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Old 5th Oct 2017, 2:59 pm   #1
peter1an
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Default Marconi TF 995 B/5 weird waveform need schematic

Hi ,
Last weekend I got a beautiful Marconi TF995 B/5 from the Hack Green Hangar sales.

Changed a few poor esr caps and it works... but the waveform is of repeating three levels.
see att image.

I need help with this.

Does anyone have the schematic for the B/5. I have the A/2 but it's so different.

thanks
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Old 5th Oct 2017, 6:22 pm   #2
Andrewausfa
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Default Re: Marconi TF 995 B/5 weird waveform need schematic

Does it do this on every range setting? Might narrow it down if it doesn't.

Can't help with a manual I'm afraid.

Andrew
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Old 5th Oct 2017, 7:38 pm   #3
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Default Re: Marconi TF 995 B/5 weird waveform need schematic

Is there a modulation on/off switch left on somewhere? Or is the modulation at 50 or 100Hz which could be an electrolytic in the power supply has gone O/C?

Christopher Capener
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Old 5th Oct 2017, 8:54 pm   #4
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Default Re: Marconi TF 995 B/5 weird waveform need schematic

The TF995 series of signal generators use a low frequency oscillator followed by a frequency tripler stage then a number of frequency doublers to produce the final output. On the range you have shown only the tripler is used.

The waveform suggests that the alignment of the tripler stage is not quite correct. You are seeing a mix of the fundamental frequency and the wanted x3 frequency.

Paula
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Old 5th Oct 2017, 10:44 pm   #5
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Default Re: Marconi TF 995 B/5 weird waveform need schematic

I recall looking at the output of a TF995 (I can't remember the exact version) way back in 1978, when I worked at Pye Telecom and we still had these available in the development lab. I saw a similar messy waveform back then - and that was on a supposedly well maintained and calibrated instrument!

I checked it in the frequency domain too - and on some bands the output was a total mess. These generators were only ever used for on-channel work - never as an interference signal for intermod or selectivity measurements because of their horrible output.

Richard
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Old 6th Oct 2017, 6:49 am   #6
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Default Re: Marconi TF 995 B/5 weird waveform need schematic

Just looks like the output waveform of a tripler which doesn't have enough removal of the fundamental.

David
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Old 6th Oct 2017, 10:02 pm   #7
peter1an
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Default Re: Marconi TF 995 B/5 weird waveform need schematic

Dear all,
thanks for the replys.
all cap are good or have been changed, dc rails smooth.
Paula and David, you're wonderful.
I'll get tweeking and post the results.
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Old 11th Oct 2017, 1:47 pm   #8
peter1an
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Default Re: Marconi TF 995 B/5 weird waveform need schematic

Dear all,
I've managed to get a B/5 version schematic. Big thankyou to Paul Jones.
Resistors and 'things' (that I can get to) are close to marked values.
Isolated external influences.
I've tweeked T2 and T3, amplitude peak for mid scale, but still getting the falling level on triple output.
All other stages are good.
see image three.jpg
Can't see the wood for the trees.
How do I improve the waveform
regards
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Old 30th Nov 2017, 3:14 pm   #9
peter1an
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Default Re: Marconi TF 995 B/5 weird waveform need schematic

I have measured component values and resistors are 10-20% higher and some caps 10% low, the tripler rather dampened.
However , by increasing the HT3a by 18v and twiddling ,
I have improved the waveform enough to say that will do.
[the dampening is not noticed on higher order outputs]
Thanks for everyones efforts and now to the next problem.
(but I'll keep an eye on this waveform )
ttfn

ps how do i close this thread?
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Old 30th Nov 2017, 4:23 pm   #10
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Default Re: Marconi TF 995 B/5 weird waveform need schematic

The thread will close automatically after a few weeks, unless you specifically want it closed now, in which case you ask the moderator to close it.
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Old 30th Nov 2017, 10:46 pm   #11
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Default Re: Marconi TF 995 B/5 weird waveform need schematic

We don't normally close threads manually except for sales and wants threads where a deal has been done and the item is no longer needed or available.

It's possible another member may wish to contribute to this thread.
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Old 9th Dec 2017, 4:38 pm   #12
peter1an
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Default Re: Marconi TF 995 B/5 weird waveform need schematic

It's good it's still open as I've had another fiddle.
The grub screws on the ganged air caps enable me to align each set of vanes.
This with increasing the HT3 has improved the tripler waveform.
phew, what a calamity.
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Old 10th Dec 2017, 12:28 am   #13
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Default Re: Marconi TF 995 B/5 weird waveform need schematic

On more modern RF kit that has frequency multipliers the output goes through a limiter amplifier before being amplified for the output circuits.
Your instrument is a bit of an early model to have such a feature.
It is a well built instrument all the same.
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