Alignment meter

[agardiner]

Hi,

So there are several ways of hooking up some kind of meter, mainly to the output, when aligning a radio. Indeed, the sister site to this forum has a simple project idea for exactly that, and I have indeed built and used this method.

Personally I like to hear the modulation tone when aligning, as it gives confidence that you are adjusting the correct signal, as it is possible to go off frequency and end up with noise which will also be able to be 'peaked' on your output meter. The problem therefore is connecting a meter that is sensitive enough to provide a useful reading while hearing the output without having the tone loud enough to annoy.

My solution has been to modify a sound level meter. There are plenty of cheap 2nd hand analogue units on ebay, and I picked up mine for £15 delivered. In my case it was a YF-20, but any similar unit should do. These meters just use a microphone to provide a sound level, and are ideal for modifying.

The modification itself is really simple. I removed the microphone and installed a BNC socket in its place, connected via a 100nF capacitor. I then adjusted the calibration preset to lower the sensitivity of the unit. Of course actual calibration is not needed as you are only after an indication of relative level. That is it.

In use, I connect the unit across the speaker and can now have a nice low level of hearing the tone, while having a sensitive meter to peak the adjustments on. The YF-20 is a particularly nice unit for the task as a) the BNC socket fits nicely in place of the mic and b) it has two sensitivity levels, ideal for working across a range of sets.

[Philips210]

That looks really good and a very useful addition to the workshop. I will modify a similar sound level when I can find the time.

Thanks for sharing your modified meter.

Regards,
Symon

[Radio Wrangler]

On sets with really good AGC, it acts to flatten the variation in audio level as the RF level is varied so it can affect your appreciation of how much things are changing by. I either do alignment by keeping the sig gen set so low that AGC action hasn't started, or I put a DC voltmeter on the AGC line (If there isn't already an S-meter). S meter calibrations are notoriously iffy, so the low RF method really is the better.

David

[Cathovisor]

Traditionally alignment has always been done with an RF level low enough to avoid the AVC coming into play, however that usually results in an AF level rather low for most analogue multimeters to display when connected to the speaker terminals. IIRC an AF level of 50mW is frequently quoted which is around 400mV on a 3-4Ω load.

The easy solution to that is to connect your meter across the primary of the output transformer with a cap in series to remove the DC component.

[turretslug]

As well as an output level meter (in my case a simple, venerable but very flexible in terms of range and loading Dymar unit), I like to hang a 'scope in parallel with it. Then, I can watch the demodulated tone rise out of the noise when using the necessarily low RF input level (as discussed above) and be certain that I'm not peaking noise, sprog or instability. Of course, one could simply listen to the output instead but I find that long periods of straining to hear low-level 400Hz peaking and subsiding with a complex receiver can get a tad brain-numbing.

[Radio Wrangler]

A really posh output level meter would be true RMS so it will read properly on noise so you can check S/N ratio.

A really really posh output level meter would also have a notch filter that can be switched in for doing SINAD and measuring S/N ratio with a signal to entertain AGC and AVC systems.

There's a whole range of possibilities from the basic to the elaborate.

David

[DMcMahon]

Originally Posted by turretslug

Then, I can watch the demodulated tone rise out of the noise when using the necessarily low RF input level (as discussed above) and be certain that I'm not peaking noise, sprog or instability.

What is sprog ?

David

[Reelman]

Sprog?

A spurious emission generated within a transmitter or receiver. IE, a signal that you don’t actually want but can be confused with one you do.

The above is my take on “sprog” but I looked it up out of curiosity. Besides the common use to describe the children it can refer to or mean very different things. I can imagine the question brought a smile to our Oz friends…..

Peter

[CambridgeWorks]

Surprised AVO is not mentioned. Looking through an old model 8 handbook, ac voltage spec is 25Hz to 2KHz and it has a dB scale for relative measurement.
Don't know about other models.
Rob

[Boulevardier]

Originally Posted by robinshack

Surprised AVO is not mentioned. Looking through an old model 8 handbook, ac voltage spec is 25Hz to 2KHz and it has a dB scale for relative measurement.
Don't know about other models.
Rob

Surprised me too!
Mike