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Old 12th Feb 2019, 9:33 pm   #1
Draker4c
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Default Replacement interconnection for Marconi Q meter

Greetings knowledgeable members this is my first post on the forum so please be gentle!
I’ve just obtained a Marconi TF 1245/1246 combination which I plan to restore but.. It’s missing the low Z lead (TM 5725), the fixed inductor set and the series test jig.
1. What type of lead could substitute for the low Z lead (is it a triax type?) 2. Is the meter usable for measurements ( and calibrable)without the inductor set ? 3. Is there a work around tho make series measurements without the test jig?
Many thanks
Hugh G1AUR
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Old 13th Feb 2019, 11:23 am   #2
WME_bill
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Default Re: Replacement interconnection for Marconi Q meter

TF1245 and TF1246 Q meter set.
Interconnection lead. Use a short 8" length of 50 ohm co-ax with BNC plugs. Not critical.
The meter is usable without the set of inductors. The set of fixed inductors are an optional extra. Helpful when measuring capacitance and provides inductance standards, to 3%.
The series loss test jig. You can make one up from copper covered PCB board, with 4mm plugs and terminals. It will work, even though not silver plated. Works with the standard inductors. Picture in the manual. Or else rig something up with bits from your spares box when needed.

At least this model does not use a fragile and irreplaceable thermocouple meter. Just two common 100ua 500ohm meters. The CS2 silicon diodes can be replaced by schottky. The planar diode EA52 seems reliable. I have a spare one somewhere.

To use the oscillator as a signal generator, you will have to make a step up transformer on a ferrite core to step up from 0.5ohm to 50 or 75 ohm impedance. I keep thinking I will make something up, but then use a conventional signal generator to save time when on some other task.

I assume you have the manuals for the bridge and the oscillator.
wme-Bill
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Old 13th Feb 2019, 11:46 am   #3
Draker4c
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Default Re: Replacement interconnection for Marconi Q meter

Hello Bill
Many thanks for your advice. Yes I have the manuals gleaned from VMARS of which I’m a recent member. The manual was’nt definitive on the use of the inductor set and the TF 1245 / TF 1246 were in reasonable condition and cheap! Was mention of it being a ‘special’ cable just Marconi being “commercial “?! Good news about the lack of thermocouples too!
73 Hugh
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Old 13th Feb 2019, 7:15 pm   #4
Julesomega
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Default Re: Replacement interconnection for Marconi Q meter

Do you need to step up the output impedance to match 50ohms? I read on Paul's thread >Success stories: Marconi TF1246< that the sig. gen. o-p is 0.8W, which would give 6V rms matched to 50ohms, possibly more than you need. I'd just put 51ohms in series to the new output
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Old 13th Feb 2019, 7:33 pm   #5
Draker4c
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Default Re: Replacement interconnection for Marconi Q meter

Hi Julian
No I hadn’t tried the Success Stories thread but will do so. My initial concern was Marconi’s
insistance that “only the special lead provided “ should be used combined. With lo z input to the TF1245. It immediately made me consider that a special low capacity cable was needed.
But you’re absolutely right there should be plenty of voltage for my purposes.
Hugh
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