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Heathkit SB-200 purchase

This last Saturday morning a post was sent out on the NLRS reflector alerting members to a moving sale. Listed with other items was a Heathkit SB-200. This was the kind of amplifier I was looking for as long as it wasn’t too abused or damaged. As it turned out, it was in fine shape actually, just a bit dusty inside, but nothing excessive. Quite humorously, all of my used HF gear has come via NLRS, which is a VHF and above weak signal club.

SB-200 front


The unit still had the original Cetron 572B tubes in it, which made it a good find indeed.

SB-200 top


I contacted the previous owner who informed me that only the supply capacitors had been changed while he owned it. As the picture above shows, this is a clean unit and did not show signs of excessive use. The Ham I purchased the unit from said it had not been on the air for probably 6 years, so I was a bit hesitant to plug it in a fire it up, but if it needed repairs it would certainly show it.

Quite fortunately, the unit had no issues when it was switched on. Further, it ended-up producing 300+ watts with about 50 watts in from my Icom 765.

There are only 2 noticeable improvements needed up front; the original fan is understandably not very quiet, and both of the doorknob caps probably need to be changed as the tuning ranges on the front panel are pretty far off from what is silk screened. I have to investigate the latter issue a bit as this may not be too far from typical experience for this model.

If/when I do some refurbishing, I’ll do some posts on that work.



Transient Voltage Suppressors (TVS)

These devices can be thought of as high-current capable zener diodes. A TVS will be rated in terms of it’s reverse breakdown voltage which in practice is the maximum voltage you want to allow before a protection device, like a fuse, kicks-in. You place a TVS shunted across a DC voltage rail with the cathode on the positive voltage rail and the anode connected to the power supply common, or the reverse of this for negative voltage rails. The protection device will normally be in-line (in series) to remove power from the circuit being protected.

ON Semiconductor makes a series called the MiniMOSORB line that has impressive current handling capabilities. For example, the SA12A part is capable of limiting the voltage to about 14V and can withstand about 25A of current. So for example, a mobile device powered by 12-13.8 volts can be protected from overvoltage or reversing the supply leads (its still a diode) even though it may be protected by a 15A fuse; i.e. the TVS will present a short and ensure that the fuse is blown. You can find TVS diodes for sale at many supplier web sites and on eBay where sellers like myself have them for sale.

In effect, if your circuits cannot tolerate a higher than normal voltage spike that is transient or permanent, caused by a voltage regulation fault for example, then a TVS is a great way to protect the circuit from an overvoltage condition.


Testing Unknown RF Transformers

Sometimes you have a shielded transformer in your scrap box, or have acquired some from a surplus distributor, and you’re not quite sure what you have. Even with a marked, currently stocked part, you may want to more closely examine its frequency characteristics or in-circuit Q factor.

There is a very basic way of determining what the intended frequency and passband characteristics were intended to be. For this you will need an oscilloscope and a signal source with a reliably close manual indicator, or a digital frequency display. Ideally the signal source should extend up to 10 or 20 MHz. The idea is to manually dial the frequency adjustment of the generator while driving a sine wave into the primary of the transformer while observing the output across the secondary using the scope display.

The transformers commonly found are going to be adjustable slug tuned or fixed, and will have 4-6 connections. Quite commonly, the primary will be a single, un-tapped winding, with the secondary being center-tapped. In the case of balanced modulator or mixer transformers, this may be immaterial because the two windings can or will serve either role. It’s really the application of the transformer that matters if the turns ratio is something other than 1:1, or a center tap is not required, etc.

You may want to check what you think is a primary or secondary with an ohmmeter just to be sure, but if the piece is really a single adjustable coil, you’ll soon discover it.

Starting at 100 kHz, you increase the frequency of your generator while looking for a peaking effect in the display of the sine wave as seen by the scope across the secondary. If the transformer is broadband, is will be a lower Q transformer that will peak, but very slowly, then decrease in magnitude as you roll pass the center of its passband. If the unit is high Q, and the turns ratio is more than 1:1, you will need to slow your frequency tuning a bit as the peak will be much more sudden as the transformer hits its point of resonance. Note this frequency. If the transformer is slug tuned, try moving the slug up or down and re-check the frequency; you should now see the shift in the passband as a result of this adjustment. Of course if the transformer is not slug tuned, your application circuit may contain a variable capacitor for fine adjustments anyway, or is broadband enough that the isolation across the transformer is all you needed.

You can also make careful measurements to determine the turns ratio of the transformer.

At HF frequencies, it is reasonable to observe the primary and secondary peak to peak voltage readings on your scope and draw a simple conclusion. If the primary P-P voltage is say 1 volt and the secondary is about 4 volts, you have stepped-up the voltage by a factor of 4, so you have a 1:4 ratio. Likewise, if the P-P voltages are about the same, you can conclude there is a 1:1 ratio. Again, the fixed transformers can be tested in the same manner. Obviously if you have some know parts from Mini-Circuits, or your testing a bi-filar wound toroid for example, then the turns ratio will be known.

Testing for Q factor takes a bit more patience and better equipment really; i.e. a scope with cursors that can help you easily find the 3dB point on either side of your transformer. Of course this is not a demanding requirement for a new or used oscilloscope today.

Follow the link for a refresher, but to calculate the Q of the transformer, we are going to need to measure its 3dB bandwidth in addition to the center or resonant frequency measurement we made above. The Q then can be calculated by dividing the 1/2 power or 3 dB bandwidth number into the resonant frequency. In this case we are going to use the (0.707 x the peak voltage) method to determine our 3 dB point in terms of voltage. If you use a convenient secondary output P-P voltage value of 1, then you are looking for the points where the voltage comes down to .707 volts P-P.

For example, if you find your transformer peaks at 9 MHz and your 3 dB point spanned 600 kHz, you would calculate a Q of 15.


Hamvention 2013

The misty view of the Flea Market early Friday Morning.


Hamvention 2013 was again a great place to meet people and find some bargains. There was a Amp Hour meetup that I could not attend, but I did run into Chris Gammell there and we had time to chat in the market. This year I found some nice used SMA jumper cables out in the flea market, and bought some great parts inside in the expo area. I also attended FDIM, and enjoyed that very much; so much I decided to return to FDIM and Hamvention in 2014.


Easy toner transfer – after preheating the board

I posted this on the Yahoo QRP-tech group as well.

Regardless of toner or transfer medium, i.e. paper or plastic film, here’s a simple way to success.

Since I already had a $29 B&D Toaster Oven for solder paste re-flow, I decided to preheat the board and then run the board and transfer through the laminator.

1 – Preheat the oven to ~300F/150C.

2 – Turn on the laminator.

3 – If using plastic film, clear or Press-n-Peel, have a piece of cover paper ready. I just cut a sheet of printer paper to mostly cover the board and then folded it about 30% from one end, but the remaining 70% or so should completely cover the plastic transfer film.

4 – I then carefully laid down the transfer onto the board, and used the cover paper to rub over and flatten the film.

5 – While holding the board with pliers, align cover sheet with 70% on top and 30% underneath board and insert into laminator.

6 – One pass through the laminator to flatten under pressure is all that is required. Run board under cold water and peel off film.

The cover sheet is just used for the same reason it is when doing badges, which is where the idea came from. It just adds a margin of protection for the laminator and seems to make feeding the board seem easier.

The result was 100% coverage for both the paper and film transfer trials. Obviously (duh..) the higher board temperature at the start was key to getting the toner stuck down well.

Your mileage will vary, but this was neither cumbersome nor time consuming; especially using plastic film since there is no wash step. You are ready to etch!


Circuit board toner transfer with Press-n-Peel

I received a 4 sheet order of Press-n-Peel to try out this weekend. I first discovered this product via one of Fran Blanche’s videos on her PCB process. At the same time, a post on the QRP-Tech Yahoo Group alerted us to a rock-bottom price at Amazon for the GBC BadgeMates badge laminator. For a mere $10.06 and a Prime membership, you get a laminator capable of being used in the Toner transfer process. Unfortunately, I had to revert to a clothes iron as the GBC unit was not quite hot enough to use for Press-n-Peel and the $29 toner I am using in my HP LaserJet 2200. Well, the toner works well with paper…

Below is a close-up of the 3rd test transfer. Very crisp all in all which should produce a fine etch result. I have many more experiments to run before I have a process, but so far the results are promising.

Fine details of transferred image

Fine details of transferred image


Why are different types of capacitors used in various circuits?

Electronic experimenters often wonder why specific types of non-polarized capacitors are specified or used in different circuit types and whether substituting one type for another will make any difference.

The short answer is yes; a specific type can be the most attractive choice in an application because its characteristics fit the type of circuit it’s being used in. Cost aside, there is no best overall type of capacitor to use in all applications.

The longer answer is that a particular type of capacitor may best suit the most important characteristic that circuit requires of that capacitor. For example, in LC oscillator circuits, frequency stability is typically an important attribute for the designer. For a similar situation in which the LC circuit is being used in a filter section, the Q factor may be the most important feature for the circuit designer.

Along with cost, the long-term stability, temperature range stability (coefficient), Q factor, voltage coefficient, and value tolerance attributes of each type of capacitor technology varies considerably. Consulting manufacturer data sheets can help you choose which type or types are the best fit for you application. Below are some common types and why one might choose them.

Ceramic (NPO) – Best temperature stability, tight tolerances

Silver Mica – High Q/ high voltage range stability

Polystyrene – High insulation/Low leakage

Polypropylene Film – low self inductance and high tolerance

Polycarbonate – High dielectric strength (breakdown voltage)

Mylar Film – Low cost

These are of course just some of the reasons you might choose one of the above types, but availability and cost are often very important qualities to consider in any choice you make.


The Softrock signal generator project, part 2

After receiving the BN-43-2402 binocular cores from Kits and Parts, I was able to add the T1 and T2 bifiler wound transformers.  I then chose to do the Band 3 inductors L10-L12 since they required the least effort to wind and install in order to test the end-to-end signal path. Lastly, I added a 51Ω 1/4 watt resistor between the junction of pins 7 and 9 on U9, and pin 9 on U6 – the only significant change to the stock design of the RX II in order for the divided output of the Si570 to flow into the ABPF section in reverse. In other words, having the signal flow out of the Ensemble RX II instead of flowing into it as a receiver.

The results were as good as I expected. From 16 MHz to 40 MHz, I measured a roughly -3db response out up to zero db and then back down again to -3db in the RMS output that peaked around 4.7 volts. The sine output appears clean on the scope, but I’d like to get it measured on a Spectrum Analyzer if I can hook up with another local Ham that has one. Lastly, I did the A and B calibrations using the CFGSR tool and was easily able to set the output to agree with my frequency counter to 5 digits after the decimal point, or 10 Hz resolution.

Next I’ll be building out the rest of the ABPF band sections to see the full response from 3 MHz up.


the softrock signal generator project, part 1

The Softrock Ensemble RX II is a good base for hacking it into a signal generator. Essentially if you build it out omitting the I/Q sampling and conversion circuit, the audio output, and flow the Si570 output into the Automatic Band Pass Filter sections, you end-up with a nice PC controlled (using CFGSR) signal generator.

In this post I’ll cover the basic starting points and progress into more detail in later posts.

Since I had nearly all of the parts to build a second RX II, I ordered a bare board and Si570 from Tony Parks, KB9YIG. I also ordered a few items from Digi-Key and Mouser that are particular to the way Tony put this design together. For example, the PCB mounted BNC, USB-B, and power connectors are each a little special in that the matching circuit board mates to specific part numbers. Of course if you simply purchase an RX II kit, you have everything you need.

The Si570 used in the Softrock kits is a grade-C part, which means it has an upper frequency limit of 280 MHz, or in the case of the CMOS output part used in the Softrocks, 160 MHz. Still, you easily get frequency coverage from 3.004 to 160 MHz if you follow the HF build instructions. Other grades of Si570 are supported in PE0FKO’s firmware, so it’s possible to go still higher; at least until the board layout begins to be a problem in the GHz range.

I also plan to use a Mini-Circuits ADE-1 to provide for modulation of the output. Once again, obviously if you just build-out an RXTX Ensemble, minus the final output, you can get most of the same functionality. However, the RX is nice because it has the ABPF switching.

One note on programming the ATTiny85, I have an Atmel STK500 development board and Atmel Studio 6.0. The ATTiny85’s come from the factory with default fuse bit settings that will need to be set to 0x5D high, and 0xE1 low bits in order for the chip to work correctly in a Softrock, therefore you must either reset the fuse bits before programming the EEPROM or use HVSP to program both the fuse bits and the EEPROM – an ISP alone is not enough.



I had an opportunity to pick-up a well cared for IC-765 from the estate of a SK. This is my first commercial transceiver purchase and it made a lot of sense for me as a starter (100 W) HF rig. I have to say that the quad-conversion super het design is as good as the reviews I read and it is a simple rig that just works as others have written. Since I already had a 20 m End-fed Zepp, I got on 20 yesterday and immediately made and logged two contacts; 1 in Ontario and 1 in AZ.

This experience certainly put QRP into perspective for me as making similar contacts with 10 watts SSB would have been much more difficult or impossible during daylight hours. This certainly brought home the idea that success in QRP is about the conditions, location and the antenna you are using; and using CW or PSK31 modes. Challenges make a hobby lasting though.

The very next project is to get my planned antenna work done before it gets dark and cold here in MN.