Output meter ?

[David Goodall]

When aligning a radio, is an output meter simply a voltmeter. Can I use an AVO on AC ? Also, are you supposed to disconnect the speaker? The manual says turn the volume, and tone up full.

[Station X]

You'll find the circuit for an output meter here:-

http://www.vintage-radio.com/project...indicator.html

The trouble with using an AVO is that the response may not be too good at audio frequencies. Try it and see. Leave the speaker connected to act as a load.

[Brian R Pateman]

AVO is perfectly acceptable.

If you disconnect the speaker don't forget to connect a load resistor in its place or you risk doing some damage especially to valve output stages.

The advantage of a decent output meter is that it will load the output correctly.

Regards,

[David Goodall]

If the speaker is left in, doesn't it risk being damaged by the full volume ?

[Station X]

Not if you inject a weak signal, which you should do to prevent the AGC operating.

[David Goodall]

Aaargh ! Of course. It says keep the output below 13.5 volts to stop the AGC coming in. All is clear. (For now) Many thanks.

[Phil G4SPZ]

For what it's worth, I've found an output meter invaluable as an aid to accurate alignment. I paid £1 for a Marconi TF893A output power meter at a junk sale, and it is so much more accurate than aligning by ear. I was surprised at the sensitivity of the meter to small changes in output, compared with the human ear. I seem to recall that one decibel is the smallest difference in sound levels that can be detected by ear; an output meter will of course register smaller changes.

In the absence of a dedicated output meter, any AC voltmeter of adequate sensitivity should suffice. As has already been said, keep the input signal low and reduce it as the alignment process proceeds. I have never encountered any problems leaving the speaker in circuit - after all, you are simply looking for a peak or dip in response.

The only difference between an output power meter and an AC voltmeter in this application is that the former is calibrated, and can provide an accurate load impedance. For radio alignment purposes, neither is necessary.

[Skywave]

Phil - your last sentence - "for alignment, neither is necessary".

So what do you use as an O/P indicator when performing alignment? Not simply a loudspeaker & your ears, surely?

Al. / Skywave.

[MichaelR]

Originally Posted by Phil G4SPZ

The only difference between an output power meter and an AC voltmeter in this application is that the former is calibrated, and can provide an accurate load impedance. For radio alignment purposes, neither is necessary.

I love those Marconi audio output meters, the other major advantage of these type of meters is that you can check the complete overall perfomance of the receiver . Follow the manufacturers control settings and RF input level injected the meters will measure the output power of the audio stage. ( with the speaker replaced by the meter set to its impedance).

Mike

[Skywave]

I frequently use my Marconi O/P power meter for assessing sensitivity, signal-noise ratios & signal / image response ratios of comms. receivers after refurbishment - plus, of course, RF / IF alignment. I wouldn't be without one.

Al. / Skywave.

[Phil G4SPZ]

Originally Posted by Skywave

Phil - your last sentence - "for alignment, neither is necessary".

So what do you use as an O/P indicator when performing alignment? Not simply a loudspeaker & your ears, surely?

Al. / Skywave.

Perhaps I should have said "neither feature is necessary", meaning being calibrated or imposing a known load. I was trying to make the point that a meter of any sort is better than one's ears.

[Skywave]

Thanks Phil; now perfectly understood; you had me worried for half a mo'

Al.

[David Goodall]

I have a little multimeter made by "Adastra" that I've had since 1976. It cost me £12. I was 12 at the time, and for the last 33 years I've not known what the "Output" socket was for ! It has a decibel scale, is 20,000 ohms per volt DC, and 10,000 ohms per volt AC. Will this be good enough? A third of a century later, am I actually going to use this feature ?!

[peter_sol]

Just use a low AC range on the analogue meter, and leave the speaker connected.

Adjust the volume as the circuits come into tune so you can see movement of the meters needle.

If you try it you will soon get the hang of it.

Regards

Peter.

[Phil G4SPZ]

It's perhaps worth mentioning that aligning using an audio-sensing output meter virtually dictates the use of a signal generator to provide a steady input as a tone-modulated carrier. Whilst most of us will have acquired a signal generator (or several!) over the years, a simple test oscillator is all that is needed so long as it is (a) stable in frequency, and (b) adjustable in output level.

Peter's right, try it and see. Like I was, you'll probably be surprised how sensitive the meter needle is to small changes that are quite undetectable by ear.

[David Goodall]

I shall give it a go. I have a signal generator. Any thoughts on my Adastra with the "Output" socket.

[Station X]

I have a cheap analogue meter which has a dB scale running from -8 to +22dB. There is no "output " socket though, the normal COM and V sockets are used. It's calibrated as 0dB equals 1mW in 600 ohms. The dB scale reads directly when the meter is set for 10V AC. For 50, 250 and 1000V DC 14, 28 and 40 dBs respectively have to be added to the readings.

[David Goodall]

Thanks Graham. By the way, I bought this meter at "Eagle Electronics" in Eagle street Ipswich ! 1975 or 6. I don't know if you knew it or not. I would spend most, if not all my pocket money there building crystal sets, and whacky projects out of "Babini" books. It was all painting by numbers so to speak. Didn't understand how they worked, (which is why most of them didn't !) I've rekindled the hobby during the last couple of years, and am trying to right the wrongs by reading Scroggie, Camm, and Scott-Taggart.