HT1250 - poor screening causing RX desense - any fixes??

[g8tzl2004]

I think I have a poor earthing/screening issue with my VHF HT1250.

If I use an external antenna the radio works 100%

Using the set top antenna I get noise pick up which is desensing weak signals. BUT if I press down just below the left "arrow" key on the front of the HT1250 (limited keypad version) , the noise disappears. This is why I think its an earthing problem??

Any views? Anybody seen the problem before? I've read the archives and seen mention of noise issues due to antenna matching circuit faults etc.. BUT I can solve the noise issue with my radio just be pressing down on the front of the case so I don't think anything to do with the antenna matching circuit.

I guess I should open up the HT1250 and check the screening and earthing points?

[mike m]

http://batboard.batlabs.com/viewtopic.php?f=1&t=16892

see the above older post, this sure sounds like your problem.

Mike

[CircleBat]

typical grounding issues on the board.. very common problem

[g8tzl2004]

CircleBat - are the typical grounding issues easy to fix? Is it just a matter of cleaning up grounding contacts? Any particular common areas that I should focus on?

mike m - I had previously read the archive including fixes involving firmware updates etc BUT I can temporarily solve my noise problem by just pressing down on the front of the HT1250 below the left arrow key....although the noise returns when I stop pressing!!

[CircleBat]

just solder the 2 tabs on the controller board where the shield touches the board ,and the single tab on the rf board on the right side on the opposite side of the PTT flex.. that should solve the problem..

Circlebat

[g8tzl2004]

Thanks.

I've read the service manual about HT1250 disassembly...but I wonder if there are any other tips or tricks? Is it obvious how you release the latch mechanism on the flex cables?

[mike m]

The UHF XTS3000 had a similar FSB for RF board tab and grounding with and without an antenna installed.

I remember when we were designing early Wulfsberg P25 avionics radios which used a modified special XTS3000 module in a guard receiver slots we had this issue rear its ugly head a few times which created numerous noise problems due to vibration in AZ state police helicopters.

I and other engineers had to sit at a bench for hours and solder around the RF boards tabs, where they contacted the chassis, on about 20 to 30 modules, after that I didn't want to ever see the insides of an XTS3000 again and I was known for dissecting radios in less than a second, not any more.

[g8tzl2004]

UPDATE: I've now fixed most of the desense. No soldering was involved.

There are 2 tabs which have been pressed out of the keypad screening. These are raised up and when both parts of the radio are clipped together, both tabs rest on one of the screening cans on the main PCB.

Over time, it looks like the tips of both tabs have very slightly "eaten" into the screening can where they touch the can. All I needed to do was clean up the area on the can where the tabs had rested, clean up the tabs and gently bend the tabs up a bit - over time, the "angle" of the tab probably gets compressed. I guess you could also add a bit of solder to the tips of the tabs ( CircleBat- I now understand what you were meaning!!) or add some solder to the area of the screening can where the tabs make contact.

My HT1250 now receives very weak signals using the set top antenna. However, I still get a bit of processor noise. This seemed more pronouced on 2m vs other frequencies. Maybe there is no fix to remove all the low level noise..but at least I've got rid of most of the noise causing desense. Maybe cleaning up the other earth tab might help..but that involves removing the PCB...and for some reason my T6 TORX screwdriver did not fit!!!...maybe I have the wrong type!!!

[g8tzl2004]

UPDATE 2 : I noticed that when I am receiving a weak signal on my VHF HT1250 , if I place a finger across the LCD display , the intensity of the processor noise increases ie. its radiating up my finger and into the set top antenna.

I decided to check the level of processor noise coming from both a VHF and UHF HT1250. I simply used an AM receiver with the antenna resting above the HT1250 LCD display. I found that BOTH the VHF and UHF set generated interference across 144-148MHz and beyond while 430-450MHz was very quiet. Infact , the UHF HT1250 generated slightly more interference on VHF than the VHF set.

So it looks like the HT1250 processor/LCD display in both a VHF and UHF set generated QRM on 2m but 70cms is very clean!!

At least I can now RX weak VHF signals albeit with some slight processor noise on the signal.