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Old 21st May 2019, 7:01 pm   #1
Deltatango
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Default Marconi TF2015A Signal Generator

Obtained a Marconi TF2015A and been cleaning and revamping it changed its ceramic control pots as they were grating (pricey) plus Carrier switch which jammed now and again plus its 10K Fine tune pot also costly.

This control was a problem, it seemed bad but on fitting the new control found it had been badly positioned mechanically so off centre causing trouble (distorting) the plastic spiral tune driver, plus because those pots do not have end stops had been set out of position in track travel.

Fitting the new control took an age trying to get it to rotate in a centered state with an easier rotation along with smooth cursor action swinging about end to end of its track.

As far as I can tell it seems to be OK 22v and 19v set without problems but my problem is calibration, its cursor and scale markings seem to be positioned right with a little overrun at each end of the widest scales.

The frequency though was out of step by a noticeable amount on every range, I checked its tune resistor ladder at 7.49K end to end that was fine, tested C7 (A19) for leakage plus a few resistors above and below the ladder for value change, all OK.

As its complete tune range track was higher than scale even with the ladder bottom at 1.73v vari-caps meant a lower voltage was required to increase tune capacitance so lower frequency, so to me that's either "lower to ground" or less top feed voltage.

It has ended with me having to lower R11 from 820R to 230R which "forced" cursor and frequencies to align with pretty good calibration across all ranges and ends.

For myself this seems a little "heavy handed" a small change OK but this seems a bit odd, can anyone suggest any other possible cause please?

Thank you for any help..........

David
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Old 22nd May 2019, 8:19 am   #2
MrBungle
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Default Re: Marconi TF2015A Signal Generator

I have an earlier non A revision TF2015 and the dial calibration is totally useless to be honest. I use it with a Racal counter. I suspect this is just a design facet as it's usually seen paired with a synchroniser. I don't have one of them so this hurts more.
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Old 22nd May 2019, 9:22 pm   #3
Deltatango
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Default Re: Marconi TF2015A Signal Generator

Thanks for the info, I will no doubt remove my mod as there is a "lot going on" at the bottom end of its tune voltage chain and tweak its top end down a bit but at the time was curious to see if the bands came right to its scale and it was an easy place to start.

Thanks again......
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