VHF Quantar in Ham Service (RX sensitivity)

[Tim]

Hi Folks,

Found the TX audio problem - one of the 6khz filters in the audio path was zotted.
Jumpered to check it out & works good. Gonna order a new part.

Anyhow, at 146 the box is pretty deaf (>2.5uV), although up in the 'real' band, the
sensitivity is quite nice.

I see the 5 adjustments on the front of the receiver, but before I start tweaking,
is there a 'sanctioned' process for this?

I remember on my old x9000, I just turned each of them 1/2 turn in.

Thanks!

Tim

[N4DES]

No real process that I have ever used. Just start turning the screws for the best SINAD.

[n7maq]

I sweep tune the front end.


Jim

[Batwings21]

Those adjustments are the preselector. That should cure it, just don't crank one of them too far, peak and re-peak for best sinad.

[sethcwilliams]

Don't know about a sanctioned process, but I do have a simplistic one. The preselector filters are "coned" if you want to think of it that way. Starting from the top, the bandpass becomes more narrow working to the bottom. Easiest thing I've found is to set it up for analog operation on the desired frequency:

- Turn all filter screws to the left (not all the way, just turn them to a point where chances are they're out of band.)

- Inject a strong signal (-60 dbm ish) and start slowly adjusting the power down until your 1 khz tone is no longer or barely audible

- Starting with the top filter screw, turn to the right until the signal reaches it's strongest point. You can adjust the power down slowly and keep working that filter until you acheive what you see as the "closest to center" you can get.

- Repeat for each filter. When your finished with the bottom filter, you should be able to distinguish a "scratchy but audible" 1 khz tone at around -125 dbm or less.

Hope this helps.

[Wowbagger]

I agree with the other posters: you need to adjust the front end RF filters of the radio to pass the amateur band, which they are not doing now.

I vehemently DISAGREE with any poster who tells you to do said adjustment without using a spectrum analyzer with a tracking generator!!!!!! You *WILL* screw it up.

Full stop.

You need to be able to see what the filter is doing in order to insure you have a proper alignment of the filters - that is, each filter element is properly set to pass the frequency of interest. Any of this ******** of "tune it for best SINAD" is going to end up with a piss-poor tuning - and anybody who says "DUURR but I didst it an' it worked so I R SMRT!" is either lying, stupid, or has never looked at the results of his "toooning" with proper equipment.

Buy/beg/borrow a proper spec-an or a service monitor with a spec-an and tracking gen. Locate the proper test points to see the filter. Do the job right - for cat's sake, you have a freaking Quantar, not some POS Hamtronics! Give the poor thing the service it is due!

[sethcwilliams]

HAHA I love this dude! (Read: wowbagger above)

While you're 100% correct, I don't think a lecture is what Tim is looking for. The way I read it, he's looking for quick and effective but I could be wrong. If I am and you happen to give him a "step by step", send it my way too. I'm always up for a good read.

Again, love reading your posts man, there's always good information to follow.

[xmo]

"... see the 5 adjustments on the front of the receiver, but before I start tweaking,
is there a 'sanctioned' process for this? ..."
---------------------------------------------------------------------------------------------------

Of course there's a sanctioned process.

It's in the manual.

Manual?

Yes. It's that big binder thingy [or CD] that came with the radio.

It's behind the "MAINTENANCE AND TROUBLESHOOTING" tab in a section named "PRESELECTOR FIELD TUNING PROCEDURE".

And - what do you know - there's a big warning on that page:

"IMPORTANT"

"Tuning for best SINAD response DOES NOT result in optimum tuning of the preselector assembly"

So...

I recommend you either follow the procedure in the manual - or - sweep tune it as recommended by Wowbagger.