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PostPosted: Tue May 03, 2011 2:50 am 
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This is an old DSO card that has worked well for me. It is the Link Instruments DSO-28264. It samples at 200MSa/s to an internal buffer, and has an 8 channel logic analyzer and very good software.

It has recently stopped working. So far I have located a short on the -5v rail, it is reading 0.0 ohms to ground. First I removed all tantalum caps on that rail. That didn´t fix it. Then I cut the ground leg of the switching regulator, on the lower left. The rail still reads 0.0 ohms to ground.

Does anyone have an idea of what is wrong?

Thanks
Alex

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PostPosted: Tue May 03, 2011 11:21 am 
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You can cut the power trace near each chip and go measuring.
Probably one chip went kaput.

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Carlos Filipe

"Any intelligent fool can make things bigger, more complex, and more violent. It takes a touch of genius - and a lot of courage - to move in the opposite direction." Albert Einstein


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PostPosted: Tue May 03, 2011 7:49 pm 
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carlosfm wrote:
You can cut the power trace near each chip and go measuring.
Probably one chip went kaput.

A more subtle way is feeling each chip with your finger. The one with a short circuit will be hot.
Also a good idea is cleaning the solder side of the PCB with some PCB cleaner and old tooth-brush. Especially if the tracks are very close a minute conducting particle can make a short circuit. Been there, done that.


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PostPosted: Tue May 03, 2011 8:28 pm 
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Thanks for the suggestions. I hope it´s not an expensive opamp that is shorted.

I have powered the rail with a small supply at 1.2v and the current maxed out at more than 2 amps. I only did this briefly and couldn´t find any hot spots on the board. Since the short reads 0.02 ohms it will dissipate much less than this small supply.

Can ceramic bypass caps short at 0.02 ohms? What components have this failure mode?

Thanks
Alex


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PostPosted: Tue May 03, 2011 9:28 pm 
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Elso Kwak wrote:
Also a good idea is cleaning the solder side of the PCB with some PCB cleaner and old tooth-brush.


Philips contact cleaner, red can.
Miraculous stuff, no joking.

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Carlos Filipe

"Any intelligent fool can make things bigger, more complex, and more violent. It takes a touch of genius - and a lot of courage - to move in the opposite direction." Albert Einstein


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PostPosted: Tue May 03, 2011 9:31 pm 
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I definitely need that.. is there a generic?


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PostPosted: Tue May 03, 2011 9:41 pm 
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IPA, isopropyl alcohol is great stuff. Even better can be to wash a board in hot (clean) water, then dry/clean it with a dip in IPA. (yes I am a compleat amateur)

Meanwhile - do you have away of applying a really low voltage at >1A, say 0.2- 0.3v and measuring current - just to check you are looking for a dead short, not just something with a forward-biased junction ?


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PostPosted: Tue May 03, 2011 9:47 pm 
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alexandre wrote:
I definitely need that.. is there a generic?


If you need a contact cleaner, this is it.
Not that you really need it for this particular case, but otherwise... it just solves problems that others don't, IME for many, many years.
Actually, my father used it too. More than 20 years at Grundig, since the late 60's.

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Carlos Filipe

"Any intelligent fool can make things bigger, more complex, and more violent. It takes a touch of genius - and a lot of courage - to move in the opposite direction." Albert Einstein


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PostPosted: Tue May 03, 2011 9:56 pm 
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Hi Martin,
Thanks for your suggestion. Good idea. I´ll take the card to the lab with me, sometime this week.

Best
Alex


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PostPosted: Tue May 17, 2011 10:07 pm 
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Finally got around to this. I have applied current and the AD9617JN opamp is heating up a bit, I hadn´t noticed this before. Looking at the circuit, the non-inverting input is grounded. What might be happening is a short inside the opamp, between the non-inverting input and the -V leg. I shall desolder the opamp and see.

Best :)
Alex


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PostPosted: Sun May 22, 2011 12:02 am 
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The short was in the small ceramic bypass next to the ad9617, the upper one. The cap near the smd ferrite.

Now I have to replace one opamp and a lot of caps. Well, this should fix this nice instrument. It even works in Win 7, in my P4 3GHz w/ isa slot.

Best
Alex


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PostPosted: Sun May 22, 2011 12:19 am 
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alexandre wrote:
Can ceramic bypass caps short


Now I can answer my own question and the answer is YES. The short was in the cap itself, I haven´t seen any short external to it.

Best
Alex


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PostPosted: Mon May 23, 2011 7:09 pm 
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This is not yet fixed. There is still some problem in the analog section or in the A/D converters, as the traces are stuck at the maximum value.

I checked the datasheet of the AD9012 converters, absolute maximum ratings:

Analog Input Voltage . . . . . . . . . . . . . . . . . . . . –VS to +0.5 V

Where -VS is -5v. This input might have been exceeded when the -5v supply to the opamps was shorted to ground. The -VS supply to the A/D was shorted to ground as well, it is the same supply.

I guess the next step is testing these A/D converters with a direct input from a potentiometer connected to ground and Vref at -2v. The analog input to the A/D is from -2v to 0v.

Best
Alex


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PostPosted: Sat May 28, 2011 4:58 am 
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I did more tests and narrowed it down to the data latches after the AD converters. Looks like both latches blew up when I ran the hardware without converters (I removed the ADs from their sockets while troubleshooting the analog front end).

The latches are 74fct2574atp (dip 20). All input signals going to the latches are switching normally, including the clock (latch) input and the output enable. But the outputs stay at zero (actually 100mV). Looks like the inputs failed.

I did a quick search but failed to find the dip package at any of the popular vendors or ebay. :confused:

Best
Alex


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PostPosted: Sat Jun 04, 2011 4:55 pm 
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I was mistaken about this. The inputs of these data registers must be fine. The real reason why it´s not working is a mystery.

(Took one 74FCT2574 out of the board and tested it, works good. Put it back in: does not work, even though all signals are switching. I put the board to work at low sampling frequencies like 10K and it still doesn´t work.)

I´m sure this thing can be fixed and I would like to, but it is a PAIN. :confused:

Best,
Alex


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