MTS2000 VHF - high level of internal processor noise

[g8tzl2004]

I recently acquired a VHF MTS2000. Using an external antenna, the radio works OK. However, using the set top antenna, I'm getting lots of internally generated interference when receiving weak/medium signals.

I had this previously on an MT2000 but it was at a much lower level.

I'm certain its due to screening/grounding issues but I'm after views on the exact areas to clean up. I've checked the service mnual but there does not seem to be any guidance on earth tags. As I press around the LCD and speaker area, I can hear the noise level change(when monitoring a weak test signal). So maybe the noise is an LCD issue rather than an RF or control board issue.

I am monitoring the noise using a low level test signal as well as an AM radio - you can't hear it just listening to FM white noise when you open the MTS2000 squelch - its still there but just masked by the white noise.

Any ideas on the exact areas within the MTS2000 that need cleaning to resolve the internal noise issue?

Thanks

[Will]

See the Jedi Noise service bulletin here on BatBoard.

The shields do not make good contact with the chassis.

[g8tzl2004]

Thanks Will.

Most of the bulletin info seems to be about TX noise (crackly TX audio) due to RF leakage...but my problem is noise on weak RX signals probably coming from the internal processor. However, I guess the solution is probably the same - bad screening and dirty earth points.

[CircleBat]

typical grounding problem.. solder the 3 tabs on the shield that are pressing against the circuit board to the circuit board... that wil solve the problem

Circlebat

[g8tzl2004]

The RF and Control boards "sit" in the chassis and then the screening cans are pushed down around the chassis to cover the PCB's...I guess the 3 tabs are on the underside of the screened cans which then press against the PCBs when you clip everything into place? BUT how do you solder the 3 tabs ...are they not hidden? Do you apply solder on the PCB and tab and then heat via the outside of the screening ? Maybe I've got it wrong!! Maybe it will be obvious once I open the radio?

[Will]

I use a ink eraser to clean the mating shield and chassis connections. I do not like the "solder fix".

[CircleBat]

g8... you have the right idea..solder the pcb against the shield...also clean the solder ob the underneath of the lower board..let us know

circlebat

[g8tzl2004]

Do I need to do anything around the LCD and Keypad...is there any screening around the LCD, Keypad and Speaker? Or is it just the RF and Control board which are the cause of the internal RX interference?

[CircleBat]

g8.. no.. just the solder on the boards touching the back housing and the shields... This will solve your problem.. Also, clean the antenna contact..These radios were plagued with grouding problems and and that was why motorola hastily discontinued these radio!!! great radios..let me know

CircleBat

[AEC]

There should also be some conductive cloth/foam pads on one or more shields, and the P.A should have a shield around it as well.
The controller board and main board can have their associated board edges built up with solder to make better contact with the upper shield.

There are no shields for the LCD, but you can use copper foil and cover both sides with clear tape, leaving sections bare for direct contact with the board, as well as a section for connection to the chassis shield assy. You can use copper braid from RG-174 as well as RG-58/9 or anything handy, just do not solder the braid, it needs to flex.
Once finished, there should be no noise heard on RX or TX, and you should be able to hear properly once more.

I had an old B/K LPE highband portable that had severe synthesizer noise, and once I shielded it to death, the noise issues ceased.
Even those add-on tone boards can be wrapped inside RG-58 braid and grounded to eliminate stray RF getting into the wiring and acting as an internal antenna on UHF.

[g8tzl2004]

Thanks very much for the info