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Old 19th Jun 2020, 2:08 am   #1
Matthew kane
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Default Metrix 310B tube tester Meter replacement

So I have one of these French tube testers from Annecy with a damaged meter. According to the schematic it is a 0.9mA 50ohm meter. The resistance when measure is around 17ohm and the winding on the coil is most likely shot somewhere due to stressed overcurrent.

I have a spare Metrix U61B meter which is identical except its rated for 200uA and has an internal resistance of 1.48 kohm’s is there anyway of converting to with a shunt circuit to match the original?

My backup is buying one of those 50mA 4 decimal point digital panel meters as the tester only measures and displays anode current.
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Old 19th Jun 2020, 10:46 am   #2
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Default Re: Metrix 310B tube tester Meter replacement

Not directly. A simple shunt to make the 200uA meter look like 50R would leave it reading about 15% of FSD in whatever circuit presents the 900uA meter with the 45mV it would need for an FSD indication.

If you can post the schematic of the tester showing how the original meter is used, it should be possible to make the 200uA one read the currents correctly though it will impose a greater voltage burden than the 900uA one. For anode current with plenty of voltage headroom this shouldn't matter too much.

You say the meter is identical, does this apply to its scale indications as well?
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Old 19th Jun 2020, 11:34 am   #3
Matthew kane
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Default Re: Metrix 310B tube tester Meter replacement

Yes the scale is identical as well as the housing.
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Old 19th Jun 2020, 6:07 pm   #4
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Default Re: Metrix 310B tube tester Meter replacement

That circuit adds confusion nicely- the shunt resistor values given make no sense for a 1mA 50R meter for what are presumably current ranges of 3, 10, 30, 100mA. However.... if you look up a "typical" 1mA MC meter, it has an internal resistance of around 150R. Add an external series 50R and those shunt values work!

So, to convert things to take the 200uA meter, you'll need to change the meter series resistor to 470R and the 3mA shunt to 139R. This will give a parallel combination of 130R and a FSD on the meter at 3mA.

Work out the other shunts by calculating what resistance will be needed in parallel with the meter path's 1950R to give a voltage across the parallel combination of shunt and 1950R of 390mV at the range current FSD required.

(390mV is the voltage needed across 470R in series with 1480R to pass 200uA.)

How "accurate" it all is, given that the actual currents will be half wave rectified AC is anybody's guess. Possibly not far out given the meter's average response to the signal.

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Old 19th Jun 2020, 8:30 pm   #5
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Default Re: Metrix 310B tube tester Meter replacement

The bias supply appears to be in anti-phase to the anode current supply. if the bias pot is adequately scaled, them measuring mutual conductance should be a simple matter.
Les.

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Old 20th Jun 2020, 1:19 am   #6
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Default Re: Metrix 310B tube tester Meter replacement

Hi there.

I would suggest a second and third look at the meter internals, to check if anything is amiss inside.
Metrix component selection, and build quality, was first class.

The meter is protected against overload current with an internal circuit breaker.

Caution that there are multiple versions of this valve tester, with different circuits.

My circuit of the M310B shows additional external resistor R9 of 40R in series with the 50R movement. This may explain some findings. I think R9 is that circled in the photo.
It may not be exactly 40R (parts list, not on schematic) as I think it is a factory S.O.T. component.

If you use the U61 meter, and after you work out new shunts etc, you still may find that you have to use half wave DC instead of steady DC for meter calibration on the bench, all as Herald has suggested. Additionally you may have to have a cal. look at any other function that the meter is used for.

Your meter does not have to be in exact cal. for tester use.

I had one of these units, but disposed of it recently.
Excellent units, and worth repairing.


Service Manual M310B en Francais, pour vous.
Bon Chance!
.
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Old 20th Jun 2020, 1:57 am   #7
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Default Re: Metrix 310B tube tester Meter replacement

I had and may still have an English translation of the Metrix 310 manual if you are interested I'll try and find it.
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Old 20th Jun 2020, 2:41 am   #8
Matthew kane
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Default Re: Metrix 310B tube tester Meter replacement

The pivot jewel screw is lost. So the meter is unusable. Unless I remove it from the U61B meter.
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Old 20th Jun 2020, 3:54 am   #9
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Default Re: Metrix 310B tube tester Meter replacement

Quote:
Originally Posted by Matthew kane View Post
The pivot jewel screw is lost.
Mon dieu!

I can imagine your consternation on finding this!

I wonder if anyone here can advise further? It is just possible that this is repairable.

(By sheer coincidence, I am today working on a 50mm 1mA 50R "dBm" meter movement).
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Old 21st Jun 2020, 2:50 am   #10
Matthew kane
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Default Re: Metrix 310B tube tester Meter replacement

Removed a pivot screw from a 100ua AWA meter and turns out the diameter of the thread is a mm too small.

Guess I might have to resort to removing the pivot jewel from the U61B meter. Unless somebody can recommend me a 1mA 50R meter replacement, I can just do the scale myself.

Also how hard would it be to use one of those 1.999v dc digital meters off ebay to convert for use in this tester? Theres no circuit for measuring gm just anode current so should be simpler.
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Old 21st Jun 2020, 3:17 am   #11
Matthew kane
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Default Re: Metrix 310B tube tester Meter replacement

Actually I’m not going to be hasty and remove the pivot screw, maybe one day I may end up with a U61B tester.

Will wait for some of you experts to weigh on how I should approach this.
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Old 23rd Jun 2020, 1:01 am   #12
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Default Re: Metrix 310B tube tester Meter replacement

Hi again,

I sense that you are certainly not a beginner, so I will dispense with simplicities.
I am not a movement expert.

Dinky suggestions-

1) Replace the jewel screw.
Try M2.0 or M2.5, or others from your box of small fine thread screws.
Drill the end, and mount something like a micro glass bead as pseudo jewel bearing.
The original jewel was probably "V" shaped.
Even consider just drilling the end of a new brass screw and use "pivot" in brass, one oil drop, with no jewel.
The pivot probably will be hardened steel.
Micro beads in craft shops; smallest sizes maybe online.
Get them with holes; use one end.
I can post you a 0.4mm HSS drill bit if needed for a "steel in brass" bearing.

2) Make up a new screw, or use existing screw with no jewel, and drill completely thru and use tiny brass tube as a bearing?
Add one micro drop of oil.
Use watch oil.
K&S Metals 1/32 inch brass tube.

3)Replace the M310 movement with one from another meter.
Make up a new needle by cutting all but 5mm from new movement and gluing something as a new needle, length to suit M310 meter scale.
Consider using 10 mm length of K&S Metals 3/64 inch aluminum tube as a joiner with micro drop of glue.

The internal movement in an Altronics MU52 meter (50 mm scale and AUD$26) is plastic cased 20mm dia, 16 mm approx deep.
I have a new spare of the meter 0-30 V DC, and the actual movement is 1.0 mA 50R with internal multiplier, easily cut out.
The jewel screw of this MU52 is to my eye about 2.5 or 3 mm dia, and about 4mm deep.
The top pivot is about 0.5mm dia.
The needle is 33mm long, and is balanced in two axes.

This suggestion retains the original M310 meter casing.

More Traditional Suggestions

1) Replace the whole M310 meter with a MU52 style meter Altronics Q0535 0-30 V DC, or similar, maybe larger, and use a painted blanking plate over the original M310 meter hole.
As mentioned before, this is a 1mA 50R movement, as I recently used one in an RF Milliwattmeter project.
If you choose this path, I can make you up a new meter scale on matte photo paper to imitate the M310 scale..
I already have this MU52 scenario programmed up in "Galva", example attached.

2)Replace the M310 meter with a digital LCD display.
You ask if difficult.
Probably not.
Zero aesthetics though, especially on a heritage valve tester.
And, ... not as useful as an analog meter in this application.

I use have just recently used Jaycar LCD meter QP5570, which is a 0-200mV display, but you would have to have a meter Power Supply which was isolated, and all able to withstand 400 V half wave DC above ground.

You may be able to re-design the circuit so that an LCD digital display reads the earth return current, and so is only a max of 200mV above ground.

My Choice?
replace whole meter with MU52 or larger.
Consider also those Student Analog Bench Meters, which are approx 100x100mm.


Musing ...

I think I read somewhere that the Metrix U61 has a superior method of measuring valve characteristics?
Maybe use it, and store away the M310?
The M310 is very good however, and compact. We even used these to test medium power transmitter tubes.

Lastly, wishful thinking, ... is the original jewel floating around in the M310 bottom metal case, maybe in a corner?
Tiny ... so worth a look?
It may not be red colour.

Encore une fois, bon chance.
.
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Old 24th Jun 2020, 6:15 am   #13
Matthew kane
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Default Re: Metrix 310B tube tester Meter replacement

Thanks for that.

Unfortunately I tried making my own pivot screw and it was a no go.

The original was nowhere to be seen and definitely not inside the meter housing.

Honestly I'm thinking going digital, that one you linked to Jaycar also come in a LED backlit version, for a dearer $5-8. Once set up and installed it means I'll forget about any wear and tear with moving parts in a traditional analog meter, especially if it starts losing its magnetism or a hard knock to the unit resulting in out of place needle positioning.

Looks like most DPM are 200mV and powered by an isolated 5-18vdc depending on the brand/unit you purchase. I'm looking at powering it with my own isolated DC circuit to put inside the unit. What shunt will I need for this to work with the selectable ranges? There is already shunts in place for the different range measurement 3, 10, 30 and 100mA currents.
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Old 25th Jun 2020, 8:19 am   #14
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Default Re: Metrix 310B tube tester Meter replacement

Working ...
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Old 25th Jun 2020, 8:59 am   #15
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Default Re: Metrix 310B tube tester Meter replacement

Hello Retailer,

can you please post the M310 Manual in English if you find it.
Thanks very much
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Old 27th Jun 2020, 7:24 am   #16
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Default Re: Metrix 310B tube tester Meter replacement

Hi again.

Sorry, but I don’t have a Metrix310 anymore, and have only access to French M310 CTR manuals and circuit. I last used one of these M310 units in about 1975.

I am surmising some of the Metrix 310 operation, and I maybe in error in places.
Always a problem with analysis and suggestion “at a distance”.

Valve Tester Experts, M310 Experts, ... “others” please step in to correct me as necessary.

(Also, there are other M310 versions, and the circuit may be slightly different).


Using the Metrix U61 0.2 mA FSD Meter in Lieu.

It is not possible to use a simple shunt, nor a mix of shunt and multiplier, to change a 0.2 mA 1480R movement into a 0.9 mA 50 R meter, and preserve existing shunts
Law of Diminishing Returns is active.

A shunt of 423R however, converts the 0.2 mA movement into a 0.9 mA meter with 329R equivalent resistance.

And so, if you wanted to use the Metrix U61 meter, you would have to shunt the M U61 meter movement with 423 ohms, and change all four shunts of R5 to R8, and remove existing M310 R9 which is probably a factory adjustment resistor, and which is a nominal 40R.


For normal DC, and Metrix U61 Movement

(I make the assumption that the magnetic response of each movements is similar, for the type of anode current which is being measured; see later).

This means resistor R5 (3mA range) , resistor R6, resistor R7, resistor R8 (100 mA), will be 141 ohms (3 mA) , 32.5 ohms, 10.17 ohms, 2.98 ohms (100 mA) respectively.

To avoid confusion, from now on I use terminology that 1000R is 1000 ohms.

In practice, remove R40, shunt the 0.2 mA meter with a 430R 0.6W 1% metal oxide resistor across the movement external contacts, and use new shunts, one 200R in parallel with one 470R (3 mA), one 47R parallel with one 100R (10 mA), one 11R in parallel with one 130R (30 mA), three 11R in parallel with one 15R (100 mA); all resistors 1% 0.6W metal oxide.

These ‘starting values’ may have to be adjusted later.

(See calibration suggestion at end of Digital Panel Meter Suggestions).


But, ... First “uncertainty” Looms.

What is the Metrix 310 actual meter movement rating of current and resistance?
The original shunt values pose a question; different to what I expected, nut probably allow for the Form Factor of the Anode Current.

It is marked as “Normal ie Steady or Continuous” DC (as distinct from interrupted DC of any form factor) on meter scale (double thick lines), and as 0.9 mA 50R on circuit. The spec is not on the meter scale, on my photos.

This normally means that it is 0.9 mA Full Scale Deflection, FSD, on “steady DC” and has an internal resistance of 50R as seen at the terminals, irrespective of any internal padder resistance to normalise the coil resistance. (There also may be an internal magnetic shunt).


Second Uncertainty

How was it calibrated, and how did Metrix calculate those original shunts?
According to circuit, the equivalent meter is 0.9 mA FSD and internal resistance of 50R and overall resistance of 50R + 40R (R9) = 90R.
The shunts then should actually be 38.57R (3 mA), 8.9R, 2.78R, 0.82R (100 mA).
These values are related to original values almost in a “1/x” relationship, not a constant.

Heisenberg at play.

What am I missing?

It is as if Metrix raised the movement sensitivity according to some empirical requirement.
And, it is just possible that the real meter current FSD is a standard 1 mA, but adjusted with R9.


Metrix 310 Operation Methodology - Perhaps

From what I gather, and partly surmise, the M310 is an “emissions” test unit, and equivalent “mu” (or gain) is “correlated” from the measured anode current, using math formula (ex Service Manual) “Ip = (Vg+Vp/u)^3/2”. This is “compared “ with M310 Tube List Spec for Anode Current.

For a pentode say, Metrix tie the “screens” to the anode externally, via switches, and use full wave AC 50 Hz “anode voltage set by switch” on the anode, and an adjustable and “individual for valve type” s full wave AC 50 Hz voltage bias on the control grid.

The valve then acts as a controlled rectifier, and the valve anode current is half wave rectified sine 50 (60) Hz current. The meter in Anode Current Mode is responding to the mean magnetic effect.

The meter in Anode Current Mode “reads” this half wave DC.

The diode bridge CR1 is only for measurement of Grid Bias Voltage.

So did Metrix calibrate the M310 meter FSD on Steady DC or half wave “DC”? And how did they work out those shunts?
I have a suspicion that their Research Lab did it empirically, ie rough guess then adjust, using real in-circuit meter.
This then calibrates for “Mean DC Current” (at approx 0.32 of peak current).
This trial and error shunt method is what is really required for our situation with a broken M310 meter.

Until someone here can make an actual DC Current measurement of say “100 mA” on another M310, we have to make an assumption, that the meter is calibrated on Steady DC.

This then means that any type of meter replacement may have some inaccuracy, but as it is all relative, it may be not critical.

If we had 1000 6V6 valves, maybe two, we could estimate the average plate current across all valves using the Metrix bias and anode volts setup. This may be adequate as a cross check.
I’ll just order them now ...


Using a LCD or LED Panel Meter as a Replacement.

The following may also settle some uncertainty about meter calibration.
There are some problems with this approach.

The Digital Panel Meter will be operating at 1.4 x Anode Voltage setting, which may be up to approx 450 volts to ground.

Some DPMs may not withstand +450 V, even though the spec sheet “infers” this rating.

Also most, but not all, Panel Meters require an isolated 5 to 12 Volt DC Supply, and this Isolated Supply itself must have a voltage rating of at least 500 V.
Some cannot use a “common ground” system, ie DPM PS neg to Measure neg.
Firstly, LED Panel Meters require approx 50 mA DC supply, due to LED segment requirements.
This may be a limiting factor, and an LCD type may be more suitable for a low power DC supply.

Ideally, you are after a Panel Meter with 200mV DC range, and when the actual M310 Anode Current is “100 mA”, your Panel Meter should show 100.0 units.

Shunts ... see later.


Isolated DC Supply in a Metrix 310.

There is no inbuilt DC anywhere in the M310.

1) Make up your favourite PSU, using perhaps a small mains transformer, bridge etc, and use a 78xx regulator for voltgae stability, to ensure Panel Meter accuracy with variations of Mains Supply voltage and temperature.
It must have a transformer to be isolated.
It must supply 50 mA for LED type or approx 5 mA for most LCD types.

2) Use a 5 volt DIL type isolated DC supply such as Altronics M7827, but check your Panel Meter PS voltage requirement first. Power this from some form of DC supply from the M310 heater circuit area?

I just happened to have some Panel Meters on the bench for other projects.


LED Type Suggestion

I have tried Altronics LED Panel Meter Q0588, AUD$34, and used a 100R shunt, to simulate a M310 situation.
Draws 50 mA for the DC supply however.
Quite a small display; check before buying.

LCD Type Suggestion
I have used a Jaycar LCD Meter QP5570 AUD$28, with 90-100R shunt, to simulate a M310 situation.
Draws 0.3 mA and suits very low power isolated PS situation.
Smallish display.

This is actually an industry standard type “CX101B” unit, and I recently bought four as spares for my other projects.
Futurlec has these for approx $8; make sure you buy the CX101B version. It may be shipped from overseas.

The Jaycar version QP5570 Instruction Sheet is confusing. I attach another, less confusing, sheet.

Note that most CX101 sheets have a typo error with the Decimal Point; connect P11 to V+ for xxx.x display (M310).


Panel Meter Tests (for M310)

I separately connected up each of the above Panel Meters, 100R shunt, with a variable voltage DC source, and included an analogue VOM in series on 1.6 mA range (Metrix 477!), and additionally in series, a DVM on auto range mA DC.

Using a 90R shunt gave 10% error (reads low at 0.88 mA) when the VOM and DVM each showed 1.00 mA.

Using 100R shunt, all meters read 1.00 mA, or quite remarkably close to it.
(I have a 1% meter checker for VOM and DVM).

I repeated the above tests with variable voltage half wave sine 50 Hz, and obtained very similar results.

I have yet to come to terms with the maths of some of this, yet, however.

Beware that if you use either type, some calibration is likely when used in the M310.

You can normally assume that these generic DPMs are reasonably accurate in themselves, but keep adjustment in mind if other uncertainties arise.


M310 Shunts with LED or LCD Meter.

As yet, I cannot determine exactly how an LCD or LED meter will respond to the original shunts, and in the actual M310 Unit.

Each DPM should be OK with the existing shunts, but I am afraid that further tests are required after you replace the original meter with a DPM.

I suggest that you replace the original meter with LED or LCD DPM, remove R9, use 100R as shunt across LCD/LED meter, then test clip an additional analogue meter (if you have it) into the M310 anode current, so you can read the current separately on DPM and Analogue Meter.
Test at 3 mA and again at 100 mA.

Adjust shunts as required.

If you don’t adjust the shunts, then I am afraid there may be up to 20% error.


For Now ...

Hopefully these are some starting suggestions for you.
My choice of DPM would be the LCD type.

If you come unstuck, you can also go back to a MU52 style analogue meter.

And if you yourself have the English version M310 Service Manual, please post it here; thanks.
I might learn more!

Good Luck.
.
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Last edited by Radio1950; 27th Jun 2020 at 7:52 am.
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Old 28th Jun 2020, 3:53 am   #17
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Default Re: Metrix 310B tube tester Meter replacement

Metrix 310 Meter Replacement by an LCD or LED Digital Panel Meter.

Just to ensure understanding of the replacement process.

We need to replace a 0.9 mA FSD 50+40 Ohm DC moving coil meter with a 3.5 digit 200 mV DPM.

The original meter is switched over four anode current ranges of 0-3, 0-10, 0-30, 0-100 mA FSD.

With a DPM, whilst you could arrange switching to shift the decimal point, the “FSD” remains at “199 units”, then normally displays ‘over-range’ of “-“.

For the sake of simplicity, it might be best to convert the DPM to be 100 units at 100 mA, and to use this same FSD of 100 mA for all current range switch positions.

Some readability will be lost, but the DPM will retain the current resolution to an accuracy of about 1 mA, ie ignoring the last DPM digit due to measurement uncertainty.

If this is not done, and if the original shunts are used, then say if Current Range of 0-30 mA is selected, and there is an actual anode current of 15 mA, then the DPM will show 50.0 units, which will be confusing.

It would be possible to arrange the circuit to allow the DPM to display 0-003.0 and 0-010.0 and 0-030.0 and 0-100.0 units, but a new three bank switch would be required for decimal point shift, and for individual new value shunts.
The result is questionable in value.



1) To convert a 200 mV DPM into a 100.0 mV device displaying 0-100.0 , shunt the DPM input with 100R 1% 0.6W metal oxide resistor.
(Also, this low value resistor may partly protect the DPM input from possible high voltage switching transients in the M310).

2) To make the now shunted DPM work as an indicator of 0-100 display units for 0-100 mA anode current, use the existing M310 100 mA shunt R8 in the M310, which is 1.01R.

3) To make this shunt used for all M310 current ranges, unsolder the “circuit LHS” end of R5, R6, R7, ie the switch S7b end.
Physically anchor the original resistors somehow to the terminal board to avoid vibration issues.
Solder a link from switch S7b positions 1 to 2 to 3 to 4.
This makes the 100 mA shunt R8 in circuit for all current ranges.

Test new meter current accuracy with a valve which has a top cap, and insert a current meter on 500 mA range, between valve top cap and top cap lead. Use a relatively high current valve such as a 6146.

Alternatively, insert your test meter somewhere in the M310 anode current measuring circuit.

Hope that all this helps, and I hope I have not made any mistakes (getting old!).
.
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Old 28th Jun 2020, 11:25 am   #18
Matthew kane
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Default Re: Metrix 310B tube tester Meter replacement

Wow, did not expect that much information! Greatly appreciate it Radio1950, I'll try absorb all that and start testing once I order some replacement Analogue and DPM's. I have a Selectest Super 50 mk2 meter so I can use that to manually test some current measurements and compare them to once I get the shunt adjustment with meter changes going.

On the other hand I'm starting to come up with some plans to add a DPM to supplement the negative grid bias pot on my vcm mk1.
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Old 29th Jun 2020, 12:26 am   #19
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Default Re: Metrix 310B tube tester Meter replacement

Please also consider using a 4.5 DPM to obtain greater resolution, esp if only one Current Range is used ie 0-100mA.
Probably not required for Anode Current of most valves, but may be useful for Grid Bias setting up fine tolerance voltages, and for low current detector valves like 6H6 and 6AL5.

Handbooks
I have just found a User Manual on the Kevin Chant website for a M310B, and which has a valve setup list at the rear. This manual is a mix of two manuals.
https://www.kevinchant.com/uploads/7...metrix310b.pdf


I have just found photos of pages of my old M310 User Manual, and I post here.
Not the best quality, as pages were photographed not scanned.


Nostalgia Corner

This User Manual was with my actual M310.
The M310 was supplied as part of support equipment for a French made A.M.E. Conventional VOR, as a ground navaid for aviation with DCA Australia in late 1950s.
RF PA used push pull QB3-300s.
Lovely warm glow!

More M310 units were then later purchased as general test equipment, as valves were everywhere.

Each Metrix 310B Unit came with three manuals, all in English.
User Manual
Service Manual
Valve List, which in turn had an addendum with valve setup data for newer and obscure valves, which were not listed in the main valve list.


I have now read the User Manual.

Error in my surmise of operation.
Metrix do not tie screen grids of multi element tubes to the anode, as I previously surmised.
All screens have separate voltage setup.


Caution with the circuit in the User Manual, as it is conceptual only.
Interestingly, the meter is drawn as 1mA 50R.


Selectest!
Sturdy Units, and memory lane time for me.
.
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File Type: pdf Metrix 310B Valve Tester R50 lo.pdf (1.81 MB, 122 views)
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Old 29th Jun 2020, 12:43 pm   #20
Matthew kane
Tetrode
 
Join Date: May 2019
Location: Melbourne, Victoria, Australia.
Posts: 66
Default Re: Metrix 310B tube tester Meter replacement

So heres a pic of the 310 meter.

And I just remembered that I salvaged and kept the water soaked and destroyed vcm mk4 meter parts in case for something. Take note of both pivot jewel screws being present just rusty, going to transplant them into the Metrix.
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