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Salvage It! BY JULIAN EDGAR The good bits in old receivers Looking for a cheap voltmeter or current meter to build into a power supply or some other project? A discarded receiver can often supply the parts required. I’ll be straight-up with you – most old receivers don’t yield a bonanza of goodies. Rather, they’re a source of a handful of good bits. Some, like the high-value capacitors from the power supply, can be put aside for later projects while others, like the fuses and machine screws, can find a home in your parts drawers. Sometimes there are also a few heatsinks worthy of salvaging and at other times the speaker output terminals are useful. But to be honest, many hum-drum receivers aren’t worth the salvage effort. There’s one main exception to that rule: if the receiver has an analog centre-tuning meter for FM reception, an analog signal strength meter or a bargraph-style LED power output meter, grab it. Why? – we’ll come to that in a moment. Another thing to look for when checking out old receivers is the rear panel marking for total power consumption. If it’s in the hundreds of watts, you’re probably dealing with a high-quality design. And that makes it more likely that some of the parts will be worth salvaging. Bits and pieces The Akai receiver pictured here (a model AA-R30 from the early 1980s) had seen much better days. The front panel was corroded and there were several gouges in the panels. The lower panel was also badly dented – probably from when it was thrown on the tip. I bought it for $5 from the shop associated with the tip but it’s the sort of receiver that’s often given away at garage sales or put out for kerbside rubbish collections. Leaving aside the meters and displays for a moment, after spending about half an hour with a pair of side cutters and a Philips-head screwdriver, I ended up with four knobs, four fuses, the nuts and washers from four pots (you know how hard these are to source), three low-voltage bulbs, two 8200mF 40V capacitors and a large folded U-shaped aluminium sheet that was used as a heatsink. OK, so those parts are not really worth the effort unless you like pulling things apart. But let’s go back to the displays – these really are useful as we shall see. LED bargraph display This Akai unit is typical of 1980s receivers that are now often thrown away. When looking for a worthwhile receiver to salvage, look for the presence of digital and analog meters – they’re very useful in lots of applications. 94  Silicon Chip The output power meter is similar to those used in many receivers and amplifiers of the era. It consists of five LEDs mounted on a PC board that’s connected to another small board via a ribbon cable. Importantly, these two boards comprise the whole of the power meter which makes it easy to remove for reuse. With this type of design, it’s also straightforward to identify what wires do what. In fact, before you’re too free with the cutters, look closely at where the wires go. In this case, there were two wire pairs that went from the power meter sub-board to the main board. They comprised almost certainly: (1) power and ground; and (2) siliconchip.com.au the signal inputs to the display (ie, the signal that causes the LEDs to light in sequence). It wasn’t hard to figure out which was which, as the main board had “Speaker A” and “Speaker B” designations next to the connections for the white and purple wires – so these were the signal inputs. The other pair of wires (yellow/black) were therefore the power supply leads, with black almost certainly the negative (ground) connection. To verify this, I connected a variable power supply to the supply leads and slowly wound up the voltage. At the same time, I fed a small DC voltage to one of the signal inputs. And it worked, the first LED coming on at 0.9V on the signal input (either Speaker A or Speaker B) and with a 6V supply. Similar LED bargraphs are used in many cassette decks – again they’re easy to salvage and hard to kill. In this case, we’ve salvaged just the heatsink, some knobs, fuses, screws, pot nuts and washers, some filament bulbs, the power supply capacitors and the dual analog meters. On-board controls In the case of the Akai power meter, some on-board controls were also provided. First, a pushbutton switch changed the display to read either x1 or x0.1 of full scale (which, as it happened, was marked at 38 watts). In addition, each input channel had a pot mounted on the PC board for fine tuning. By adjusting the pots and/or the pushbutton, it was possible to set the full-scale deflection to anything between 3.4V and 18.4V. Further testing showed that while the response of the five LEDs wasn’t perfectly linear, it wasn’t far off. So what good is this display? Well, it’s ideal anywhere you need an indication of DC voltage! Voltmeter & ammeter The two analog meters (one for signal strength and the other for tuning) on the old Akai receiver are even more useful. These are both contained in the one housing and are ideal for showing both current flow (positive and negative) and voltage. The application? – anywhere a battery is charged and discharged, as in a solar-powered battery bank or in a low-voltage electric vehicle with regeneration. The signal-strength meter is easily converted to read a voltage from 3.4V upwards – simply wire a 1MW potentiometer in series with it and adjust siliconchip.com.au MAKING A VOLTMETER Fig.1: here’s how to recalibrate the signal-strength meter so that it can be used to measure higher voltages. All you need is a 1MW potentiometer, a variable power supply and a multimeter. The pot is used to set the fullscale deflection voltage, while the variable supply and multimeter allow the meter to be calibrated. MAKING A CURRENT METER Fig.2: this circuit shows how to convert and calibrate the tuning meter for use as a current meter. The shunt resistor (easily made from fencing wire) is wired in parallel with the tuning meter and its effective length adjusted until the reading on the meter agrees with the reading on the ammeter. September 2006  95 The receiver’s signal and tuning meters can easily be adapted to show voltage and current. This photo shows the signal/tuning meter combination equipped with the new scales, while in the foreground is the ammeter shunt, made from a length of salvaged stainless steel rod taken from a discarded garden leaf rake. This set-up is ideal for monitoring a battery bank (eg, in a solar panel installation). the pot to give the required full-scale deflection – see Fig.1. The tuning meter can be converted to a current meter simply by placing a shunt in parallel with it. A shunt is easily made from steel fencing wire or, as I did, from a length of stainless steel rod salvaged from a garden rake! It’s also easy to make new scales for the meters using a PC, scanner and printer – see “Salvage It!” for June 2005. To calibrate the signal-strength voltmeter, you will need a variable power supply. Set the full-scale deflection with the pot and then calibrate the markings against the input voltage as measured by a multimeter (see Fig.1). Calibrating the ammeter is only slightly more difficult. Fig.2 shows the basic scheme. In this case, a highpowered battery charger (complete with built-in ammeter) was connected to a flat car battery. The shunt was inserted in series between the charger and the battery and the tuning meter (now acting as an ammeter) wired in parallel with the shunt. By sliding one terminal along the shunt (effectively altering its length and thus its resistance), the reading on the new ammeter can be calibrated to match the battery charger’s ammeter. If the battery charger has a 2-position charging switch (ie, to alter the charging rate), it’s a good idea to check the meter calibration at the two different charging current values. Alternatively, you can use a current clamp to measure the current flow and then place varying loads on the battery. So there you are – any salvaged receiver will give you a handful of useful parts. And if you get hold of a receiver with analog or digital meters, it takes relatively little effort to convert the meters for use in a wide range of SC applications. Rat It Before You Chuck It! Many older receivers, amplifiers and cassette decks use LED bargraph meters. In this case, the meter’s electronics are completely separate from the main PC board, making it easy to salvage and use in another application. Note how only two pairs of wires connect to the sub PC board at bottom – one pair for the power supply connections and the other pair for signal inputs. This particular circuit also uses two pots and a high/low switch. With these adjustments, it was easy to set the full-scale deflection to anywhere between 3.4V and 18.4V. That makes it a pretty useful voltage meter for use in all sorts of applications. 96  Silicon Chip Whenever you throw away an old TV (or VCR or washing machine or dishwasher or printer) do you always think that surely there must be some good salvageable components inside? Well, this column is for you! (And it’s also for people without a lot of dough.) Each month we’ll use bits and pieces sourced from discards, sometimes in mini-projects and other times as an ideas smorgasbord. And you can contribute as well. If you have a use for specific parts which can easily be salvaged from goods commonly being thrown away, we’d love to hear from you. Perhaps you use the pressure switch from a washing machine to control a pump. Or maybe you salvage the high-quality bearings from VCR heads. Or perhaps you’ve found how the guts of a cassette player can be easily turned into a metal detector. (Well, we made the last one up but you get the idea . . .) If you have some practical ideas, write in and tell us! siliconchip.com.au