BU406D (was Wintron install: Does this look right?)

Hey,

I'm following up to my own post.... I found the following
on the archives (after my sig), seems applicable. I'm just
not sure if that 21.6V on the 7824 is too low and I have
other problems besides (possibly) the bu406d.

In summary, the posting below states, even though a bu406d
may test good, it may not be good and that the best way to
check is to first get good readings from the 7824/7924 and
then, if the HV still seems to be dead (as mine seems to
be), try replacing the bu406d.

My main question is this: is the -33 V from the 7924 and
the 21.6V on the 7824 too far out of range for me to
consider them good?

If it makes sense to try a known good bu406d now, can
anyone around here (Boston area) help me with a BU406D
(Joel!!!)... My 9YO son's birthday party is Saturday and
I'm trying to get it back up for it. If someone can lend
me one, I'll replace it as soon as I can get a hold of a
new one.

Thanks.

Dan Rasmussen

From: jwelser@ccwf.cc.utexas.edu
Date: Tue Oct 21 1997 - 13:40:05 CDT

On Tue, 21 Oct 1997, Joel Rosenzweig wrote:

> How have you all gone about testing the BU406D?

The BU406D is a wierd one. There shouldn't be a diode drop
from C -> B, or from C -> E (I think I got that right...the
case where the diode across C and E is reverse biased
should be infinite impedance, of course, is what I'm
saying.)

Even on good BU406D's, the B -> E and E -> B will show low
impedance, because there is an internal resistor (in
addition to the diode) from B <-> E

The way I fixed a bunch of HV units: Test the BU406D. If it
tests bad, throw it out (obviously) if it tests good,
you're still not out of the woods yet, because I had a
couple that caused no HV, even though they tested good.

> When yours have failed, do they open up, or short, or ???

Short from C <-> E just like most other transistors...
  
> I have three more Amplifone HV units that I'm trying to
repair. I
> suspect the HV transformer in all cases, but I'd like to
know how to
> tell if the BU406D is at fault or not.

No HV, heater, etc. Same symptoms as the bad HV
transformer, shorted regulators, etc.
  
> One last thing .. I have 4 HV boards in total, all with
red Hv
> transformers. One of the HV units actually works. (I
know, a rare
> breed) Similar to what Clay mapped out with a DMM for the
Wells Gardner
> HV transformer, I performed similar type measurements on
the Amplifone
> HV transformer units. Every last one of them (including
the working
> one) had the same measurements. Yet, only one of them
actually works.
> The ohm test seemed pretty logical to me, and it sounded
like it would
> be a surefire way to detect an open or shorted winding
... but
> apparently, that's not the whole story. If the windings
appear to be
> good, what else is wrong with these transformers? The
BU406D's that I
> can't test? :-) :-) :-)

I measured the same on good and bad Penn-Tran/Wintron HV
units. I think what sometimes happens is that the
insulation between turns breaks down, and the effect is
that the transformer has fewer windings, so the HV drops.
From the schematic of the HV unit from Wintron, it looks
like there is a diode IN the HV transformer, so maybe that
breaks down too, but I'd guess you could test for that
<shrug>
   
> I have not removed the HV transformers and swapped them
into the known
> good HV board, but I have checked every component except
for the BU406D
> and MC1, whatever that is?? Aside from that, these units
have basically
> been rebuilt! Any ideas?

I just repaired a half-dozen of these boards, and the most
common failures are the 7824 and 7924 regulators (The 7924s
failed in all but one instance) If you're getting good +/-
24V, then try the BU406D. I found a bunch of creative
replacements that previous techs had done (but all
incorrect) for the BU406D, so check to make sure that the
transistor that's there is actually a BU406D. If you've got
those cheesy brown jumpers (W1, etc) on the board, check
them out, because they DO open up. If you're not getting
+/- 24V (or the +/- 30V input to the board) check the brown
jumpers on the deflection board. I had this happen in 1
case.

Joe

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Received on Wed Oct 24 06:27:38 2001