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Old 23rd Jul 2021, 12:01 pm   #1
Julesomega
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Default TF 2100 A.F.Oscillator

I was delighted to pick up a TF 2100 recently: it looked like a unit I already have and it was working perfectly at switch-on. I was thinking that now I had two units I would investigate modifying the output impedance of one to about 4Ω for driving low-impedance loads at high levels, in the manner of the Farnell LFP1 which has long been on my wants list, but on opening it up I found that my two units are quite different models, the previous one being the TF2102M1

The MI catalogue for 1975 shows the TF2102 AF Oscillator (3Hz-30kHz) specified as having no amplitude bounce, making it useful for vibration testing setups. This is because a diode detector with an arbitrarily fast response is used for amplitude stabilisation rather than the usual thermistor or tungsten bulb. The output amplitude is unusually high, using a Class-B output stage with series output transistors to give 10V rms into 600ohms, or 20V emf. By comparison the catalogue describes the TF2100 as having "extremely low distortion" which was by then discontinued and supplied only in a double-unit case with a "monitoring attenuator" as the TF2000, or as the TF2005R two-tone signal source, being a triple-unit with a second oscillator unit plus the monitoring attenuator, for intermodulation testing on high quality AF equipment. I am now even more delighted with this purchase and will certainly not be modifying it in any way: the 2102 would be the better candidate for modification

Regarding the thermistor for the Wien Bridge oscillator, it is shown in the parts list as an "A54" thermistor which measures 4mm 22mm. This is NTC, and according to an old catalogue is nominally 5kΩ at 20C, reducing to a minimum of 80Ω when hot. The only modification I am now considering on the 2100 is to replace the rear panel O-P connector which is one of those rare "miniature co-ax connectors" with something more easily found, like an SMB

The 2102 can now be modified for the low-impedance output: it would seem prudent to replace the original Ge TO18 devices with Si TO39s with suitable heatsinks. It seems a shame that this first generation of solid-state MI signal sources is so seldom encountered: these pre-microprocessor analogue instruments are eminently restorable and of superb electronic- and mechanical- design
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Old 23rd Jul 2021, 12:29 pm   #2
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Default Re: TF 2100 A.F.Oscillator

Picture 5 !! Wow

Very good,now thats what I call a servicable item wonderful and practical to work on simply impressed
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Old 23rd Jul 2021, 12:29 pm   #3
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Default Re: TF 2100 A.F.Oscillator

With two units you're good for two-tone tests like intermodulation distortion, or running up an SSB transmitter for PEP measurement.

It might be more versatile to leave the units original and just build a little power amp. In the extreme a box with a power amp and speaker as well as a combiner. THen you have an amplified test speaker, a low impedance driver and a combiner all of which can be exploited separately as needed.

David
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Old 23rd Jul 2021, 1:14 pm   #4
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Default Re: TF 2100 A.F.Oscillator

I'm not sure that co-ax output was intended to be used apart from when the oscillator was built into a frame such as in the Transmission Measuring Set. There it fed the adjacent MF Output Attenuator.

In normal standalone use wouldn't output be taken via the 4mm sockets at the front?
I think that coax connector is probably a mini Belling Lee - like a scaled down version of the 75 Ohm TV plug. Not all that hard to find and better than modifying an otherwise "mint" unit.
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Old 23rd Jul 2021, 1:16 pm   #5
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Default Re: TF 2100 A.F.Oscillator

I won't need to mod it if I can find a Farnell LFP1

The designers were certainly designing for servicability, maybe in response to pressure from the repairs department! Thankfully someone on the Marconi Instruments forum sent me a full scan of the manual
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Old 24th Jul 2021, 6:50 am   #6
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Default Re: TF 2100 A.F.Oscillator

I wonder when the fold out design first came out, the TF2331 distortion analyser used the same fold out construction, whoever thought of it is a genius. Makes assembly easier too.

Andy.
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Old 24th Jul 2021, 7:08 am   #7
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Default Re: TF 2100 A.F.Oscillator

It appeared in 1963, and was designed by W.D. Cain, the chief mechanical engineer at the time. He was quite a free-thinker.

https://worldradiohistory.com/Archiv...ni-1963-06.pdf

It's also worth browsing the other issues at this time, as they contain details of of other instruments that crop up on the forum.
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Old 24th Jul 2021, 7:44 am   #8
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Default Re: TF 2100 A.F.Oscillator

Thanks for that link Richard, and hooray for Mr.Cain! The issue was devoted almost entirely to the TF2000/2100 and the completely unknown "MF" variants which extend the range to 550kHz (they were not in the later catalogues I have). It's hard to imagine what market these 600Ω instruments were intended for, and carrier telephony systems must have been into the MHz region by then
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Old 24th Jul 2021, 9:03 am   #9
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Default Re: TF 2100 A.F.Oscillator

The TMS I referred to earlier went to 550kHz and the oscillator was marked MF not AF.

Low capacity multiplexed links were probably still operating below 550kHz. I am thinking of units such 1+1s etc.
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