H7816-BA power supply (Was Re: H7821 power supply in MicroVAX 3100, SCSI disk enclosures and others)

[Peter Coghlan]

A few months ago, I mentioned a faulty H7816-BA power supply in a rackmount
DEC 3000/600 Alphaserver.  I finally got around to looking at it again
recently.  I hoped I had an identical working power supply to compare it to
but I didn't.  It turns out what I had is two working H7816-AA power supplies
in tabletop models of DEC 3000/600 Alphaservers.  These are very similar but
not identical to the H7816-BA.  The case layouts are different and the -AA
has four integral fans while the -BA has a extra board mounted under the lid
to power a large external fan instead.  There may be differences on the main
circuit boards, or there may not be.

The faulty H7816-BA was emitting an audiable click just after power on as
if it was trying to start and tripping out.  If I put a meter on the +3.3V
(orange), +5V (red) or +12V (brown) outputs, there was a very brief voltage
kick visible coincident with the click.  However, there didn't seem to be any
activity visible on the -12V (blue) output.  This was the first good clue to
where the fault lay.

With them switched off, I tried comparing resistance readings between the
-12V outputs of the -AA and -BA power supplies and 0V (in both directions,
in both diode mode and non-diode mode).  There were visible differences.
However,  when I did the same for the +3.3V, +5V and +12V outputs, the
results were pretty much identical between the -AA and the -BA.

So, I looked where the blue wire is soldered to the main PCB.  It is very
difficult to trace where it comes from but there is a 7912 voltage regulator
IC (E10) mounted on a heatsink close by and continuity tests show that this
is it's source.  Further continuity tests show that the other two terminals
of E10 seemed to be shorted together.  Aha! This is it I thought.  It wasn't.

After I unsoldered the 7912, the short was still on the board.  Suspicion
then fell on a 1000uF capacitor C45 connected across the same two points.
After unsoldering this, it was found to be innocent too.  Following the PCB
traces further back turned out to be nearly impossible put poking around
randomly with the continuity tester brought me to D21 (FEP16DT), a double
rectifer in a TO-220 case mounted on a heatsink.  This item seemed to have
all three terminals shorted.  After it was unsoldered, I found it to be
shorted on one side only, the other side giving reasonable readings for
a diode.

After refitting E10 and C45 and throwing everything back together (because
it is pretty much impossible to operate the power supply without it being
fully assembled) I found that it didn't click at power on any more and it
now stayed operating long enough to light my headlamp bulb dummy load on
the +5V output for a few seconds before cutting out, presumably when the
control circuitry noticed the lack of the -12V output due to D21 being
missing.  The green LED didn't come on.

I didn't have anything like an FEP16DT to hand.  I tried looking in a H7821
which I have already been scavanging parts from and all I could see was a
UTG5346 (I think) which might be similar but I couldn't find any information
about it anywhere so I decided not to risk trying it and breaking it.
Instead, I tried refitting the faulty FEP16DT with the lead going to the
shorted side bent up out of the way.  I thought that maybe if it was being
fed from a centre tapped winding on the chopper transformer, it might work
with only one rectifier provided the loading was not heavy.  On test, there
was some negative voltage on the -12V output but the power supply still cut
out after a few seconds and the green LED still didn't come on.  On further
reflection, the double rectifier is not being fed from a centre tapped
winding because I can't find the centre tap anywhere on the transformer
so having both rectifiers present is probably essential after all, depending
on how the circuit is supposed to work, which I can't quite figure out.

I had some used MR760 high current fast recovery rectifiers left over after
repairing a battery charger so I decided to try bodging in one of them in
place of the faulty half of the FEP16DT.  I guess the worst that would
happen is the power supply would trip out.  However, this time the power
supply continued working, my dummy load lamp glowed gently and the green
LED came on.  Yay!  I even got 5V on the white wire which I am guessing
by a process of elimination is probably the "power good" output.

Looks like I need to put an FEP16DT on my next component order.

Regards,
Peter Coghlan.


Previously I wrote:
> Flushed with success, I moved on to look at a H7816 from a DEC 3000/600
> Alphaserver.  This one shows little or no signs of life except for a barely
> audible click at switch on.  The fuse is not blown and there are no obvious
> signs of distress anywhere.
> 
> This one is a real pig to work on.  Firstly, the incoming earth connection is
> made to the lid of the power supply so when the lid is removed to work on it,
> there is no longer any earth connection to the rest of the unit.  A sticker is
> thoughtfully placed on the underside of the lid to warn about this situation.
> There are also two sub-boards attached to the underside of the lid.  One is a
> fan controller circuit (partially obscuring the sticker) which is easily
> removed and put out of the way.  The other sub-board contains the mains input
> fuse, bridge rectifier, a 110V/220V autosense circuit and what seems to be
> either a soft start circuit or less likely, a circuit to cut the power under
> computer control.  It is connected to the main board via a plug and socket and
> five leads which are only barely long enough to reach.  The plug has to be
> removed in order to take the lid off and then 300V DC is no longer fed to
> the main board meaning that measurements cannot be taken on the main board
> to investigate what might be wrong.  The real doozie is that the sub-board
> has a heatsink on it which is directly connected to the live side of the
> incoming mains for no good reason that I can see other than to make it
> difficult to work on.  Trying to plug the sub-board back in without the lid
> in place would leave this live heatsink flopping onto the components mounted
> on the main board below.  As well as all this grief, a whole bunch of low
> power components are mounted on another board which clipped under the main
> board about 10mm away from it and connected to it using three multipin
> plug/socket combinations denying access to the print side of the main board
> and the component side of the low power board whenever the unit is able to
> operate.  There seem to be 20 Ohm load resistors across all of the outputs,
> making it difficult to check the output rectifiers and smoothers without
> desoldering stuff, in the event that the power supply turns out to be
> tripping due to overcurrent.
> 
> On a positive note, there are component references on the boards, the
> minor boards are single sided, the unit is clean inside and one side
> is completely open allowing some access with the lid on (while keeping
> away from the live heatsink...)
> 
> I took on this machine knowing it had a power supply fault.  How hard
> could it be to fix it, I thought :-)  I'm hoping I have an identical
> power supply in another machine which I can use to make comparisons to.
> 
> Regards,
> Peter Coghlan.