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Old Nov 14, 2005, 03:00 AM
PLD
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Tuning GWS RX's

Is there a 'simple' *cough cough hahahah I know* way to peak/tune a GWS R6NII receiver?

I ask because I often find myself with some receivers working great and others barely get out of their box before they start having some spasms. What I have noticed with the GWS RX's is that neither of their tunable pots are potted/sealed, hence it's possible for them to drift due to vibration.

So, back to the question, what's the easiest way to peak/tune the RX? (have on hand an oscilliscope, multimeters (at least 3 handy)... no spectrum meter sorry ... can get a fq counter soon enough.

Paul.
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Old Nov 15, 2005, 07:36 AM
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I bought a few of them after being very happy with the first one I bought. One among the batch turned out to have poor range compared to others. Since sending it back would cost me more than a new rx, I decided to open it up. It was just lower range compared to others, no other issues. So I suspected coil tuning just like you. I marked the original slug positions and started tuning them. Found out they were already peaked, any change was only making the signal worse. So I returned slugs to their original and best position. There is a RSSI pin in this IC, so I started monitoring its level. After some poking around for voltages at different points to see if I can spot anything abnormal, I suddenly found the signal level has improved. A range test confirmed that it was as good as all the others now. I haven't changed anything, so this points to a loose contact somewhere. Could be dry solder joint, crack in PCB trace, crack inside filter etc. I haven't found out the exact cause yet. The problem is I cannot reproduce the issue now . I need to stress the board a bit to see if the sensitivity goes down again.

-- Rajesh
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Old Nov 15, 2005, 08:00 AM
PLD
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Rajesh,

Wonderful story ---- though a real shame you don't know the true cause

I might actually mark the pot and turn it back/forth a bit, perhaps it's the coils need shifting a bit or something *shrug*. I endeavor to find the cause of these mysterious "poor range" GWS RX's, especially since I sell quite a few of them. I use them a lot myself as well and have (as you have) got one or two which just don't seem to be 'right'.

Comparing two RX's side by side today, monitoring pin 9 on the FM chip, I was getting 1.60V vs 1.77V (bad RX vs Good RX), not sure if that's enough to be worried.

Checking the PPM out signal on a CRO indicated very little difference and both waveforms seemed "fine".

Guess it's back to the workbench for me too.

Paul.
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Old Nov 15, 2005, 01:56 PM
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Paul,

I brought that receiver out and guess what, it had low range again. Time to debug......

I could see the difference in RSSI (BTW RSSI pin is 12, not 9) means the problem is in the RF section upto the IF part, not after that. I made sure it is not the crystal socket. Poked the components in RF front end, crystal oscillator and IF section lightly with a tooth pick. The only place that showed a difference was the ceramic filter. If I push it outwards, signal seems to improve. Checked for pcb trace problems in the IF stage, checked voltages at both ends of trace and checked continuity - no problems. I can almost toggle between good and bad states by just pushing the ceramic filter. There is nothing wrong with the PCB, the transistor and IFT. So that leaves only the ceramic filter as suspect. I don't have an oscilloscope to check the signals, so I can't be 100% sure, but I am 95% sure it is the ceramic filter in my case.

You could compare the signals at the input and output of ceramic filter with a good receiver to see if it is the same problem you have. If the signals are similar at the input but not at the output, then we can be sure.

You could also try pushing the ceramic filter lightly while monitoring the RSSI voltage. I see about 0.6V difference between the two states. Good signal is over 2V.

Rajesh
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Old Nov 15, 2005, 08:44 PM
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Rajesh,

I'm assuming the /large/ black plastic block is the ceramic filter?

Could it be that the crystal filter is cracked?


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Old Nov 16, 2005, 12:47 AM
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I checked the output on pin 12, both receivers I have were well over 2.0V (more like 2.3~2.4V).

I'm going to do some range testing now to see how they compare (I think people at my home-offce here are going to look at me strangely

Paul.
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Old Nov 16, 2005, 01:01 AM
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Paul,

Yes, you have identified the part correctly. It is big black part on top right of your picture.

A crack is what I suspect, they are fragile devices like crystals. This was a brand new receiver just out of the box. Never flown. So this could be a manufacturing defect or the receivers were really thrown around during transportaion. A loose contact like this would explain why they passed GWS QC but showed up at other times. Loose contacts give "now its there, now its not" symptoms.

If you have a chance to look at the waveforms, do let me know. The input to the filter is from pin 3 of 31136. There filter has a row of pins connected to the ground. There are two pins on the other row, the one near the crystal socket is the input. Other one is the output. See if you can spot any differences at the output compared to a good receiver for similar input levels.

-- Rajesh
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Old Nov 16, 2005, 01:05 AM
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Paul,

If the RSSI levels in both receivers are the same, then you have a different problem. Or you could be seeing the contact acting up. Waiting for your results.

-- Rajesh
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Old Nov 16, 2005, 01:26 AM
PLD
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Rajesh,

I mounted both units onto a piece of cardboard, both with their own battery pack (no ESC) and both with one servo connected into one channel (same channel on both).

I must have walked over 50m away from the transmitter (located inside my home-office) before I started to get glitching. Both units glitched within 3m of each other. I'm starting to suspect the following

- Crystal was damaged in the 'faulty unit' (but I don't have the crystal on hand)
- Like your unit, perhaps the ceramic filter was intermittent and is now "good".

Both units are comparable for RSSI and PPM outputs.

Paul
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Old Nov 16, 2005, 01:32 AM
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Just wanted to note also that I'm having a hard time with the waveforms, dratted PC scope still seems to have a loading effect on the circuit (Despite the opamp being used).

Could be time for me to spend $400 on a dual-trace 20MHz cro

Paul
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Old Nov 16, 2005, 01:51 AM
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Paul,

I get way over 3m difference between good and bad ones. Guess your filter was working during the test.

My suspicion is getting stronger with your results. You are also getting intermittent good operation which points to a loose contact.

May be you can monitor like I did. I soldered a small piece of wire to the RSSI output and left it connected to the multimeter. Then slightly push the filter from different sides or tap it lightly. See if things change.

I have ruled out crystal problem. I interchanged the crystals but the bad rx was always bad. I spent a lot of time making sure it was not the crystal socket either.

I can't afford a good CRO either. May be I'll test at a friends office in the weekend.

-- Rajesh
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Old Nov 16, 2005, 04:48 AM
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Hi,

When the Rx is in "bad condition" try reflowing the solder joints on SMD capacitors. One of my Hitec Rx presented such a defect cap in the local oscillator. Fluctuated between 7pF(bad) and 22pF(good). I was on the wrong path searching around coils and filters.

My 2c.
Daniel
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Old Nov 16, 2005, 01:46 PM
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I asked my friend about the oscilloscope and he said I could use it at night today. The results proved conclusively that the filter is the bad part.

The first picture shows my probe points at the filter input and output. I soldered wires directly to filter pins to avoid any confusion.

There is some high frequency noise signal riding on the waveform, it can be ignored. It is probably the 72MHz signal induced in probe wires. The "Good" state waveform is first. RSSI level was about 2V for this. Then I pressed the ceramic filter slightly until it went into "bad" state. The RSSI level dropped to about 1.4V. You can see that the output signal is pretty much gone. The input signal amplitude has also gone up, I guess this is because the filter is no longer loading the IF out from 31136.

-- Rajesh
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Old Nov 24, 2005, 05:45 AM
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Paul,

Any update on your rx ? Or gave up ?

-- Rajesh
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Old Nov 24, 2005, 05:56 AM
PLD
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No, I haven't given up I'm just very busy with my Aerial Photography plane at the moment and I'm trying to see if I can get a 10MHz single-trace CRO for Christmas... lots of begging.

Okay, those CRO outputs definately do look conclusive, the problem now is though, how do you replace that filter, can you obtain them there? Is it even worth trying to obtain them?
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