Simpson 260 vs. AVO-8

[MartinMarris]

I'm gradually completing my workshop prior to getting to work on 405-line valve TV restorations. Among my vintage gear from the 1950s I have a scope (which needs fixing), a VTVM and an RF signal generator.

I don't have a good-quality VOM however and am dithering between an old AVO-8 and an American contemporary model, the Simpson 260, which is a lineal descendant of the U.S. equivalent of the AVO. The Simpson is a more expensive alternative but very high-quality and would last a lifetime (but then, so would a good-condition used AVO).

The AVO needs no introduction here, but for those unfamiliar with the Simpson, the details are here: https://simpsonelectric.com/wp-conte...heets/2608.pdf.

The VOM will be used not only for UK TV restorations but also for my ham radio work, much of which involves building ARRL designs from the 1950s and 60s.

Advice?

[chriswood1900]

I have used both and would say there is not a lot to choose. It does depend on which model you are referring to as there are differences in range and some view later Simpsons as having better meter protection.
If you need help you will probably get more with the AVO here as it is better known.
Good luck

[stuarth]

The Simpson 260 is indeed the American equivalent of our AVO, along with its contemporary, the Triplett 630. All have been through several versions over the years.

The Simpson predates the AVO by more than a decade. The very early ones used carbon composition range resistors which will by now have drifted well off spec, and “pin jacks” not 4mm banana jacks (as the Americans call them). I think precision film resistors came in with the series 3, along with 4mm sockets. Later versions of the Simpson (Series 5 onwards) were available with an excellent cut-out, using an amplifier running off the Ohms battery which released a latching relay, these models are identified with a “p” suffix after the series number, eg the 260-8p for a series 8 with a cut-out. Later models also moved onto a taut-band movement rather than a jewelled pivot movement, these are frictionless and more robust then jewelled pivots. You need to go to a 260-6p to have a taut-band movement, a cut-out, and 4 mm sockets - for me this is the model to go for because with the series 7 and 8, Simpson had changed from 4mm banana jacks to “reverse Banana jacks”, a bit non-standard over here.

Simpson 260s have a 10/50/250 voltage range sequence, giving a more cramped scale than the AVO 10/25/100/250V sequence. They don’t have AC current ranges, but theses days you’d probably use a clamp meter for that.

Apart from early models, the Simpson uses a PP3 for the high resistance range rather than the increasingly rare 15V battery of the AVO. The Simpson cut-out (if fitted) is somewhat superior the the AVO cut-out, it trips before the needle has time to move, rather than tripping when the needle hits the end stops, and it trips if AC is applied to a DC or resistance range.

The Triplett 630 range developed over time more or less in parallel with the Simpson, with taut-band movements, optional cut-outs, and reverse banana jacks etc. Of the Tripletts, I prefer the Model 60, it has a 10/30/100/300 voltage range sequence.

The European contemporary of the AVO is the Unigor 3n, which in my opinion is superior to the AVO and its US contemporarion in almost all respects, but that’s another epistle.

Stuart

[pmmunro]

First let me say that I should declare an interest here because my enthusiasm for Avometers is well known.

I largely agree with what Stuarth has written and it has to be admitted that a major disadvantage of the Avometer is that it has been out of production for about 8 years. However, good used models are still available and of these the best are probably the Model 8 Mk III and the Model 8 Marks 6 & 7 (the latter are condsiderably more robust than the Model 8 Mark V which has come in for some criticism although, in my experiance a good Mark V is a very nice meter).

In my judgement, the consturction of the Avometers is superior to the Simpsons and the Avos are more repairable, should that be necessary. It has to be remembered that the last Model 8 Mk IIIs date from 1969, so some maintenace is likley to be necessary.

I also agree that the Unigor is the best of those discussed, partly because the Avometer was marketed to a possibly very conservative cleintele. There were other models of Avometer, mostly for export which were in some ways more advanced than the Model 8.

The modern equivalent of the Unigor is now sold under the name GMC (Gossen, Metrawatt, Cammille Bauer). https://www.gmc-instruments.ch/?mid=...d=T0RFd01nPT0= and I would say, from its specification and reputation, these are probably the best analogue multimeters currently available.

If it is a new meter you are looking for, it would also be worth considering Mertix or Chauvin Arnaux. https://uk.rs-online.com/web/c/test-...e-multimeters/

None of these modern analogue multimeters is cheap, but you get what you pay for, as usual.

PMM

[MartinMarris]

Thank you all three for the advice! To clarify, if I buy a Simpson it will be one of their current models since they are still being made. Among other things, that should take care of the calibration issue.

[stuarth]

Buying new!

Is that allowed on this forum?

Seriously though, I’ve found good but quite elderly AVOs, Simpson’s, Unigors etc mostly have errors no greater than 1 or 2%, not much more than a needle width, and well within spec (my very old series 1 Simpson 260 with carbon composition resistors with errors of over 30% is obviously an exception). I’ve not found Far Eastern meters as good though, their errors seem twice that of their European/American counterparts.

For the difference in cost between a brand new Simpson and a decent secondhand “good” meter, you could buy a “good” digital with far higher accuracy to check it against and for the odd occasions when you need better accuracy and/or resolution.

Stuart

[Radio Wrangler]

If you have a job to do, it can be done well with either AVO or Simpson instruments... Or Triplett. They all made fine instruments. But also take a look at the GEC Selectest so you could do a good job with any of them. HP and Marconi made some nice bench-top multimeters and valve voltmeters.

There are plenty of choices, not just two. You could pick something suitable of any of several makes, and it won't matter which. See what turns up, see what you like. It's a personal thing. Have fun. There is value in having a few different ones too.

David

[John_BS]

Another vote for the Unigor series. I have two. Mirror scale, taut-band suspension and excellent accuracy. More importantly for me, smaller footprint than either AVO or Simpson.

[Electronpusher0]

I've never seen the Unigor before, how sensible having the input terminals at the top of the meter.

Peter

[Radio Wrangler]

THe British 'Selectest' also had terminals at the top after the early models. Very sensible for a meter to be used on its back.

Note that AVOs are calibrated for use on their backs, so they would have benefitted from terminals located this way.

David

[turretslug]

The Unigor represents the sort of thing that the AVO 8 could have evolved into- even the excuse of keeping to the accustomed 20k/V applies to the Unigor 3, though the Unigor 4 went to a respectable 100k/V. I expect that there was an attitude of expecting meters to be reassuringly big and heavy with the long-running AVO 8 and it was usually bought with someone else's money by its principal customers anyway.

If you can find a decent Simpson 269 for a sensible price, it's a nice 100k/V meter with a taut band movement- I bought one as NOS from M + B Radio long ago for what I now appreciate was a bargain price (I was working with a colleague who is also a forum member at the time, I regret not snapping another up for him, I'm reasonably sure it would have been appreciated!). It does combine the AVO 8's vice of a clunky Bakelite case with the Selectest 50's of a large and vulnerable glass panel though, so gets used for occasions when it does score over DMM and workaday analogue meter- in particular, its input resistance of 160 megs and 400 megs on 1.6 kV and 4kV ranges is good for 'scope work.

[John_BS]

The Unigor 4N was also sold under the Venture brand-name by Smiths Industries.



John

[stuarth]

I think it's a very good plan to have a decent analogue meter for probing around inside old TV and radios. Analogue meters have more than enough accuracy for the job, with analogue you can see at a glance whether the reading is about right, you don't have to think what the display is telling you, and I'd be uneasy probing about with all those high voltage spikes with a digital meter because either I'd have a cheap DVM which would be very likely be destroyed by the spikes, or a nice expensive DVM which I wouldn't want to risk in such a hostile environment, especially an auto-ranging one which starts on its 2V range.....

The question of AVO vs Simpson depend to some extent on with version of each is available.

The AVO 8 spec stayed pretty constant over the years, with only some detail changes such as adding 4mm sockets in the middle of the terminals early on, and deleting the kV ranges on later models. The innards changed a lot, and have been discused on here many times. Of those mentioned in this thread, my favourite AVO 8 is actually and AVO 9 MkII, a version of the 8 MkIII but with 10/30/100/300 ranges and a less cluttered scale plate. Probably the last and most refined of the solidly constructed early versions.

The GEC SEI Selectest Super 50 is a clone of the AVO 8, but the "50" in the name is crucial as there is also a Selectest Super K which is a low impedance meter, a clone of the AVO 7.

The Simpson 260 also stayed basically the same over time, but the changes they did make had more impact on users. Early models used 2mm test terminals, and did not use precision resistors. From the Series 3 onwards they had 4mm sockets, precision resistors and several other improvements. From Series 7 onwards, they used "reverse banana jacks" instead of 4mm sockets, impractical if you want to use the meter with anything but the supplied test leads. A full description of all Simpson models including circuits is available on simpson260.com. I have a few Simpsons including a 260-6p and a 260-6xlp. I used to have a couple of 260-7p's, but found the non-standard connectors for the test leads a hinderance if trying to use anything other than the Simpson supplied test leads. I suspect you would find the same with a new 260-8p.

The Triplett 630 range was a competitor to the Simpson 260 (rather than a clone) with different ranges and switching arrangements, and a similar development path over the years.

One feature of the Simpson 260 (and Triplett 630) which I'm not keen on is the 3 different fsd values (10, 50, 250 for the Simpson) all sharing one set of scale markings. The sub-divisions on the scale plate make reading one or other of those fsd values awkward. I prefer the AVO arrangement with 2 fsd values (25, 100 and decades up or down from those) each with a dedicated sale.

Any of the above would be good for all sorts of applications, including 405 line TV restorations. So which is my go-to meter - I have all the above in my collection, but my go-to meter would be a Unigor.

Unlike the AVO and Simpson meters, the Unigor range had a couple of complete re-designs with significant changes. Unigor meters are not often discussed here. Early models (1950s ?) were the 25kohms/V Unigor 3p/3s, and the 100kohms/V Unigor 4p/4s. These were beautifully built, but had limited resistance ranges using an R10 cell and an external supply to measure high resistances. These were replaced by the 31.6kohms/V Unigor 3n (think sqrt10) and the 100kohm/V 4n which were bigger than the previous models, had an improved cut-out, and powered all the resistance ranges from a built in DC-DC converter to measure up to 50Mohm using a single D-Cell (so no 15V battery!). The 3n and 4n were replaced in the 1970s by another new range, the slightly smaller A42 and A43. The A42 was a passive meter with a current transformer for AC ranges like the 3n, and a good substitute for the AVO 8/9, the A43 was a high impedance meter, 100kohm/V AC and DC, passive on DC but active on AC, the whole instrument being powered by the C-Cell used for the resistance ranges.

The Unigor 1p, 1s, 1n, and A41 are low impedance meters.

Unigor meters are found badged Goerz, Metrawatt, ABB, BBC, etc in various combinations, and in the UK, I've seen Unigor 3n (or 4n) meters badged as "Venture Multimeter 3 (or 4)" for Smiths industries. I have a Normameter Model T, which is clearly a Unigor 4s in a more rounded case, DC voltage ranges to 5kV at 100kohm/V, excellent for chasing EHT lines in oscilloscopes.

My go to meter would be a Unigor 3n, 4n, or A43 depending on mood.

So what are the advantages of the Unigors?

Well, many including a very effective passive cut-out, for example it trips with around 2V AC or DC applied to any resistance range with hardly a kick from the needle. The resistance ranges cover a wider range than the AVO or Simpson, and all with a single D-Cell or C-Cell depending on model. The current ranges are closely spaced with steps of 3.16:1, rather than decades. They have robust frictionless taut-band movements. They have clear uncluttered scales and range switches. They also have forward reading resistance ranges, the needle goes to full scale with the test leads open circuit and zero deflection with the test leads shorted. This has two advantages, seeing the needle at fsd reminds you to switch back to a (safer) voltage range, and any resistance in the test leads cannot be zeroed out, so you're aware of the problem.

All the meters mentioned here more than adequate for TV restoration, and most of them will be in spec for accuracy despite their advancing years. The final choice will be based on individual requirements, availability .. and personal preference.

Stuart

[The Philpott]

- A couple of other little functional details also separate the Avo 9/II from the 8/III:

-It can measure very low AC currents (even down to a small neon lamp)
-It did not benefit from the upgraded Germanium rectifier. (I doubt this is a problem in the real world)
-It seems to be marginally less popular so may cost a few quid less- not everyone likes the international picture symbols on the facia.
-The 300vAC range is arguably more suitable for UK mains than a 250vAC range.

Dave

[MartinMarris]

Thank you all, much to digest!

[MartinMarris]

(For those VOMs that have them.) What is the purpose of the mirror embedded in the scale, is it something to do with parallax?

[deswradio]

Originally Posted by MartinMarris

(For those VOMs that have them.) What is the purpose of the mirror embedded in the scale, is it something to do with parallax?

It is indeed. It's to help make sure that you are reading the scale vertically above the pointer.

Best wishes
Des

[Simon Gittins]

Yes, the most accurate reading is when you are directly above the needle so that you don't see the reflection to one side or the other.

[The Philpott]

- But you have to get the background lighting 'just right' to take advantage of this feature!

[chriswood1900]

Just spotted this in relation to the Simpson 260 meters a useful run through by Alan Wolke (W2AEW)
https://www.youtube.com/watch?v=eXQLq4X47hk
explaining the differences in the models and which are the best buys.