Linstead M2B AC/DC FET Millivoltmeter (1975)

[Phil G4SPZ]

I bought this neat-looking instrument on impulse as I passed the Bring and Buy stall on my way out of the NVCF in 2007. I paid the princely sum of £3, and so I wasn’t too disappointed when I got it home and found it wasn’t working. Forum member Bill (WME_Bill) kindly replied to my request for circuit information and not only sent me a copy of the user’s manual and circuit, but also his own notes and circuit sketches which subsequently proved to be very useful. A quick look inside revealed a broken solder joint on one of the meter amplifier transistors, and a dab of solder got it working again. However the calibration wasn’t good, but the wide range and sensitivity on AC made it handy for basic tasks such as alignment, where only a peak or a relative reading was required.

Fast forward nine years to last week, and I recently needed a sensitive and accurate AC voltmeter to set up a tape recorder. I started using the Linstead M2B, but it soon became apparent that not only were some of the AC ranges miles out, but the darned thing was highly frequency-dependent. I wasted some time chasing a non-existent equalisation fault in the tape recorder before I began to suspect the meter. This is the second time I’ve been led up the garden path by faulty test gear, so I thought “never again” and started on this repair.

The Linstead M2B was also marketed by RS Components as their “Wideband Millivoltmeter stock no. 610-512” and my example was clearly from that source. The meter came with an accompanying sheet of hand-written notes and calibration measurements, together with a message dated 2007 from a previous owner which read: “I made a case for this meter, it was left over from a student project in my college”. His measurements showed that the calibration was indeed miles out, varying between 16.7% low to 5.3% high on the DC ranges and anything up to 12% high on the AC ranges. It’s obvious why the meter ended up on the Bring & Buy stall the same year!

I started to chase the cause of the frequency dependency. On AC, the input attenuator operates in three stages and includes two pre-set trimmer capacitors, so as I had no setting-up instructions I found by experimentation - using an audio oscillator source - that adjusting these trimmers markedly improved the frequency linearity. I also experimented with the setting of VR2, the ‘set gain’ control in the AC amplifier, and quickly managed to obtain excellent accuracy on all ranges. I don’t have access to lab grade instruments, but I based my calibrations on a Marconi TF1370A wide-range R-C oscillator and I managed to get the Linstead to agree to within a couple of percent. The makers claim an accuracy of +/- 2.5% from 10Hz to 500kHz. Patting myself on the back, I started to put the meter back in its case whilst it was switched on, and noticed the needle swinging wildly as I moved it. In fact, almost any movement or flexing of the chassis caused the meter reading to change. I found and re-soldered two dry joints but the problem persisted. The design is to some extent flawed in that several connections to chassis rely on mechanical tightness of nuts and bolts associated with the wafer switches, which are not of the most robust type. In the end I added three additional wires to electrically bond the crucial points solidly to chassis, and the fault was cured.

Having fixed the AC side so quickly, I rashly assumed that the DC side would be just as simple. Famous last words... on test, the lowest 120mV range was spot-on, but all the other DC ranges read low between 1.5% and 17%. As the instrument was accurate on AC and the two-transistor meter amplifier was common to both AC and DC, I studied the DC attenuator circuit in more detail, and here Bill’s notes came in very handy. The resistive attenuator is again in two stages, an input multiplier and an output attenuator each having four steps, and they operate together in a rather elegant manner to cover the eight DC ranges. I started checking the values of the associated resistors. The input multiplier resistors were within half a percent, but those fitted in the output (meter movement) attenuator differed markedly in value from the circuit diagram.

I worked out that the basic accuracy of the instrument was set on the lowest 120mV range by the input multiplier, as the output attenuator is shorted out entirely. On higher ranges, the output attenuator works by switching various resistors (R34, R37 and R38) in series with the 50uA movement’s negative lead: zero resistance on 120mV; 5.4k on 400mV; 22.8k on 1.2, 12 and 120 volts; and 76.8k on 4, 40 and 400 volts. The input attenuator is a simple resistive multiplier of total resistance 11.23 Megohms, operating in four steps of x1, x0.99, x0.099 and x0.0099. Between the two attenuators the eight DC voltage ranges are covered. The 120mV range was perfectly accurate, and the input multiplier resistor values were all correct, so I tried experimenting with various resistances shunted across those fitted in the output attenuator. Working upwards through the ranges, I temporarily linked a one-megohm pot across R34 and adjusted the pot to give accurate FSD on the 400mV range, then measured the value of the pot at that setting and soldered in the nearest preferred value fixed resistor. I did the same in turn with R37 and R38. By the end of the process I had all ranges within less than 2% of FSD, and I have noted on the circuit diagram the resultant resistances that I used in the output attenuator. Success!

With so much space available, the layout of most of the components is crammed around the two range and function switches S1 and S2, making repair tricky. Also the switch wafers are rather delicate and flimsy - I think these are in the style of ‘Maka-Switch’ components - but using the long tie rods behind S1 to double up as the sole mountings for the AC amplifier PCB leads to a very flimsy construction. The photos show the components strung around the switch assemblies as well as the grounding wires that I added. The resistors that formed the original DC output attenuator and that I had to change are shown at the end of S2.

I can’t deny I was happy to put the instrument back together! It’s amusing to speculate about this meter’s history, as it was obviously hand-built from a kit by a student, albeit with a fair degree of skill. Did RS Components market these instruments in kit form, I wonder? I feel sorry for the student who built it, because the component values supplied for R34, R37 and R38 would have produced gross inaccuracy on the DC ranges. My copy of the circuit diagram also omits a crucial wire link between R34 and the 400mV switch position, which makes understanding the operation of the circuit doubly difficult. Perhaps the student thought that a constructional error had caused the fault, and failing to find the reasons for the poor accuracy, eventually gave up? Anyway, it’s a very useful instrument and one that I can now rely on for accuracy.

Phil

[Brian R Pateman]

Another excellent job and a good write up, Phil.

It's years since I've had anything to do with them and I have to admit that they were not among my favourites. I agree with you about the layout.

The other thing was that in the days when they (and we) were younger PP9s tended to be heavier than the modern ones which made them a bit top-heavy. I well recall having spent most of an afternoon repairing one I left it at the edge of the bench, caught the top of it with my elbow and it tipped forward onto the floor.

That said they were reasonable performers especially as they were at the lower price end of the market.

The clustering of components on the range switches does minimise wiring length and probably helps to keep noise problems to a minimum.

[Phil G4SPZ]

Hello Brian and thanks for the comments. It's nice to know that someone else remembers these meters!

Originally Posted by Brian R Pateman

...PP9s tended to be heavier than the modern ones...

That's something I forgot to mention. Weight-wise, I'm sure you're right, but a 'modern' PP9 won't actually fit properly inside the battery holder as it's a bit too fat. I've found this sometimes with old equipment, and I think the cardboard used for the casings on old batteries was thinner than on their present-day counterparts.

I certainly can't complain about this meter's performance now it's been fixed, it's a very useful bit of kit.

[Radio Wrangler]

Phil, when you're setting the hf response trimmer capacitors on the input attenuators of things like AC voltmeters and distortion meters, you can cheat by applying a squarewave and prodding with a scope further down the amplifier. Twiddle for best waveform, and only a slight final twiddle is needed when it is swept.

David

[ronbryan]

Phil

I believe I bought this Linstead meter at Harpenden in the same lot as a Heathkit V7-AU VVM, a Telefunken Teletrans 1 Germanium transistor tester and a Robin multimeter. I could find no information on it so I put it and the multimeter on the Bring and Buy at the NVCF in 2007. It certainly had a hand made outer case (in primer?) and was part of a student project.

Congratulations on your persistence on fixing it, a really good effort and a nice write -up.

Ron

[Phil G4SPZ]

Thanks for the tip, David, I would never have thought of that! And I'm sure it's the same meter, Ron, still in its primer-coated case. What a coincidence. Thank you for your kind remarks.

All we need now is for the student who built it in the 1970s (who will be the same age as me now) to come forward and finish the story

[WME_bill]

Linstead M2B. Thanks for the write up. I have sales leaflets and the circuit dated 1975, by Linstead Manufacturing Co, which implies that they actually made these instruments, rather that marketing a kit, so your experience is interesting. May be they did both. I have a sales leaflet dated 1988 showing that Linstead products were being manufactured by Masterswitch Ltd, selling power supply units, the meter unit and an audio oscillator, clearly all aimed for the schools market.
My version of the M2B is still sitting under the bench, so you have encouraged me to get it out for checking and repair. Thanks also to David "Radio Wrangler"'s suggestion of using a square wave to set up the AC ranges. Is that a HP trick? wme_bill Bill Nichols m0wpn

[Phil G4SPZ]

That'll be interesting, Bill. Can you please post a picture of your Linstead's innards so we can compare the professionally-built version with the kit version?

Cheers,

[Phil G4SPZ]

Originally Posted by WME_bill

Linstead M2B. ...you have encouraged me to get it out for checking and repair...

Bill, the attached sketch of the detailed operation of the DC input multiplier and meter attenuator may be of help. Apologies for the quality. The resistor values in the meter attenuator are the ones I determined experimentally, not the values shown on the circuit diagram nor the values I found fitted in the meter. Good luck.

Phil

[gezza123]

HI.Phil-g4spz.
Nice write up, I have got an original Linstead M2B I think as its in a case
made by MAGNAVOX.

I did get the circuit a few years ago from a member but not the manual, I
would be interested in a copy if you have it. I have a few pics if you want them.
Gerry.

[Phil G4SPZ]

Hi Gerry,

That's interesting. Your meter looks a lot older than mine. I think the Magnavox label is actually an asset number confirming that Magnavox owned the meter and to deter theft. A nice historic artefact.

Yes, it would be good to see more photos of your meter. If you PM me your e-mail address, I will send you what I've got - a very brief user guide and specification plus circuit, which originally came from Bill, plus my notes.

[gezza123]

Hi Phil,
Thanks for the information, I hope its not a borrowed one.I bought it of that well known shop on tinternet a few years ago, it was faulty and I got it cheap.

I had to replace the two front end transistors Q9 2N3906 and Q10 ME4266
and have had no problems with it so far.

If you need any more let me know.

Gerry. PM sent

[Phil G4SPZ]

Thanks Gerry. It's nice to see the detailed photos of the inside of your meter. Yours looks to have some much older transistors in the AC amplifier and larger components generally. I presume these meters were made in smallish quantities over a fairly long period of time, and they updated periodically as newer (or cheaper) components became available. For example, in my meter Q9 is an ME0412 and Q10 is an ME4102. I don't think either type is critical.

I've e-mailed you scans of the remaining information that I have, and I'm also attaching it below for the potential benefit of others searching for data on this meter.

Phil