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Last month we described how this all-in-one AM radio test and alignment device works and gave the PCB assembly instructions. Now we have the details of how to wire it up, test it, calibrate it and finish the assembly by mounting it in a diecast case. The H-field Transanalyser Part 2 – by Dr Hugo Holden I f you’re building the Transanalyser and have been following along, you should have a fully assembled PCB. But it is not quite ready to be powered up yet. So let’s get onto wiring up the remaining components that are not mounted on the PCB. Chassis wiring You can do the chassis wiring, plug everything together and test the unit before fitting it into its case. It may not perform brilliantly due to the lack of shielding, but if there is something wrong, it will be much easier to fix it at this stage. But before you can test it, you need to wire up the DC socket, three chassis-mounting pots, the three input and output sockets and the LED frequency meter. As the wiring is somewhat complicated, in addition to the following de84 Silicon Chip scription, we have produced a wiring diagram (see Fig.5). This includes approximate lengths for each cable run, but note that you may need to make some adjustments depending on the exact location you’ve mounted the parts in your chassis. Also note that the terminal arrangements for VR4 & VR5 may be different depending on which exact parts you’ve purchased. Start by cutting a 150mm length of light-duty figure-8 cable and solder it to the two live pins of the DC socket. These sockets usually have three pins, one of which is open-circuit when a plug is inserted. If you aren’t sure which is which, plug in the plugpack, power it up and probe the pins with a DMM set to measure DC volts until you get a sensible reading. If the reading is positive, the red probe is on the + contact, Australia’s electronics magazine whereas if it’s negative, the black probe is on the + contact. Once you’ve soldered the wire at that end, crimp and/or solder the other end to a pair of polarised plug pins and insert these into a two-way plastic shell. When plugged into the DC input on the board, the wire from the + side of the DC socket must go to the side marked + on the PCB. Next, cut three lengths of shielded wire: 120mm long for METER IN (CON1), 150mm long for 1kHz OUT (CON6) and 220mm long for RF OUT (CON7). Solder these to the appropriate plugs, ie, BNC for RF OUT and either RCA or BNC (depending on your preference) for the other two. The shield braids go to the outer shields of the connectors. Attach two-way header plugs to the other ends of these cables in a similar siliconchip.com.au manner as you did for the DC input. In each case, the inner conductor goes to the side that matches the + symbol on the PCB when plugged in, with the shield braid to the other side. Make sure none of the shield braid wires are floating around so that they might short to something; if they are, cut them off. That just leaves the wiring for the three pots. You need a two-core (three conductor) shielded cable for the 1kHz output adjustment potentiometer; the type often used for stereo audio is fine. Cut a 120mm length and solder the shield braid to the anti-clockwise end of the 5kΩ potentiometer, VR6. The inner two conductors each go to one of the two other pins. Crimp and/or solder pins to the three conductors at the other end, and insert them into the three-way plug shell. Ensure that the wire going to the clockwise end of the potentiometer (viewed from the front of the pot) goes to the side marked with a + on header CON5. The shield braid goes to the opposite end of the plug, with the third wire (from the pot wiper) to the middle pin. Solder wire off-cuts from the central wiper connection to the anti-clockwise end terminal on each of the two remaining pots, so that they become variable resistors which decrease in resistance when turned clockwise. Then cut an 80mm length of figure-8 cable, and solder one end to a pair of Repeated from last month’s issue, this is what your completed PCB should look like. We used brass strips for shielding; strips of tinplate should work but will rust over time. pins which are then inserted into a twoway polarised plug. It doesn’t matter which pin goes where. Split the wires apart at the opposite end and solder them to the wiper terminals of VR4 and VR5. Then run a short length of medium-duty hookup wire between the clockwise terminals of VR4 and VR5. The only part left to wire up is the LED frequency meter. Cut a 50mm length of shielded cable and a 100mm length of light-duty figure-8 cable. Crimp and/or solder these to pairs of pins and insert them into two-way plugs, either way around. The shielded cable will go to the signal input on the back of the frequency meter, and the figure-8 cable to the power input. These cables then meet at a single three-way plug to go to CON4 on the main PCB. The positive wire for the figure-8 power cable goes to the end marked + on the PCB, while the sig- Scope1: this shows the RF output signal from CON7 when the 1kHz signal going into the modulator is disabled, resulting in a pure carrier wave. The frequency setting is around 1800kHz (ie, at the upper end of the adjustment range) and you can see that the sinewave is quite pure. siliconchip.com.au nal input goes to the middle pin. Both ground wires must be connected to the third pin, at the opposite end from the + symbol. Testing and calibration If you’ve used IC sockets, make sure all the ICs are plugged in now, with the correct orientation and in the right locations. Now is also a good time to pop the plastic cover off the analog meter and replace the 0-1mA scale inside with a 0-1mV (or similar) scale. Temporarily attach the analog meter to the front of the PCB by removing Scope2: the same signal as in Scope1 but the 1kHz signal has been re-enabled, so it is now 30% amplitude modulated. If the output of your unit does not look like this, adjust trimpot VR3 to get the correct modulation level. Australia’s electronics magazine June 2020 85 Next, connect a sinewave of known amplitude to the meter input, set S1 to select the correct range (fully anticlockwise = 10V, one step clockwise = 1V etc) and then adjust VR1 to get the correct reading on the analog meter. Final assembly Only four holes are required on the rear “panel” (which happens to be the base of the diecast case). Position is not particularly critical but the locations shown make sense. the nuts from its two rear screw shafts, feeding these through the holes on the PCB marked “To meter”, “CON2” and attaching the screws to these pads using a nut on either side (you need nuts just behind the meter to space it off so that it clears the solder joints under it). Plug all these cables into the appropriate headers on the main PCB (see labels and the text above for an idea of which goes where), prop it up in a convenient location on a non-conducting surface and make sure none of the floating components and wires are shorting together. Since you were careful to connect the plug wires correctly earlier, once you’ve made sure the right plugs go to the right headers on the board, all the connections should be right. That just leaves the two plugs which go to the frequency meter. As the headers on that board are not polarised, they can go either way around. So check the labelling on the back of the frequency meter carefully and ensure that both plugs go into the right sockets (the shielded cable carries the signal) and that they have the right orientation, with the shield braid and ground wire connecting to ground. Once that’s sorted out, set rotary switch S1 on the board fully anticlockwise and S2 (at top) fully clockwise. Adjust VC1 and VR1-VR3 to 86 Silicon Chip their halfway points and flip toggle switch S3 up. Apply 12V power to the floating DC socket; nothing should happen since the power switch is off. Flip S3 and check that the frequency meter lights up. Adjusting floating potentiometers VR4 and VR5 should change the frequency reading. Rotate VR4 and VR5 fully anti-clockwise and adjust VC1 to get a reading close to 205kHz on the frequency meter display. Now rotate both fully clockwise and check that the reading goes up to at least 1.8MHz. For proper calibration, you need an oscilloscope or spectrum analyser. Connect this to the RF output on your instrument, set its input impedance to 75Ω (or use a 75Ω terminator) and adjust VR2 for a maximum carrier amplitude of 50mV RMS (141mV peak-to-peak). Adjust VR3 to get a modulation depth of about 30%, which means a carrier amplitude at the troughs of 35mV RMS (100mV peak-to-peak). Scope2 shows what the unit’s output should look like with 30% modulation, while Scope1 shows the carrier with the modulator disabled (eg, with Q1’s base shorted to its emitter). Both grabs were taken with the loop connected, so the output is correctly loaded to give a 50mV RMS signal. Australia’s electronics magazine If you were able to complete the above calibration, then it seems that everything is working correctly and you can start preparing the case. Fig.6 shows the holes that need to be drilled and cut. You may need to enlarge the hole “A” at the far right of the case, depending on whether you’re using a bezel for the LED and how big it is. To make the rectangular cut-out for the frequency meter, drill a series of small holes inside the perimeter, join them up with a file, knock out the piece inside and then file the edges to shape. Don’t worry about getting it perfect since we’ll be fitting a bezel over the top later, but the meter needs to fit into the hole, and you don’t want any huge chunks missing from around the edges. You can make the large round hole for the analog meter in a similar manner, but it will be easier if you use a 44mm hole saw, which cost around $8 at most hardware stores. As the hole size is specified as 44.5mm, if you find your meter won’t fit through, file around the edges until it does. You also need to drill four holes in the rear of the case, close to the bottom edge. We haven’t produced a drilling template as their exact locations are not critical. Just make sure to drill them along a line parallel to the edge of the case, so it looks neat, and space the three on the left side apart evenly. Try to get the positions reasonably close to ours, as the cable lengths given earlier are based on those locations. When finished, deburr all the holes. You can then consider painting and labelling the case. While not necessary, it gives a more professional-looking result. After drilling and cutting my box, I first treated it with Bondrite, which is an Alodine-like etching agent. I then painted it with VHT spray paint from a can, and baked at 93°C in a home oven for an hour. You don’t need to go to quite that much trouble; a few light coats from a can of decent spray paint suited to aluminium should give an acceptable result. siliconchip.com.au Using the Transanalyser with valve radios ing transformer so that the chassis can be Earthed for making measurements and injecting signals. Like most professional-grade RF generators, the Transanalyser’s RF OUT is DCcoupled and has a low impedance (75). So in many cases, you will need to insert a high-voltage series capacitor (say 10nF) + + 3.9k 5819 18k 1 F CON6 1kHz out IC3 TL072 2.2k CON5 To pot E VC1 Q1 MOD1 ITB0505S 10F C L2 + VR6 4 330 H Q1:2N2222 6 ~ 120mm ~ 150mm ~ 120mm 1 2 10F + + B ~ 150mm 100nF 100nF 15 F 2.2k 5.6k 100nF IC2 TL072 2.2k 510 220 F 2.2k 3 100 BAT46 IC1 TL072 680pF D1 4148 4148 D2 430k 3x 10nF 10F 2 12pF D3 CON2 VR3 100nF 500 + 180k CON1Meter in 18k 1 100k 100nF 4 D4 BAT46 L1 330 H – + + 10nF 1.8k 1.8k 180nF 100nF + 180k 10F + + 12 5 A VR5 + 12V DC in 100nF + 6 11 CON8 To meter VR1 500 7 10 1 F + 9 10 F 100nF ~ 50mm CONNECTS TO PIN 3 (TOWARDS FRONT) + 06102201 RevA H-field Transanalyser Dr. Hugo Holden 8 100nF REG1 7805 1N5819 + 10 F + + To counter CON4 + 1k MAX038 10k 1 F 220 F (LED1) 390pF CCW ~ 80mm CON3 Freq adjust 2k CW 12k 27pF IC4 510 1 F 100nF 300 100nF D5 100nF 100nF 100nF 1k 100 5.1k 3k 78L09 100k 5.6k 10 2k 7.5k 27k 5.1k 75 75 VR2 500 REG2 IC5 MC1496 1k 5 VR4 100nF 5.1k 100nF 6 3.9k 4 1.8k 3.9k GND + 1.8k 100nF 1.3k 3.9k 100nF 100 IC6 AD8056 1k 7 100nF 1.8k 1 F 300 1.3k 75 110 3.9k 8 10 F 100nF RF INPUT ~ 60mm 3.9k 2k 110 3.9k 110 75 75 1.8k 75 1.8k 3.9k 150 110 3.9k 9 75 CON7 3 110 1.8k 75 110 A 110 3.9k 75 1.8k 2 10 + 110 3.9k 1 75 75 150 RF out 3.6k 11 + A + 12 3.6k to couple the signal into various points in a valve circuit. You may also need to include a series resistor to increase its effective output impedance to suit the circuitry being tested. For example, add a 220series resistor to couple the signal into a circuit expecing a ~300source impedance. PLJ-6LED-AS FREQUENCY COUNTER MODULE (REAR VIEW) Fig.5: use this diagram as a guide when you’re wiring up the unit. The wire lengths are based on our – prototype; measure yours to verify they’re right POWER before cutting (remember to leave extra for the + stripped sections at each end and also some slack for case assembly/disassembly). The panel meter is not shown here. It mounts on the opposite side of the PCB to the two large pads either side of VR1, with M4 nuts ~ 120mm on both sides of the board in each case. 100nF As noted in the text, the Transanalyser is intended mainly for use with transistor AM radios. But the 1kHz OUT and RF OUT terminals are provided so that it can also be used with valve-based gear. If you are making any sort of direct connection to a valve radio with a hot chassis, you need to use an isolat- ~ 220mm REAR OF CASE CON1 siliconchip.com.au CON6 CON7 Australia’s electronics magazine CON8 June 2020 87 37 37 C C A 25 B 18.5 A 27.5 A A 42.5 WINDOW 20 x 76 42.25 18.5 50 18.5 A A 38.5 18.5 27 CL 44.5 DIAMETER 24 42.25 A A A 18.5 29 4 75 A A C 37 37 C C Fig.6: most of the holes that need to be made in the case are in the lid. The large rectangular cut-out for the frequency meter can be made by drilling a series of small holes inside the outline, filing them together until the middle section falls out, then filing the edges out to match the outline. If you don’t have a suitable hole saw, the 44.5mm diameter circular hole can also be made this way. Note that this diagram is reproduced slightly less than same size – case size is actually 222 x 146mm. HOLES A: 3.0mm DIAMETER ALL DIMENSIONS ARE IN MILLIMETRES HOLES B: 6.0mm DIAMETER HOLES C: 9.0mm DIAMETER CL I made the labels with a Brother tape label machine, with white text on transparent tape. Use whatever labelling method you prefer. Once the labels are attached, mount the frequency meter by feeding in four 88 Silicon Chip machine screws through the bezel, then the holes around the rectangular cut-out, and screw them into the spacers which come pre-fitted to the counter module. Make sure it’s the right way up, with the display deciAustralia’s electronics magazine mal points towards the bottom. Next, put the LED bezel into its hole and attach the PCB to the inside of the case using the two rotary switch nuts on the right-hand side and a tapped spacer and two machine screws siliconchip.com.au This photo shows how the PCB “hangs” from the front panel, supported by standoffs and the controls. Note that this is a photo of an early prototype board – the final PCB will look somewhat different. through the PCB mounting hole and corresponding front panel hole at left. We’ve specified a countersunk machine screw for the PCB mounting spacer through the front panel so that it sits flush, but you could use a panhead type if you don’t want to countersink the hole. Make sure the LED goes into its bezel as you bring the PCB up to the inside face of the case; note that you could get away without a bezel if you make the hole the same size as the LED lens. The Transanalyser’s case was mounted on 12mm thick tilted plastic feet attached with machine screws, so the front face adopts a 9° backwards tilt, to make it easier to view on the bench. If installing feet, do so now. Then fit all the chassis-mounting components and wire them up to the main board, as you did before for testing. That includes the frequency meter. Leave the rear-panel components until last, as once you plug them in, access to the PCB will be limited. Then join the two halves of the case together using the supplied screws. Attach all the knobs to the various shafts and the main unit is finished. The final step is to make up the cable that will be used to deliver the signal to the radio’s antenna. You can see my arrangement in the photo on p91. I soldered the bare ends of the coax to a small piece of PCB material and attached two tiny thumb nut terminals. These allow the thin wire loop to be connected and disconnected as needed. You will need to come up with a similar arrangement, although there are different ways you could achieve it. For example, the wire only needs to be disconnected at one end, and you could use a spring clip or some other wire connection device. The loop should be made from thin wire-wrap wire or similar, so it can be threaded through a narrow space. This siliconchip.com.au may be necessary where ferrite rods are mounted close to the radio case. Wire wrap wire works very well as it is delicate and easy to thread around a rod coil, easy to twist and doesn’t put excessive force on the sometimes delicate ferrite rod coil wires nearby. Using it Disconnect the small loop from the end of the test lead and thread it once around the radio’s ferrite rod antenna. The flying leads with alligator clips that lead to the Meter input circuit are connected across the radio’s volume control outer terminals. The loop has a very low reactance over the operating frequency range and acts like a dead short until the loop is placed around the ferrite rod. The resonant frequency of the tuned circuit on the rod then matches the applied frequency, and at that point, the loop’s impedance increases. The signal level at the volume control connection (detector output) is measured on the millivoltmeter in the Transanalyser. Why this is the preferred place to measure the radio’s response and not at the speaker output is explained later. Some calibration protocols and test instruments rely on monitoring the power level at the radio’s speaker, with the RF input sensitivity quoted for say 50mW at the speaker. However, because there is a wide variation of speaker impedances, this sort of testing is fraught with difficulties and pitfalls. Also, consider that depending on the volume control setting, the output stage could be driven into clipping, giving a false reading across the speaker. So I think it is better to test and analyse a transistor radio by monitoring the RMS voltage from its detector (or top leg of the volume control), rather than by a connection to the speaker. The audio amplification stage of Australia’s electronics magazine the radio can be checked separately by using the variable level 1kHz test tone provided by the Transanalyser. It is unlikely that the audio amplifier in small transistor radios would have to be checked at different frequencies, so the fixed 1kHz test tone should be adequate. The transformers and speaker largely determine the frequency response in most vintage transistor radios, along with the capacitors in the output stage on later transistor radios. Any such electrolytic capacitors can be checked for ESR, leakage and capacitance easily, to verify that they are not having any adverse effect on the output frequency response due to ageing. For radios with transformer-less audio amplifier designs (like the Hacker Sovereign and others), the only way to be 100% sure about the audio amplifier functionality is to do a full audio frequency sweep; however, a good listening test manipulating the bass and treble controls would show any significant fault. The Transanalyser could be modified for its frequency synthesizer IC to produce an audio sweep, but in the interests of simplicity, I thought that to be unnecessary. IF alignment For IF alignment, you just need to set the Transanalyser to the correct intermediate frequency and feed the signal in via the loop as usual. The modulated IF signal will easily break through the mixer to the IF stages (even with the local oscillator running). This is preferable to injecting a 455kHz signal into the mixer output, as this alters the tuning. Many transistor radios have a combined mixer-oscillator, so it is not possible to deactivate the oscillator without altering the operating conditions of the IF amplifier. In cases where the June 2020 89 Similarly, the early PCB from the opposite side. Very clear here are the brass shields on the top of the board. radio has a separate oscillator transistor, it can be unplugged if it has a socket, or its base and emitter temporarily shorted out to deactivate it. A lower IF signal level will then be required to be fed into the antenna. If the local oscillator is not (or cannot be) deactivated, it is best to have the radio tuned to the low end of the band for IF alignment. Regardless, use the weakest possible IF signal to peak the IF stages, but keep it above the noise floor by observing the effect on the millivoltmeter. Strong signals and AGC action can alter the IF tuning and make the tuning peaks more difficult to observe. In addition, the test protocol for aligning IF stages (typically around 455kHz in most transistor radios) involves peaking them on the one centre frequency. The design of the IF transformers themselves determines the bandwidth. This is one reason why a ‘wobulator’ or frequency sweep of the IF amplifiers in transistor radios has limited utility. They are not meant to be stagger-tuned to any specific bandpass characteristic (unlike the video IF stages in TV sets). The IF bandpass response can be easily measured with the Transanalyser. You just adjust the Transanalyser’s VFO up and down in frequency until the millivoltmeter reading drops to about 70% of its peak value, and subtract the two frequency measurements to determine the -3dB bandwidth. Aligning transistor radios Fig.7 shows the adjustments typi- cally available in AM broadcast band transistor radios. Rarely, some radios (such as the NZ-made Pacemaker) have a three-gang capacitor and an additional radio frequency stage. There are many variations, so it pays to check the manufacturer’s alignment instructions. The information here is a general guide. Twin-gang variable capacitor VC1 & VC2 are often 6-160pF and 5-65pF respectively, or similar value. If the gang values are the same, a padder capacitor is used to lower the overall value for the oscillator. VC1 tunes the antenna coil and TC1 trims the antenna circuit to set the high-end of the band to around 1200-1500kHz. A sliding coil on the ferrite rod is typically used to set the low end of the band to around 550-600kHz. VC2 tunes the oscillator coil. A slug in the oscillator coil is used to set its lowest frequency to match the dial calibration, while TC2 sets the maximum oscillator frequency to match the upper dial calibration. All IF transformer slugs are usually peaked on the specified centre frequency, typically 455kHz, although 465kHz is not uncommon. Very old transistor radios such as Regency TR-1 had 262.5kHz IFs. This is why the Transanalyser VFO output goes so low. The oscillator is arranged to tune over a set of frequencies which are above the AM broadcast band by the intermediate frequency. So if the radio tunes stations from 550-1650kHz and the IF is 455kHz, the oscillator tunes over a range of (550+455)kHz to Fig.7: this shows the typical adjustments that are available in a transistor AM radio. VC1 & VC2 are the elements of the tuning gang. These are trimmed by TC1 and TC2 (and sometimes a moveable coil on the ferrite rod) to adjust the tuned frequencies at upper and lower ends of the dial, and to set the tracking. The IF coils usually have slugs which can be rotated to peak their response at or near the intermediate frequency. 90 Silicon Chip FERRITE ROD (1650+455)kHz, ie, 1005-2105kHz. The mixer then generates a difference signal at the same intermediate frequency for all stations. Therefore, it is important that the tracking is correct. This represents the range of the frequencies tuned by the antenna coil on the ferrite rod versus the range of tuned frequencies selected by the oscillator frequency minus the IF frequency. The tracking can only ever be correct at three points; normally near the upper and lower ends of the band, and right in the middle. Tracking errors occur on either side, but they are usually small, so the bandwidth of the IF stages is wide enough to let signals through that are slightly off due to these tracking errors. Generally, the IF is aligned first to the correct centre frequency. Then a low-end signal at around 550kHz is used to adjust the oscillator slug; so the low end of the dial calibration is correct. If there is a padder capacitor, this is used instead of the oscillator coil slug, radios that use padder capacitors often have no adjustable slug in the oscillator coil. Then a high-end signal around 1200-1500kHz (often specified in the alignment instructions) is used to adjust TC2 to make the dial calibration correct. The above process is then repeated a few times, as one adjustment affects the other a little. This ensures that the IF and oscillator are correct and that the received frequencies are over the correct range and match the dial calibration as best possible. LOCAL OSCILLATOR ANTENNA COIL VC1 IF COILS (x3) OSCILLATOR SLUG TC1 TWO GANG VARIABLE CAPACITOR Australia’s electronics magazine IF SLUG VC2 TC2 PADDER IF PRESENT SC 2020 siliconchip.com.au TABLE 1: H-FIELD TRANSANALYSER TEST RESULTS – THREE RADIOS 0dB –10dB –20dB –30dB –40dB –50dB mV OUTPUT 50 20 16 14 13 10 SUBJECTIVE N0 N0 N0 N0 N1 N3 LEVEL: HACKER SOVEREIGN (2N2084) –60dB –70dB –80dB Meter fluctuations due to noise N4 N5 N5 CLIP RATIO = 5 mV OUTPUT 120 160 165 100 70 SUBJECTIVE N0 N0 N0 N1 N2 Meter fluctuations due to noise SONY TR-72 N3 N4 N5 N5 CLIP RATIO = 7.5 mV OUTPUT NORDMENDE CLIPPER SUBJECTIVE 300 180 95 80 76 N0 N0 N0 N1 N2 Meter fluctuations due to noise N3 N4 N5 N5 CLIP RATIO = 7.5 N0: No significant noise heard, just modulation N1: Audible modulation >> Noise N2: Audible Noise = Modulation N3: Audible Noise >> Modulation N4: Modulation just audible in Noise N5: Noise heard only The numerator for the Clip Ratio can be read right off the Transanalyser’s voltmeter with its output attenuator set to 0dB, but the denominator is a bit more tricky. You can measure this by connecting the Transanalyser’s 1kHz audio output between the radio’s volume control pot wiper and ground, with the volume control set to mid position so that the control itself does not load the applied signal. You then adjust the 1kHz output level and measure its amplitude at the onset of clipping. This is easily determined without an oscilloscope by the sound from the speaker. The ‘soft’ sound of the sinewave suddenly becomes ‘sharp’ with a ‘zinging’ sound at clipping, due to the high-frequency harmonics created. Other notes Finally, the antenna circuit is peaked. TC1 is used at the high end. The low end can only be peaked by sliding the antenna coil on the ferrite rod. In many cases, it is completely sealed with wax and attempting to move it would damage it, so it is best to leave it alone and tolerate low end tracking errors. Subjective performance tests Listening to a radio receiver with a 1kHz modulated RF signal, I have found it that is very easy to subjectively grade the noise into five categories without too much ambiguity. I label them as follows: • N0 – no significant noise heard, just the loud and clear demodulated signal • N1 – modulated signal level is greater than the background noise • N2 – the modulated signal and noise levels seem equal • N3 – noise is dominant, but the modulated signal is still audible • N4 – the modulated signal is barely audible in heavy noise • N5 – only noise is heard. I tested three radios, and the results are shown in Table 1. Note how the Hacker Sovereign (on the AM broadcast band) has relatively low detected audio voltage levels, but as it has much more gain in its audio amplifier stages, the subjective results are better than the other two radios listed. This radio had been re-populated with 2N2084 transistors, as the origisiliconchip.com.au nals failed from tin whiskers. Clearly, in the noise department, the 2N2084 transistors are superior to those used in the 1956 TR-72 or the OC44/45 or similar used in the Nordmende Clipper. The “Clip Ratio” numbers given are the ratio between the output of the detector with a strong antenna signal and the voltage at the wiper of the volume control pot just on the edge of clipping. Another way of looking at this is that the higher the Clip Ratio, the weaker a radio station can be and still give you full volume at the speaker. This number is a good way of doing a quick ‘health check’ of a radio even if you know little about it. If you get a figure in the range of 4-10, that indicates that the radio’s front end is more or less healthy and providing enough signal to the audio stages for it to be useful even with weaker (eg, more distant) stations. In general, when feeding the radio a test signal from the Transanalyser (or any source for alignment purposes), the audio signal (recovered modulation) should be enough to hear clearly above the noise, but not so high as to induce significant AGC action. The AGC action minimises the visible peaks on the output meter, and AGC also alters the tuning. For the three radios I tested, a good level was with the Transanalyser’s attenuator setting at either -30dB or -40dB. It is also possible to use the Transanalyser to determine the signal level where the radio’s AGC becomes active. If the radio (or the Transanalyser’s) tuning frequency is manually adjusted across the tuned carrier, the millivoltmeter momentarily passes to a higher value before settling to a lower one, which is easy to see on the analog meter. This is due to the time constant of the radio’s AGC filter. SC This small PCB, with a 75 terminating resistor, has screw terminals allowing the loop to be disconnected and threaded around the ferrite rod. An RCA-to-crocodile clip connector can tap into the signal for the millvolt meter or apply signal from the 1kHz tone generator. Australia’s electronics magazine June 2020 91