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Fixing Crook Pvr's.


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#1 Suzygs1000

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Posted 08 March 2010 - 09:56 AM

HI,

Ok, so "crook" probably isn't really a technical term, but most people who have had these "generic" brands of PVRs will know what I am talking about:
Loss of recorded programmes
Unable to play recorded programmes on startup from cold
Scheduled recording doesn't stop, but just keeps going, and not able to be played back
and many other problems.

Mine gave up the ghost for good just after xmas, after having problems for about the previous 6 months.
I purchased an Avedia Media Centre with 800 gig hard drive, and this worked great for a month, then also had problems.

I sent it back for warranty, but in the meantime, had no way of recording programmes.

I recalled reading a year or so ago about the problem being failure of capacitors, so, not having dumped the troublesome unit, pulled it out of the cupboard, and took the cover of it, figuring that I couldn't make it any worse.

Now, I know nothing about electronics, but I do know what a capacitor looks like.

There are about 8 or 10 in a bunch in this unit, on a small board which is held in by three screws. It is a very simple matter to unplug the board, and remove it.

Two of the capacitors (both the same) seemed to be slightly domed, and had a bit of brownish stuff on the top of them.
All the rest looked ok.

These capacitors were 2200uf 10v 105'c.

I checked on Ebay and bought 10 on a card for $10 including postage.

When they arrived, altho they were the same specs, the were much larger!

Anyway, even tho I can't solder too well, I found it relatively simple to unsolder the two I suspected to be crook, and replace them with two of the new ones. Even tho they were much bigger, I still managed to fit them in ok.

The result was a unit that performs faultessly now, and in fact, even performs better than it did when it was new!

So, if anyone out there has a crook unit, probably of any brand, guys, don't dump it, just pull the cover off and see if any of the capacitors seemed to be slightly domed. If so, check for equivalents,(or a higher voltage even, so I am told) on Ebay, and have a go at replacing the ones you think are crook.

It is a very simple task, really!

Oh, and I have 8 capacitors left over, which should keep this PVR going for the next 20years!

Dave Morgan

#2 tonymy01

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Posted 08 March 2010 - 10:41 AM

On the Topfield forums, it is well known to replace the caps on the 5V lines in order to fix the issue of the 5V dropping quite low and causing every other voltage to go sky high (and doing things like frying the HDD with upwards of 15V rather than 12V, and frying the tuning agc transistor with upwards of 36V instead of 30V). You *must* replace these caps with LOW ESR caps, else you will get the same issue very quickly. And not all caps are made the same. The Jaycar so-called LowESR caps fail again in a few months (only one of the brands they sell, the other brand has been fine for me, but good luck getting the right ones, often the bins you collect them from end up mixed up from people and or the work experience kid putting the wrong ones in the wrong bin). They can't cope with the constant heat and current demands placed on them (the 5V circuitry cops the most load). Highly recommended is the RS Components and/or Farnell Components Panasonic LowESR capacitors, they last well and are high quality, and Farnell has very good shipping rates (I think free!!!). Topfield originally had 16V 1000uF caps in the 5V filtering circuitry (2 caps and an inductor). The fix now is to put a 2200uF cap into the section closest to the switch mode transformer, and keep a 1000uF or 1200uF at the output. Often caps are larger than the original manufacturer installed caps (esp if you double the capacity of it!). This is the trick, to get good quality ones that aren't huge in size.
Another trick rather than increase the cap value is to get one designed for higher voltage. Again, these are larger of course, but tend to last longer as there is more chance of heat dissipation if they are over engineered for the job (rather than the originals getting pretty cooked in warm cabinets and warm Australian conditions, esp over the life span of the cap).

Generally why they die is caps only really have a limited life span. High currents increase internal resistance, which decreases the filtering capability. High currents increase heat, and as internal resistance increases, increases the heat even more. High heat from poor air flow in enclosed cabinets makes things worse again. So, while a cap should last 10years or more, very rapidly declines once it has to suffer from these conditions. The idea is to get very good quality caps with very low internal resistance (Low ESR), and this will reduce the impact of these poor conditions (heat & current loads). And not all caps swell and/or leak/explode. Many Topfields die (crash lots, kill HDDs and/or have HDDs disappearing from the player and/or crash on bootup as the HDD tries to spin up) long before the caps fail to swell and/or leak mode. I guess this is because the internal ESR has increased to the point the switch mode PSU can't get the duty cycle any higher to get 5V happening, and the ever increasing duty cycle starts making every other voltage line not under a heavy load increase way too high and cause other issues. For Topfields at least, this doesn't happen at the point of cap explosion, it happens quite a bit before this.

Regards
Regards

#3 ozbeaker

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Posted 08 March 2010 - 08:17 PM

On the Topfield forums, it is well known to replace the caps on the 5V lines in order to fix the issue of the 5V dropping quite low and causing every other voltage to go sky high (and doing things like frying the HDD with upwards of 15V rather than 12V, and frying the tuning agc transistor with upwards of 36V instead of 30V). You *must* replace these caps with LOW ESR caps, else you will get the same issue very quickly. And not all caps are made the same. The Jaycar so-called LowESR caps fail again in a few months (only one of the brands they sell, the other brand has been fine for me, but good luck getting the right ones, often the bins you collect them from end up mixed up from people and or the work experience kid putting the wrong ones in the wrong bin). They can't cope with the constant heat and current demands placed on them (the 5V circuitry cops the most load). Highly recommended is the RS Components and/or Farnell Components Panasonic LowESR capacitors, they last well and are high quality, and Farnell has very good shipping rates (I think free!!!). Topfield originally had 16V 1000uF caps in the 5V filtering circuitry (2 caps and an inductor). The fix now is to put a 2200uF cap into the section closest to the switch mode transformer, and keep a 1000uF or 1200uF at the output. Often caps are larger than the original manufacturer installed caps (esp if you double the capacity of it!). This is the trick, to get good quality ones that aren't huge in size.
Another trick rather than increase the cap value is to get one designed for higher voltage. Again, these are larger of course, but tend to last longer as there is more chance of heat dissipation if they are over engineered for the job (rather than the originals getting pretty cooked in warm cabinets and warm Australian conditions, esp over the life span of the cap).

Generally why they die is caps only really have a limited life span. High currents increase internal resistance, which decreases the filtering capability. High currents increase heat, and as internal resistance increases, increases the heat even more. High heat from poor air flow in enclosed cabinets makes things worse again. So, while a cap should last 10years or more, very rapidly declines once it has to suffer from these conditions. The idea is to get very good quality caps with very low internal resistance (Low ESR), and this will reduce the impact of these poor conditions (heat & current loads). And not all caps swell and/or leak/explode. Many Topfields die (crash lots, kill HDDs and/or have HDDs disappearing from the player and/or crash on bootup as the HDD tries to spin up) long before the caps fail to swell and/or leak mode. I guess this is because the internal ESR has increased to the point the switch mode PSU can't get the duty cycle any higher to get 5V happening, and the ever increasing duty cycle starts making every other voltage line not under a heavy load increase way too high and cause other issues. For Topfields at least, this doesn't happen at the point of cap explosion, it happens quite a bit before this.

Regards
Regards


In order to test any capacitor in circuit, the best thing to do is buy or construct an ESR / Low Ohms meter. Otherwise you are just guessing as to which capacitors are really faulty. Bob Parker designed such a meter; see http://members.ozema...ar/esrmeter.htm

As I am constantly repairing switch mode power supplies, it is my opinion that you need to ensure any replacement capacitor is rated at 105 degrees C, NOT 85 degrees. This will ensure that they can withstand the higher temps in a power supply area for longer. Also, ensure all replacement capacitors are made in Japan; not China. The Chinese don't seem to have got their act into gear when it comes to electrolytic caps as yet. I also agree that Farnell / RS Components are the best place to buy replacements; for one, you can check the dimensions before you buy and they don't sell rubbish.

#4 iann

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Posted 26 March 2010 - 11:32 AM

Our spare PVR, a DVX-555U, has always emitted a high pitch buzz, couldn't really hear it when the PVR was in the living room, but now it's in my son's bedroom (where it's quieter), you can hear it buzzing when it's in standby mode.

I took the cover off and visually checked all the caps, all were visually OK, then I rolled up a sheet of paper to make a funnel, and put the big end to my ear while I moved the other smaller end over the components so I could hear which was buzzing.
Turns out it's an Inductor, the board location is L101, the Inductor is 10uH 3A radial, and covered with heatshrink.

I can't seem to find this part anywhere, RS have a 2.6A one, then the next one is 5A, but no 3A, and I'm not even sure if this is even the correct part as they describe it as a Choke, but from what I've read, there is a difference between an Inductor and a Choke, so I'm not sure if I need an Inductor or a Choke.

Is there any place in Sydney where I could take the PVR and they could identify the part and just sell me the part?

Edited by iann, 26 March 2010 - 11:35 AM.