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Thursday, 24 August 2017

Radyne ComStream QAM-256 QAM Modulator


I got myself a new toy to play with: a Radyne QAM-256 modulator!

Of course I already have three Dektec modulators in my laboratory:

But still I wanted to have a professional grade modulator for continuous operation. The Radyne QAM-256 was offered at eBay at a very modest price, one of the reasons being that the seller did not manage to get it to work. That didn't necessarily mean it was broken, though one never knows.

Anyway, I won the auction, the seller was very pleasant to talk to (his hobby is to actually run an amateur TV channel - check it out HERE) and when the device finally arrived, I wasn't able to produce a QAM signal, either.

After a lot of trials, using two Deviser field meter (S7000 and S7200 - one to output an ASI stream, the other to analyse the RF signal at the test port of the Radyne QAM-256), I came to the conclusion that several things were wrong. I could see the transponder being generated at the correct frequency, but no constellation diagram could be rendered.

I therefore decided to open it up and see what was the problem.

Inside look of the Radyne QAM-256.

First problem I detected, was that the fan had slipped from it's support frame and did not cool the device properly. This is not good! Fortunatly, it was an easy fix, as the only thing I had to do, was to snap the fan back into the support structure.

Next I saw that this device had been tampered with in the past! Not by the seller who sold the device to me (he didn't open the device at all), but by someone in the past... They must have replaced one of the capacitors and soldered a new one at an 90 degree angle, probably due to lack of soldering equipement for surface soldering...

I used the opportunity to look for further issues with capacitors, especifically electrolytic capacitors. And indeed, on the modulator board, one capacitor was swollen up. I replaced both of them, just to make sure... This certainly was the problem with the absence of a constellation in the RF signal.

Because the modulator PCB was screwed from the back side to the casing and due to the fact that the two screws could not be removed, I had to solder the two capacitors slightly tilted from the top side of the PCB. This probably explains why the other capacitor was replaced in a similar way.

Next I noticed the culprit for the fact that the Radyne QAM-256 wasn't storing the settings. Every time I turned the device off, all settings were lost, including time.
I found this odd looking yellow component - and what a find! It is a "Timekeeper Snaphat".

The datasheet can be easily found on a Google search and from the description, it makes sense, why the Radyne QAM-256 is not keeping the settings: the internal battery of the Timekeeper has simply run out.

Diagram of the SNAPHAT. It snaps on top of a RAM IC. While I ordered a replacement part (5-15 Euro, depending on where you buy it), it has not arrived, yet. So I did a small hack: I just connected two thin wires to VBAT- and VBAT+ and connected them to a CR3032 Lithium battery. It worked like a charm and now the settings are correctly stored, when the device is turned off.
This is of course a temporary fix, until the SNAPHAT arrives.

Finally, I noticed that the flat cables connecting the main PCB with the modulator PCB were not properly inserted. Again, a quick fix, as I just had to press the connectors down.

After all these small repairs and fixes, I was able to produce a propper constellation diagram, but though I got a lock on the signal, I was not receiving any data. I was disappointed and mentally preparing myself for the fact that the device was somehow defective beyond repair (at least without spare parts and documenation), when I got the idea to look at the constellation diagram with my Kathrein MSK33.

This field meter, despite being a little old by now, features the best constellation diagram I ever seen on a field meter. This is because of two reasons: 1) the constellation diagram is rendered without a lock on the signal and 2) it is really fast and by this I mean fast as in lightening fast!

This video shows a bit of my Kathrein MSK33.

Suddenly I noticed that the Kathrein MSK33 would correctly lock the RF signal produced by the Radyne QAM-256 when using a DOC64 modulation, while failing to lock the same signal with a QAM64 modulation. Now that was strange, as DOC64 means that a DOCSIS modulation was being used.

Well, that could only mean that some parameter on the Radyne QAM-256 was not set correctly. I read the manual again and eventually, at 2 a.m. I found the wrong parameter!

So finally, I got the Radyne QAM-256 to work properly:

Test setup: DekTec DTA-2115 is producing a QAM-256 signal, which is read into the Devisor S7000. The Devisor S7000 outputs the TS on the ASI port. From here a cable connects to the ASI Input port of the Radyne QAM-256. Finally, a cable goes from the RF output port of the Radyne QAM-256 to the RF input port of the Kathrein MSK33, which correctly renders the selected channels!

I have the Radyne QAM-256 now integrated with my other professional rack-sized devices:
What else can I say? I am happy - great purchase with satisfying repair: what else can one ask for?


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