robert.jarratt at ntlworld.com
Fri Apr 10 10:07:40 CDT 2020
I took a break and while I was doing that I thought the same thing, I am going to see if I can see a reason for the failure.
I did wonder if the tester’s current would be too low to show the nominal forward voltage on D11 and D12. I won’t change those.
From: Mattis Lind <mattislind at gmail.com>
Sent: 10 April 2020 15:16
To: rob at jarratt.me.uk
Cc: General Discussion: On-Topic and Off-Topic Posts <cctalk at classiccmp.org>; Matt Burke <matt at 9track.net>
Subject: Re: VAXmate PSU
I have found the problem and a possible other problem too.
But that is great!
I have discovered that D24 is shorted (no visible damage), although its twin D23 is not. I will replace both. I know it is shorted because I lifted one end.
Well, sometime there are no visible damage but still a smell.
I have also checked the rectifiers, D11 and D12. I am not sure if there is a possible problem here too.
D12 is an MBR3045PT. It tests correctly as a common cathode diode network. However, the forward voltage seems to be 0.19V. The datasheet (https://pdf1.alldatasheet.com/datasheet-pdf/view/53622/FAIRCHILD/MBR3045PT.html) would suggest it should be 0.76V at room temperature. I can see no physical damage to it though.
The point is that the measurement in the data sheet is with 20A forward current. You meter perhaps give 10 mA forward current. This is a Schottky rectifier it has very low Vf.
0.19 is completely normal.
D11 is marked PHS 2402. It too tests correctly as a common cathode diode network, with a forward voltage of 0.43V. I can't find a datasheet for it though, but I believe the equivalent is MUR1610CT (https://pdf1.alldatasheet.com/datasheet-pdf/view/82128/ONSEMI/MUR1610CT.html) and I think the forward voltage for that should be 0.9V. Again no physical damage that I can see.
Is it possible both rectifiers are marginal? Or at least D12? Should I replace one or both?
No. At this point replace just D24 and possibly D23. Actually I don't think it is necessary. But to be on the safe side it might be a good idea.
But the question is why it failed. Sometimes they fail because of just nothing. But it could be something in the network following it that had made it fail. Check that there are no short circuit in the LM317 regulator. Check output capacitors in this area. ESR and value.
> > I did check Q2 before, but perhaps I should check again. I have
> > checked for shorts on the actual outputs, but there don't seem to be
> > any. I have checked one of those rectifiers, I think one of my next
> > tasks is to desolder all of them and check them.
> I have put probes on Q2 with the probes connected as follows:
> Ch1. D19 anode (for triggering)
> Ch2. D19 gate
> Ch3. Q2 anode
> Ch4. Q2 gate
> The result is here:
> It doesn't look like Q2 is being triggered.
> > >
> > > Matt
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