Reprinted from August 1992 QST Lab Notes
Copyright 1992 American Radio Relay League, Inc.
All rights reserved.

Thank you for requesting the following information from the ARRL 
Information mail server. ARRL HQ is glad to provide this information free 
of charge as a service to League members and affiliated clubs.

For your convenience, you may reproduce this information, 
electronically or on paper, and distribute it to anyone who needs 
it, provided that you reproduce it in its entirety and do so free 
of charge. Please note that you must reproduce the information as 
it appears in the original, including the League's copyright 
notice.

If you have any questions concerning the reproduction or 
distribution of this material, please contact Mark Wilson, 
American Radio Relay League, 225 Main St., Newington, CT 06111 
(mwilson@arrl.org).

Lab Notes - Substituting Parts

Conducted By Steve Ford, WB81MY, Assistant Technical Editor and 
the ARRL Laboratory Staff.

You need to replace a 47ohm resistor in your 2-meter power 
amplifier. Any 47ohm resistor you have in your junkbox should do 
the trick -- or will it? Did you check to see if the substitute 
resistor was a carbon-composition or wire-wound type? Contrary to 
what you may think, it makes a big difference in RF applications! 
ARRL Laboratory Engineer Zack Lau, KH6CP, offers many important 
tips to consider before you start swapping parts in your 
equipment. -- WB8IMY

Q: Many construction articles say that I can substitute higher-
wattage resistors for lower-wattage ones. Are there cases where 
this doesn't work?

A: The most obvious problem results from the larger size of the 
higher-wattage resistors: They may not fit in the same space as 
smaller resistors. Inductance is also related to size. At VHF and 
above, larger resistors may have too much inductance for circuits 
to work properly.

Q: How does the inductance of metal-film  resistors compare with 
that of carbon-composition resistors?


A: The metal-film resistors made today seem to be quite low in 
inductance, and are comparable to carbon types. I've used them 
well into the VHF range with little difficulty. However, these 
should not be confused with wirewound resistors, which are 
probably too inductive even in the MF/HF spectrum.

Q: How about capacitors? Why is a given value available in so many 
types?

A: Manufacturers usually use the cheapest capacitor that will work 
well in a given circuit. Depending on the application, you end up 
with a menagerie of capacitors made from various materials, 
including ceramic, mica, polystyrene and so on.



Q: I can't seem to find the polystyrene capacitors specified in an 
oscillator project. Is there another type of capacitor I can use?


A: If it's an RF project, NP0-type (that's zero, not "oh") ceramic 
disc capacitors work just as well or better. For audio or very-
low-frequency projects, polyester capacitors are a good 
substitute. If you manage to find polystyrene capacitors, take 
care when you solder them in place. Polystyrene capacitors are 
easily damaged by defluxing solvents and heat.

Q: The mica capacitors in my transmitter filter are getting 
awfully warm, yet I used the 500 volt capacitors specified in the 
article. What's going on?

A: When you're using capacitors in power circuits, the current 
rating of the capacitor is just as important as the voltage 
capacity. Unfortunately, current ratings are not usually 
specified. I look for larger capacitors with thicker leads when I 
have an application that requires the capacitor to handle high 
currents. An alternative is to use several smaller capacitors in 
parallel.

Q: I want to use disc capacitors to filter my ac line cord. How 
can I convert the dc voltage rating of the capacitors into an ac 
voltage rating?

A: Rather than use the conversion approach -- which is a bit risky 
-- the better course of action is to use ac-rated capacitors 
intended for across-the-line use. Another option is to use 
filtered line-cord connectors. They're often very cheap on the 
surplus market.

Q: I know that the tuning range and plate spacing of air-
dielectric variable capacitors is important. Is there anything 
else I should worry about?

A: Size can be an important consideration -- especially if the 
capacitor has to fit inside the box you're using! Loss is 
sometimes a factor as well. For instance, small tuned loops for 
transmitting require low-loss tuning capacitors to work 
efficiently. Expensive vacuum-dielectric variables are often used 
in these applications. However, if the author got away with an air 
variable, chances are you can, too. Some types of equipment, such 
as VFOs, also require capacitors with a high degree of mechanical 
stability.

Q: How come an author merely specified "ferrite bead " without 
specifying its size or material type?

A: In many cases, the ferrite bead was added as insurance against 
RF intrusion and the author merely grabbed whatever was available. 
It's difficult, but not impossible, to differentiate the various 
ferrite materials by color and texture. For example, type 43 
material has a pronounced metallic sheen compared to type 72. Type 
75 material usually appears to be dull and dark.

Q: I 've purchased some mystery toroids at a recent hamfest. I 'd 
like to substitute them in one of my amplifiers. How do I 
calculate the proper number of windings to use?

A: First, you have to determine the inductance, and possibly the 
Q, of the inductors you're replacing.

Once you've determined the inductance of the inductor you want to 
replace, make your best guess at the number of windings required 
on your mystery toroid. Wind the wire through the toroid and 
measure the resulting inductance.

If you find that it's impossible to maintain the proper turns 
ratio to get the inductance you need, your toroid is unsuitable. 
Also, if the loss is excessive, the Q of your toroid probably 
isn't high enough.

Q: I checked my parts-substitution directory and found that a 
IN5767 PIN diode is similar to a IN914 small-signal diode. I 
thought they were completely different. What gives?

A: A PIN diode is a specialty device that shares some 
characteristics with small-signal diodes. However, the publisher 
of your directory failed to take the special characteristics of 
PIN diodes into account. To add to the confusion, 1N914s can be 
used as RF switching diodes, but they have much higher losses 
(often approaching 50-70%) compared to a few percent or less for a 
properly designed PIN diode switch.

Q: Why do some circuits require a specific transistor while others 
operate with just about anything?

A: Good designers add extra parts to compensate for device 
variations. This allows a wide variety of devices -- transistors, 
for example -- to function in the circuit. In some cases, 
designers find that optimum performance can only be obtained by 
using a specific device. In other cases, cost calls the tune. A 
circuit designed to accommodate a variety of substitute parts 
requires extra effort...and extra expense!

Q: The project I 'm building specifies a certain wire gauge to use 
when making the inductors. Can I get away with using a different 
wire size?

A: Maybe -- if the size of your substitute wire is reasonably 
close. Much thinner wire will result in more inductance and lower 
Q -- even if you're winding a toroid. This can make the difference 
between a working circuit and a dead circuit! Too much Q, on the 
other hand, can result in unwanted oscillation. Broadband circuits 
are usually less critical with regard to circuit Q.

Q: I want to replace the tubes in my radio with solid-state 
devices. Isn 't there a company that sells replacements?

A: Sorry. The last manufacturer of solid-state tube replacements 
closed up shop a long time ago. Even building your own isn't easy. 
The necessary high-voltage field effect transistors are difficult 
to find these days.

Q: Why can't I use the fifth overtone of a cheap microprocessor 
crystal for my microwave transverter?

A: Nothing says you can't. However, the microprocessor crystals 
I've examined have strong spurious responses. If you attempt to 
use the fifth overtone of these crystals, be prepared to see your 
oscillator unexpectedly jumping frequencies. Unlike your 
microprocessor crystal, a crystal made for a particular overtone 
is cut to keep spurious responses to a minimum.

Q: I have a bad crystal in an FT-243 case. Can I replace it with a 
crystal in an HC-6 case?

A: Possibly, but be careful. The HC crystal is a more modern 
vintage. Older crystals generally can handle more power than most 
modern crystals. This means you have to be cautious when using a 
modern crystal as a replacement in an older piece of equipment. 
Crystals will fracture if you apply too much power to them!

Q: What about using plastic-insulated wire when copying antenna 
designs that use bare or enameled wire?

A: Insulated wire will make a difference in the antenna's resonant 
frequency, though for HF antennas the antenna's placement has just 
as much effect. Besides, if the finished antenna isn't resonant on 
the desired frequency, a little shortening or lengthening will do 
the trick. For VHF and microwave antennas, the effect is more 
pronounced, and correcting the problem is more difficult.

Q: Does it make any difference whether I use phenolic or glass-
epoxy circuit board?

A: Yes! Phenolic boards are junk compared to glass-epoxy boards. 
Assuming that you'll have to make a modification or repair to the 
circuit at some point, glass epoxy is a wise investment. A 
phenolic board is brittle and will break easily.

Q: Is it really necessary to have my PC boards tin plated?

A: Not at all. They'll look better and won't oxidize as quickly as 
unplated copper, but you can always scrape the copper clean if you 
need to do a modification in a couple of years.

Q: What can I use for coil forms?

A: Polystyrene and Teflon make great coil forms, even though 
polystyrene cracks and melts easily. Fiberglass, Delrin and wood 
have also been used, apparently with good results. Some hams have 
tried PVC, but I've heard several reports of problems with this 
material. (One ham reported melting an antenna coil wound on PVC!)

Q: I can 't seem to find audio-taper potentiometers. Can I use 
linear-taper potentiometers instead?

A: It depends on the circuit and how fussy you are. The wrong 
taper can make the 9. difference between a smooth control and a 
touchy control. Audio tapers provide the smoothest adjustment for 
audio circuits, assuming the designer used them in the right 
place.

Q: Can I use anodized aluminum for my antennas?

A: Anodizing is an electrochemical process that has one huge 
disadvantage -- it forms an excellent insulator on the surfaces of 
metal objects. I once encountered a TV antenna that seemed to be 
mysteriously nonfunctional -- until I sanded off the coating to 
establish electrical contact!

Q: When is it appropriate to use IC sockets in a circuit?

A: Let the author's design be your guide. Sockets are usually safe 
for RF applications, although the author may have a perfectly 
valid reason to avoid them (too much inductance, high reliability 
requirements and so on).

Q: Can I use UHF connectors instead of those N, BNC, or SMA 
connectors so often specified in UHF and microwave projects?

A: No. Despite their name, UHF connectors have poor SWR 
characteristics at UHF and higher frequencies. They should not be 
used above 420 MHz. However, if the rig in question uses UHF 
connectors, its circuitry is probably designed to accommodate the 
mismatch they cause.

Q: Can I substitute a 6146B RF power amplifier tube for an 
ordinary 6146?

A: In most cases you can, although the B-suffix version's slightly 
higher grid-to-plate capacitance may cause problems if the range 
of the neutralization circuit is marginal. In addition, while the 
plate dissipation of the new tubes may be greater, don't try to 
squeeze more power out of the circuit. Your power supply may not 
survive the additional stress! For tube-type amplifiers, it's 
often the power supply that limits the amplifier, not plate 
dissipation.


Q: I found some high-quality Teflon cable at a hamfest. Can I use 
it in place of regular coax to reduce loss?

A: This is actually two questions. Yes, you can usually replace 
regular coax with its Teflon dielectric equivalent in applications 
where the velocity factor doesn't matter. But Teflon cable 
generally isn't less lossy than standard cable of the same 
impedance and diameter.

Q: How can I determine the impedance of the cable I bought?

A: Unless it has an RG-type number, forget about looking it up. 
It's easier just to measure the dimensions and calculate it. You 
can confirm your calculation by attaching a long piece of the 
cable to a dummy antenna and measuring the SWR.

Q: I copied an antique transmitter almost exactly, except that I 
used aluminum instead of steel for the chassis and shields. How 
come it's full of unwanted oscillations?

A: Assuming the original design didn't have oscillations its 
builder didn't know about, the aluminum is probably the problem. 
The use of steel, a lossy material, effectively lowers the gain of 
RF circuits, making it difficult for oscillations to occur. This 
is why old steel chassis were often copper plated -- and one 
reason why old rigs on bare-steel chassis were relatively easy to 
tame.

We welcome your suggestions for topics to be discussed in Lab 
Notes, but we are unable to answer individual questions. Please 
send your comments or suggestions to: Lab Notes, ARRL, 225 Main 
St, Newington, CT 06111.
