Using Homemade Attenuator for Valve Radio AM Alignment

[MrVa1ve]

With my capacitors winging their way from the far east, I've been contemplating my post-recap tasks. The set is a Philips 310A variant (I'm not quite sure what model exactly, since all labelling has been lost). I'll most likely need to do a realignment of the set, since I've repaired the tuning capacitor blades (see other thread) and accidentally changed a trim cap during an over-vigorous cleaning session (yes, I know - I should have marked them!)

Anyway, I'm pretty new to all this and research I've done on this site and elsewhere would suggest that I would find a decent signal generator advantageous. I do have some basic electronics gear including a cheapo DDS arbitrary waveform generator that's capable of generating sine waves to over 20MHz. I'm not sure how much restoration work I'll actually do, so I'd be cautious about splashing out on expensive equipment at this stage.

So, what I'm wondering is if it were possible to use the DDS generator I have and build my own step attenuator, something like the one here (https://www.arrl.org/files/file/Tech...Attenuator.pdf). It sounds like it might be a useful piece of kit and I like building stuff anyway. So, my question is has anyone else tried this and what are the pitfalls?

[Nuvistor]

You will need to have the sig gen in a good screened enclosure or the signal will bypass the attenuator.
Frank

[ms660]

If the DDS isn't modulated you can monitor the DC at the detector or AGC line with a DMM or scope for maximum signal when aligning.

Lawrence.

[G6Tanuki]

I've never found it necessary to connect a local signal-source directly to the input of a receiver - unless there's something horribly wrong with the receiver you'll get adequate (and often excessive) coupling from a test oscillator on the bench to a receiver by virtue of free-space.

Remember the useful "square law" attenuation effect of distance.

In the past I've 'attenuated' the fundamental and harmonics from a battery-powered 100KHz crystal calibration oscillator by putting it in the summer-house, some 100 yards or so from the receiver I'm working on.

With the osc. feeding a few feet of wire as an antenna [hint: a typical DVM test-lead plug fits a SO239 coax socket], and a load-resistor fitted across the antenna/earth terminals of the receiver I was aligning, the harmonics were easily detectable at a level which didn't activate the AGC.

[MrVa1ve]

That's a very good point G6 and you've reminded me of a little project I completed a while back whilst following Ron Quan's book "Build your own transistor radio". In the early sections of the book he recommends you build a simple variable square wave oscillator covering the AM broadcast band modulated by a 1KHz triangle wave. The antenna was a simple wire which is exactly as you described.

I'd forgotten I'd built it! I just rummaged around and found it, but I've noticed it stops at about 760kHz. The square wave would provide a fundamental and a series of odd numbered harmonics, so perhaps that's why it's limited to such a low frequency. At any rate, it probably isn't sufficient/accurate in its current form. But you've given me some ideas for an improved circuit.

[G6Tanuki]

My "100KHz calibration oscillator" uses a 7400-series logic-chip as the oscillator itself, which generates pretty much a 5-volt peak-to-peak square-wave: I then feed this through two parallel-connected high-frequency switching diodes [1N4148 or-similar] to generate maximum amounts of clipping.

The harmonics are well-audible to 30MHz. It all runs off a single PP3 battery (via a 7805 regulator). In times-past I would 'tweak' the 30pF trimmer across the xtal to zero-beat it with Radio4 on 200KHz. Then they went and shifted R4 to 198KHz so this was no longer possible. WWV on 5.000MHz became the answer!

[Skywave]

Originally Posted by MrVa1ve

It sounds like it might be a useful piece of kit and I like building stuff anyway. So, my question is has anyone else tried this and what are the pitfalls?

During my days of employment in government R & D during the mid-1970s, I built such an attenuator and subsequent to that, as the years have passed and other attempts have been made, I have learnt the following.

1. The standard of the mechanical construction must be above reproach; thorough screening without any gaps is essential.
2. Use carbon resistors; do not use ones made with a spiral track (stray inductance). A good design approach is to consider using SM resistors.
3. Dependent upon the accuracy you require for each attenuation section, you will need to measure the resistance of each resistor carefully. That, in turn, requires an Ohm-meter instrument of known calibration accuracy that is commensurate with the required accuracy of the attenuator.
4. Where a value of resistance is required but that value is not readily obtainable, thus requiring a combination of two resistors, do not put resistors in series; use the parallel connection. Aim at using resistors from the E24 range as a minimum; ideally E48 or 'higher', or a combination of both, if necessary. Never use more than two resistors to obtain the required resistance value. (Increased capacitance to metalwork case & screening). However, see 3 above.
5. Capacitance from the resistors to the attenuator's screening can cause a frequency-dependent attenuation at high frequencies, especially in the high attenuation sections. Adequate spacing is the usual remedy.
6. The inter-contact capacitance of the switches can be problematic for same reason as in 5 above and with same consequences. Select low inter-contact switches; slide-type switches I do not recommend for that reason.
7. All connections inside the attenuator must be made with short wires (Stray inductance).
8. Theoretically, the contact resistance of each switch is important, but with relatively high values of the resistors in each attenuator section, that rarely causes a problem.

And finally, just to add to the fun, you then need a method and a suitable instrument to measure the attenuation of the finished article!

From all of that, you can readily understand why switched attenuators designed for use, say, to 50 MHz (or more) are expensive!

Al.

[MrVa1ve]

Much as I like a challenge, I think you're right. It's all too easy to make a mess of it. Which is why G6s approach is looking more attractive. Essentially, I need an accurate local transmitter (preferably with the ability to add a modulating signal) and let the distance do the attenuation for me.

I'd still like a homebrew attenuator, but it looks like I'll be spending some time picking up the right parts.

Originally Posted by Skywave

During my days of employment in government R & D during the mid-1970s, I built such an attenuator and subsequent to that, as the years have passed and other attempts have been made, I have learnt the following.

1. The standard of the mechanical construction must be above reproach; thorough screening without any gaps is essential.
2. Use carbon resistors; do not use ones made with a spiral track (stray inductance). A good design approach is to consider using SM resistors.
3. Dependent upon the accuracy you require for each attenuation section, you will need to measure the resistance of each resistor carefully. That, in turn, requires an Ohm-meter instrument of known calibration accuracy that is commensurate with the required accuracy of the attenuator.
4. Where a value of resistance is required but that value is not readily obtainable, thus requiring a combination of two resistors, do not put resistors in series; use the parallel connection. Aim at using resistors from the E24 range as a minimum; ideally E48 or 'higher', or a combination of both, if necessary. Never use more than two resistors to obtain the required resistance value. (Increased capacitance to metalwork case & screening). However, see 3 above.
5. Capacitance from the resistors to the attenuator's screening can cause a frequency-dependent attenuation at high frequencies, especially in the high attenuation sections. Adequate spacing is the usual remedy.
6. The inter-contact capacitance of the switches can be problematic for same reason as in 5 above and with same consequences. Select low inter-contact switches; slide-type switches I do not recommend for that reason.
7. All connections inside the attenuator must be made with short wires (Stray inductance).
8. Theoretically, the contact resistance of each switch is important, but with relatively high values of the resistors in each attenuator section, that rarely causes a problem.

And finally, just to add to the fun, you then need a method and a suitable instrument to measure the attenuation of the finished article!

From all of that, you can readily understand why switched attenuators designed for use, say, to 50 MHz (or more) are expensive!

Al.

[kalee20]

I usually do alignment with a signal generator, set to give a healthy output, positioned a reasonable distance away. Shorting the output, but with a wide loop, and positioning said loop in line with a ferrite rod aerial, frame aerial, or input tuning coil, gives sufficient signal to do the job.

It means that the radio is disturbed as little as possible, so any detuning effects of the signal generator on the circuitry, is negligible.

If I'm trying to get a nice IF passband response with more exotic equipment, then this approach is not applicable because the input needs to be guaranteed constant as the frequency is swept. But that's almost outside the scope of the OP's request. (Incidentally, I would suggest using 2 or 3 cascaded step-down transformers, each 10:1 turns ratio, wound on 20mm ferrite toroids).

[Mach One]

If the radio is an old LW / MW / SW AM set then the only bit that you should not do this way I would argue is the aerial input circuitry. I would adjust this connected to the antenna that you would use for the set. My reason for this is that I believe that there is an interlinking effect of the generally simple coil input and the antenna on an old radio.

For this reason, and the fact that most of the transmissions that the set is likely to pick up are very stable, known transmissions on a known frequency, I have always and successfully aligned my sets on broadcast transmissions with the aerial attached.

The only time that I would think differently would be if I ever attempted to adjust the FM circuitry - I have never had the courage - or the equipment...

[ms660]

There used to be dummy antenna pads available for signal generators.

Lawrence.

['LIVEWIRE?']

In the service data of many AM radios, IIRC, a loop, connected as kalee20 says in post # 9, is often specified for MIxer/Oscillator alignment, although I often do as Mach One says, using known broadcast frequencies - in this area there are strong signals on 1413, 1215, 1053, & 693kHz MW, plus others on 1296, 1089, 909, 882, etc., which allow accurate setting of most radios, certainly all those with analogue scales. Dummy aerials are, or were, supplied with RF Generators, although the one I use with my AM/FM Genny is a home-made one, built into the case of an old motor from a cassette player. This is optimised for car radio Aerial circuits, and, IIRC, consists of a series & shunt capacitor, with a BNC socket at the input end to connect to the SG, and about 0.5M of car aerial co-ax cable terminated in a non-Iso Car aerial plug. The SG's o/p Z is 75 ohm, whilst Car Raio aerial i/p Zs are, or were 150 ohm ( Those with ISO aerial connectors are nominally 50 ohm, but the setup described works well enough with both types, although I've never understood how the Non-ISO to ISO aerial adaptor reduces the impedance from 150 to 50 ohms.