SG-9120B repair

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N0QBH

Thread Starter
SatelliteGuys Pro
Jul 13, 2006
204
53
Central MN
My year old SG9120B dish mover quit moving to the west earlier this winter, so I replaced it with a DG-380.
Put the non working mover on the bench to try figure out what went wrong, here's what I found.
The relay K2 was inoperative. It's driver transistor Q2 and protection diode D4 were also bad.
I didn't have any luck finding an exact replacement relay so I substituted with a pair of Omeron G5V1 mounted upsidedown and wired to circuit.
I had no way to ID the transistor and diode, so I used a 2N3904 NPN transistor and a 1N4148 diode.

Attached is a photo showing the new relays (I replaced both as a precaution) and a schematic of the relay "H bridge" and driver circuit on the pcb.
sg9120b_schem.JPG sg9120b.JPG

Seems to work fine now.
Mike
 

AcWxRadar

SatelliteGuys Pro
Apr 26, 2006
4,575
2
40 miles NW of Omaha. Omaha?
N0QBH,

Kudos from me too. Really nice to see the schematic attached.

These control relays do seem to be a common failure in some H-H motors. Looks like a really good repair from you.

There is one major thing that caught my eye, and it isn't anything related to your workmanship, but it is the really crappy and "cold" solder joints on some of the other factory devices like IC5 and the LED. Not to mention the can of the one cap leaning so badly. Particular components on this board were obviously wave soldered at the factory and the circuit contact made on the other side of the board, but it really looks lousy from the top side, don't you think? Boy, you would think that the motor mfg company would have better QA measures than that. I personally wouldn't have let such defects go out the door. Back when I was repairing avionics equipment, if I turned out solder joints like that, I would be laughed at first and then fired in the same breath they used for the laugh!

I was really proud of my soldering quality back then. I still am, but I don't do 1/100th as much component soldering today. I can still out-solder any one of my coworkers and that is doing it with very lame eyesight and shaking hands. I used to solder meter springs and meter coil wires to repair some items. I don't know if I could manage that practice any longer. That was a challenge even when I was young. Soldering a wire finer than a human hair to a terminal post the diameter of the thinnest sewing needle takes a lot of patience, control and steady hands. I doubt I could ever do that again. I am not that old, but I have lost those good elements of control and eyesight, especially sight.

RADAR
 

migold

SatelliteGuys Pro
Dec 2, 2006
553
70
Pearisburg, Virginia
I think it would be good to archive information on successful repairs of FTA equipment for future reference for those of us who are into 'self-repair'. This is an excellent example.
 

northgeorgia

SatelliteGuys Pro
Nov 14, 2011
1,537
194
North GA
It'd really be great if a few people with experience in repairing some of these H-H motors might consider examining/repairing them for a fee :) I'm one of those people that has never soldered before or know the basics of electronic components (used to know a little when younger, then forgot everything *duh*). So my 9120B is just sitting around out of service for now (no problem, I don't have a new dish for it anyway). I know for many people, this wouldn't wouldn't be worth the time or money, though. Shipping costs back and forth are probably the most prohibitive costs.
 

migold

SatelliteGuys Pro
Dec 2, 2006
553
70
Pearisburg, Virginia
OK, I'm experienced in repair and would be willing to do repairs on electronics of most any kind. I'm retired and would not need to charge much at all to do a repair. Only thing is, that my 'workshop' in the garage is right cold this time of year, so I would be limited to fair weather repair. Shipping would be a problem on heavy items, as prices have increased in recent years.
 

Cham

VE4GLS
Pub Member / Supporter
Dec 19, 2008
2,246
807
Boonies
I had problems with the board, there were some cold solder joints that were intermittant. I thought one of the relays was toast but re-soldering it fixed the trouble. Been working well for a while now (and in -20F weather too). Re-did a bunch of the joints when I had the board removed from the rotor assembly. Should be a thread on that somewhere already..
 
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