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RADIO RESTORATIONS BRINGING OLD RADIOS BACK TO LIFE Marc Ellis marcellis@monitoringtimes.com NC-57 Electronic Restoration Completed We finished the last NC-57 work session with about half of the capacitors replaced before I ran out of time and almost out of capacitors. There’s an unusual amount of bypassing in the various circuits, which is an indication of the excellent quality that was built into this fairly low-priced radio. I think I’ve already mentioned how impressed I was by the professional construction of this set as compared to the Hallicrafters S-40 (restored earlier in this column) – which had a similar price, but which was built more like a household broadcast receiver. ❖ Mica or Paper? At any rate, with a fresh supply of capacitors on the bench, I went back to work and completed the job. This took quite a bit of time because of the tight construction of the radio, but the result was very satisfying. I know I’ve told you in the past about the disappointment some folks feel when they open up a recently-purchased receiver to find that all of the original paper caps have been replaced by modern equivalents. They feel that the radio has been somehow violated and, understandably, wonder if the circuitry has been changed, either deliberately or accidentally. However, I see a radio chassis with a complete set of meticulously-installed modern capacitors as a thing of beauty. (As long as it’s a set of capacitors that I put in!) I know that there’s an excellent chance that the radio can be aligned to perform like new. It should also give good service for many years without suffering a sudden and disastrous short circuit that might ruin hard-to-replace components. As you know, most of the paper capacitors in the NC-57 were Bakelite-cased units that look like mica caps, which hardly ever need to be replaced. However, as discussed last time, the Bakelite cased paper caps are no more to be trusted than the waxcovered tubular units. They all tested leaky on my capacitor checker. So it’s important that you find and replace any such units present in your restora- Fig. 1b. Six-dot coding pattern. See text for details. tion project. Generally, the Bakelite-cased paper caps look much larger than similarly-cased mica ones – which makes sense because their capacities are much larger. The paper caps, used as they are for bypass and coupling purposes, typically range from .01 to .1 uf. Mica units, often used in r.f. frequencydetermining circuits, might run from 10 to 500 pf (uuf). So, if in doubt about whether a capacitor is paper or mica, check its size and function in the circuit. Also check the parts list. The NC-57’s list helpfully indicates which caps are which. ❖ Reading Capacitor Color codes The color codes used to identify Bakelitecased paper or mica caps can be confounding because so many versions have been used. Eventually the system was somewhat standardized by the RMA (Radio Manufacturer’s Association). Following are the more common styles. If the capacitor is the standard garden variety (500-volt; 20% tolerance) as found in most receivers, including the NC-57, then a simple row of three dots is used (Fig. 1a). Make sure you read along the row from left to right with the manufacturer’s name right side up or in the direction of the arrows. The first dot (A) is the first significant figure of the rating; the second (B) is the second significant figure; the third (C) tells how many zeros to add following the second figure to give the capacity in pf (uuf). For the specific values to use for the different colors, see the table in Fig. 2. For mica capacitors, the capacity is usually left in uuf. However, the capacities of paper caps are usually so much larger that it they are converted to uf. This is simply a matter of moving the decimal point six places to the left. For example, a very common Bakelite-cased unit found in the NC-57 has a three-dot color code of brown-black-orange. Since it has just a 3-dot code, we know that it is a 500-volt, 20% tolerance unit. The first (black) and second (brown) significant figures are 1 and 0. The number of zeros after the significant figures is three (orange). So the capacity is 10000 (same as 10000.0, of course) pf (uuf) or – moving the decimal point six places to the left – .01 uf. You might sometimes see a row of five dots (less common and not illustrated). This format is used when voltage (fourth, or “D,” dot) and tolerance (fifth, or “E” dot) must be specified. See Fig. 2 for the values associated with various colors of these dots. The fourth and fifth dot might be found, instead, on the rear of the capacitor or above the basic row of three. Finally, you might come across a capacitor coded with two rows of three dots (Fig. 1b). This might be (but see next paragraph) a unit with a capacity requiring three significant figures to express (let’s say 325 uuf, just to make up an example). For this unit, the top-row colors would be Orange- Red-Green. The third color (labeled “B1” in the figure) is read from column “B” in Fig. 2 (or the identical column “A.”) The number of zeros to be added (labeled “C” as before) is the last dot in the bottom row. In this case it would be brown. The first two dots in that row represent voltage (“D”) and tolerance (“E”). Just a few caveats! If you see a six-dot coding Fig. 1a. Two examples of three-dot coding. If no direction arrows are present, as at left, read the dots from left to right with the manufacturer’s name right side up. Fig. 2. Chart shows values corresponding to the various dot positions and colors. 64 MONITORING TIMES January 2005
Purists hate to see new capacitors in an a vintage set. But there’s no doubt that a meticulous recapping will give a fine old radio a new lease on life and ensure that it will operate for many more years doing what it was designed to do. where the first dot (“A” position) is white, black, or silver, you are looking at an RMA post-1948 revised coding (white) or a unit made to military standards (black represents mica; silver represents paper). In these capacitors, the first and second significant figures are represented in positions “B” and “B1” (using figure 2 as a position reference only) and the number of zeros is shown in position “C”. Tolerance will be found at “E”. Position “D” is called “characteristic,” and I can find no reference to what might be specified here – perhaps voltage would be included. Can a reader help me out on this? Oh, and if you find a six-dot coding with the bottom center dot blank, you are really looking at another version of the five-dot code, with the outer two bottom dots representing values “D” and “E.” ❖ Other Restoration Issues With the recapping now complete, the electronic restoration of the NC-57 has essentially been accomplished. But there are a few issues connected with it that are worth mentioning. In spot-checking the values of some of the resistors, I came across a 2200 ohm unit that measured over 6000 ohms. I really couldn’t believe my eyes or my ohmmeter! It’s true that the values of carbon resistors tend to increase with age. But the problem is generally found in resistors in the megohm range and the percentage increase is usually not so extreme. Quite concerned that the effect might be widespread, I did my best to check every resistor in the set. Of course one can’t obtain true readings on many of the resistors without disconnecting one end. That’s because they are wired in parallel with other circuit elements – which would reduce the resistance shown on the ohmmeter. Such wholesale disconnection would not only be too time consuming, but could also compromise the integrity of the radio. So I decided to adopt a compromise philosophy learned from a friend who has been restoring radios much longer than I have. Here’s his technique for a rough check (keeping in mind that resistors may increase, but do not decrease, in value with age). He puts his ohmmeter across each resistor in turn – leaving it connected in the circuit. If it shows a significant increase over the marked value despite being connected to other circuit elements, he’ll replace it. If it shows significant decrease, the reading is likely to be an artifact caused by circuit interconnections. So he reserves judgment and leaves it alone. Of course some resistors are essentially disconnected at one end because they are connected to tube pins that have no other wiring on them. These can be checked accurately. I did replace the 2200-ohm resistor of course, but noticed no other outlandish values during the rough check. Later, when the set is powered up and operating, I’ll check the voltages at various circuit points against the published readings. Any that are significantly low would be a symptom of an associated resistor that had drifted up to too high a value. The only other resistor I replaced at this stage was a crudely-wired-in 12,000-ohm, 10-watt wire-wound power resistor that had been installed in place of the dropping resistor for the voltage regulator tube (intended to be 3900 ohm, 2 watt). No sign, so far, of what caused the original to go bad. The two replacement electrolytic capacitors (left) are mounted on a terminal strip installed under the disconnected capacitor can, left in place for cosmetic reasons. Light-colored rectangular component at right is replacement VR tube dropping resistor (see text). I also got a chuckle over a shielded cable connected to the wiper arm of the volume control and routed out of the radio through the keyway of the accessory socket. Intended to be a phono input I guess. But in this receiver the volume control is between the first and second audio stages – so there would have been very little gain. Even funnier is that there was a proper factory-wired phono input available at the accessory socket. After removing the cable, I installed a new line cord to replace the deteriorated one I had cut off early in the game and, electronically speaking, the old girl was ready to go. ❖ How Do You Like Us So Far? Many of you have no doubt noticed that Monitoring Times has recently been taking a hard look at itself in the form of a short survey posted on the Grove Enterprises web site. I know that some readers of this column have responded to it and I appreciate the positive comments that have been made. As it happens, this month marks the beginning of my sixth year of writing “Radio Restorations.” So it seems appropriate for me, with your help, to take a personal hard look at the past five years of “Radio Restorations” to see what has gone over well and what hasn’t. The format of the column, so far, is simple and obvious to all. I simply let the reader look over my shoulder as I restore vintage radio receivers and test equipment. I have been stressing the restoration of communications receivers because I figure that’s what MT readers would like most. These restorations are not done in advance and edited for publication later. Everything is done in real time – so you share my triumphs and frustrations as they happen. Recently, in response to a reader suggestion, I’ve begun to look back and develop some columns on general restoration topics synthesized from my experiences with various previously-discussed projects. So how do you like us so far? What would you have me do differently? Are you happy with the type of gear chosen for restoration? What about the pace of the projects – too drawn out? – too fast? – about right? Would you be interested in my including mention of vintage radio clubs and radio events in different parts of the country? How about the inclusion of an occasional radio history tid bit? Anything else I may not have thought of to ask? Finally, I’d be curious to know what role this column plays in keeping you as a Monitoring Times subscriber or reader. Do you find it a major attraction? – or an interesting sidelight to your work with modern, high-tech receiving equipment? – or would you not find it much of a loss if we went away? Take a few minutes right now and write me a note! If you don’t happen to be e-mail equipped, send snail mail to me at PO Box 1306, Evanston, IL 60204-1306. I’ll look forward to hearing from you, whatever your opinions. January 2005 MONITORING TIMES 65
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