Marconi Instruments TF-428B Valve Voltmeter

[RogerWalker]

I turned to this as a break from trying to fix my Cossor scope - which is not going well.

The TF-428B is a real old-timer, built like a battleship with a hefty front panel and housed in a paxolin case with a lift up lid that provides access to the valves, chassis and the [EA50] diode probe. Despite the massive mains transformer, it relies on just a single 6Q7 [DDT] for the voltmeter circuit. Easy-peasy I hear you say . . .

Unfortunately this dear old Marconi delights in multiple hand-wound precision resistors, some individual, some on a long multi-sectioned bobbin about 5 inches long. The wiring is in a limited number of colours and tightly bound with twine making it extremely difficult to trace the circuit. I have so far replaced 3 open circuit bobbin resistors and 3 more old carbon jobbies that were miles out of tolerance [with 1% metal film resistors for the moment]. I have tested the 6Q7, replaced the rubber mains lead and cleat and have done routine voltage and resistance checks - the meter looks to be fine, thank heavens. That was the easy-peasy bit.

The fault appears to be an open circuit in the 6Q7 triode cathode circuit - which happens to be on the 5" bobbin multi-resistor - Apart from unwinding the o/c resistor and measuring both halves, I have no way of knowing its value. This is not a trivial task as there a seven resistors wound on the bobbin and a similar number of connecting wires tightly bound together and passing through a hole in the chassis directly under the bobbin.

I need a circuit diagram before going much further. There's nothing available on the web and nothing anything like this to refer to and get an idea of circuit topology. If anyone has a manual, circuit - even of something similar - I'd really appreciate a copy. I've added a couple of photos in case they jog someone's memory.
Regards
Roger Walker

[WME_bill]

Marconi TF428. Roger, all I can find is the attached description from the 1948 Marconi catalogue. About your failed cathode resistor, the voltage each side of the meter will be the same. So look at the balancing resistor chain on the right, measure the voltage across the bottom resistor, guess an anode current for the 6Q7 triode, say 1 ma, and that will give you a ball park figure for trying out a few resistors for the replacement cathode resistor. See which value makes the unit work. By about 1960, the concept of two balanced triodes gave much better results, as in the TF1041 or Avo ETM and lots of others.
For ideas about this style of meter, Scroggie: Radio laboratory Handbook discusses them, but you need the earlier editions for any detail: 2nd of 1944 is helpful. 6th of 1958 is very brief. 9th of 1980 has nothing. I'll copy the pages from 2nd if you want them. wme_Bill m0wpn

[RogerWalker]

As usual you have come up trumps Bill! At least it gives me a starting point.
I have spent today trying to trace the circuit, but the limited palette of wire colours, the [unmarked] multi-resistor and the tight bindings make it very difficult. The rest of the time was spent going through AVO, Heathkit, Taylor and other manuals looking for something similar - to no avail; pretty well all went over to the twin triode bridge circuit at an early stage. There are a couple of early designs using a single triode AC coupled to a bridge rectifier and thence to the meter, but that's not what I've got.
The meter is stationary at zero except when range 1.5V is selected, at which point the meter needle hurtles backwards alarmingly. The set zero control does nothing, though the pot is working OK and is sending a varying voltage off I know not where. The voltage drop across the HT smoothing resistor [5k] is 70V suggesting 14mA total current useage.
I will do some more ohms law and calculate the value of the cathode resistor - the DH63/6Q7 data suggests 3mA anode current. I'd love the early Scroggie pages if you have time. Thanks as usual for the constructive advice!
Roger

[WME_bill]

TF428 valve voltmeter. I'm glad that info was a help. I have now remembered that the CT54 (or EIL22A) of 1955 vintage uses the single triode valve, and the manual explains the principles quite well. There has been some discussion about the CT54 on this forum over the past couple of years, so you should be able to find a copy. Else VMARS I seem to remember. wme_bill. m0wpn

[Dickie]

I've dug out a handbook. Drop me a PM with your email and I'll forward a scan in due course. I've also got a manual for the original TF428 (not B) dated 1939, which shows how old it is.

[WME_bill]

TF428B. Spurred on by "RogerWalker's" original enquiry, I have found a 1957 catalogue, when this model had become the TF428C, which uses the modern circuit of a balanced triode pair. Attached. And even more interesting is the note from "Dickie", confirming the original design was 1939. So Roger, you have a real vintage piece of gear there. For Dickie, can I beg a copy of the manuals you are doing for Roger please. I will send a PM. wme_bill m0wpn.

[RogerWalker]

After a day of frustration, Dickie's news is the electronics equivalent of Andy Murray winning Wimbledon. The PM went off at about 130mph - the speed of the last serve.
I am waiting with bated breath.

WME_Bill - I actually used a 428C 'in anger' some years ago.

Thanks all
Roger

[RogerWalker]

The manuals arrived and were quite a revelation. The circuit diagram relates closely to what I have - but in common with other old equipment, the physical layout of the components is markedly different. The manual lists all the bobbin resistors as separate entities whereas on mine, a number of the resistors are combined on one long bobbin. I can't get my head around the wire gauge needed to wind a 90,000 ohm resistor on a tiny bobbin! No wonder they are mostly open circuit.

I have attached the page containing the circuit diagram and component values for anyone interested in the troubleshooting that will follow.
Regards and many thanks to "Dickie".
Roger

[Dickie]

Just for fun, here's an original TF428 from around 1940, which demonstrates some of the early history of Marconi Instruments. It's in a polished wooden box with nice dovetailed joints.

In the early 1930’s test gear required for the manufacture of radios, etc., was made by the engineers who needed it. A company in Southend - E.K.Cole Limited - began producing test gear for its own use in manufacturing radios (EKCO to you and me), and in the early 1930’s Coles agreed to sell test gear to other manufacturers, in particular Marconi's Wireless Telegraph Co.. Business between E.K.Cole and Marconi grew until, in 1936, the two companies jointly formed a new concern - Marconi-Ecko Instruments Limited - with premises at both Southend and Chelmsford.

In 1939 Europe entered into war and in 1940 the British government decided that the Southend factory was in too vulnerable a position to continue to house such a vital link in the country’s telecommunications industry. At 24 hours notice the Company moved to High Wycombe and later also took factory space in St. Albans. Owing to legal complications of dual ownership in wartime the shares in Marconi Ekco Instruments held by E.K.Cole were acquired by Marconi’s Wireless Telegraph Company in 1941, thus making the Company a wholly-owned subsidiary of the Marconi parent company. It was around this time that the name was changed to Marconi Instruments Limited.

Looking at the photo, you can see it was made by Marconi-Ekco, and the meter says "E.K.Cole, High Wycombe."

[RogerWalker]

Ahah! THAT'S why the first 428 used american valves, 75, 955 etc!
When I worked for the British Radio Corporation [later Thorn Consumer Electronics] the radio factory at Chigwell and the TV factory at Enfield designed and manufactured pretty well all their own factory testgear and jigs, customised to provide exactly what the factory needed, nothing more - nothing less.
Roger