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XTL5000 2.85v regulator issue

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mac1_131
Posts: 10
Joined: Mon Dec 08, 2008 3:20 pm

XTL5000 2.85v regulator issue

Postby mac1_131 » Sun Jul 16, 2017 4:41 am

Any component level XTL techs out there?

I have a couple VHF main boards that were fried by connecting a bad O5 head to them. Being VHF, I'd really like to fix them.

At first I thought it was an sb9600 bus issue, but when I found sb9600_enable was not even turning on I looked elsewhere.

I am finding the 2.85v regulator at about 1.3 volts. It's not the regulator chip, I replaced that same result. The other regulators are also low, but since they are enabled by the 2.85 vcc that is understandable.

5v is good by the way. Filtered 5v is low but that is due to current draw from 2.85v regulator through a dropping resistor I think.

2.85 vcc goes lots of places. Short of unsoldering every 2.85 vcc chip lead, how can I troubleshoot this?

Another thing that puzzles me is I measure about 5k ohms on the 2.85 vcc both on the bad radio and on a good radio. I would have expected it to be lower on the bad radio but it is the same.

Something is not letting the 2.85 come up, and that kills everything else down the line.

Head says fail 01/90 btw.

Thanks for any ideas.

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jackhackett
Posts: 1400
Joined: Tue Jun 10, 2003 8:52 am

Re: XTL5000 2.85v regulator issue

Postby jackhackett » Sun Jul 16, 2017 5:58 pm

Looking at the schematic, I see a couple of things to consider...

I don't see where the other regulators are enabled by the 2.85V one, they all appear to be independent except that their /error outputs are tied together. How low are they?

You say the filtered 5V (VCC5_FILT) is low, you think maybe because of the drain from the 2.85V reg through a resistor, but the schematic shows the 2.85V reg powered by VCC5, not VCC5_FILT. The 1.55V and 1.85V regs seem to be the only things connected to VCC5_FILT.
There should be some drop across that 10 ohm filter resistor, how much will depend on the current draw, E=I*R. If you can, see what the filtered 5V is in a good radio.

A static resistance check on the regulator outputs isn't necessarily going to show anything useful. The circuits will act differently when they have power on them to bias the semiconductor junctions on.

mac1_131
Posts: 10
Joined: Mon Dec 08, 2008 3:20 pm

Re: XTL5000 2.85v regulator issue

Postby mac1_131 » Mon Jul 17, 2017 1:13 pm

thanks for your reply. I made a couple mistakes in my original post. VCC2.85 is the pull up source for the LV_DETECT signal, so that is what I meant to say. If the 2.85 is bad, the LV_DETECT will never go high which I assume will keep the nautilus from booting. On my bad radios, LV_DETECT stays low, while on my good test radio LV_DETECT goes high.

The other regulators are about half-voltage too. The 1.55 is about at .45, and the 1.85 is at about .8 or so.

I don't think the 1.55 and 1.85 draw enough current to show much voltage drop across R0519, if I remember correctly the good radio had 5V on the filtered 5V side of R0519 - it wouldn't be filtered 5V if it wasn't 5V. But mine is low, about 3.3V.

So I am just trying to get the 2.85 to come up first and deal with the others later, but it is strange they are all running about half of what they should be, but I assume you the 5V regulator is fine at full voltage.

thanks for your comment that reading static resistance may not be valid, I wondered about that.

I am just dreading pulling up the VCC lead on every chip that touches 2.85 - which includes the daughter board and the nautilus chip I think. If the daughterboard is bad, that is the end of things.

I knew a guy who used to be at the depot, he said they would get a certain amount of time to troubleshoot something like this then the board got replaced. I was hoping someone has seen this before and might know the magic fix as it was simply caused by plugging a bad head into a dash mount configuration and fried something immediately.

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jackhackett
Posts: 1400
Joined: Tue Jun 10, 2003 8:52 am

Re: XTL5000 2.85v regulator issue

Postby jackhackett » Mon Jul 17, 2017 6:44 pm

If all three are low something would have to be loading each of them down, the one place I see them all connected appears to be at the main processor.


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