On 2017-Aug-21, at 8:46 AM, Dominique Carlier via cctalk wrote: >> 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. >>
> > 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. I'm not acquainted with Power Physics as a transistor manufacturer, they seem a little odd in that I believe most of the smaller transistor manufacturers had faded away by the 1970s compared to the early 60s. The 7540 is (far more likely) date code however (40th week of 1975). I expect you can trust the 2N6254 on the schematic. As per Anders comment, you're not going to get reliable sub-ohm readings with a common DMM. Another way to test a small-ohm R is to put it in series with another R, say around 5 ohms, apply V from a bench supply, measure I's and/or V's and apply Ohm's Law. Or, for example, put all four 0.1 Rs in series, drop a volt or less across the bunch and measure the V across each R. The voltage should be evenly distributed across the four. But no, I don't expect they're the problem. The 47Ω Rs are 5% tolerance, 50Ω is fine. The 0.44 does look anomalous compared to the three 0.52s (as long as there is no in-circuit effect distinguishing those readings). If one transistor has a leaky BE junction and it's hfe (gain) has dropped it would be upsetting the regulator abilities. I see the schematic specs 12A @ 5V for each regulator. That puts ~ 6A through each pass transistor and each 0.1 Ω R if things are balanced, and 0.6V across the 0.1Ω Rs at full load. I recall you may have said you don't have access in operation, but measuring the voltages across those 0.1Rs could be informative, for both the V level and imbalance between the legs.
