Thank you very much for these valuable detailed information! :-)
Also following the suggestions of Curious Marc, I focus now on the
comparison between the two almost identical circuits that deliver the
regulated + 5V on G2.
Following your explanations, I started by checking the transistors and
resistors on the heatsinks A1 and A2, and also checking the resistors of
0.1 Ω (R10 to R13)
http://www.zeltrax.com/classiccmp_forum/psu_g2_test/cheked.jpg
But first I see on the schematic that these transistors are supposed to
be "2N6254", but I see on A1 and A2 that the 4 transistors are "7540"
(full name: POWER PHYSICS 001554 7540), I do not find The datasheet of
this model.
http://www.zeltrax.com/classiccmp_forum/psu_g2_test/serie_pass_transistor.jpg
The results of the test on the pass transistors:
By doing the various checks between B E C for NPN transistor, no
problems. No unwanted open or closed circuit at wrong place.
However if I measure the value between B and E, on all the transistors I
read + - 0.510 except for one where it is + -0.440, it is Q2 on A1 which
is precisely the board that delivers the + 5V which collapse. Is this
transistor defective?
The small resistances which are between the bases and collectors of each
transistors all show + -50 Ω (they are supposed to be 47 Ω but I imagine
it's OK like that)
But about R10, R11, R12 and R13, these resistors are supposed to be have
a value of 0.1 Ω , right?
Because for these 4 resistors I read values ranging from 0.2 to 0.4 Ω!
http://www.zeltrax.com/classiccmp_forum/psu_g2_test/resistor_check.jpg
Strange thing, today I take the measurements again (with two different
multi-meters to be sure) and the values today are different, located
between 0.8 and 1 Ω !! I do not understand this variation, I had
unwrapped only one leg, I removed both now, same type of result: 0.9 Ω !
Unfortunately because these 4 resistors are in the same state, these are
probably not the cause of the collapsed +5V only on the side of A1, What
is your opinion about that?
Anyway, I'm going to change those resistors for sure. I will also change
the transistor which only passes 0.44 instead of 0.52, I intend to put
an equivalent of 2N6254 hoping that the actual 7540 are also an
equivalent of the 2N6254 mentioned in the schematics.
Thanks a lot for your help !
Dominique
On 20/08/2017 09:08, Brent Hilpert wrote:
On 2017-Aug-19, at 12:10 PM, Dominique Carlier via cctalk wrote:
Maybe it's better to give us all the useful information these power
supplies, moreover it might be useful to other people with the same
computer.
An overall bloc diagram of the D-116 power supply including G1 and G2.
http://www.zeltrax.com/classiccmp_forum/psu_overall_bloc_diagram.jpg
The complete schematics of the part of the power supply named G2.
http://www.zeltrax.com/classiccmp_forum/g2_schematics.jpg
A drawing of the regulation board of the power supply G2 with the
physical locations of the components.
http://www.zeltrax.com/classiccmp_forum/g2_regulator_board.jpg
The schematics of this regulation board.
http://www.zeltrax.com/classiccmp_forum/g2_regulator_board_schematics.jpg
And a bit of literature concerning the principle of operation about the
regulation with this PSU (you will understand better why I am a little bit
lost ;-) This principle of regulation with a panoply of verification and
Protection systems everywhere is unusual for me)
http://www.zeltrax.com/classiccmp_forum/psu_regulation_principe.jpg
I have already tried without the CPU board: same symptoms. Next step,
try to check the capacitors in operation.
Results of the observations:
- This is definitely the regulated +5V of the G2 power supply. More I
add boards more the + 5v level goes down. +5v, +4.8v, +3.6v, +2.9v. It
remains stable however with just the CPU and the three core memory boards,
it becomes difficult for the power supply when I add boards in addition to
these.
- This is definitely not a problem at the level of the Power Fail
circuit.
- The big capacitors are not in fault (I rechecked twice).
- So this maybe a problem at the level of the regulation itself, the
+5V balancing system ?
Question: a faulty voltage regulator can behave in this way? I always
thought it worked or it did not work, but not between the two states
depending on the charge.
(In answer to the question, yes, a faulty regulator can produce
'in-between' output voltage.)
As is typical for power supplies of this type and era, this power supply
includes current limiting circuitry.
The current-limiting circuitry will throttle down the output voltage
(not shut it off completely) as the output current draw goes above a design
limit.
This would appear to fit the symptoms you describe.
The current-limiting circuitry works by placing a small-value resistor
in the current path after the main regulator transistor(s) (aka pass
transistors) but prior to the voltage-regulation sense point.
A transistor senses the voltage across this R.
As the output current increases, the voltage across the current-sense R
increases, at some point the transistor starts to turn on, and the
transistor is connected in such a way that as it turns on it reduces the
drive to the pass transistors, throttling down the output voltage.
Arbitrarily using the "A2" heatsink half of the two +5 supplies in this
power supply for component references, the current-sense R is comprised of
a series-parallel circuit formed by the BE junctions of the 2 pass
transistors (A2.Q1, A2.Q2), the two 0.1ohm Rs on the emitters of the pass
transistors (A3.R10,A3.R11), the 47ohm Rs between B&E of those transistors,
A3.R23, A3A1A1.R8, 376.R9, along with additional influence by 376.R8 and
A3A1A1.R10.
The current sense transistor is inside the LM376 (see internal
schematic), 376.Q16 between pins 1 & 8, controlling the 1st-stage driver
transistor 376.Q14.
There are various things that could go wrong on this circuitry.
Anything that upsets the current-sense resistance network to cause the
net R to increase will lower the output current that can be drawn (that is,
the current limiting circuitry will start kicking in too 'early').
A likely scenario is one of the pass transistors has failed open.
This would take out a parallel leg of the current-sense resistance.
All current would be forced through the good pass transistor and it's
emitter resistor, raising the current-sense voltage for a given output
current.
The current-limiting circuitry would kick in at a lower current than the
design intention.
This is beneficial inasmuch as it would work to save the good pass
transistor.
The 0.1 ohm 9W emitter resistors are critical, although they're probably
wirewound and fairly reliable unless quite overstressed.
One of these resistors being open would result in the same operation as
an open pass transistor.
You'll have difficulty measuring them with accuracy but you could do
some sanity checks for continuity around both the pass transistor circuit
legs,
as well as checking the pass transistor BC/BE junctions.
(In principle, the 'proper' thing to do in this area is look at what the
current-sense voltage is doing, but that requires knowing what the target V
is. Could compare with the other half of the supply.)
None of this is to suggest this area is necessarily at fault, or rule
out other areas of the supply.
Another possible fault that would fit the symptom (decreasing voltage as
current draw increases, if that's what's going on) is inadequate (partially
failed) drive to the pass transistors.
In short, anything reducing the power gain of the regulator error
amplifier could produce this symptom.
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