reoccurring power problem with Echostar LT-8700 for large dish turning and skew

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paulvr

SatelliteGuys Family
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Hi guys,

Over the past years, like many C-Band crazies here in Europe, I've been using an Echostar LT-8700 receiver to turn the dish (using an AJAK H2H mount with 10 ft. Unimesh dish, range from 65 West through 75 East) and also to turn the polarizer for ultimate perfection receiving extremely weak stations.

However, there's one thing that keeps occurring time and again: something goes terribly wrong in the 230 Volt power part, causing not only the fuse to blow but also some kind of power surge most likely between C18 and R18, causing the entire track on the motherboard between R9 and T1 to melt
away.

After replacing the melted line and of course the fuse the receiver then works normally for up to a year when the same mishaps reoccurs. This has been happening in a number of copies of the LT-8700 in different locations and leads to the assumption that there must be something in the power part that slowly looses it's value over the years and thus causes a voltage to become too high at a certain point.

Most recently we thought we were lucky because only the fuse had blown, but once it was replaced a few days later the receiver blew up once again. But then again, the mere replacement of the melted track and introduction of a new fuse somehow resolved the problem again for the time being. But of course we would like to resolve this problem permanently but we have no idea what to replace. The elco's don't seem affected, and also it seems rather illogical that replacing the fuse would cause this major shortage while repairing the erased track that was wiped out by that shortage suddenly makes the receiver work OK again for up to a year. We really have no idea where to start searching for a solution.

Does anyone else have such experiences or an idea what the cause might be? The receiver is about 20 years old and it seems likely that there is some part that, after 20 years, somehow looses it's original value even though that doesn't explain why after reparing the track the receiver will then work again for up to a year.
 
I assume that you put the right size fuse in whenever it blows? If the fuse was too large of a value, it'd blow a trace off the board.

If you can open the files that Fat Air gave the link to above and post a schematic of the supply, I may be able to help.
 
Hi,

I'm sorry, I should have added the schematics and a picture of the place where the track on the motherboard melted away. As the LT-8700 and it's predecessor the SR-8700 were just about a DX-er's dream in the analog period, it hadn't occurred to you that this type of receiver didn't have the same reputation in Echostar's homeland.

And to reply to the other question: yes, the fuse of course was replaced by a correct one, 2.5 Amp, slow, 250 Volts. No mistake possible as Echostar was so smart as to print the value right on the motherboard.

I hope I did OK uploading the two illustrations, the Dutch word "voeding" means "power supply", I see a question mark over the schematics of the receiver which is a pdf file, if I need to alter it into something else please let me know?
 

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Without a lot of studying, I'd replace the inrush limiting device, RT1. Then "See how it goes'.
But I'd also check all the electrolytic caps for bulging and/or leakage. (may require removing from ckt board)
An arcing C8?(shorting) or any of the output filters???
 
Without a lot of studying, I'd replace the inrush limiting device, RT1. Then "See how it goes'.
But I'd also check all the electrolytic caps for bulging and/or leakage. (may require removing from ckt board)
An arcing C8?(shorting) or any of the output filters???

Thanks for your good advice. I'll transmit it to my good friend who has been helping me out with the technical problems, I'm not at all knowledgeable on electronics myself. One problem of course is that once repared like right now, it might take up to a year before the problem reappears. It's however something one wants to avoid desperately, first of all because it's very destructive to the motherboard and further it happens with such ferocity (burn, flash, bang) that I don't know if it might actually be a fire hazard when it happens at a moment that no one is in the house. It does each time cause for the ground security switch of the house to short out so far, making sure the whole house is powered off immediately but what if somehow it doesn't cause for the main fuse of the house to switch off....?

Question remains: how can one be (relatively) sure that it wouldn't reoccur? Furthermore, perhaps the photocrgaphs shows a little of it: no bulging caps but of course a gerneral replacement might be a good thing anyhow.
 
Variable. Last week I woke up, heard on the news about an earth quake and tsunami in Chile, went to the sat receiver to see if I could see something on Chilean TV and found out that there was no display on the Echostar. I had been using it the evening before, turning the dish from 11 West to 49 East without any problem and later on also changing polarity from LHCP to RHCP so I know that turning the dish wasn't the cause. I had left the receiver on standby during the night.

At that time it appeared that only the fuse had blown. I went out to purchase another fuse of exactly the same kind and value, put it in and as if I had a hunch about what was to happen I first connected the receiver to AC (nothing was yet connected to it at that moment) and then the bang, flash and smoke came as a reaction.

Perhaps a conclusion could be that while the receiver is on standy (showing only the clock) only the fuse blows, and if the receiver is connected after having been unplugged the big bang happens? I don't know for sure because I don't really remember any situation in which the receiver actually blew up while being used. But I do know of a number of receivers of this type that somehow out of the blue suddenly decided to blow and erase the aforementioned strip on the motherboard so it sounds like some component that reaches "end of life" after about two decades.

Another thing worth mentioning: my receiver that blew last week had been in daily use for a number of years, the other one had been stored by it's owner a number of years ago while still in great condition. While it didn't blow immediately when first powering up after that many years, it did within say 15 minutes. Not while actually in service, my friend had powered it up in order to read the exact voltages in order to compare it with my defective one. But because the other one blew up also this was never carried out.
 
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If the bridge rectifier, CR1, is good, I would suspect the following: C2, RT1, C3, RV1. Either of the capacitors can short, and the VDR (RV1) is likely to fail with age, or following a power surge. Years ago, I saw a lot of these type power supplies being replaced in satellite receivers, usually from electrical storm damage - power line surge, which occurs frequently in the summer in our area.
 
Thanks, the electrical storm but isn´t really likely to be a cause over here, especially since last two blow/ups I described occurred without anything being attached to the Echostar receiver, and as far as I know these things are rather rare in the Netherlands and if they occur it´s generally in Summer time. The RV1 failing with age sounds like the prime suspect, and considering the little cost and trouble to replace capacitors along with the things you and FaT Air suggested.

PS It probably is of no relevance but perhaps I should mention that the dish it turns is a 10 ft Unimesh with AJAK H2H mount. I have been searching for a newer or anyhow less (ab)used AJAK H2h mount than mine but so far no luck, I guess they aren´t so readily available as they once were, at least not right now. Also the purchasing price is an issue because of high transport cost to Europe and equally high importation taxes costs easily run higher than I can afford. If anyone knows of a good one somewhere to replace the one I have please let me know).
 
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Is the trace that's burning off at the spot where there looks to be a green jumper wire connecting two resistors? Can you tell me the location numbers on the board for those resistors, if that's the spot? I can't really tell for sure if it is from looking at the picture, if that's where it is or not, the picture's not really clear on my computer.

Sorry about the fuse question. You never know for sure until you ask, scary as it is, some people think a fuse is a fuse and that the value doesn't really matter. :D
 
Its me helping PaulvR with this problem .
Paul already wrote : [causing not only the fuse to blow but also some kind of power surge most likely between C18 and R18, causing the entire track on the motherboard between R9 and T1 to melt away. ]

looks to me the track did not burned away by overcurrent but by the lightning .
The green wire connects again R9 and T1
 
Well, there are some important things that speak agains lightning. First of all I've been using the Ajak mount for well over 20 years on the exact same location and on a pole that's very solidly grounded without ever experiencing this problem. Furthermore, both I and Trust last week experienced a melt-down of the track while the receiver was attached to nothing at all, in my case also with beuatiful sunshiny weather. Afaik lightning effects cannot be delayed a few days :)

About the Ajak mount: it has been greased recently and doesn't - as far as I know - put any special strain to the Echostar. It's just that it would be good to have a back-up, I'd even consider using it with my 6ft5 Ku-Band dish that now is mounted upon an EGIS Robothead. The Robothead as such is a beautiful and perfect dual axis mechanism (was originally sold much less developed in the USA under the name JRC NITEC) but unfortunately it has too much free play which causes it to move more back-and forth in the wind than the small aperture of a large Ku-Band dish allows for.
 
Its me helping PaulvR with this problem .
Paul already wrote : [causing not only the fuse to blow but also some kind of power surge most likely between C18 and R18, causing the entire track on the motherboard between R9 and T1 to melt away. ]

looks to me the track did not burned away by overcurrent but by the lightning .
The green wire connects again R9 and T1

Typo? Did you mean R9 and T2? The R16 and 17 that Fat Air posted about above are right in that area, I'd start with checking them, also R18 and CR6,5,4, C1[or maybe C18-schematic's a little blurry] and other components in that area. Also check for bad solder connections, especially on the pins of T2, but also across the whole board.
 
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