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Old 25th Apr 2008, 1:40 pm   #1
Kitski2008
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Default Marconi TF2002

G'day all,

With help from member "wavesolder" on finding service manuals for the TF2002, I set about fixing up my sort-of working beast.

After repairing a faulty oscillator coil on Range F (ask me what I found inside the ferrite sleeve shorting out the coil !) and mechanically recalibrating, all the RF stages were working.

Functional checking then began, but after finding 6 or more coils o/c in the DC filter boxes 7 & 8, I made the decision to replace all 22 coils regardless to claw back some kind of reliability. Finding suitable coils with acceptable DC resistance is another story.

I then noticed some intermittent RF o/p level variations when the instrument was knocked. Again, another long story, but the fault was one faulty o/p transistor in the wide band amplifier. The only substitute transistor that did the job was a metal-cased 2N2222A. (Plastic cased versions caused LF oscillations on Range F.)

Many other dead transistors were replaced. (2G401's anyone !)

Cal adjustments on the ALC etc then proceeded as per handbook. But it's here that something very odd became apparent.

My TF2002 is definitely a base model 2002, not a /B or /AS - however both the -15v supply & -13v oscillator supply contain IC's and other minor cct variations.
Additionally, my front panel has no silk screening noting that the manufactuer was (Mr) Marconi or that it was made in England ! And further, there's no front panel headphone jackor audio i/o binding sockets!

The only conclusion I can arrive at is that this instrument (Marconi logo: TF2002 & s/n 54798/47) was made by the (now defunct) AWA factory in Sydney, Australia. AWA were Marconi agents in the 60's - I suspect the 2002 was locally made in Sydney but with a "few designer approved" modifications. Nothing else makes any sense to me. Any other ideas ?? Has this instrument any other siblings around the world ?

All up, a good and stable instrument, but as heavy as a tank. I think I'll keep this one in the lab. Now to start restoring a HP-608D after finding a manual !!

Cheers,

Kitski
Sydney
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Old 25th Apr 2008, 9:26 pm   #2
Phil G4SPZ
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Default Re: Marconi TF2002

Kitski, what did you find inside the ferrite sleeve?!

I'm lucky enough to have been given a TF2002 in full working order, along with a full service manual. I've not yet learned how to use the wobbulator function though!

Phil
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Old 26th Apr 2008, 12:33 am   #3
Kitski2008
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Smile Re: Marconi TF2002

Hi Phil,

Thought no-one would ever ask ...

The winding wire was natural (pink-ish) copper wire covered with a clear insulating "coating" (unlike the normal dark-red enamel colouring). The coil itself was ~9 turns spaced over 50mm. The end turn has some sleeving covering it as this wire doubled back on itself to come out the same end cheek as the start turn.

The far-end sleeving has reacted with something (the clear wire insulation, perhaps corrosive atmosphere) and melted into a glob of sticky green goop that lay across 2 adjacent turns. This partially shorted turn resulting in lowering the coil's Q to the point where oscillation stopped.

The fix was to cut the wire "lead" off at the end turn, brush off the green goop with alchohol & toothbrush and a short time in the ultrasonic bath to finish it off. I resoldered a new length of leadout wire with kynar wire of the same wire diameter and put it all back together.

Interestingly, a similar green goop (although not as sticky) was found on almost half of the filter inductor pot core wires. The wire insulating sleeving used had become so brittle that a mere touch caused it to disintigrate. This was the main reason to change all inductors.

The metalwork, soldering etc is clean and showed no other signs of atmos corrosion, so I'm drawn to the conclusion that there was a "material incompatibility" used within the coil assy that caused the green goop to grow.

I'm wondering when the next range will stop working and I find an identical problem.


Regarding 'wobbulator' functionality, are you refering to what the manual describes (on page 16) as "external frequency shift" ? These external frequ shift binding posts are missing on my model ...


Kitski
VK2LL
ps - how do you insert or get rid of these icons ?!!?
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Old 26th Apr 2008, 11:49 am   #4
Skywave
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Question Re: Marconi TF2002

From Post #1:

"Many other dead transistors were replaced. (2G401's anyone !)"

ISTR that these can be replaced with AF11x germanium.
Can anyone confirm this?

Al / Skywave.
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Old 26th Apr 2008, 1:17 pm   #5
XTC
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Default Re: Marconi TF2002

Quote:
Originally Posted by Skywave View Post
ISTR that these can be replaced with AF11x germanium.
Can anyone confirm this?

Al / Skywave.
Yes.

This
http://www.datasheetarchive.com/cros...nce/2G401.html

suggests AF117, AF124, 2N990 and GT320A.

I'm sure there are others, depending on where the transistor is in the circuit. Before fitting a replacement, it's a good idea to look at the circuit and the datasheets of the transistors.

Given the problems reported with AF11x transistors, AF124 might be a better choice, if possible.

Pete.
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Old 26th Apr 2008, 1:26 pm   #6
Skywave
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Question Re: Marconi TF2002

I own a TF2000B which is slowly coming to the top of the overhaul list.

So - since this topic of replacement transistors has arisen for this item - is it feasible to contemplate replacing defective AF11x germaniums with silicon transistors, taking into account the requirement to change the biassing arrangements?

Al / Skywave.
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Old 26th Apr 2008, 7:02 pm   #7
XTC
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Default Re: Marconi TF2002

Judging by some of the threads on transistor radios, certainly feasible and sometimes, oddly, a plug-in replacement.

OTOH, most common silicon transistors have Ft's of two or three times those of the AF11x series and you just might find you have horrible problems with parasitic oscillations, because the circuit layout isn't up to it. With a sig-gen, you're more likely to be sensitive to problems like that than in a transistor portable.

You may have to choose the silicon replacements carefully, and they may not be what are usually thought of as RF types.


Pete.
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Old 26th Apr 2008, 8:43 pm   #8
jim_beacon
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Default Re: Marconi TF2002

For anyone that is interested, the details of my battle with a 2002 are here:

http://www.g1jbg.co.uk/tst10rep.htm

I found suitable replacement filter inductors from Farnell.

Jim.
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Old 28th Apr 2008, 12:54 pm   #9
Kitski2008
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Default Re: Marconi TF2002

Couple of points to add:

Thanks to Jim (above) who spurred me on to attempt the repair on my 2002 !! (Pictures on this web site show his 2002 in much better cosmetic nick than mine !).

I just used a PNP Si transistor to replace the dead VT1 (PNP, Ge) in the audio oscillator. I figured the bias resistors could take care of themselves, particularly in this non-critical application. But earlier comments by XTC on using "better" Si transistors and parasitics should be kept in mind.

Regarding the inductors in the filter banks - there's a few "gotchas" lurking. The RS (or Farnell) 1mH series resistance (3.8 ohm) was OK. However, 22mH devices were 120 ohms and 100/120 mH rose to 420 ohms. In practice, this caused excessive voltage drop, particularly in rails carrying substantial DC. I made no attempt to substitute 48mH coils in the ALC-filter feeds using 1 or 4.7mH instead.

The general feeling was that the filter TC's were optimised to the 'nth' degree to maximise isolation performance. I felt this "boots 'n all" approach was not warranted because rewinding the original toroid cores with 38(?!) swg wire was a fruitless task and altering the filter TC would have negligeable affect on overall performance except for the ALC filters noted earlier. (I'm unlikely to want to AM modulate a 50 kHz carrier at 10 kHz in the near future ...)

Kitski
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