Re: Testing BU406D's

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
Received on Tue Oct 21 11:41:15 1997