Cossor 1044K Valve Voltmeter.

[Joe]

I seem to have sorted this out. I changed the rectifier for a BY133 and hundred ohm resistor and reset the calibration.

I'm curious about a couple of points though. The Heathkit instructions say that for DC tests the positive probe should be connected with a screened cable and have a one meg resistor at the prod end. What I'm curious about is why, if these things are needed for DC, aren't they necessary for AC? Also, given that AC and DC use different sockets on the meter, couldn't the resistor be in that? It certainly seems to work fine with normal test leads so perhaps these extras aren't needed on this Cossor one.

Cheers

Joe

[Skywave]

I think that the reason for the screened lead when measuring d.c is because the VTVM has a very high input impedance: an unscreened lead will pick up stray a.c. / r.f. fields and thus produce misleading indications on the meter. However, when a.c. / r.f. is being measured, that screening will become a capacitor - and thus shunt some of the signal away (the magnitude of which will depend on the cable: length and permittivity of its core insulation, and frequency of the signal) - again, producing misleading indications.
The use of different sockets for a.c. and d.c. is simply a way of simplifying the otherwise necessary complicated switching arrangements when changing between these two modes of measurement.
As for the 1 meg-ohm resistor being needed on d.c. but not a.c., I can only assume that the intrinsic design of the VTVM produces an 'ohms per volt' sensitivity on d.c. that is far higher than that when on a.c. - and this could be due to the necessary rectification internal to the VTVM when measuring a.c. / r.f. As I am sure you are aware, 'ordinary' moving coil multi-meters always have a lower ohms/volt sensitivity on a.c. than when on a d.c. measurement range: the inclusion of the rectifier circuit is usually at the core of this difference.

Al.

[Joe]

Thanks, Al.

It came with various test-leads, and bits thereof, so I'll get a correct pair sorted out.

- Joe

[Alistair D]

The purpose of the 1meg resistor is twofold.
Firstly,the resistor isolates the point being measured from the the capacitance of the cable and the instrument itself. I am sure I remember reading an article where someone was testing an audio amplifier and was sick of the loud clicks in the speaker every time he made a measurement with his DMM. The solution was to dig out the valve volt meter which reduced the sound level.

Secondly, the combination of the resistor and lead capacitance acts as a low pass filter to prevent r.f. entering the meter and giving false readings.

Al

[Skywave]

Just a small snippet of trivia - but relevant to this thread.

Many, many years ago (early 1970's) I had a good friend who was a highly skilled and, it must be said, a very gifted electronics engineer. He designed a whole raft of electronic equipment for his own hobby use and some of it was quite impressive too. Anyway, the only multimeter he ever owned & used was a Heathtkit VTVM. I'll never forget the way he used to gently tap the meter transparent plastic cover now and again, because the needle would occasionally stick!   I never did discover if he cleared that minor, but irritating, fault. I recall asking him if he ever used, or had owned, an AVO, or a passive multimeter, to which his response was something like "Why should I use one of those? This meter tells me everything that I am likely to need to know - and it doesn't load the circuit."
To my mind, that just about sums up the usefulness of a VTVM (especially if it reads resistance as well.)

Al.

[bigtom]

just to add my tuppence worth to the discussion, I have one of those Cossor 1044k meters ,obtained as a non worker. The glass was loose also.I discovered that the set zero and the set infinity were unstable, to make along story short both 10k pots had increased in value to around 20k that cured that. I now notice the meter does not always stay on zero on ac dc or resistance but will slowly drift upwards on Dc + Ac and ohms and downwards on Dc- ,I suspect that the drift is related to the ambient temprature and is more noticeable in really cold weather. Has anybody got any ideas onthat
Thanks

[Skywave]

That drift you're experiencing might just be due to the HT rail moving 'up and down': easy to check that out. Only a few + or - volts about a central figure could produce that drift. I don't know if that item has a stabilised HT rail - but I would certainly expect to find one in there.

Al.

[Yorkie]

Just a small snippet of trivia - but relevant to this thread.

Many, many years ago (early 1970's) I had a good friend who was a highly skilled and, it must be said, a very gifted electronics engineer. He designed a whole raft of electronic equipment for his own hobby use and some of it was quite impressive too. Anyway, the only multimeter he ever owned & used was a Heathtkit VTVM. I'll never forget the way he used to gently tap the meter transparent plastic cover now and again, because the needle would occasionally stick!   I never did discover if he cleared that minor, but irritating, fault.

Al.

I had one for a number of years, with the same apparently defective meter. I was lucky enough to get a brand new boxed meter movement for just £3.00 at a rally at which Heahtkit UK spares were being sold off. I fitted the new movement and it had exactly the same problem. I later discovered that it wasn't the meter itself, but the clear plastic front that was the culprit. Static built up and caused the meter needle to freeze. Take the front off and all was well. I used to just wet my finger and thumb and put them on the dial and to the case, and it removed the static. It was most likely that tapping the plastic cover would have the same effect in removing the static.

A valve Voltmeter, with an input impedance of several MegOhms, is far superior to an AVO, which - though better made - at 20,000 OPV, is no better in terms of accuracy than many cheapo analogue meters which generally have 20,000 OPV input (assuming it's stated, which it often isn't). Fine, if users know the limitations and in what circumstances the AVO will load the circuit under test, but I suspect that many don't. Extremely well made, designed and built, jolly expensive in their day, and the standard instrument quoted on many radio and TV service sheets. Nice to have on display on the shelf, but of limited use on the workbench IMHO. AVOs and DAC90As deserve each other, but time has moved on. Certain names - AVO, Fluke and TEK come to mind - seem to create in the minds of some, an almost evangelical appeal. Avo is to multimeters what Thermos is to vacuum flasks, and Hoover is to vacuum cleaners. All have become genenric terms.

[Skywave]

Another co-incidence you mentioning VTVMs David. Very recently I had a full uphill struggle with a Hewlett-Packard VTVM model 400H; dates from 1961. The fault was that the needle was extremely unstable: either no deflection at all or banging hard against the end stop - with occasional fleeting pauses at all points in between. One very strange symptom was that the entire chassis was 'hot': just a touch on any part of the metalwork - no matter how gentle the touch - and off it would go. (Yes, the mains earthing was 100% intact - as was the 'cold' end of the input wiring - which was joined direct to chassis.) A 'scope on it showed that it wasn't going unstable - which looked favourite; the fault was partly due to badly seating valves. So I replaced two valve sockets which effected some improvement - but it soon became apparent that all of the eight sockets (B7G and one B9A) would have to be replaced as well! And they were riveted to the chassis. At that point, after having changed several R and C, and having spent several days on it (I don't give in easily) - I called "time up": it simply wasn't any more effort. I only started to look at it because it had been hanging around in my 'to do' pile for about 4 years and was getting in the way. And I was unlikely to ever seriously use it: I use a 'scope for such measurement. So it was dismantled for salvage spares - which included a very nice mains transformer and a 2 uF 400-v non-polarised capacitor that I know I'll need to repair my HP 1715A 'scope!

Having said all that, one day I might just get around to designing and building my own VTVM based on F.E.T.s.: a much smaller unit, battery-powered and much more versatile - and stable. When - and if I do - the GVR will be the first to see it! But don't hold your breath folks!

Al.

[Skywave]

. . . Anyway, the only multimeter he ever owned & used was a Heathtkit VTVM. I'll never forget the way he used to gently tap the meter transparent plastic cover now and again, because the needle would occasionally stick!   I never did discover if he cleared that minor, but irritating, fault.
Al.

I had one for a number of years, with the same apparently defective meter. . . . . . I later discovered that it wasn't the meter itself, but the clear plastic front that was the culprit. Static built up and caused the meter needle to freeze. Take the front off and all was well. I used to just wet my finger and thumb and put them on the dial and to the case, and it removed the static. It was most likely that tapping the plastic cover would have the same effect in removing the static.

Y'know, now you come to mention it, I do have a kinda vague recollection that that 'wet finger and thumb' technique was what my aforementioned colleague used to do.
As we all know, you could but Heathkit products as a kit for the home-constructor to build. Since Heathkit offered a 'we'll fix it for you' facility for the head-scratching home constructor, I just wonder how many of those VTVMs were sent back to their repair facility with that fault! And what did the Heathkit technicians did to fix it . . . .

Al.