Transformer Ratio Meter

[Nick]

If someone is going to do radio servicing/restoration and/or homebrew radio builds I would say a variable frequency RF sig gen is more of a priority than a variable frequency AF sig gen in the test gear stakes, that and a couple of multimeters, a decent analogue one and a reasonable or cheapo digital one.

The point about the analogue multimeter is important - most service manuals (all of the antique genre) had voltage readings taken with a 20,000 ohms/volt meter (or similar), e.g. an AVO, which would potentially give very different readings when measured with a typical DMM at 1,000,000 ohms/volt input, i.e. the DMM will read higher as it's presenting far less load to the the device under test (DUT).

The second point about analogue meters is that they "average" readings due to the mechanical and thermal inertia of their workings - digital meters take a snapshot ever few milliseconds or so and display that, which means that on some signals they can appear erratic.

Know your test instruments  Don't blindly trust what they say they're doing - they may be being "economic with the actualité" as well as adversely effecting the DUT's behaviour...

[AlanBeckett]

If someone is going to do radio servicing/restoration and/or homebrew radio builds I would say a variable frequency RF sig gen is more of a priority than a variable frequency AF sig gen in the test gear stakes

Indeed, and most rf Generators will have a simple af Output anyway.

Alan

[SurreyNick]

The item of test gear that I sent you Nick wasn't a signal generator (used for RF/IF alignment) - it was  a signal injector/tracer that I'd made for you for fault tracing, based on a Velleman K7000 Signal Injector/Tracer kit.... No doubt it's packed away for the house move Nick, along with the instructions on how to use it.

Oh, of course, that's what it was  It is, as you rightly suspected David, packed away and has been for several months. Gosh my memory really is rubbish nowadays!

I've decided there is little point in worrying about buying additional kit before the move. SHMBO would probably castrate me if I brought something new into the house just now, as it is her eyes roll every time she sees my ratio meter project and associated paraphernalia taking up one end of the kitchen table. Until we move I also don't know what working space and storage I'm likely to have. Better to wait I think. I will also want to understand far better what the various items of equipment can and will be used for on the radios I'm likely to work on. So I'm likely to have quite a few questions to ask of you guys.

In the meantime I will stick to building the four projects I have in hand. The ratio meter being one. Details of the other three to follow in due course.

Thanks for all the feedback so far though.

Nick

[Keithstv]

Now that sounds like an ideal oportunity to sneek a few extra items in during your move.

(If you can get away with it!)

Keith

[SurreyNick]

Now that sounds like an ideal oportunity to sneek a few extra items in during your move.

(If you can get away with it!)

Keith

Ha, ha, ha  
Droll Keith, very droll.
If I thought I could get away with it.....but no,
My beautiful wife has a nose as sensitive as a bloodhound, eyes as sharp as a hawk, hearing as acute as an owl, and the intuition of a, a, a.....oh, yeah, of a woman!
Not a hope in heck mate
Nick

[Radio Fixer]

I tend to disagree on the analogue meters versus digital. Never get the AVO 8 out and gave 2 to the BVWS B&B. I want to know what the accurate voltages are now and the only ones that matter are HT and valve cathodes and neither is affected by the impedance of the meter used. From the cathode voltage its easy to calculate anode +screen current and that is the best health check for each stage.

I wouldn't buy to cheap a digital meter either unless you are strapped for cash. My White Gold has given years of service and when I was working on a project that needed true RMS readings I treated my self to a Uni-T 70D. This is really clever and has digital signal processing and does area under the curve measurements (remember those from college, all integration stuff, loved it) far from quick averaging.

The original thread was about measuring transformers and Yes! if you are working on radios than an RF genie maybe more useful. But to measure transformers than an AF one is what you need. Ever rewound inter-stage ones? or looked at some of the US ones that are supposed to do from DC to annoying dogs as equivalent? Without an AF genie you will never know if its 3dB down at ...

Gary

[pwdrive]

With analogue and digital meters each can have it's own merits to the users of such, I mainly use a digital either for accurate low voltage checks in high impedance circuits and for checking very low value or very high value resistances, for most other measurements I use an Avo 8 especially when monitoring an AGC rail or the voltage in to a dummy load (sound output) when aligning RF/IF circuits, I've tried several times to align an IF stage to a crystal filters frequency or establish a crystals frequency by manually sweeping the sig gen using both a digital and an analogue meter by monitoring either the AGC line or the detector current, the analogue was the best for me for sure.


As always with all this test gear preference/priority malarky it's down to the user and what they intend to do most of and what they feel comfortable with.

One could always build an AF oscillator with two RF oscillators and a mixer like wot they used to do....

Lawrence.

[Nick]

Most DMMs that claim to be "True RMS" are AC coupled when computing RMS values so they don't include any DC component that may be there.

One of the reasons I chose a Tektronix DMM916 some years ago is that it gets this right - it gives you the choice of AC true RMS or AC+DC true RMS...

[Radio Fixer]

Analogue meters (AVO 8) are useless at monitoring AGC line voltage, they simply load it too much. Another reason that filter caps on that line are so important. Need > 10M Ohms else loading occurs. Use digital here or a scope with a 10X probe. I do use an analogue meter across loudspeaker terminals when doing alignment sometimes but if the radio has an eye tube or shadow meter that's all that's needed and is very handy. My analogue meter of choice here, if used, is a Level AF / RF voltage, acquired cheaply, that has a large vertical, easy to read meter and a big easy to adjust range knob in a quite small box. Sits on the window sill.

Some Fluke meters have the ability to compute True RMS with or without a DC component but were more than I wanted to spend. With the UNI-T you just measure them separately and do a quick sum with a calculator, no problem. Great auto ranging meter, good price.

Gary

[pwdrive]

An ordinary analogue meter will load the AGC line (obviously) but how much it loads depends on where the connection is made.

AGC diode loads can vary quite a bit depending on the circuit design, eg: a megohm or greater down to 200k or less.

The point I was trying to make is that for myself and lot's of other folks it is easier to interpret a moving needle against a background scale as opposed to a purely numerical and sometimes erroneous representation with a digital voltmeter display in the context of obtaining a peak reading when adjusting tuned circuits etc.

Using an analogue meter was/is a quick way to establish if the local oscillator is working in a valve receiver although in most cases the impedance of the oscillators grid circuit will less than what's typical in an AGC line.

If I had one I would use a valve voltmeter.

But hey ho each to their own.

Lawrence.