{"id":3150,"date":"2014-03-02T19:02:33","date_gmt":"2014-03-02T19:02:33","guid":{"rendered":"http:\/\/www.bartola.co.uk\/valves\/?p=3150"},"modified":"2014-03-02T19:02:33","modified_gmt":"2014-03-02T19:02:33","slug":"refurbishing-an-lcr-bridge","status":"publish","type":"post","link":"https:\/\/www.bartola.co.uk\/valves\/2014\/03\/02\/refurbishing-an-lcr-bridge\/","title":{"rendered":"Refurbishing an LCR bridge"},"content":{"rendered":"

Recently I acquired a second-hand and untested TF2700 LCR bridge from Marconi. This is a fantastic piece of engineering and a superb LCR bridge that can only be beaten by an expensive impedance analyser, something I can clearly not afford.<\/p>\n

\"Matching<\/a>
Matching Russian NOS capacitors<\/figcaption><\/figure>\n

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This bridge is perfect for selecting accurately capacitors for a RIAA network with extreme accuracy. In fact, the 1% precision can be improved down to 0.25% by replacing the main bridge 0.5% resistors with 0.1% ones as suggested by Morgan Jones in his “Building Valve Amplifiers” (Second Edition) page 330 and 331.<\/p>\n

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I was looking to build my own LCR bridge but given the Marconi’s circuit is so simple and has a great reputation, I decided to venture on the repair of this old unit.<\/p>\n

\"LCR2700<\/a>
LCR2700 by Marconi<\/figcaption><\/figure>\n

Firstly, you have to replace the PP9 battery connector with either a six AA battery pack or a PP3. I added a PP3 connector for a rechargeable battery. \u00a0You have to clean all the switches and parts. This takes a significant part of time, but is worth doing it.<\/p>\n

The old electrolytic capacitors must be replaced. Is not a great deal but you can do that very easily without dismounting the PCB boards:<\/p>\n

\"Replacing<\/a>
Replacing the old electrolytics<\/figcaption><\/figure>\n

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\"1kHz<\/a>
1kHz Oscillator Circuit<\/figcaption><\/figure>\n

Newer capacitors are much smaller in size. Despite being axial types, the voltage requirements are very low (16V) so there are still easy to find:<\/p>\n

\"IMG_0561\"<\/a>Once all electrolytics have been replaced, you can proceed to test in isolation the 1kHz oscillator. You can do this by measuring with the scope the output of the T1 secondary. You wouldn’t expect any\u00a0calibration needed unless C1 or T1 are damaged.<\/p>\n

You don’t have to upgrade the range multiplier 0.5% resistors. But doing so, will improve the accuracy of the bridge down to 0.25%. These days, 0.1% metal film resistors are not that expensive. You have to get a set of 1M, 100K, 10K, 1K and 100\u03a9 ones but the 10\u03a9 one should be a non-inductive 0.1% one:<\/p>\n

\"IMG_0554\"<\/a>Hopefully the standard 100nF 0.1% capacitor is ok, otherwise getting hold of one of those it may prove to be real headache. The real challenge are the potentiometers. I was lucky that the balance (fine) RV1 wirewound potentiometer was in good condition, but unfortunately discovered that the sensitivity pot RV2 (dual gang) was seriously damaged. I was testing the detector\u00a0amplifier\u00a0when I found that the error signal wasn’t amplified by the second stage transistor (VT3). So dismantled this potentiometer and tried to repair it\u00a0unsuccessful\u00a0as part of the sliding arm carbon broke when I was cleaning it:<\/p>\n

\"failed<\/a>
failed attempt to repair the ganged potentiometer<\/figcaption><\/figure>\n

The end result was that I had to put on hold this project until I could source a 2K5\/25K LIN\/LOG ganged potentiometer. I could have attempted in building this from a ganged 2K5 and 25K pair but I was keen to get this pot somehow.<\/p>\n

Luckily someone on the forums recommeded trying Blore Edwards <\/a>potentiometers from Wales.<\/p>\n

I was surprised to see that I could specify the potentiometers for a custom build at a very reasonable cost. Within a week I had a pair of these ganged\u00a0potentiometers<\/a>\u00a0with me ready to replace RV2.<\/p>\n

I completed the refurbishment\u00a0successfully\u00a0and\u00a0re-calibrated\u00a0the unit by using a ladder of 100K\/200K 0.1% resistors as recommended by Morgan Jones in his book.<\/p>\n

The unit now works like a charm, it’s stable from initial power up and can accurately measure capacitors and even chokes\/primary inductance of SE transformers using an external CCS and coupling capacitors. Easily to set up and drive with an external oscillator to ensure the inductance is measured at correct operating voltage.<\/p>\n

I’m planning to refurbish a second TF2700 I have in the future if you are interested please let me know.<\/p>\n

cheers<\/p>\n

Ale<\/p>\n