Recently I performed a restoration of a Heathkit SB-102. The transceiver appeared like new and all tubes tested good but it didn't work. All I could get from it was a static hiss from the speaker. The hiss was adjustable via the audio and RF gain controls so I figured that it wasn't too far from full function. I was not quite correct in that assumption. As this was my first experience with the Heathkit SB-10x product, I was fortunate to have a complete manual. I don't think it would be very practical to attempt a diagnosis and repair of such a complex unit without a manual. What follows are my thoughts and observations on what it took to repair the unit. My hope is that it will help you with your project. With a little effort, these units are fun to work on and success is very satisfying. It should probably be mentioned here that if you are in a similar situation, you may be working on a unit that never was working to 100%. Maybe that's why you got it for such a good price.
After spending a little time with the manual, I came to the conclusion that although it was well written it could use improvements. The transceiver is built with 6 different circuit boards. The manual includes pages showing a bottom view of each board and includes individual component identification. Given that the tube sockets are a type that provides top access to each pin, I thought that a top view of each board would be useful. This is especially true because it is easier to identify parts when they can be seen. Therefore, I scanned the bottom view images, reversed them and re-identified each component. This made it easier to make resistance checks and a bit safer to make voltage checks. While I was at this, I decided that a parts list for each board would be beneficial especially for the resistance checks. One glaring deficiency in Heathkit manuals is that the parts lists don't identify the part as denoted on the schematic. For example, the schematic will show RFC101 as a 300uH choke but there is no mention of RFC101 in the parts list. One can find a 300uH choke in the parts list and then get Heathkits part number (40-487). We have to assume it is RFC101. Fortunately this isn't a show stopping problem and the manual is otherwise a model for others to emulate.
After preparing myself with the new documentation materials, I went to work trying to locate problems. My first pass consisted of making voltage checks. Most results were within 10% of specification. Some, however, weren't even close. They were so far off as to make me unsure of the accuracy of the manual. As Heathkit is no longer available for support, I had to get down and dirty with the circuitry. This process consisted primarily of making resistance checks. Like with the voltage checks, most readings were within 10% of specification. Some, however, were way off. For instance, the manual says that the resistance to ground on pin 2 of V1 should be 1 megOhm. My meter read 400K. That's a 40% difference which is a lot more than the 10% specification. At first I suspected the 1 meg resistor R3 was bad. Seemed obvious. I disconnected one end of the resistor and found that it actually was within 5% of correct value. After much head scratching and chassis poking, I determined that there was a wiring error going to the microphone jack. Correcting the error fixed the problem. As this error was in the speech amplifier input, my receiving problem was still to be solved.
There were a number of other resistance measurements that were far from correct. I decided that the best way for me to proceed was to measure each resistor. This is not as hard as it might sound. Many resistors are not in parallel with other resistances so the measurements should be valid. If a resistor measures higher that its marked value, it's not in parallel with another resistance. Most of the resistors are of the carbon composition type of construction. In these types, carbon particles are mixed in a chemical binder. The amount of carbon in the binder determines its resistance. As these devices age, they tend to drift up in value. This drift can take them far beyond their tolerance specification. My tests found 15 resistors that exhibited this problem. These resistors were replaced with new carbon film resistors and the resistance readings were then well within spec.
A number of resistors had other resistive components in parallel with them so the only way to accurately test them was to disconnect one lead.
The following table illustrates what I found while doing resistance checks. Although some resistors were within the 10% spec, they were so close to the edge that I decided to replace them. Carbon composition resistors do not improve with age.
|Correct Value||Measured Value||Percentage|
The capacitors measured as follows:
|Correct Value||Measured Value||Leakage (ma)|
Measured / Allowable
|500uF||876uF||1.5 / 0.67||0.03|
|20uF||28.8uF||3.6 / 0.5||1.1|
|20uF||27.3uF||0.3 / 0.5||1.0|
|10uF||17.4uF||0.38 / 0.073||36|
|10uF||17.8uF||2.3 / 0.073||55|
I also found that the 100kc crystal calibrator had a serious problem. When the Mode switch was placed in the Cal position, a loud scratchy sound came from the speaker. Lightly taping anywhere on the chassis (or even the bench) induced crashing static sounds. The calibrator circuit is very simple. After eliminating all other components in the circuit, the only part left was the crystal. Fortunately I had a good replacement. This solved the problem. Here is a picture of the defective parts:
I also discovered that the filament of V17 was open. Other than the above, the transceiver was fine
As these transceivers are more than 35 years old, other components need to be checked for age related problems. This is especially true of electrolytic capacitors. Although there are many instances of old electrolytics still working fine, I think it is not worth keeping them when replacements are relatively inexpensive. Some like to try to reform electrolytics to bring them back to some degree of original performance. I have done this myself using my Sprague TO-6A capacitor tester and an ESR meter. In this case, I don't think it is worth the effort. The SB-102 only has 7 electrolytics and comparable modern devices are readily available at low cost. I paid $8.13 for all 7 new devices. Out of curiosity, I tested the old removed electrolytics and found that many were indeed defective as shown in the table above. Capacitor C304 (20uF 350V) had a leakage current of 3.6ma at its rated voltage. I tried to reform the device with no luck. Actually, the capacitor started to feel slightly warm as it was dissapating 1.25 watts. This is not something you would notice in actual use as the capacitor is mounted close to tube V14 which radiates plenty of heat to the surrounding area. All of the other electrolytics were also excessively leaky (although not quite as bad as C304).
The ESR meter previously mentioned proved useful in checking the quality of grounds. I attached one lead to the chassis and the other to various points that should be grounded and found slight discrepancies. For example, the shell portion of the antenna jack measured 0.04 Ohms. I tightened its mounting screws and saw the display immediately drop to 0.00 Ohms. A few other locations had similar resullts.
Given the amount of effort involved, it is probably wise to check the values of all resistors and voltages. Carbon composition resistors and aluminum electrolytic capacitors do not last forever. In fact, tubes can last longer. Although most resistors are 10% units, the circuitry will probably work adequately with parts that are within 20%. Keep in mind, that depending on circuitry, out of tolerance errors can be cumulative which could result in a much larger out of tolerance condition. Carbon film resistors can be purchased for 6 or 7 cents in unit quantities so why not replace them? I gave some thought to replacing all of the resistors with 1% devices - cost be darned. Then I realized that the tubes were such a variable that would probably be kidding myself. Because ceramic capacitors have such good long term reliability, I didn't individually test any of them. I hope that performance testing of the transceiver will prove this wise. I think most problems associated with these old kits will be one or more of the following in order of probability:
Somewhere in the above list should probably be included the dreaded undocumted modifications. It would be a better world if you would somehow include documentation for any mods you make so future owners won't mutter oaths to you.
Test equipment used
In the above list, I consider the VTVM to be the most important single piece of test equipment for this project. It does just about everything a DMM does but it is hard to beat an analog meter for detecting noisy potentiometers or fluctuating signals. Sucessful repair of one of these old transceivers can be quite satisfying. Please do not assume that I am an expert on the SB-10x product. I am a "seasoned" engineer but I'm learning (or re-learning) this old stuff just like you probably are.
Here are the documents I created to make the testing process a little easier. More will be added as soon as possible.
|RF Driver||RF Driver|
Schematics are available SB-100 ~ SB-101 ~ SB-102. I spent some time making the schematics really clean so they can be enlarged. I took the SB-102 PDF file to Kinkos and had a 3x4 foot enlargement made. They also do 2x3 foot prints which are just about the size of the original manual foldout. Looks perfect.
Don't forget to check out the Service Bulletins SB-100 ~ SB-101 ~ SB-102.
You may run into a unit that has one or more documented modifications ~ SB-100 ~ SB-101 ~ SB-102. This is probably a good thing but you need to know about it.
If you want to change the rubber belts (O rings) for something much better, see the following:
String diagram - String pictures