EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: Jbliss on August 28, 2020, 12:49:44 pm
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Hi All
Looking for some input. Working on repairing a SMPS for a Large format mixing console. The power supply is a MPS14 looks to be a PWM SMPS (Allen and Heath)
Under no load the the power supply measures correctly all with in spec, on all rails. Once connected to a load (The Mixing Console A&H ML5000) The Supply goes into protect mode. I have replaced all electrolytic Capacitors as a matter of course as the unit is about close to 20 years old. There are some large MKP Capacitors that are currently stock (C62, C63, C64) and Ceramic caps have been left untouched. I know the console is not the problem as I have an Identical PSU that works correctly.
PWM IC = SG3525AN
looking for any tips for further trouble shooting and can provide any further information required.
Link to User manual (not very helpful)
https://www.manualslib.com/manual/23470/Allen-And-Heath-Mps14.html (https://www.manualslib.com/manual/23470/Allen-And-Heath-Mps14.html)
Any Help very much appreciated
Cheers
JBliss
SCHEMATIC BELOW ! (Updated with a higher res PDF)
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Just Attached Schematic... forgot |O
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That's a nice PSU! Lots of post-regulation.
Hopefully it suffers from the same ills as other SMPSs. The prime candidates for failure under load would be the Low ESR Electrolytics on the outputs of the SMPS, before the post-regulators. Check the condition of C65, C66, C67, C68, C74, C75, and maybe C71. The pre-reg is taking it's voltage sense from the rail with C65 and C66 as reservoirs but the others are likely to be in the same condition.
Check for bulging or vented lids, electrolyte leakage and if possible measure capacitance and ESR. If you find any of these defective, replace all of them.
Damn, I should have read the OP more carefully. :palm: Did you use low-ESR parts in the above locations? Did you also replace the small electrolyitics around the SG3525AN, ie. C96,97, and C49 etc, the small ones tend to dry up in high temperature environments.
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Have to ask the obvious: Have you positively identified that the fault is with the power supply and not with the console?
My next step would be to increasingly load the power supply with resistivve loads and determine the point at which it fails (or not).
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Yep, replaced all electrolytic Capacitors :)
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Yeah, I have an identical working PSU which operates perfectly with the console. So reasonably confident that the console is not issue. :)
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Just a thought - is there time to see if one of the green LEDs goes out before the others (or not come on), or does it just not start up at all (apart from the fault LED LED4)? It would be nice to definitively isolate the problem to the main SMPS or the Post-regs / Aux PSU.
Maybe you could scope the output of IC5, the fault comparator, on a scope as the supply comes up.
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Hi,
I would suggest checking the values of the current sense resistors. If one of these failed, either high in value or open (there are two resistors in parallel in some locations on the schematic), the current limit for that channel will be lower than it should.
These are R6, R7, R8 and R9
I don't see the current limit for the 11V rail.
It probably isn't the 48V rail that is taking the supply down.
You need to isolate which channel is causing the problem.
Do you have any big resistors you can use as dummy loads?
Jay_Diddy_B
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good Idea! Just Checked and looks like the -17v Shuts down first
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Hi Jay_Diddy_B
Thanks for the reply!
Just checked those resistors they all seem to measure the same, hard to get a stable measurement as they are only 0.1 ohm.
Looks like the issue is with the -17v channel
Thanks
JBliss
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Update.
Probing around on the -17v channel post regulator and I have discovered that D32 I am reading a 0.4v drop across in both directions. On the +17 Channel at D33 I get 0v (and 0hms). Furthermore in my second working power supply I get a 0v (and 0hms) reading across D 32.
Out of circuit the D32 has a 0.6v drop
Could D32 or TR9 be the culprit.
Tested both D32 and TR9 and they look good. What am I missing here |O |O |O
Hopefully that makes some sense.
Thanks again
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Hi,
Let us try this. I have analyzed the -17V regulator circuit. I have marked the schematic with letters A - I.
[attachimg=1]
(https://www.eevblog.com/forum/repair/smps-repair-fail-under-load-(mixing-console)/?action=dlattach;attach=1056258;image)
In standby these should be the voltages all these points. All are with respect to ground.
A -19.5 to -20V from the SMPS section
B -17V output voltage
C approximately -16V (voltage at A + Vgs TR9)
D approximately +1V ( Vgs (TR9) / 8 + Vbe Q2)
E 0v (if the voltage is in regulation)
F 0v
G 6.15V (depends on +17V rail)
H 5.62V (depends on +17V rail)
I should be positive approximately +12V so that D26 and D22 are not conducting.
As the load current is increased H will be become more positive. At about 11A, H should equal G, D22 and D26 will conduct reducing the output voltage.
Regards,
Jay_Diddy_B
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Huge thanks! Let me go and check will let you know the results.
Cheers
JBliss
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A = -20.2
B= -19.5
C= -16.65
D= 1v
E = Aprox 0v
F= 0v
G = 6.3V
H = 2.2V
I = 5.77
Just Measured all the points and only discrepancies are H and I
Once again Thanks for you assistance !
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Hi,
Next step:
[attachimg=1]
(https://www.eevblog.com/forum/repair/smps-repair-fail-under-load-(mixing-console)/?action=dlattach;attach=1056264;image)
Check the resistors that I have highlighted. You should lift one end to get an accurate reading.
The resistors R60, R61 and R56 should be 1%
Check or replace C24.
If they are all good, change IC4.
I think the problem is IC4, but you need to confirm the resistors and capacitor first.
Jay_Diddy_B
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Great ! Shall do will confirm all Resistor values and caps and get back
I also reckon I have couple of 072's around !
Cheers JBliss
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Hi Posted by: Jay_Diddy_B
Tested all of the Resistors and C24 and they where all good. I changed IC4 however the problem is still present unfortunately :'(
D32 is still reading 0.4v Drop in both directions in comparison to my working supply where there is 0v and 0ohms.
JBliss
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If I understand what I've read correctly, I would say that TR8 is shorted. The -17V supply tracks the +17V supply via R55 & R59. Since the -17V supply output is too high per the measurements that were posted, that makes me think the +17V output is high as well. Also it was stated that D33 measures 0 ohms, I assume in-circuit. All of that points to TR8 being shorted IMO. I haven't seen any output measurements so without that information, it's difficult to draw a conclusion with 100% confidence.
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Hi Squoip,
Correct TR8 was a dead short on all pins! Yikes! How did I not see that! chasing red herrings on the -17 Channel. :palm:
So I have replaced TR8 however the problem still is still present. I guess that TR8 took some components with it when it went!
Further, I have tested the supply with a load. I am able to load the +17v channel without any issues. Load on the -17v Channel still puts the supply into Protect Mode. When the +17v Channel is under load TP-9 Measures 26v !
Thanks to everyone who has helped so far !!
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I would put a minimal load, maybe .5 - 1A on the 12V output since it's the only one that is regulated solely by the pre-regulator. Then I would troubleshoot it with the rest of the outputs unloaded. Since the rest of the outputs have secondary linear regulators, they should all run fine unloaded. The reason for the load on the 12V output is we don't want to load it fully but don't want it to run discontinuous either. With a small load on 12V, measure all the outputs and post the results. Since TP9 is higher than expected, it seems to indicate a problem with the pre-regulator. You'll need to make sure the pre-regulator works right first, then +17V, then -17V. Given that TR8 was shorted on the +17 output, I suspect you may still have issues there.
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I'm Back, got caught up with work.
So I have loaded the 12v Rail down with 0.6A or so and the results are
+17 Rail Output = 17.60
-17 Rail Output = -17.60
TP8 = 19.85
TP9 = -20.1 (drops half 0.5v when 12v is unloaded)
12 = Drops to 2v when loaded
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Ok, So After replacing some out of spec resistors and the SG3525AN the PSU no longer goes into protect mode and is stable under load, GREAT !!
However...Under load (Mixing console) TR1 makes a screeching sound when starting up first time after the supply caps have been drained. The power supply also seems to slowly ramp up over a second or two.
The two 33Ohm resistors R33 and R76 Get extremely hot.
I have verified that TR1 is not damaged. As all the electrolytic Capacitors have been replaced could it be the larger film capacitors such as C61 C62 C63 C64 that are to blame ? or possibly the inductor (IND1) 1mh
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Sample of start up noise. TR1, R33 and R76 all getting extremely hot under load after just several seconds. Hard to discern where the noise is coming from.
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Has been a while, replaced IC 1 and D6 and D28, both diodes where shorted. PSU is stable under no load however, now under load sounds like IND1 is constantly humming.
TP 3 no load is 320 and drops to 270 under load looks like something is still pulling it down.
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Sounds like the PFC is probably going into overcurrent. Could be due to a damaged component causing the overcurrent setpoint to be too low or, obviously, something is actually pulling too much current. Since it's running, it may well be due to a damaged resistor that's making U1 think it should limit current. My guess is the latter. R13 & R76 are .33 \$\Omega\$ resistors that are in parallel. You should be able to put a scope on IC1-4 and see if it's hitting the current limit.