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Old 10th Aug 2022, 4:43 am   #1
QQVO6/40
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Default Marconi TF2701 In Situ RCL Bridge.

Greetings everyone.

I have recently been given a Marconi TF-2701 In Situ RCL Bridge.
I have looked and not found any information on this bit of kit other than a mention in an old Marconi catalogue of 1963 - 1964 vintage.
Looks to be in unmolested condition. So far so good.
The only change spotted so far is a previous owner has wired the battery connections to a low voltage concentric connector on the rear panel.
Also written on the rear with a felt pen is that it is to be run on 9 Volts DC.

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I think this voltage might be correct however I would like to make sure.
There are numerous electrolytic caps on 2 boards with the highest voltage of 12 Volts so I guess the 9V could be correct.
There is two battery holders on the rear apron. No connectors or information about battery type.
The approx dimensions of each holder.
65mm wide, 55mm high and 85mm deep.
I am not sure if they are 2 x 3V batteries in series or 2 x 4.5V batteries in series.
My memory from a long time ago seems to be of a 4.5V battery that would fit each box.
I am thinking of putting a small regulated supply into one of the battery boxes and feed it from a well insulated 12 V AC plug pack and so keep the isolation from the mains supply.
Just a thought.

Not having the book on this doesn't help.
I have looked and found pictures of this but nothing else.

If anyone could help it would be very much appreciated.

Thank you in advance for reading this.
Bit long winded but it gives me good typing practice.

Cheers all.
Robert. VK2BNM.
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Old 10th Aug 2022, 6:02 am   #2
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Default Re: Marconi TF2701 In Situ RCL Bridge.

Sure its not two by 9 volt supplies ??. Bipolar supply ?.

Nice looking instrument though.

Joe
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Old 10th Aug 2022, 7:25 am   #3
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Default Re: Marconi TF2701 In Situ RCL Bridge.

Hello Joe.

Nah. Single supply. Just the 2 wires going in.
I was told it was in working condition when I picked it up.
I do remember it had a very small plug pack with it but I can not find it anywhere.

Somewhere I have a thick folder of "Eveready" battery technical specs from years ago but do you think i can find it? NAH!!! Couldn't be that lucky.

Maybe if I offer a couple of cream filled lamingtons as an incentive for some kind person who just might know where I can get a copy of the book.

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Thanks for your fast reply

Cheers,
Robert, VK2BNM.
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Old 10th Aug 2022, 7:26 am   #4
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Default Re: Marconi TF2701 In Situ RCL Bridge.

Looks like a TF2700 but with two phase controls instead of a loss/balance control. The TF2700 runs on one 9v battery Joe, a PP3. You might find the TF2700 manual worth a look Robert your 2701 is probably very similar circuit wise but with a few tweaks. Service manual here - https://pdfhow.com/downloads/service..._generator.pdf
Edit, that link is about the only half way kosher link I could find. I got about 50 hits for "TF2701 service manual" but with very dodgy addresses, it's being used as click bait essentially. The pics of the TF2701 I found are nothing like yours Robert, intrigueing.

Andy.
Edit #2 member GMB also has one and may have a manual, see - https://www.vintage-radio.net/forum/...ad.php?t=60329 he's still here with us, logged in yesterday.
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Old 10th Aug 2022, 8:39 am   #5
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Default Re: Marconi TF2701 In Situ RCL Bridge.

https://worldradiohistory.com/Archiv...ni-1963-06.pdf says 2*9V batteries!
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Old 10th Aug 2022, 9:30 am   #6
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Default Re: Marconi TF2701 In Situ RCL Bridge.

Hello Andy.

Thanks for the reply.
The TF-2700 is a totally different machine. Born of the same period but different.
I have the book for the 2700 and it has 5 transistors.
My TF-2701 has 11 transistors and is switchable 80cs or 1kc inverter supply for inductance or capacitance measurements.
When I was told about it originally it was described as a TF-2700 so I went ahead and printed off the manual. That is how I have ended up with the manual for the 2700 but not the bridge.
Too quick I guess!
The link to GMB has promise! I will follow that up soon. Just finished our evening meal. About to make a cup of tea so I will then chase up the link you kindly provided.

Thanks again for helping.

Cheers.
Robert, VK2BNM.
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Old 10th Aug 2022, 9:54 am   #7
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Default Re: Marconi TF2701 In Situ RCL Bridge.

Hello wd40addict.

Thank you for that very informative document.
Magic!!

It does not give the circuit but it provides the information on the theory of how it works which is very important to me to understand it.

Just a quick look at the book also gives info on some other Marconi kit I have. Namely audio sig gens and switched attenuators. I have saved it and will print it off tomorrow morning.

Two 9V batteries so must be in parallel as the electrolytic caps are rated at 12V.
A previous owner did write on the back to use a 9V source.
I always check to make sure and thanks to you I now know for sure.

Looking forward to using a workshop power supply tomorrow to see if it works and what it will do.

Cheers.
Robert, VK2BNM.
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Old 10th Aug 2022, 7:19 pm   #8
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Default Re: Marconi TF2701 In Situ RCL Bridge.

Hi Robert, from what I see the circuits are basically the same but additions on the 2701.

If you look in the 2700 handbook there are additional circuits that can be added to extend the usefulness of the bridge (DC in a choke, high voltages to check ecaps etc), well worth adding to your 2701

Ed
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Old 13th Aug 2022, 7:37 pm   #9
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Default Re: Marconi TF2701 In Situ RCL Bridge.

I have something similar to this( liberated from college when they were throwing it out) and ISTR mine used two 9 volt batteries. I made a power supply to go in it, but I can’t remember whether it was for +-9 or 18 volts. Still have it but haven’t used it in years, but last time I did I seem to recall it worked and was reasonably accurate.
I think there is a probe thing that goes on the front. Service info link will be handy. Thanks
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Old 13th Aug 2022, 8:33 pm   #10
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Default Re: Marconi TF2701 In Situ RCL Bridge.

I think I remember removing two PP9 battery holders …….leaving just two wires. I have a feeling batteries were in series. I was worried about leaving the thing switched on and flattening the ( then) expensive batteries so I built the mains supply for it. Wall wart type supplies weren’t as common then.
Pretty awesome specs for those days! Not bad now actually and perfectly good enough for anything I’ll ever need to measure.
I’m on hols right now, but I’ll try to dig mine out and have a look at it. I have the probe thing that goes on the front terminals as well, so might be able to open that up if you want to reverse engineer one.
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Old 13th Aug 2022, 9:40 pm   #11
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Default Re: Marconi TF2701 In Situ RCL Bridge.

The manual I sent clearly shows two PP9 batteries in parallel.

And yes, the TF2701 is completely different to the TF2700.
In my opinion the TF2701 is brilliant!! I use mine all the time.
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Old 14th Aug 2022, 2:45 pm   #12
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Default Re: Marconi TF2701 In Situ RCL Bridge.

Greetings everyone.

I would like to thank everyone for your kind help with my little project.

Especially GMB for sending me the manual for my TF-2701.

Hello Ed. The 2700 and the 2701 are worlds apart. The 2700 uses resistors and capacitors in the ratio arms to achieve a balance. A long proven way of designing an RCL bridge. The conventional bridge does not cope with extended leads such as those needed for an in situ bridge.
The 2701 has transformers located in the ratio arms and to achieve a balance the taps are changed along with coarse wire wound pots. This is why the internal oscillators must be used to drive this beastie and not introduce any external ac source.

Hello Tim. The two 9V batteries are in parallel and not in series. Mentioned earlier that the highest voltage read on the many electrolytics was 12V so 18V would end in tears.
A point to remember when you go to get your 2701 going again.
The leads are nothing special. Just a fancy little box with 4 banana plugs that engage the 4 terminals on the front ( 2 active and 2 earth). The leads will be just some RG-58C/U flexible coax. Easy for me to fabricate.
Thanks Tim for the offer of investigating the box on the front but not needed. The offer was very much appreciated.

As a side note. I have had for many years a device from our old government electricity distributor. (Transgrid. The really big stuff 160kV, 330kV 500kV and others). They sold a lot of stuff at auction.
My box is used to measure the capacitance from large transformer bushes (the big insulators on the top) to the frame or case. A record is kept of the capacitance values at each regular service and is graphed. A change upward is an indication of a developing problem and is addressed before any catastrophic failure can occur.
My bridge for this has two test leads each 15 metres long made of RG-58C/U coax with an alligator clip on the end.
Even with these long leads the bridge will read down to 1pF.
This bridge is a balanced transformer type of machine. There is two transformers inside with dozens of taps.

Anyway.
Thank you to everyone who read this post and even more so to those who have taken the time to respond.
I can assure you your efforts are very much appreciated.

Cheers all.
Robert, VK2BNM.
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Old 14th Aug 2022, 2:59 pm   #13
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Default Re: Marconi TF2701 In Situ RCL Bridge.

Thanks Robert. I don’t think I have used the 2701 for about 20 years, so my memory might not be correct. I’ll check though just incase I wasn’t as clever as I thought I was 30 years ago!

GMB, may I throw myself upon your mercy and beg a copy of the manual for the 2701 too please? It would be nice to know how to work it properly, rather than trying to work it out from the controls.

I also seem to remember one of the functions was graphically represented by a resistor and capacitor in series. Does that mean I can use it to measure ESR of electrolytics?
Of course, when I “rescued” mine circa 1987, I didn’t even know what ESR was!

Batteries were/are known as PP9 over here. They are still available but becoming harder to find.
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Old 14th Aug 2022, 8:10 pm   #14
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Default Re: Marconi TF2701 In Situ RCL Bridge.

Hi Robert, thanks for that, yes a transformer ratio arm bridge is a class above the normal RCL type and is not always suitable to use with DC voltage/ current Injection.

The insulator/ bushing test set is possibly a high voltage Schering bridge. These were popular for this sort of work. I believe my son's group at Citicor/ Powercor in Vic have some of these available.

For those that are interested there is an old, but excellent book "Alternating Current Bridge Methods" bu B Hague 1923, updated 1942
I think it is on the web as a scan somewhere.

Ed
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Old 15th Aug 2022, 11:21 am   #15
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Default Re: Marconi TF2701 In Situ RCL Bridge.

Quote:
I also seem to remember one of the functions was graphically represented by a resistor and capacitor in series. Does that mean I can use it to measure ESR of electrolytics?
Of course, when I “rescued” mine circa 1987, I didn’t even know what ESR was!
I might experiment by measuring a cap on the resistance range.
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Old 17th Aug 2022, 1:13 pm   #16
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Default Re: Marconi TF2701 In Situ RCL Bridge.

Hello Tim.

Sorry to spoil your thoughts but all of the in situ settings on this bridge have a resistor in parallel with the capacitor.
None of them show a resistor in series with the capacitor.
So the idea of measuring the ESR of a cap is not going to work with this device.

What would be nice for myself and a good friend is to be able to somehow predict the failure of Tantalum caps in situ. My friend and I both have devices that contain hundreds of "the devil's spawn" Tantalum caps.

Cheers. Robert.
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Old 17th Aug 2022, 2:29 pm   #17
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Default Re: Marconi TF2701 In Situ RCL Bridge.

Quote:
So the idea of measuring the ESR of a cap is not going to work with this device.
The only reason that may not work is because ESR is very low so the equivalent parallel version of the circuit will have it very high. I haven't tried it (yet).

The fact that it shows a resistor in parallel with a capacitor is because it is for in-situ use and the direct parallel case is not resolved by the transformer so it must measure them both. It does this using the phase adjustment pots and then looking the results up in graphs (hence you need the manual). You can do the maths or find a web site that will convert parallel to series for you - but remember it depends on frequency. It also measures resistance with a capacitor in parallel so it depends on the various values and frequency you choose as to how accurate the result will be.
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Old 12th Dec 2022, 1:32 pm   #18
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Default Re: Marconi TF2701 In Situ RCL Bridge.

Hello everyone.

Just an update.
First of all I would like to thank everyone that contributed to this topic.
Especially GMB for his valuable help.
Fantastic!!

I have now been using this TF-2701 bridge for some months on a regular basis.
Enjoying it immensely.
Accuracy is not fantastic especially the high resistance range however it gives me a good idea of ballpark values where without it I was only guessing.
If I need accurate values I have other test gear I can use but it is quite fiddly to set up.
The TF-2701 is so much easier.

Cheers all.
Robert, VK2BNM
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Old 16th Dec 2022, 11:42 am   #19
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Default Re: Marconi TF2701 In Situ RCL Bridge.

To add to the comments above. Marconi made some excellent "for the time" test equipment. I still use my FT2700, especially for small inductors. The "cheapie" chinese box is not trustworthy. Looking at that article posted above "world history etc".. I didnt know about the other equipment available , but looked the part. Its a pity Marconi equipment didnt keep up with the 20 / 21st century .
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Old 17th Dec 2022, 9:14 pm   #20
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Default Re: Marconi TF2701 In Situ RCL Bridge.

Quote:
Sorry to spoil your thoughts but all of the in situ settings on this bridge have a resistor in parallel with the capacitor.
None of them show a resistor in series with the capacitor.
So the idea of measuring the ESR of a cap is not going to work with this device.
Thanks Robert.
I thought about measuring caps on the resistance range, but of course the low frequencies used by the bridge, probably won’t be very effective.
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