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Old 4th Jun 2018, 7:52 pm   #1
ColinTheAmpMan1's Avatar
Join Date: Jan 2012
Location: Wimbledon, London, UK.
Posts: 808
Default Upgrade of Marconi Universal Bridge TF 2700

Some time ago, I purchased a Marconi Universal Bridge TF 2700. This instrument is mentioned in "Building Valve Amplifiers" by Morgan Jones and he describes it as an excellent instrument. He furthermore goes on to say that the accuracy can be improved easily by replacing the main range switch resistors from the original 0.5% to 0.1%. These are mainly metal-film types, except for the 10R, which should be a non-inductive wire-wound resistor. He also describes a method for reducing any error in the dial on the fine balance variable resistor. This is achieved with a series of 0.1% metal-film resistors, comprising four 200K resistors and one at 100K. Wiring these in series with the 100K at one end, values from 100K to 900K in 100K steps can be picked off. With the main range switch set to 10M, these are measured and a plot of measured value against known value can be drawn.

I purchased the required resistors and considered the task of improving the instrument. I was initially alerted by the fact that the 0.1% resistors came in static-control envelopes from the supplier. It had already occurred to me that they might be damaged by excessive heat and that finger-grease on the surface might compromise their values. With these thoughts in mind, I opened a thread on the “Components and Circuits” section on the handling of close-tolerance components. It was suggested by several contributors that apart from the use of clip-on heat-sinks to avoid excess heat on the resistors and the use of alcohol to wipe any contaminants from the outside of the components after assembly, that I might also use the normal static-control techniques, such as a work-mat, wrist-strap, solder-sucker and soldering-iron.

I looked at where the main range switch was mounted and wondered how I might access the resistors to replace them. This wasn’t as difficult as I initially thought, as removing the knob (two 1/16” hex screws) and then taking off the mounting-nut and crinkle-washer enabled the switch to be carefully withdrawn into the case and turned through ninety degrees to facilitate this access. In removing the original resistors, I was careful not to damage the wafer-switches, using gentle force to remove them and using the solder-sucker to remove most of the solder. Luckily, the resistor leads weren’t wound several times around the tags and so the job wasn’t too difficult. When I installed the new 0.1% resistors, I soldered the inner ends first, while using two clip-on heat-sinks and not removing these heat-sinks until I was sure that the joint had cooled adequately. I then went through the other ends of the resistors in similar fashion. After making sure that all the joints were good, I swabbed all of the new resistors with IPA on a cotton-bud.

Finally, I checked the old resistors with the bridge to make sure I hadn’t made any silly mistakes. All checked good. I then went through Morgan Jones' method of calibrating the variable balance control and found it to be totally OK.

Finally, I re-made the battery-leads as they looked rather frayed.

Result, one Marconi TF 2700 fully sorted.

I hope this might be of some use to other owners.


PS: I should point out that one of the leads of the oscillator transistor had never been soldered!
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