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Transmitter repair.

Posted: Tue 05 Jan 2016 3:51 pm
by andrewinpopayan
Hi all.

Has anyone successfully repaired a transmitter? The transmitter on mine flashes the LED every 48 seconds (or so) but doesn't put out any RF, I am watching an SDR radio tuned to 433.9 Mhz and have the bandwidth opened right up, nothing being sent out when the light flashes. I could get a spare for £15 from Maplins but as a *very* last resort. The RF stage appears to be an ST4021 capacitively coupled to the "antenna".

Any replies most welcome

Andrew

Re: Transmitter repair.

Posted: Wed 06 Jan 2016 10:50 am
by AllyCat
Hi Andrew,
andrewinpopayan wrote:I could get a spare for £15 from Maplins
Sorry, but no you almost certainly can't. :(

About 3 years ago Fine Offset changed the transmitter hardware and transmission/wireless protocol, so the transmitters now sold by Maplin (and AFAIK everybody else) is completely incompatible with your Console. It was very much a "cheapening excercise" because I believe the old stations used FSK whilst the new system is simple OOK. The only advantage of the new hardware/protocol is that it's much easier to hack or intercept. ;)

AFAIK there is no solution apart from trying to find a an old "second hand" transmitter. But that's difficult because there are no external differences between the versions. See this very recent thread (and there are others on the forum) and the sticky thread linked there.

If you're an "expert" in electronics you might be able to fix the transmitter. But I've never even succeeded in seeing any "wireless data" waveforms on a 'scope (with those early stations), because I believe it's all done via an internal (SPI) data bus.

Cheers, Alan.

Re: Transmitter repair.

Posted: Wed 06 Jan 2016 1:31 pm
by andrewinpopayan
Hmmm, thanks for the heads up on non compatability issue Alan.

To this point now, I have stripped the PCB out, given the board a flush with "Servisol" and scrub with toothbrush. I left it on a radiator overnight and it's now in a sealed jar that's full of dried out silica-gel. I figure if water has crept in anywhere, then the silica-gel should pull it out, if it does work again, I will paint the whole PCB (except sensors) with encapsulation epoxy resin.

I am going to have a snoop around the board with a multimeter and oscilloscope to check that the 4.00 and 10.00 Xtal oscillators are running and that everything is getting power. I also have a USB microscope, so should see anything obvious.

Andrew

Re: Transmitter repair.

Posted: Wed 06 Jan 2016 3:02 pm
by AllyCat
Hi,

At least you have the advantage of working with a "real" ic package. The majority of the Fine Offset boards use "chip on board" technology (under a blob of black epoxy) which are less reliable and impossible to repair.

I did open up one FO "transmitter" which has the same chip, but it also has a RTC antenna, so I think is the Clas Ohlson 868 MHz version (which stayed with FSK longer than Maplin, but is now also OOK). I remember probing around the board (and the corresponding receiver) but never saw any "millisecond" type signals consistent with the RF data.

The duration of the "old" transmissons is only around 50 ms (much longer with the later protocol) and I think the LED has an independent control (i.e. it's not just across the supply/enable pin of the TX). I'll be interested to hear if you manage to get it working again. But working with the later models is much easier, with the protocol documented by for example Kevin Sangeelee (who has posted on this forum) on his website here.

Cheers, Alan.

Re: Transmitter repair.

Posted: Wed 06 Jan 2016 3:12 pm
by andrewinpopayan
Image

Hopefully a picture of the board is here.

Re: Transmitter repair.

Posted: Wed 06 Jan 2016 3:31 pm
by AllyCat
Hi,

Yes mine looks very similar (but with an antenna "rod" for the RCC). I've only just noticed the "433." on the PCB near the antenna, mine says "868" although the printed antenna is identical!

I hadn't previously noticed that Kevin's web page has a link to the Raspberry Pi forum, where they have apparently hacked an "old" Clas Ohlson WH1080, which confirms the FSK and SPI bus. The thread dies about a year ago, probably when FO also changed the CO protocol.

Cheers, Alan.

Re: Transmitter repair.

Posted: Wed 06 Jan 2016 4:04 pm
by andrewinpopayan
Hi Alan.

So yours is up on 868MHz, which nowadays I find odd because OFCOM seemed to have grabbed every frequency up from 500MHz for yet MORE mobile fone space. There it is on my board 433.92, I am spoilt for receivers from 100MHz upwards to 2GHz-ish.

The board is back in the desiccating jar, I have also pulled a vacuum in there, as well as stood it on a radiator, so I can be pretty sure it isn't water causing the problems. :ugeek:

Andrew

Re: Transmitter repair.

Posted: Wed 06 Jan 2016 6:25 pm
by andrewinpopayan
Hi Alan.

The Si4021 is not a friendly little chip to investigate, however looking at the "standard" usage application notes, I can see a little bunch of SMD inductors around the RF O/P stage, candidates for the multimeter, especially L3 which supplies power to the >finals< :geek:

I have 3.12 V Vdd, the 10.0MHz Xtal is too high an impedance to measure with my frequency counter.
Image

Image

Re: Transmitter repair.

Posted: Wed 12 Jun 2019 11:12 am
by leccyrob
Hi Guys,

I'm probably a couple years too late to post on this but came across your posts while investigating a very similar fault......
I've got a bit of experience at soldering SMD's (Surface Mounted Devices) so I had a look into my WS1080 which started failing to transmit intermittently and more and more frequently not transmitting.
Moving the transmitter within a few feet of the receiver didn't help the issue so I opened it up and found a SI4021 transmitter IC with it's 10Mhz crystal.
The PCB appeared to have a lot of gunge/hot melt glue type compound all over it so I decided to remove the Si4021 (with a hot air desolder tool) and the crystal too, cleaned up the PCB and resoldered the components.....
This appeared to do the trick and all worked well for about 3 weeks. Then it started to not transmit more and more often once more......
I've now ordered a new 10Mhz crystal and a Si4021. Will try just changing the crystal first and then the Si4021 transmitter IC if it still fails.
Will post my results back here in about a month with (hopefully good) news.