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Old 11th Apr 2008, 11:52 am   #1
Oldtimer74
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Default Marconi TF868 Universal Bridge

I have one of these and need a copy of the manual or at least a circuit. I notice that 2 years ago there was a similar request dealt with by Joe (Hexode) but it is now closed. Could he help me please?
Pat
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Old 11th Apr 2008, 10:04 pm   #2
WME_bill
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Default Re: Marconi TF868 Universal Bridge

I have extracts of the manual. The various versions are differ, mainly with the chopping for the DC amplifier. and the valve types used. TF868, TF868A,TF868/1, TF868B. I will see what I can copy. Can you let me know which particular model you have. Dates 1954 to 1959. WME_Bill.
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Old 11th Apr 2008, 10:08 pm   #3
Oldtimer74
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Default Re: Marconi TF868 Universal Bridge

Hi there. I have the early model, plain TF868 using octal valves. I have already downloaded the TF868B manual from BAMA but clearly very different. My current problem is that the 1000c/s oscillator isn`t working. Valves all checked OK.
Pat
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Old 11th Apr 2008, 11:30 pm   #4
AVO_VCM163
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Default Re: Marconi TF868 Universal Bridge

Herewith a circuit for the 868/1. If you require any other data. pease let me know.
Regards, Roy
Attached Files
File Type: pdf TF868.pdf (33.1 KB, 441 views)
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Old 11th Apr 2008, 11:51 pm   #5
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Default Re: Marconi TF868 Universal Bridge

Hi Pat

If you click on search and enter TF868 in the search box you will find the circuit and parts list is still there.

Joe
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Old 14th Apr 2008, 2:53 pm   #6
Oldtimer74
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Default Re: Marconi TF868 Universal Bridge

Gentlemen. Many thanks for your replies. I guess they had problems with the 868 as the 868/1 has a different oscillator circuit using a double triode as against the single valve in the 868. Will let you know how I get on!
One slightly puzzling point is that the meter in mine is centre zero yet the brief instructions on the top of the case seem to refer to a left hand end zero meter.
Pat
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Old 15th Apr 2008, 3:04 pm   #7
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Default Re: Marconi TF868 Universal Bridge

Interesting debate. The centre zero meter was used on all earlier versions, with the DC operated bridge on Resistance ranges, with no amplifier. The TF868B (1958/59) used the AC amplifier also for Resistance, using a chopper relay, and a lefthand zero meter. It therefore did not indicate which side of balance you were.
As there also seems some debate about the versions, I attach circuit diagrams for: TF868 - 1946, TF868/1 - 1955, TF868/A - 1957 the same as TF868/1 except that the voltage applied to the bridge could be measured, and TF868B of 1958/59, a very different design. A virtually identical amplifier + chopper design for DC bridge operation on Resistance was also used for the later 0.1% accuracy bridge TF1313A of about 1966. The amplifier certainly makies balancing much easier when measuring high resistance. I hope this will help you with getting the Bridge going. Also a sheet about the arrangement of the dial discs and washers. Most confusing to solve without guidance. When I was repairing a TF868, I got very confused until I found this. Apologies for the copies, I made them very hurriedly 20 years ago from original manuals, now lost and gone away, so I cannot get fresh copies. Bill.
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File Type: pdf TF868_cir_all ver.pdf (1.22 MB, 374 views)
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Old 27th Apr 2008, 7:29 pm   #8
Oldtimer74
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Default Re: Marconi TF868 Universal Bridge

Hi Gentlemen. Well I got the TF868 oscillator working - 1 faulty paper capacitor and it works OK on capacity measurement but not on inductance. High values give a good null but I suspect not at the correct value and lower values give only a vague or no null at all. Have used Marconi test inductances intended for their Q meter in the 25 mH down range. Any ideas?
Pat
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Old 28th Apr 2008, 1:22 pm   #9
Oldtimer74
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Default Re: Marconi TF868 Universal Bridge

Well gentlemen I have found the fault. The QMB switch Q/delta Q had a high resistance which was affecting the null. Cleaned and replaced now works well.
I would still like to obtain a copy of the manual for calibration purposes.
Pat
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