RE: Still trying to solve this K6100 HV

WHOOHOO J

Thanks a ton William, that is a great description of what is going on in
that circuit. I almost makes sense now J

I learned a couple of things:

Pay attention to the parts you get in "get well kits" I had a zanen kit
that I bought back 10 years ago. Used the MPSA06's from that to replace Q902
& 901. Following your suggestions I pulled Q901 (shorted). Then 902 (hmm,
that's odd the HFE is not reading unless I put it in PNP.. Oh bugger).
Let's look at that under the magnifier, MPSA56.. Don't trust the parts you
get in a kit and verify the part before you put it in.

Also learned that just because R908 tests to the right value in circuit
doesn't mean it works correctly.

Ended up replacing Q901,2 & 3 (again) and R908. Now HV cage is working and
I can adjust the B+ to 181.

Now my only remaining HV cage to get working again is one with a half inch
black char area where Q901 and his brothers live. Not sure it is worth
trying to fix that one.

Thanks,

David

From: owner-vectorlist@vectorlist.org
[mailto:owner-vectorlist@vectorlist.org] On Behalf Of William Boucher
Sent: Saturday, October 15, 2011 9:27 AM
To: vectorlist@vectorlist.org
Subject: Re: VECTOR: Still trying to solve this K6100 HV

Your schematic with marked voltages is helpful. One red flag is the values
shown for Q901 base/emitter being 23.1/14.5. The base/emitter junction
forward drop is that of a diode so it would never exceed 0.5V to 0.7V and
0.6V is typical. The excessly large difference of 8.6V indicates a blown
transistor.

The job of Q900 is to reduce the power voltage delivered to the HVT primary
winding to the level required to achieve the proper output voltage. It is
obviously conducting too hard at present. The negative feedback loop for
the regulator begins with a string of components D901, R912, ZD902, R910,
R918, R911. Since you measured 36V at the junction of R910/R918, it would
appear that the entire voltage divider string is conducting okay. When the
output voltage goes up, the base voltage of Q903 should also increase. This
increases conduction of Q903. As Q903 emitter outputs more current, the
voltage drop of R909 increases and the Q903 base/emitter voltage maintains
its typical forward biased diode voltage drop of approximately 0.5V to 0.7V.
This results in an increase in base current on Q902 (if resistor R915 is
okay). As the collector of Q902 conducts more, it pulls down the voltage at
the base of Q901 by conducting more current through resistors R905 and R904.
As the base voltage of Q901 falls, its emitter voltage also falls by the
same amount, again because the base/emitter junction, when forward biased,
will always be that of a diode, about 0.5V to 0.7V. The same rule holds for
the base/emitter junction of Q900 and so the output voltage of Q900 also
will fall. This delivers a lower supply voltage to the primary winding and
so the output voltage will fall.

Of course when the output voltage is too low, the opposite happens. The
base voltage of Q903 falls, it conducts less collector/emitter current, the
base voltage of Q902 falls and it conducts less collector/emitter current.
The base voltage of Q901 rises and its emitter follows. The base voltage of
Q900 rises and its emitter follows and the supply voltage increases.

I suggest based on your measured values that Q901 is fried and is leaking.
It also appears that the feedback loop is not working at Q902. Note that
the collector of Q902 has a very high voltage close to the supply level,
indicating that it has turned completely off. It is trying to allow Q901
and Q900 to conduct more in order to increase the voltage output. Since the
output voltage is already much too high, it is obvious that there is a
problem with one of the following components failing to conduct current:

    - wiper of R918

    - Q903

    - R909

    - R908

    - Q902 (already replaced so maybe okay)

Q901 may be failing soon after powerup due to prolonged excessive base and
collector current caused by failure of of Q902 to turn on. Replace (or
resolder and confirm condition of) the above listed parts as well as Q901.

Good luck,

William Boucher
http://www.biltronix.com

----- Original Message -----

From: David Shoemaker <mailto:davids@oz.net>

To: vectorlist@vectorlist.org

Sent: Saturday, October 15, 2011 2:24 AM

Subject: VECTOR: Still trying to solve this K6100 HV

I am still digging into this HV unit which is giving me some troubles. Here
is what I have observed.

The collector of q901 is way off of spec and I don't understand the circuit
well enough to understand what is causing it. And the base / emitter of
Q900 are about double what they should be.

I have checked the values of all the resistors and they are fine.

I have replaced q900 with a bu406d (verified good in another HV unit).

I have replaced q901 & q902.

Any suggestions? Thanks.

I have attached a copy of the schematic with my noted values in red.

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Received on Sun Oct 16 01:46:31 2011