FT101ZD MkIII - S Meter adjustment question

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FT101ZD MkIII - S Meter adjustment question

Postby VK7ADQ » Mon Dec 26, 2016 8:26 pm

Hi all,
My first post here and a question that I've also recently asked on the QRZ forum.
I bought my 101ZD MkIII about a year ago and really love it. It's in very good condition and works really well.
Luckily there are a couple of amateurs locally who know a thing or two about this rig, but as it's Christmas and I've got the lid off it, I thought a question here, rather than annoying them at this time might be the better thing to do.

Ok, on checking another rig the other day that had an obviously incorrect reading S meter on receive (which I fixed), I also ran a check on the 101 and found it to be a bit out as well.

I've got a Sark 110 antenna analyzer and luckily it's got a sig gen that can put out the correct levels to calibrate this rig's meter.

I've looked at the manual and it points to 3 particular pots on the 'IF Unit' board, ie, VR402, VR403 and VR405.

Now on first tweaking these as per the manual, I just got into a mess, until I realised the pots VR403 and VR405 need to be set up in reverse to the instructions. - well it worked for me and finally brought order to chaos as I totally lost control over the S Meter initially.

So it's now back in the ballpark and going by the manual I can align S9 on the meter to a 50uV signal and the +60db position to a 50mV signal, but everything else in between doesn't quite track correctly when the correct sig gen levels are applied, ie, 158uV= (approx) +10dB, 501uV= (approx) +20dB, etc, etc.

So I'm not meaning to be anal about this, but just wondering if it's possible to get better linearity with the S meter as it heads up into the red, or am I stuck with this and it's an inherent problem with my or indeed any analogue S meter, or is there a particular way to line this sucker up to get it to calibrate a little better at each main dB marker, all the way up to +60dB?

Anyway, I did have a reply from W1GCI, he said...

At 50mv, which is -12 dbm, you're pretty close to, if not at, the output intercept point of the receiver and probably seeing some compression. I don't know the design of the s-meter for the 101zd, but I expect that it was designed for relative measurement rather than as a lab instrument. If you can get steps close to the 6 db per s-unit and something pretty close to accurate up to around S9+ 30, you'll probably meet the design expectations. Just my thoughts.

...and I accept his reply in principal and certainly don't expect my meter to be of the quality of a 'lab instrument', but still idly wonder if I can do better.

I can tweak the alignment and get it to passably correspond to S9 up to +30, but after that it is wildly inaccurate.
So at the moment i've either got a nicely calibrated S meter at S9 and +60db with everything in between totally wrong, or I settle for the second option and at least have it sort of ok from S9 up to +30 and forget about the rest!

And BTW, could someone tell me how I should set the level correctly of VR402 as it has some impact on the level and position of the meter and of course the level of the received signal.

Many thanks and hope you're all having a great Christmas, it's shorts and thongs here in Tasmania (;->
"Little do they know the littles that I know are little, little do they know!"- Spike
Posts: 1
Joined: Sun Dec 25, 2016 6:59 am
Location: Tasmania, Australia

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