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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment.

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Old 7th Aug 2022, 3:08 pm   #1
its ur aerial
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Default FDK 750 Help required.

I bought a FDK 750 from a SK`s daughter a few years ago, but have only just got round to testing it properly.
It appears to receive ok but here is very little RF out put on any mode, on removing the top and bottom, it appears to have had a fair amount of work done, the PA transistor in situ is a 2SC2094, but according to a not too good circuit diagram on the net its listed as a 2SC1729.
What RF there is sounds ok, on FM at least.
13 V on the PA collector nothing cooking up, not been in there with the scope yet.
Any body had similar issues, is that 2SC2094 the right equivalent ?
Any info appreciated .
Ken, G6HZG.
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Old 10th Aug 2022, 6:02 pm   #2
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Default Re: FDK 750 Help required.

My FDK750 has a 2SC1729 fitted, I think I replaced it, there was no output when I acquired it, The biggest problem was intermittent feed through pins on the boards without proper PTH holes, I assume that when they hand soldered the component side, the solder side "desoldered", (or vice versa), The plating on the pins wouldn't tin easily, perhaps that is why yours looks "worked-on". But it would run ok for weeks sometimes. some of the skeleton presets were a bit flimsy too.
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Old 11th Aug 2022, 7:17 am   #3
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Default Re: FDK 750 Help required.

G6FLYneil thanks, for the reply, you past experience, sounds and looks like mine. I will now have a closer inspection of that PA output PCB, initial checks suggest the PA transistor could be leaky base emitter.
Replacement 2SC1729`s all seem to retail north of £25, and I cant find any data to suggest a 2SC2094 is a equivalent,
Its just that I don't want to spend 30 quid on a PA transistor, then find other multiple faults, and then have to condemn the Rig to the dustbin.
Thanks again,
Ken G6HZG.
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Old 11th Aug 2022, 8:50 am   #4
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Default Re: FDK 750 Help required.

Hi Ken, a simple diode probe can be useful in this situation. Don't know if you have one?
I will have a look through my box of rf devices today and get back to you. None are 2S numbers I do know. The flange fitting is also scarce in what I have, except for some very high power FET's.

Rob
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Old 11th Aug 2022, 9:27 am   #5
G6ONEDave
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Default Re: FDK 750 Help required.

I have a couple of these sets, an older version and a newer version as replacement for the first one after it was stolen. The set was recovered and since it had been replaced I was lucky enough to be able to buy back the recovered one. The old old one had suffered before I got it back, as someone had physically broken the PA output transistor, so that had to be replaced. I have had issues with the 2 protection diodes across the aerial input to the front end. The only other issue that I've had is the reports of synthesizer whine being heard on the outgoing transmission, when being received at the other operators equipment.

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Old 12th Aug 2022, 6:38 am   #6
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Default Re: FDK 750 Help required.

Quote:
Originally Posted by robinshack View Post
Hi Ken, a simple diode probe can be useful in this situation. Don't know if you have one?


Rob
Rob, not got a Diode Probe, but do have a half decent 220 Mhz Scope and other test gear, but no Bird 43 Meter either.
It does put out some RF, possibly just from Driver and the Modulation sounds fine, the integral S/Power, is not reading any power at all.
And I will look for and check those Diodes.
Thanks again for all the advice.

Ken G6HZG.
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Old 12th Aug 2022, 9:13 am   #7
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Default Re: FDK 750 Help required.

Ok Ken. The diode probe is simple and very useful, be it tx driver/pa or rx local oscillator.
I have found some 2sc2237, but only rated at 5 or 6w out at 175Mhz.
I think they were used in an airband am mobile transceiver maybe.
https://www.alldatasheet.com/datashe...I/2SC2237.html
Rob
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Old 19th Aug 2022, 8:47 pm   #8
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Default Re: FDK 750 Help required.

No reply, so I have put the transistors away again. Now buried. Wife is fed up with bits hanging around in the lounge! Understandably I think.
Rob
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Old 22nd Aug 2022, 8:56 pm   #9
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Default Re: FDK 750 Help required.

The ‘750 was a multimode rig I believe. If so, the PA stage has to be linear for SSB, ie biased for class AB. You could try similar “looking” devices but they will probably be from FM radios (class C) so therefore won’t be suitable for linear service.

I have read somewhere that not all power devices capable of working at VHF etc are suitable for linear biasing. Most commercial applications (then) were mainly FM and using devices designed for that use.


If you want a cheap way to use just FM and possibly CW with the 750, a PA from a scrap high band PMR radio could be grafted on. You’ll need to identify driver output on the FDK and also any tx/rx switching on the donor PA. I have done this with a vintage Standard multimode and FM PMR PA. Routing the output back through the original board restores the transmit metering. Bit messy though

Happy days.
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Old 5th Sep 2022, 4:34 am   #10
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Default Re: FDK 750 Help required.

Thanks Rich, that's a idea I didn't think of, I have a scrap 4M Phillips PMR.
Might have a feasibility study.

Ken G6HZG.
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