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Old 2nd Jan 2022, 5:00 pm   #1
sentinel040
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Default KDK Konundrum

This synthesised KDK FM740 70cms rig dating from the 1980's 'landed' from a friend with the fault that it "drifted".

That turned out to be the VCO, which was not quite in lock and the lock detect circuit had not quite noticed, perhaps not the best of designs.

Anyway, a slight tweak of the VCO settled that, with it stable it then became apparent that the transmit was circa 14.5 kHz low on transmit, but spot-on an receive. For example, 433.5 = 433.4865. My pair of brain cells surmised that the TX reference osc may be off frequency, so it was time to go away and try to find a manual. Well, I found the user manual but not a workshop manual (OK, that would probably stretching luck a bit far) or even just a circuit diagram. Having so failed I returned to the tried and tested technique of last resort, 'poke and hope'.

There are a pair of crystals and associated oscillators, one at 22.088 MHz and one at 23.277. I assumed these might be reference oscillators. The old Racal frequency counter showed the former was active on receive and the latter on transmit, with both oscillators running at the crystals marked frequency. Out of interest, I adjusted the core of the tuning inductor for what I assume to be the TX reference crystal and with the radio set to 433.5, the transmit frequency varied from 433.4870 to 433.4755. This however, does not answer the first question, which is if the crystal is oscillating at the marked frequency how comes the synth output on TX is off frequency?

On a very remote chance I also had a careful tweak of what I assume to be the VCO and there is only the single lock point.

Without a circuit I am now officially up the creek without a paddle. So, suggestions please.....

Ian
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Old 2nd Jan 2022, 6:19 pm   #2
Jon_G4MDC
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Default Re: KDK Konundrum

We can do some guessing...

At the x18 multiplication point these are 21.4 MHz apart.
A very likely IF frequency for a 70cm rig.

RX PLL Mix frequency is probably 397.584MHz (397.6 MHz actually intended?)
TX PLL Mix frequency is probably 418.986MHz (419 MHz actually intended?)

In both cases the digital part would be counting 14.5MHz for 433.5MHz.
RX LO would be low side at 412.1 which with +21.4 =433.5MHz

I think your TX crystal has aged low by more than you can presently pull it.
Maybe check if the seals are intact? Could have gone to air if it was an evacuated one.

Another possibility is that the digital side works in finer steps than 25kHz. Now if you had a 12.5kHz program offset
which appears only on TX, you might be within 1kHz of where you wanted to be and the pulling range is enough. (Actual error is 13.5kHz)

I doubt the KDK supports 12.5KHz steps - but if it does...it could be down to a rogue programming bit.
Further reading says it probably does support programmable steps including 12.5kHz and further there is a QSY function which allows you to slide off to another frequency and return.
It might also be possible to insert TX offsets. Check all the tricky uP stuff before blaming that crystal I reckon! I thought this might be a simple 40 channel switched job but it isn't.

Last edited by Jon_G4MDC; 2nd Jan 2022 at 6:44 pm.
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Old 2nd Jan 2022, 7:47 pm   #3
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Default Re: KDK Konundrum

With some more cogitation..

I begin to think it unlikely that this uses digital division to do all the fine frequency steps that I think it has. Bad for loop filter.

The main divider might work in 25kHz or 12.5kHz steps.
Anything finer is more likely to be interpolation by shifting these crystals in a VXO.

The coils you tuned for experiment - are they the only ones nearby?

Others in the area switched by diodes would make me think caramba! Quite often they switched in more coils for progressively more frequency shift.

Bad diode - fixed offset.

Last edited by Jon_G4MDC; 2nd Jan 2022 at 8:12 pm.
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Old 2nd Jan 2022, 8:48 pm   #4
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Default Re: KDK Konundrum

KDK manuals were a *** to try to find even back in the day.

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Old 2nd Jan 2022, 8:52 pm   #5
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Default Re: KDK Konundrum

Jon;

Thank you.

By way of a first response. The bit that confuses me is that the crystals are oscillating at their marked frequency. Of course, that is assuming KDK did not pull a crystal to a different frequency that they wanted; although that seems odd for an OEM.

I can see no other obvious candidates for tweaking, but I will have another perusal on the morrow, if her Ladyship permits! There are the pair of rocks, with adjacent coils and associated components and nowt else obvious. I am a little cautious about madly tweaking without a circuit to guide me.

The radio has 25khz steps with a standard 1.6Mhz offset available for repeaters, if you want a different offset you can use 2 of the 10 memory channels for a single non-standard offset, but I digress.

Cheers

Ian
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Old 2nd Jan 2022, 9:18 pm   #6
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Default Re: KDK Konundrum

OK Ian. Best of luck. What I found covered several models and the 740 was not explicitly covered - more it was lumped in with other sets 2m, 1.5m etc.

When you know what it should do for frequency steps etc please post more.

Didn't we have a 4m Hudson discussion going on some while back? How is that?
Maybe that's OT here and needs reopening for 2022 if there is more news.
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Old 2nd Jan 2022, 11:54 pm   #7
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Default Re: KDK Konundrum

Depending on how much you are prepared to pay for a manual:-

https://www.vintagemanuals.com/manua...er=KDK&center=

3 up from the bottom.
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Old 3rd Jan 2022, 9:48 am   #8
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Default Re: KDK Konundrum

Thanks all so far.

RW - I know, I have had problems before and had that feeling of impending doom when this one landed. I must learn to say no....

Terry - Blimey, robbery with violence, thats probably more that the radio is worth - and it seems to exclude the 740 ("-740"). Thank you anyway, it's the thought that counts.

Jon - The radio only seems to do 25kHz steps, based on the user manual that I did find. Yes it was / is me with the growing Hudson collection and I was considering an update post.

Cheers all

Ian (G8KSZ).
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Old 21st Jan 2022, 10:33 pm   #9
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Default Re: KDK Konundrum

Rather than leave this up on the air, a final report. After a delay due to caring duties intervening I decided to have a further look at this, sans manual. Removing the logic board, they slipped up as there, only three wires going off a different side to the rest, I had a peer at the rear of the main RF pcb. As I had guessed, there is an inductor in series with the TX reference crystal and at the other end of the rock a 30pF series capacitor, mounted on the rear of the board. Nice.

The RX reference crystals series capacitor showed signs of having had 'attention' and as I was considering playing with the value for the TX rock, I went ahead. after a bit of experimentation replacing the 30pF with a 15pF brought matters into the range of the slug in the series inductor. Bingo, 433.5 = 433.5 out. I am not sure if this is realy the way to do it, but it works.

Just for a check, I went and checked other random frequencies and it seems anything ending in 5 (433.525 for example) ends up just over 2kc/s off, yet anything ending in 0 is dead on. I think I will let that one rest for the moment!

Ian
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Old 22nd Jan 2022, 9:58 am   #10
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Default Re: KDK Konundrum

I had a TR2400 that did that from brand new. A certain dealership thought I should pay to return it and have it fixed. Solder bridge on the back of the uP. I probably voided the warranty by clearing it but then the warranty was worthless in my view anyway!
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Old 22nd Jan 2022, 11:23 pm   #11
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Default Re: KDK Konundrum

I have had a KDK FM 2030 since the early 80's that still works but is very touchy on the antenna - its needs to have a good 50ohm antenna - I also have the manual with schematic if that's any good to you.
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Old 24th Jan 2022, 9:37 pm   #12
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Default Re: KDK Konundrum

Jon;

The concept curomer focused seemed to have gone missing that day, although in those days suppliers would often try it on more than today. I did seem to think during that period that you sometimes felt as if you should be in awe to even be allowed to buy something off some suppliers, with the result I took up modifying equipment.

Whilst I had the logic board hanging off I did look around for anything obvious, but it seemed OK, as did all the interconnecting wiring and connectors.

Majoconz;

Thank you for the kind offer. About the only KDK manual I have found is - wait for it, the 2030. Murphy's Law and all that! I do actually have modified 2030 I was given some years ago, for some reason that is now lost in the mists of time someone has modified the tuning arrangement with a PIC based 25kHz step up / down counter. It does seem to perform quite well.

Cheers both.
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Old 24th Jan 2022, 11:20 pm   #13
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Default Re: KDK Konundrum

I hope you can dig into it some more, the answer is in there somewhere.

I think you have made a perfect hit on the dealer philiosophy.
I was about to write about put your name down now, pay up front and get one when we please.

But then I thought better of it. Let's fix your KDK if we can.
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