Transformer Ratio Meter

[Refugee]

I have a restored valve volt meter.
It is like an AVO7 with valves in it and loads the circuit no more than a DMM.
http://golbornevintageradio.co.uk/forum/...p?tid=3358

[Radio Fixer]

Yes! an excellent restoration Ref. But too big for me but most people have bigger benches. My Level is tiny in comparison and stands vertical with the meter on the front face. I only use it for alignment across the LS terminals.

You were right though if the meter, in your DMM, was working and linear it would have made good money for an AVO valve tester MK1 and Mk2.

Gary

[AlanBeckett]

How about the best of both worlds?

   

It really is Electronic but it drives an Analogue Meter. The LCD changes to match the Range selected.
Quite amusing really

Seriously, this topic is circled on most Fora at some time or other. It tends to generate more Heat than Light.
I use both - my AVOs for old Sets where it was what was used for the Service Manual and modern ones for modern Equipment.
I've even got a splendid Bench DVM with fearsome accuracy if I really want to measure something.

I've long been of the view that the fundamental question is 'Do you know what you're trying to measure?' and 'What variation is acceptable?'. To put that into a simple context most old Radios, and a lot of Test Gear for that matter, were built with 20% Resistors. With luck there'll be two or three Mains Taps so you'll get withing a few Percent. The Valves will age but still give acceptable Service. So, if the Manual says the Anode Voltage should be say 220v what will you accept? with which Meter? before you start changing things.

Alan

[Mark Hennessy]

Some wise words there. In summary, you have to be pragmatic. Which is easy with experience, but can be confusing at first. Especially if you have a nice new shiny DMM that is giving you figures like 2.9381V DC - we as humans like that sort of thing, even though it might be really quite a way off from reality (there is a massive difference between precision and accuracy).

Most well-designed circuits are very forgiving - from the ground up they would be designed to work well with the expected spread of component tolerances (and then some), plus the expected spread of supply voltage (and then some), plus the expected reception conditions. No experienced designer would sign off mass-production of anything that is super-critical or in any way fussy - that's commercial suicide. At the very least, detailed R&D would be needed to quantify the effects of the expected variations so that an informed decision can be made (and warning made in the service documentation). Far better and more economical to just design the circuit differently in most cases. Fortunately, negative feedback is extremely helpful in many situations like this

With most repair jobs, the voltage in question usually adopts one of two conditions: either "about right", or "miles off". The meter used shouldn't really be a big deal here, but of course, loading effects might be a concern in high-impedance areas.

When investigating more specialist equipment, or when doing circuit design, then yes, more detailed results are often needed. But by the time you're doing this sort of work, you'll have a much clearer idea in your own mind about what is going on, and will be able to account for any limitations in your gear (which, by this stage, you'll know intimately). But at this stage, try not to be put off by the "Volt Nuts" with their 7-digit multimeters (I have one, but rarely use it, and never for radio repair). A basic Uni-T or similar will do just fine. Rather than an analogue meter, I'd prefer to get an old-school analogue oscilloscope to supplement the DMM. My AVO 8s are sadly languishing in the attic - I feel rather guilty about that, but that's how it is...

[pwdrive]

I haven't got a 'scope.  

Lawrence.

[AlanBeckett]

I haven't got a 'scope.

Well get your finger out

Looking at Signals with a 'scope opens a whole new Can of Worms, but that's another subject.

Alan

[pwdrive]

I haven't got a 'scope but I know how to use one, one day maybe I'll get one.....Now then...Digital or analogue  

Anyways this is what I like about analogue meters when it comes to servicing radios etc...The intermittent varying voltage/pulse bit:

https://www.youtube.com/watch?v=j1UVd3dBRq0

Lawrence.

[AlanBeckett]

By the way Nick - Surrey version - this sort of deviant rambling happen all the time of this Forum. It makes it more interesting

Alan

[Mark Hennessy]

I haven't got a 'scope but I know how to use one, one day maybe I'll get one.....Now then...Digital or analogue  

If buying second-hand, stick with analogue.

If buying new, you'll struggle to find analogue 'scopes, and those that exist are from obscure brands that cater for the educational market. And probably NOS these days. For less money, a second-hand unit would be better.

So new would be digital. In which case: http://www.rigol-uk.co.uk/Rigol-DS1054Z-...s1054z.htm

There is nothing at this price point that gets anywhere close to it. It is absolutely staggering. Just two stand-outs: it has 4 channels, and it has analogue-like intensity grading (unheard of at this price).

But for a laugh, I've just ordered one of these: http://www.banggood.com/DIY-Digital-Osci...?bid=12658

OK, it's obviously a toy, but it might be a bit of fun

Anyways this is what I like about analogue meters when it comes to servicing radios etc...The intermittent varying voltage/pulse bit:

https://www.youtube.com/watch?v=j1UVd3dBRq0

Unfortunately, that video did nothing to convince me of the advantages of analogue meters. It does demonstrate how good the Fluke 87V is - once Martin remembers to use manual ranging mode (notice that he forgot to switch it off for later in the video?). And it's reminded me how bad analogue meters are for measuring resistance - it's easy to forget just how cramped and horrible the non-linear resistance scales are.

It's really important to remember that different analogue meters have different "ballistics" - rise and fall times - and these really affect the observations. Honestly, for a time-varying signal, a meter is not the tool, but if the 'scope isn't warmed up, I'd choose a DVM with a good bargraph over an analogue meter every time.

But each to their own - we are all different. When I was still at school, I worked with an engineer who would *never* fire up the Hameg because he was trained to get by with an Avo. It frustrated me because even at this age (16), I had a 'scope at home, and was using it all the time. He missed ripple on power supply rails and all sorts of similar problems because of this approach.

[Radio Fixer]

I'm with Mark here.  Its years since I got out the last AVO 8, it was good in its time but I find it clumsy (bench space again!) and error prone in reading,  but that's just because I don't use it enough.

Like the scale changing meter Alan, a neat piece of design by someone.  As to measuring anode volts and impedance loading well rarely do that ... HT and cathode volts tell me all I need to know.  Its sometimes useful to measure (digital meter is easier) the primary resistance of the output transformer (or halves of for push pull) so that from the voltage across them, the valve current(s) can be calculated for stages that don't have cathode resistors.  These are often biased by resistors or a choke or field coil in the -ve HT rail.  Old hands will have been doing this for years of course but I put it in just for those new to the art.


 Its handy to look at the Trader Sheet or Service Info voltages and currents but I don't really need those either.  Interestingly all the radios I look at have valve parameters the same as the AVO valve tester settings (another piece of kit, never stops does it  ).  Just use those figures after testing the valves.

Gary