MTS2000 - RX instability at very low signal levels

[g8tzl2004]

I have recently acquired a mint UHF front LCD full keypad MTS2000 - type PJ302H - model H01RDH9PW1BN - manufactured in late 2005 - so its a relatively "new" version.

At "typical" squelch levels, the MTS2000 works perfect. However, if you open the squelch (to get white noise) and inject a very low signal, there appears to be some RX instability with the reception of a unstable fluttery signal below a certain threshold. As you gradually increase the signal, the RX then suddenly becomes stable rather than a gradual increase in quieting.

The effect is more noticeable using 12.5Khz spacing. Using 25KHz spacing provides a much more stable effect when lowering the injected signal strength ie. the noise noise level gradually increases in a normal way and visa versa.

When using the Tuner/ Service Mode to lower the squelch levels, you DO NOT get any RX instability when injecting a weak signal - just a normal increase/decrease in noise level.

So is it something to do with CPS settings which are NOT used when in Tuner/Service Mode? Its like the MTS2000 is trying to "process" the weak signal which causes some instability?? Is it something to do with de-emphasis? Or something to do with HearClear (or whatever its called..but do UHF MTS2000's have HearClear??..the option is not enabled in CPS anyway). I think I also read that 12.5Khz spacing had not been properly implemented in early firmware. The version I am using is 5.44.

I'm aware that MTS2000's have an number of issues with regard to Mixer/IF/AGC problems. I have a few other Jedi's with similar but "worse" issues ...but they are 10 years older than my 2005 model. Surely any design issues have been resolved on later models.

My experience is that "30%" of all MTS2000's have issues especially if you start lowering the squelch threshold to RX very weak signals!!!

Any thoughts?

Thanks

[g8tzl2004]

I've done some more experimenting.

Using the Test frequencies in Tuner / Service Mode (for adjusting the squelch level), a weak signal received on the MTS2000 is 100% clean and stable - as you increase the signal level , the white noise reduces and visa versa. No RX "chuffing" or instability. This is on all bandwidths. If you adjust the test signal 5KHz away from the test freq, the signal is no longer RX'ed - just white noise.

However, when you are using the MTS2000 in "Normal" Mode , you continue to get the RX instability on very weak signals - its most noticeable on 12.5 KHz spacing.

I think it might be something to do with AFC .. as when you shift the test signal by 5KHz the signal continues to be RX'ed ..which is NOT the case when using the Tuner software and the fixed test frequencies.

The MTS2000 is definately TXing spot on freq. Also the RX squelch does not open on a weak test signal which is 5KHz off freq, so I think the RX is more or less on freq ...BUT with 12.5KHz spacing , a signal which is 5 KHZ off frequency does NOT seem to have the same degree of instability as a spot on test signal when you are increasing and reducing the signal level???????...so maybe its RXing, say, 2.5KHz off freq...BUT using the Tuner RX test freqs its all perfect???

Any ideas?

Thanks

[Andreas]

This is well known Jedi problem.I had this often and always changed the RF board.
I think it is the mixer IC (bad solder joints).Its a BGA type and you can´t reflow it.

[g8tzl2004]

Hi Andreas.

Yes, I have the "common" problem on several of my other Jedi's - but this US firmware MTS2000 seems different.

RX instability is most noticeable on 12.5KHz spacing.

If the squelch is set to "typical" (relatively high) levels, you might not know there was a problem. Instability is only noticeable when you open the squelch (white noise) or set the squelch level very low and RX a very weak signal.

Using the Tuner software (in squelch setting mode) on the 7 test frequencies, there is NO problem - very weak signals are RX'ed 100% perfect on all bandwidths.

Why is the MTS2000 RX 100% perfect using the Tuner alignment software on the test frequencies??

Is the Tuner software not using all the MTS2000 receivers features - so the faulty side of the RX is not switched on in Tuner mode?

Could it be bad firmware which cannot implement 12.5Khz spacing correctly (version 5.44)??

Thanks

[Andreas]

This is what I often had:
Deaf receiver (comes with a grumbling sound).Sometimes the sensitivity changed (together with the sound).When I opened the squelch (with no signal),the noise change with a grumble from white to pink and back.90% are portables and 10% mobiles.
You can try,to press on the components and look for bad solder joints.Mostly the BGA components have them and you have no chance to fix it.
The firmware is not the problem.you can change the controllerboard and try it again,if you have some doubt about it.

[g8tzl2004]

I have UHF MTS2000's with the following:

- sensitivity is OK - always WITHIN manufacturers specs BUT big variation in sensitivity between each Jedi portable - so its luck of the draw if you get a good one.
- powering up and opening the squelch gives "normal" white noise BUT if you then BRIEFLY inject a signal and then stop, the tone of the white noise changes and stays that way???
- at very weak signal levels, you get a sort of instability on the signal. As the signal increases, the instability stops. If you open the squelch and gradually vary the level of the injected signal, there is not a "clean" stable increase and decrease in "quieting" but lots of instability but only at a certain low threshold. If you set the squelch high , you might think there is NO PROBLEM!!!
- using the Tuner test frequencies , all signals RXed are 100% clean - this is what I cannot understand - why is the signal 100% clean using the Tuner test frequencies (when setting squelch thresholds)???

I have 2 VHF Jedi's which are PERFECT - great radios
I have 6 UHF Jedi's and 4 are FAULTY!!!!!!!