Recently I've been trying to fix one of these lovely old meters. Designed for military use in the early 1960s there were two versions, the CT471 with a regular meter and the CT471C which has a centre zero meter. Transistorised, they can measure voltage with frequencies up to 1 Ghz with the probes provided. Power is provided by 3 x 1.5V 'D' cells. From the calibration stickers on one of mine they seemed to still be in RAF use in the 2000's. These are substantial meters and weigh about 11lbs a pop. Despite its size I use my '471' quite a lot.
These meters use OC170 transistors on the amplifier board which seem to suffer the same problems as our favourite AF11X. Indeed this was part of the problem with my '471C' model, a snip of the screen lead and a new capacitor across the meter got it working again. The other issue was the meter scale had come very slightly unstuck causing the needle to stick at FSD. The repair meant the meter had to come out and its casing was very carefully taken apart and the meter scale glued down again.
My '471' has had one of it's OC170's replaced with an AZ1 which I'd not heard of before - RAF service unit seals were intact on purchase - and it works perfectly.
While I had the CT471C apart I thought I'd take some piccies. Incidentally, the smell when you open the case is glorious.
The CT471C. The 'Lethal Warning' sticker is to do with rated insulation of the case
Power supply board
Amplifier board with three OC170 vertically. The lower one is the problem one.
Attenuator board
Point to point wiring and wafer switches, with the spring metal battery connections at right and left
RF Probes; 50 and 75 ohms at 4V terminated and 4V and 400V unterminated.
The CT471 with a PP9 for scale.
Andrew
That does look a very nice meter Andrew, I don't think I have seen those before.
Thanks for showing the pictures 
Nice meter, there is a lot to it, more than just the normal collection of shunts and dropper resistors.
Mike
Indeed nice 
Though not very cuddly in bed otherwise the missus would be sleeping elsewhere. I've got several AVO's but despite the bulkiness I use both my CT471 a fair bit, the 'C's centre zero facility being useful at times.
Luckily, one came with an original user and maintenance manual, an older printing is on the VMARS site. It has a very lengthy list of equipment needed for calibration.
The meters come with standard 'AVO' type leads and probes - though one of mine had Hirschmann probes - and Novedi croc clips. The RF probes connect through a co-ax lead with a BNC on the end and then there's a second co-ax lead with sockets you can plug your 'AVO' probes in to.
It's quite interesting sticking it on the lowest RF range, sticking a probe on, waving it near the pc or whatever and watching the meter needle move around.
Thanks for the interest.
Andrew
Very nice meter, high component count.
Picked up a CT471C today at Harpenden from the bring and buy.
One of the battery leaf springs is broken, hopefully I should be able to fashion one without too much effort.Was really pleased to find that the 4 probe units terminators were still present, although the leads are missing.
Andy could you post a picture of the coax lead assy? Does the terminator connect directly to the probe, or at the meter end of the coax?
Found this picture in an old HMSO publication, distrubuted to the residents of Medway towns in the mid 70's, when HM Dockyard Chatham became a refit complex for nuclear submarines.
The photo is taken in one of the Dockside Test Organisation (Weapons) workshops.
Note the CT471 on the shelf, to the right of the Marconi sig gen.
Rob.
Thursday night is shed night.
Connected it up to the bench supply, but totally dead. 
Oh well, one more for the 'tuit pile. 
Rob.
Thursday night is shed night.
Hi Rob,
I saw the CT471C...but didn't see you! I ended up with the Marconi TF2600 Valve Voltmeter which looks like eight valves worth of fun.
There's two sets of leads, three including the standard 'AVO' leads - same as AVO spade ones just branded 'GEB'. Set A for the probes is like this, co-ax terminated in a BNC. Co-ax is c. 1/4" and about 30" long.
Set B is same but two 'Y' splitters and sockets for the familiar rod style 'AVO' probes or crocs.
Shame you couldn't get it running, do you have anything at all on the 12V calibration feed? The manual if you don't have it already.
http://www.vmarsmanuals.co.uk/archive/1 ... imeter.pdf
Regards _ Andrew
Hi Andrew, no nothing at the +12 Vdc tp.
I did see you at the bring and buy, but you seemed far too occupied with your purchase.
I sure it will be something simple, around the chopper circuit I suppose.
Thanks for the link.
Rob.
Thursday night is shed night.
Made a bit of progress now.
Removed the case and connected the bench supply directly, check battery position gave the correct deflection of the needle so not a bad start.
Checked the power supply section, 2.5 Vdc section adjusted ok, but could not get the 20 Vdc supply to adjust above 19.5 Vdc. I left it like that as I guessed something else was pulling the rail down, rather than a fault in the regulator.
After a quick functional test, it was apparently dead on all ranges.
Thanks for the link to the diagram, I printed out the relevant pages and after a bit of sticking and glueing started to get to grips with how the meter works.
I decided it was probably best to get the ac ranges working first, as this part of the circuit is common to all ranges.
With a suitable ac source connected, the input to the ac amplifier seemed about right, and could be varied by adjustment of the range control so seemed in order.
Probing around VT16, 17 and 18 produced some very odd results, so following Andy's success I snipped the shield connection to VT16, this increased the signal at the base, but still nothing on the meter.
I moved onto VT17 and again snipped the shield connection, this gave a kick from the needle of the meter, and the 20 Vdc rail rose to the correct value. Finally snipping the shield connection to VT18 gave an indication from the meter, however the response is very non-linear and some very strange waveforms on the scope. Looks like one (atleast) of the transistors (OC170) is breaking down and will require replacing, any ideas on a suitable sub?
TBC....
Rob.
Thursday night is shed night.
Hi Rob,
Well done! My CT471 has an AFZ12 (not AZ1 as I said above) in place of the OC170 at VT16.
Type Designator: AFZ12
Material of transistor: Ge
Polarity: PNP
Maximum collector power dissipation (Pc), W: 0.083
Maximum collector-base voltage |Ucb|, V: 20
Maximum collector-emitter voltage |Uce|, V: 10
Maximum emitter-base voltage |Ueb|, V: 0
Maximum collector current |Ic max|, A: 0.01
Maximum junction temperature (Tj), °C: 75
Transition frequency (ft), MHz: 135
Collector capacitance (Cc), pF: 2
Forward current transfer ratio (hFE), min: 20
Noise Figure, dB: -
Package of AFZ12 transistor: TO72
Wow, that's pretty good stats.
One of the OC170's was u/s bad leak Iceo, the other two marginal (and of course with whisker shorts to case), so I replaced all three with 2G403's which are a pretty good match.
A bastorial job to solder in place, but it's now working on the dc ranges too, the 12 Vdc ref reads 11.85 on the scale, so we're getting close!
Not bad for an evenings work.
Cheers,
Rob.
Thursday night is shed night.
