Repair, Teardown: Unbranded SF-95 Analog & Satlink WS-6903 Digital Satellite Finder

As a RF hobbyist, getting into satellites was quite a natural progression. First, it was amateur radio satellites, and then later progressing onto broadcast TV satellites. To the inexperienced, satellite TV seems rather complex, but it really isn’t provided you have a bit of hardware knowledge and geometric sense.

With a dish, one of the most important things to get right is the pointing. While you can use services such as DishPointer to find the relevant azimuth, elevation and skew values, in reality, that is only a start. To capture the optimal signal requires refining the dish’s position to counteract errors in your heading, elevation and LNB positioning.

If you’re patient enough, you can spend a decent amount of time trying to follow these degree values, or you can use one of the most basic tools known as a “satellite finder”.

What is a Satellite Finder?

Despite the suggestive name, most satellite finders doesn’t “find satellites” per-se. Instead, it’s a meter which connects in-line with your LNB, powered by your receiver, which indicates very roughly the signal energy in the 950-2150Mhz LNB IF section. Think of it as a peak envelope detector of some sort. This is often indicated through an analog scale, or a digital LED display, or an LCD bargraph along with an audio tone feedback (either pitch, intensity or beeping frequency).

By having it connected, and adjusting its variable attenuator, it is possible to swing the dish left and right, and up and down to maximise the signal received, and then refine the LNB position. It will often provide good results if properly used, although, it cannot distinguish whether you have targeted the right satellite, or if the LNB is faulty or receiving other sources of RF noise. As such, using it can be quite a frustration.

More expensive satellite finders are standalone units which provide power to the LNB and can receive the actual carrier signals, decode them, and present a display similar to a set top box. Professional units go further and can display a spectrum readout to positively identify the satellite and provide SNR/CNR and bit error rate readings to best optimize the position.

This particular posting will be dealing with the most basic satellite finders you will find on the market.

The “Unbranded” SF-95 Analog Satellite Finder

Back in 2013, I wanted to realign my dishes, and I took out my trusty analog-scale satellite finder. This particular unit is very popular, low-cost and can be found at virtually every satellite shop, or online.

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This unit features an analog scale display, with mirror to reduce parallax error. The variable attenuator is controlled by a knob on the front of the unit. The cables are attached to the connectors at the end.

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The whole unit is enclosed in black plastic, with holes in the rear to let the audio buzzer tone out of the enclosure. No further switches or options are provided. Despite the basic nature of this unit, it was enough for basic “first alignment” by just literally swinging the dish around almost randomly around the area you’d expect the satellite to be.

Unfortunately, the unit suddenly stopped working in October, after serving me well for a few years. The scale would light up, but no signal or tone would be shown regardless of the attenuator setting. As it was an inexpensive unit, I decided to retire it and take it apart for a quick inspection.

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The rear cover pops off with a gentle pry to reveal the internal PCB, marked JXSF-9502 and produced in week 35 of 2007. This particular PCB appears to have provisions for power and or 22khz tone indication, but it wasn’t fitted to this unit. Surprisingly few components are in here, mainly built around a quad LM324 op-amp. The meter movement coil in the centre connectors proved the coil was fine. The common ground is connected on the right, with the outer connectors providing power to the scale lamp, through a 1W resistor from the 13/18V supply. The lamp itself produces a lot of heat and consumes a lot of power.

I had hence concluded that it was likely that the opamp may have succumbed to static discharge, or some other malady, and there was no obvious reason visible for the unit’s demise.

As a result, I decided to order a whole host of different satellite finder units to replace this. One of them would be an analog unit, similar to that above, another one of them would utilize an LED display, and the final one would have an LCD display.

The Satlink WS-6903 Digital Satellite Finder

One of the replacement units was the Satlink WS-6903, which is an LED based unit. At the time, I had purchased three different satellite finders, I didn’t find them noteworthy enough to post anything about them.

But then, this unit also failed on me just a few days ago while I was trying to align a dish, much to my dismay. During an alignment, the receiver connector got slightly loose and then the finder and LNB lost power altogether. I swapped over to my backup LCD based unit, which eventually got me through. But that gave me a good reason to take it apart, to better understand what’s inside, why it broke and probably even try to fix it before it hits the bin.

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I didn’t have the sense to take a photo of the unit itself before I started taking it apart. For the record, the first step in disassembly is to unscrew the two halves, which are held together by four screws in the corner underneath the front cover label. In order to get to them, a sharp hobby knife makes easy work of cutting out those segments.

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The model number and specifications are provided on the rear, and the connectivity is much the same as well. Holes are provided for the buzzer.

Removing the top cover of the unit makes it obvious why the unit stopped working. See if you can spot why …

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The PCB is marked 6903BMC and has silkscreening which indicates most of the components on the board, which is nice. The same goes for the underside of the board, which has a trimpot which is covered in wax, probably used for alignment at the factory.

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The design involves the use of an unmarked main IC, likely an ASIC of some sort. The remaining supporting components include:

  • 78M09 9V 500mA Voltage Regulator
  • 1SS88 Schottky Barrier Diode
  • 1015 PNP Transistor
  • AMS1117 Adjustable Voltage Regulator
  • JRC4558D Dual Op-Amp
  • UPC2709TB Silicon MMIC Medium Power Amplifier
  • HCF4051 Analog 8-channel Mux/Demux

So, what was the problem? The problem was an improperly secured F-connector thread section, which rotated and cracked the centre pin connection to the board. After de-twisting the centre pin connection, this is how it looked when the pin rotated.

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Such a problem can be remedied with a lot of heat, and a little solder. Getting out my soldering iron, I was able to get enough heat into the connector so I could get some solder into the voids to secure the outer threaded barrel, and add some more solder into the inner pin to help support it against rotation.

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I also made a little modification and soldered down the barrel from the other side as well, although the amount of heat caused the plastic dielectric disc in the F-connector to deform somewhat. It’s still functional though.

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As a pre-emptive repair, I’ve also done a similar thing to the connector on the other side, to help strengthen it. Hopefully it will continue to live for a few more uses.

Conclusion

Satellite finders are relatively useful pieces of inexpensive equipment. Unfortunately, they also seem to break like inexpensive equipment, so it always pays to have a few units on hand. Sometimes it can be fixed fairly easily, so don’t go tossing them away just yet.

While they are generally not notable, and I didn’t decide to publish an article when they were purchased, from my experience, I prefer the use of analog display units as they provide more immediate feedback which is easily visually comprehended. The next in preference is the LED display, as it is also fast (unlike LCDs) but only refreshes a few times a second.

About lui_gough

I'm a bit of a nut for electronics, computing, photography, radio, satellite and other technical hobbies. Click for more about me!
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2 Responses to Repair, Teardown: Unbranded SF-95 Analog & Satlink WS-6903 Digital Satellite Finder

  1. Vince Foley says:

    Hi Gough, thanks for the write-up. Just thought I’d pass on a helpful tip which I use when soldering… before adding a lot of heat to a connector, connect it to a mating cable or socket. The cable or socket will help keep the connector aligned correctly and reduce the deformation of melting plastic. The mating cable will also heat up though, so it’s better to use scrap connectors for this purpose.

    I use this technique when soldering board-to-board headers, since ground pins can require significant heat to achieve solder flow. The plastic which holds header pins in position is very easily deformed, but always comes out correctly if I have attached a scrap header cable before soldering.

    • lui_gough says:

      Thanks for the tip Vince – I also do a similar thing when soldering other sorts of connectors and that didn’t come to mind at the time. Also would be advisable to have a short “stub” of wire rather than a full cable to reduce the heat losses into the cable to make soldering easier (especially for users of more delicate electronics style 30W irons).

      – Gough

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