Marconi TF1313A LCR bridge

[Refugee]

I got one of these from a forum member a year or so ago.
As it has been waiting for a while I am doing a full check over before powering it up.
It is a classic Marconi instrument with that great build quality.
On the bench with a cap reformer on the grid coupling caps check out fine and the smoothing caps came up until the bleeder resistor formed a potential divider with the dropper in the cap reformer.
I had to use waves of label killer and Ambersil FE10 to get some label residue off. There is a tiny scratch on the panel legend by the edge of one of the knobs where a previous label has been scraped off some time ago but otherwise it is in good cosmetic condition.
The call for dinner came so it is first light tomorrow.
It is looking like a cal check and it will join my collection of test equipment.

[Refugee]

More photos.

[Diabolical Artificer]

Nice bit of kit mate, are all those blue Philips caps OK? What is it like at testing big inductors like chokes and tfmr's?

Andy.

[Refugee]

The blue caps did all the right things on reforming before I was called away. The black ones with bright coloured resin in ends are like new ones.
It is going to be brilliant.
The inductance goes right up far enough to do smoothing chokes with an external input to bias caps and chokes as they would be in use.

[turretslug]

Lovely piece of classic test gear from a sweet spot in time when passives were at last really good (plastic film caps, metal oxide/film resistors, improving electrolytics) but the construction quality and mechanical ingenuity hadn't yet been pared back. Some Marconi, H-P, Tek etc. things of this era should qualify as cultural heritage!

Hope it gives long and dependable service.

[Refugee]

The range selector is like a turret tuner with all the resistors mounted on the rotating part with the smaller steps done by more resistors mounted on a side assembly.
The main dial is the classic enclosed type seen on many Marconi instruments.
It is an AC bridge by default to such an extent that a neon oscillator is used to make the resistance section work with the metering.

[Refugee]

I have started doing a cal check and it is looking very good.
I checked the 100K resistor that I had been using to reform the caps and it came in at 101.4K and 101.5K on a good DMM so top marks there.
A 7.5mh inductor came in at 7.85mh on both both the Marconi and a component tester.
Capacitance read a little bit low but not by anything that would put me off the component tester so I will have to dig out a very high spec capacitor if I can find one in my bits box.

There is a little bit of mechanical tinkering to do to get the ranges to appear in the center of the three windows in the dial. A hex key that is a kid glove fit for the grub screw needs to be found. I have got plenty to dig through to find it.

It reads range in the little windows and then the black pointer on the outside and then the little red one closest to the knobs in the middle. It has 4 concentric controls including the function flags so it is a complicated dial.

[Refugee]

More photos.

[Refugee]

During all the testing and making sure it is worth putting the panels back on I found a dead spot on the balance pot.
It is a 3-gang and as such it would be made of unobtanium and if that were not bad enough it is situated behind the mains transformer.
The problem was on all balance ranges so I figured that it would be RV5 as it is in circuit on all ranges of the balance selector.
Isolating the end of RV5 was easy enough but then I had to change the low ohms battery on my AVO as that had been bought and put by to be fitted. RV5 is 51 ohms so there was no choice with that.
RV5 came back no fault found so there was a bit on an anticlimax to be had.
RV3 was next and it had a dead spot on it and then RV4 with two dead spots on it.
I got my fat free cleaner out as it is even compatible with Tektronix switches and is safest.
It worked on RV3 and RV5 but RV4 was a problem. It has a plastic cover with a bearing cap over it and there is no space where the wiper conductor goes in like with the other two.
The bearing plate would not come off without getting the whole assembly out so I tried the fixing screws for the tags.
The things have got nuts on the other side so great care was needed to avoid them dropping inside. I got some cleaner in and the two dead spots on RV4 resolved.
With forceps and a screwdriver I got the nut to catch enough to tighten the screw again.
I am going to do a full round of testing again and see about getting the covers back on again.
You bet that my everything tester is going to get through a few charges during testing.
It is accurate enough to be a calibration standard for all the measuring devices we get from China.
The date code looks like 1977.

[wd40addict]

Attached is what the Marconi in-house magazine said about it. Note they abandoned the AGC balancing of the TF1313 (no A), when only a few years before they were trumpeting it!

[turretslug]

Whilst I respect olde-worlde Marconi stuff for its sturdiness and precision of construction, they didn't half like to use pots with funny values and the ganging/concentricity often used just makes the repair substitution even more tricky. I wonder if they had a tie-up with Colvern, or maybe they just produced sufficiently large quantities of test-gear that bespoke potentiometers were worthwhile?

Interesting to see components with 1976/77 datecodes in a piece of valve test gear when e.g. the thoroughly solid-state and portable TF2700 had been available for some 15 years by then. Perhaps the TF1313 was a stipulated "lab" reference for some major user like the MoD that featured in many legacy manuals.

[Refugee]

It is interesting about the AGC being dropped. Perhaps like me many other users at the time liked manual so back it came with this 0.1% model.
The standard resistors are marked 0.025%.
The pot I had to clean is just a triple gang. The main dial has four concentric parts to it that operate all the flags to show the selected ranges and a shaft that carries many of the precision resistors.
During my internet searches one of the Marconi sites says that it was the last valved instrument they produced.
They would have been produced in hundreds as opposed to thousands based on the serial number ending in 520 on mine. They must have got a reasonable price break on the fancy pots. The latest date codes I have found are late 1976 so it would have shipped early 1977.
They also changed from a square dial to the pear shaped one we see on mine.
If I ever put my old bridge up for sale it will be priced for the nice bright EM34 plus a bit more.

I can vaguely remember one sitting on a rack of lab gear at college back in the 1970s.

[Bomber county]

I have just got one of these with a load of old test gear from a friend who is retiring. What were they used for, more to the point are they worth anything?

[TonyDuell]

It's used to measure the value (electrical size) of resistors, capacitors and inductors quite accurately. It's essentially an more complex version of the Wheatstone Bridge that you may have covered in physics lessons at school.

As to whether it's worth anything, you are not going to retire on the sale of this instrument. But there are plenty of people here who would like such an instrument and would be prepared to pay a reasonable amount for one.

[Refugee]

It is a very accurate instrument with good quality components throughout.
The only down side it that it is a little bit large.
You have an instrument that can to some extent act as a calibration standard.
Compensating for the Q of coils needs a little bit of getting used to.
If you are experimenting with RF circuits you can just test through piles of odd coils and fine exactly what you want or just wind your own by guess work going a little high and then take the wire off a turn at a time.
I like mine and am proud of it.

[Radio Wrangler]

They seemed to be replaced by the Wayne-Kerr autobalance transformer-ratio bridges which were faster to use, more accurate and still delightfully well made. Smaller too.

A golden era.

David

[Refugee]

You are right about there being more modern instruments coming on stream at the time.
It was one of the last valve instruments Marconi made.
It is easily accurate enough for anything I want to do so now that I have restored it I am keeping it.

[Radio Wrangler]

A gorgeous bit of treasure.

But your grandkids will ask "Isn't there a phone app for measuring those things?" whatever you have.

David