Ham To Ham #18 - March 1997
73's Ham To Ham column
c/o Dave Miller, NZ9E
7462 Lawler Avenue
Niles, IL 60714-3108
This month's Ham To Ham has an interesting potpourri of ideas, so let's get right to them without any further introduction:
Stud Power
Mark Marholin KE6JJR of San Jose, CA offers this suggestion for easier stud-mounted power transistor handling: "If you're faced with having to replace a stud-mounted RF transistor in your commercially made rig, or perhaps you're building an add-on "brick" amplifier for your VHF or UHF low-power transceiver, here are some points to keep in mind before starting the job. I've hard-won these ideas over my own years of building RF amps in the 50 to 450 MHz. range.
Before mounting the ceramic-bodied, stud-mounted "final" transistor(s), try to form the flat copper leads (bands) on the transistor as closely as you possibly can to fit the board foils and the opening in the circuit board which the transistor body occupies. This may require first bending the flat leads up at an angle, then down somewhat again for the right "fit." It's much easier to do all of this pre-fitting ahead of time. Once the leads are pre-formed, trim off the excess to avoid unexpected additional inductances...especially in UHF circuitry.
Now pre-tin the undersides of the flat leads, so that when the transistor is finally in place, solder will flow smoothly and easily underneath the flat leads without the need to apply excessive amounts of heat (which might be bad for both the board and bad for the transistor itself).
A good quality thermal-conducting grease (silicon grease) should be applied to new transistors, and any mica washers or insulators that might be involved, but don't overdo it...too much thermal grease can actually hamper heat transfer, or worse yet, can deform the flanges on some devices when they're finally torqued down.
And speaking of that final torquing down of the nut on the transistor's mounting stud, try not to apply counter pressure by holding the device by its leads, they're not meant to stand that degree of strain. Even after soldering the leads in place, don't depend upon them to prevent the transistor from twisting during the stud-tightening procedure. Stud-mounted devices usually have a flattened end on their threaded studs, hold the stud in place by that flat, that's why it's included. I hope that some of these tips prove to be worthwhile for your next power transistor installation."
Free test signals
Phil Salas AD5X of Richardson, TX sent in the following nifty suggestion: "Those little MFJ SWR Analyzers also make great little portable signal generators for general testing purposes. In order to do their job as SWR analyzers, the units need to simulate a very low power CW transmitter, along with an equally sensitive SWR metering circuit. You can therefore use these little gems for general signal injecting or troubleshooting as well antenna and transmission line analysis.
In order to achieve a variable RF signal output, I've used a Radio Shack (reg. trade mark) #15-578 variable 75-ohm TV attenuator, along with the appropriate adapters, to reduce the signal level to whatever I've needed for the particular troubleshooting job at hand. I also decided to open up the Radio Shack attenuator's case and replace the F-connectors with BNC connectors. Just another twist to make the attenuator even handier. And speaking of Radio Shack, I've found that their #21-506, 50-ohm dummy load, can be expected to work well up to about 500 MHz. It will handle 10 watts continuously, or even 100 watts for a few seconds at a time. Iíve put 100 watts into mine a number of times, getting it so hot that I couldnít touch it, and it doesn't seem to have damaged or degraded the unit's SWR in the least. Try not to overdo it though!"
Nix on hard-to-find variable caps
John Nix of Foley, MN offers this suggestion for helping you to get that antenna tuner construction project off the ground: "If you find yourself in need of a fairly large, fairly expensive variable capacitor for that low frequency antenna tuner project you've been putting off, here's an idea you might want to consider adapting to your own requirements.
Just as an example, let's assume you're looking for a 1000+pfd. variable cap...not the easiest thing to come by...but don't let that stop you. Variable caps in the 365 to 420pfd. range are fairly common, so all you really need to do is to switch-in additional 350pfd. (or so) good quality (meaning stable) fixed capacitors, ones that will put you in the right range, until you have the value over the maximum value of your variable cap on hand. In other words, if you need a maximum of 700pfd., use a 365pfd. variable and a 350pfd. fixed cap in parallel...this will give you something in the area of 380 to 715 pfd. variable. If you'd like to get up to a 1000+ pfd. variable as mentioned before, use a 365pfd. variable, along with two paralleled 350pfd. fixed capacitors for a total of 730 to 1065pfd. variable. Figure 1 shows how to do it, using just a 2-pole, 3-position rotary switch. Of course you can substitute whatever actual values you wish for the various capacitors, and add more fixed "jumps" by using a rotary switch with additional poles and additional positions...another pole and another click-stop position are needed for each fixed capacitor "jump" above that shown in Figure 1. It's worth keeping in mind for future use, even if you don't have an immediate need right now...it can save you some money that might have otherwise been spent on high-priced, hard-to-find high-picofarad variable caps!"
Hot stuff!
Richard Measures, AG6K, of Somis, California, explains why the electrolytic capacitors, used in many ham linear amps, can often be subjected to life-shortening heat...much of it unnecessarily: "The explanation that follows will help you to understand why the electrolytic filter capacitors in the popular Heath SB-220 linear amplifier, and others of similar design, are subjected to a higher-than-healthy degree of heat during "normal" operation. The SB-220 is typical of many linears in ham shacks around the world, so these tips may well find applicability in yours, regardless of its actual brand-name and model number.
A major source of heat applied to the electrolytic filter capacitors in the SB-220 is from the eight 30K, 7 watt voltage-equalizing resistors that are directly adjacent to the eight electrolytic filter capacitors used in the high-voltage power supply. A minor source of capacitor heating results from a 60Hz ripple-current flowing through each capacitor's inevitable internal resistance. In the SB-220, the filter capacitor heating problem is compounded by the fact that air cooling from the amplifier's fan doesn't directly reach the capacitors' metal cans...this is because the molded plastic covers that surround and insulate each electrolytic also act as thermal barriers. Some amplifiers use paper slip-on covers, but all use some form of insulating medium because of the high-voltage on the metal cans themselves...since they're above ground in almost all cases. In the SB-220, I've seen this plastic insulating material actually melt in the areas nearest the eight 30K, 7 watt resistors! One can only imagine what the temperature inside of the cap must be! It's worth mentioning that wet-electrolytic capacitors will have their life-expectancy reduced by a predictable amount for every degree rise over normal room temperature (70 degrees F). Many electrolytics are also marked as having a maximum permissible operating temperature...a temperature that should never be exceeded under any circumstances. By the way, that doesn't mean that the capacitor can be operated near that maximum without life-expectancy degradation as mentioned before.
It's interesting to note that the heat produced by these eight 30K resistors can be reduced by 570%, simply by changing the 30K resistors to 100K, 2 or 3 watt metal-oxide-film resistors instead. This modification alone should greatly prolong the expected life of the eight filter capacitors in an amplifier like the SB-220. Other resistance values may be used, up to about 150K, provided that the resistors themselves can withstand the voltage across them. Metal-oxide-film resistors of the 2 to 3 watt variety are often only rated at 350 volts maximum across the element, but this can easily be doubled by using two 50K resistors in series (in place of one 100K unit). Spiral-film resistors, if available, can also be used, since they'll withstand higher voltages across them than any other variety of resistor. By the way, I don't recommend using the older carbon composition types, they don't maintain their tolerances well with heat and age.
It's important to note that increasing the value of the equalizing resistors across the filter capacitors will also increase the "bleed-down-time" of the capacitors when the power is turned off. Therefore, be especially careful to allow enough time for the charge on the caps to bleed-off by watching the voltmeter on your amplifier drop to zero, then apply a reliable jumper lead from the HV anode terminal directly to ground to provide a sure ground path for the high tension point.
Here's another caveat that can occur in some linears with an automatic shorting interlock switch - such as the Heath SB-220. It will spill the HV filter caps charge directly into the grid-current metering shunt when a short is applied by the interlock switch action or by an external jumper to ground, if the charge on the caps has not been allowed to bleed off normally. If the meter happens to be in the grid-current position when this happens, it can be destroyed in short order under these conditions! So just opening the HV compartment can potentially (no pun) fry your amplifier's metering circuit. Something you wouldn't normally think of unless you've seen it happen. That's another good reason not to leave the multimeter switch in the grid-current position and to not even go into your linear until the caps have had a reasonable time to bleed-off their high voltage charge. So again, put the meter in the high voltage voltmeter position and keep an eye on it until it's safe to enter."
Moderator's note: One source for the metal-oxide-film resistors mentioned in Rich's piece is Digi-Key Corporation, Tel: 1-800-344-4539. As one individual, Rich has done an amazing amount of research into amateur HF amplifiers, their peculiarities and their shortcomings. I've found his "fixes" to be practical, and his procedural outlines should be manageable by the average ham radio hobbyist. As Rich said, please remember that anytime you're working around high voltage components, take time to think about the safety procedures that must be followed. We tend to become somewhat complacent since most of our servicing is done on low voltage electronics these days...but the high voltage present in tube equipment is just as deadly as it ever was...maybe more so because of our current tendency toward complacency. I know that I have to keep reminding myself to go slowly and cautiously when tube anode potentials are nearby.
Easy in & easy out
Mark Marholin KE6JJR of Richardson, TX offers this handy tip: "For TS-50S and IC-706 (and probably most other rigs) mobile users, the standard screws used to attach the radios to their mobile mounts are 4mm X 20mm phillips head screws. This makes removal of your radio from your mobile mount inconvenient and time consuming. Sometimes getting even a 'stubby' phillips screwdriver alongside the radio's mounting bracket can be challenging.
For a major improvement in this area, purchase 4mm X 20mm steel hex head screws and #4 thumbscrews from your local hardware store or home center. Cut off all but 3 or 4 threads on the threaded portion of each of the thumbscrews and screw them into the hex head of the 4mm X 20mm screws. This will be simply a friction fit, but it will hold the thumb screw part to the hex head screw part temporarily. Now solder the thumbscrew part to the hex head part with silver solder, acid flux (not rosin flux) and plenty of heat. Clean the screws well when cool to remove all traces of the acid flux. Now you have 4mm X 20mm thumbscrews that make it a snap to install and remove your mobile rig from its mobile mount by hand."
Moderator's note: This sounds like it might be an excellent opportunity of a budding machinist/ entrepreneur...supplying metric thumbscrews for a variety of uses in the amateur radio market. These are the types of recognizable business opportunities that Uncle Wayne has spoken of so often in his editorials. There are many such "gateways of opportunity", supplying simple but necessary accessories to the amateur marketplace. They can sometimes lead to eventual independence from the need to get up every day and trudge off to the same job at someone else's place of business..
Photo opportunity
Thomas Hart AD1B of Dedham, MA came up with this neat little remote tune up switch and keyer interface idea: "Being active on all bands from 160 through 2 meters, on a variety of modes, I decided to add a PK-232 terminal unit to my station to augment both CW as well as the various digital modes of operation. Since converting over, I've found myself using my straight key and keyer very infrequently, relying instead on my computer and TNC for virtually all non-voice communications.
For those times when I need to manually key the transmitter (for adjusting power output and for antenna tune-up), I've found the simple device shown in Figure 1 to be more than adequate. Basically, it consists of a discarded 35mm film canister, holding a momentary push-button switch, that can be used use to key my transmitter, allowing me to make the necessary final adjustments to the transceiver/transmatch combination. There's also a miniature female 1/8" phone jack in the side of the canister for inserting a straight-key, bug or electronic keyer if desired...just for nostalgia, HI.
The few parts needed are readily available from any Radio Shack store and the layout is non-critical, quick and easy. Simply cut the line that presently connects your TNC (Terminal Node Controller) to your rig's keying input, and insert the circuit in Figure 1 in parallel with that line. The entire project should take only a few minutes to complete...not counting the time needed to go to the store to buy the roll of film! The converted film canister stands on the shelf next to my Kenwood transceiver and has been very useful for my own operating set up...I hope that some variation on it will work for you just as well."
More small bulbs
Andrew Gretchen of Warm Mineral Springs, FL suggests that a good replacement bulb for meters and dials in modern transceivers can be found in the type 2182 miniature lamp. The 2182 is rated at 14 volts at 80 milliamperes and has an average bulb life of 40,000 hours (that equates out to nearly 5 years of constant running!). If operated at somewhat less than 14 volts, the life expectancy will naturally increase. For instance, when operated at 12 volts, the 2182 lamp draws 75 milliamperes and will probably last 20 to 25% longer yet. Long life for our transceiverís bulbs is important, especially if the equipment is left running 24-hours-a-day...as in the case of most packet BBS's, digipeaters and network nodes. The 2182 lamp measures about 3/16" in diameter and 7/16" long and comes with sufficiently long wire leads. It would be a good replacement choice for spots where that physical size doesn't preclude its use. Check out the Mouser catalog as one source for this lamp and keep a few of these bulbs on hand.
Don't put off sending in your own favorite tips, ideas, suggestions or shortcuts to my address at the beginning of the column. To keep Ham To Ham lively and interesting I need more continuous input...your input. Just jot down whatever you'd like to pass on to others from your own experiences in our hobby and I'll do the rest. I'm waiting to hear from you.
Murphy's Corollary: Whatever it is that you want to do, you must always do something else first!
Finally, a special thanks to all of this months contributor's, including:
Mark Marholin KE6JJR
1588 Four Oak Circle
San Jose, CA 95131
John Nix
9123 Highway 23 NE
Foley, MN 56329-9501
Phil Salas AD5X
1517 Creekside Drive
Richardson, TX 75081
Richard L. Measures, AG6K
6455 La Cumbre Road
Somis, CA 93066
Chuck Steer WA3IAC
3446 Shelmire Avenue
Philadelphia, PA 19136-3525
Thomas M. Hart AD1B
54 Hermaine Avenue
Dedham, Ma 02026
Andrew Gretchenuk
12233 Rosaro Avenue
Warm Mineral Springs, FL 34287
Note: The ideas and suggestions contributed to this column by its readers have not necessarily been tested by the column's moderator nor by the staff of 73 Magazine, and thus no guarantee of operational success is implied. Always use your own best judgment before modifying any electronic item from the original equipment manufacturer's specifications. No responsibility is implied by the moderator or 73 Magazine for any equipment damage or malfunction resulting from information supplied in this column.
Please send all correspondence relating to this column to 73 Magazine's Ham To Ham column, c/o Dave Miller, NZ9E, 7462 Lawler Avenue, Niles, IL 60714-3108, USA. All contributions used in this column will be reimbursed by a contributor's fee of $10, which includes its exclusive use by 73 Magazine. We will attempt to respond to all legitimate contributor's ideas in a timely manner, but be sure to send all specific questions on any particular tip to the originator of the idea, not to this column's moderator nor to 73 Magazine.
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