10 wpm, a lot less than the 20 wpm I needed to get my Extra “back in the day.” I figure the best way to raise my code speed is to simply get on the air. Good luck with your OHR 100A!

  7. Richard Hull says:

    Hi Dave,
    Nice review, I built mine about a year ago now in preparation of building a K2 but I found the OHR 100A so good that I left the K2 alone for a while. I have also built a frequency counter which does makes things easier. I’m so impressed I fancy getting the 3om kit as well, so I’ll have a family of OHR 100A’s!! Did you ever get the keyer option? I never I’m still using a straight key I’d be interested to know what its like? – 73’s Richard M0AUW

  8. Ruben says:

    will be great to have an schematic diagram with signal wave forms and levels at key points in the circuits… at Tx and Rx…………

  9. EA8BVP says:

    Thanks Dave. I’ll do that.

    72

  10. Dave says:

    Hi Bal, I had the thumping problem for awhile myself, but with further adjustment, it went away. I do recommend emailing Marshall—he is patient and helpful.

  11. EA8BVP says:

    Hi Dave,

    After rechecking all the transmitter chain components and reorganizing toroids turn I’m getting 1,86w at 12vdc and 3,8w at 13,8vdc. I can’t get more. I have an oscilloscope to make rf measurements but there is no voltage table or something like that in the manual in order to troubleshooting. I guess i’ll have to email Marshall.

    Another annoying problem I have is an audio thump every key lift. I think it must be related to power supply filtering. I can’t use the headphones more than 15minutes w/o having a headache.

    Thanks Dave. I’m looking forward for your second part!
    72 de EA8BVP, Bal

  12. Dave says:

    Bal, I don’t get a full five watts either, even with the power control turned up all the way. My MFJ-993 IntelliTuner measures 4.3 watts. It’s daytime, so my photovoltaic system is running about 13.5 volts. And the tuner isn’t really designed to measure power amounts that small. But I can still get QSOs. I really should update the post with “the rest of the story,” to discuss what happened after the rig failed and I had to send it back to get it fixed. Long story, but with a successful outcome. 73, Dave, KEØOG

  13. EA8BVP says:

    Hi Dave. Thanks for your review. It’s helpful.
    I have a question about power output. I’m getting
    2W at 12VDC and 2,6W at 13,8VDC. I can’t get 5W
    as manual says.
    How much are you getting?

    Thanks.

    72 de EA8BVP, Bal.

  14. Matt Nassau says:

    Many thanks Dave for the review of your OHR 100A. I am also building this kit from Marshall Emm, and it’s also for the 20m band. Thanks for posting your experiences and photos. It’s a bit reassuring that my built-up board and my own hand-made toroids look like yours!

    Just a note, I’ve also built the OHR DD-1 digital display/frequency counter that I intend to use with the 100A once completed. Both kits are first class products in my opinion, with great customer care from Marshall.

    I hope to work you on the air some time, though I am still learning CW from the ground up.

    73
    Matt
    2E0MTT, UK.

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