TTi CPX400A teardown/repair - Page 1
Forum section: electronics / repair

author: ovnr
author_rank: Frequent Contributor
timestamp: July 09, 2016, 12:16:17 pm
content: So I accidentally blew up my trusty TTi CPX400A PSU (2x 20A/60V limited to 420W). Was playing with a DIY 3-phase motor drive, and ended up regeneratively braking into the PSU. With a motor that - when connected to a 240V SLA bank - regens at around 1 kW. Ended up with an overvoltage trip, and upon restarting, it failed to regulate at anything over 15-20V at zero current, and substantially lower if there was a load connected.Oops.So off I went trying to find the fault. Surprisingly the pass transistor was fine; the topology tripped me up for a good while. It's a bit unusual (even for a SMPSU with a linear post-regulator):It consists of a PFC stage with a 390V output, which is fed into two forward converters (for isolation) running at a fixed duty cycle. These do not regulate based on the set output voltage; unloaded they output 350+ volts. That is then fed into a magamp."What's a magamp?" I said.Turns out it's a magic inductor that essentially acts as a buck regulator (it's the large toroidal inductor w/ stranded wire in the pictures). There are no switches driving it; it takes the raw switching signal from the forward converter, and by adjusting the reset current, regulates it to whatever you end up setting the PSU to (plus a volt overhead for the linear post-reg). Hadn't even heard of one before this; it seems like a nifty technology, but it'sweird.Anyway, it's then rectified, filtered, and the final regulation is done by a beefy NMOS. Nothing unusual here.Once I stopped looking for a broken feedback path, I figured that the magamp wasn't being driven correctly. Turns out the control transistor - a MJE350 - was toast, as was most of the overvoltage shutdown components. Ended up replacing Q8, D22, R27, R28 and R34. Works perfectly well now; total cost was something like $5/channel to repair it.Anyhow, attached photos because yay teardowns, right? Also,here's the service manual(PDF, 5.8 MB).

author: ovnr
author_rank: Frequent Contributor
timestamp: July 09, 2016, 12:18:09 pm
content: More pictures. Damaged components highlighted in #2.

author: Vgkid
author_rank: Super Contributor
timestamp: July 09, 2016, 02:25:35 pm
content: Thsnks for the teardown. Interesting failure, parts even look ok. Expected the resistor to look crispy.

author: ovnr
author_rank: Frequent Contributor
timestamp: July 09, 2016, 02:53:44 pm
content: Quote from: Vgkid on July 09, 2016, 02:25:35 pmMmh. Should maybe point out that the parts had already been replaced by the time I bothered getting the camera.But yeah, the original parts had no visible signs of damage; the fusible resistors were all open circuit, the zener was shorted, and the transistor was shorted in some configuration too (it's been a while since I actually fixed it).

author: imamophead
author_rank: Newbie
timestamp: April 13, 2018, 09:01:28 am
content: Hi,Really sorry to resurrect an old post, but i have one of these bad boys and one of the outputs keeps going straight into CI mode (constant current). Just wondered if you or anyone had any clues on this? Its auto switching so i may try to isolate that circuit as a next step.Cheers.

author: ovnr
author_rank: Frequent Contributor
timestamp: April 13, 2018, 09:23:28 am
content: Hmm. Question time!* Is italwaysin constant current mode, even with no load attached?* When the output channel is off, does the current display show a sane value when you adjust the current limit pot?* What's the output voltage when it's in constant current mode? Measured with a DMM on the output, not the built-in meter.* When in CC mode, does adjusting the voltage or current pots do anything at all?* If it's not always in CC mode, does it regulate voltage properly?Quote from: imamophead on April 13, 2018, 09:01:28 amBy auto-switching, do you mean between 110/220V input? Because this is a true universal power supply; it doesn't so much "switch" between modes as not give a shit about the input voltage (within reason).Also, note that there's +350V DC present on everything but the front panel board at all times, even when the power switch on the front is off.Again, the topology consists of a boost converter straight off of mains (probably PFC, can't recall), going into a forward converter for isolation. The output of the forward converter is dumped into a magamp, acting as a buck converter, which is then post-filtered by a linear regulator. If something is wrong, it's very likely on the post-regulator or magamp controller side.

author: imamophead
author_rank: Newbie
timestamp: April 13, 2018, 09:53:43 am
content: wow thanks for the swift reply!Quote from: ovnr on April 13, 2018, 09:23:28 am- Yes always in constant current mode. (im assuming here CI is constant current mode? im usually used to CC)- Yes current pot shows what you would expect when adjusting with the output off and on.- Measured output 0.00, 0.01 if i crank the current up a bit (but didnt want to go too high).- As aforementioned voltage pots do something current does nothing.- Its always inSo as writing this response i thought id check for shorts directly across the outputs....There IS A dead short somewhere. Could be the issue or just a symptom not sure.Thanks again for the help

author: ovnr
author_rank: Frequent Contributor
timestamp: April 13, 2018, 11:53:28 am
content: So your issue is something else, not the same as I had.Unplug it, wait a couple minutes, then open it up. Disconnect the output channel that's acting up from the front panel by unscrewing these lugs and separating the wires:Measure the front panel jacks again. If it's still a dead short, your issue is likely that the reverse protection diode has blown up:If the front panel isn't shorted and it just reads like you'd expect a capacitor to (steadily increasing resistance), the issue is on the power board. Measure the drain-source resistance on the output MOSFET; it's accessible from the top while assembled:(I really don't expect that to be the issue, tbh.)

author: gslikker
author_rank: Newbie
timestamp: March 14, 2025, 02:27:05 pm
content: This is an old topic, but I want to use it, as I have about the same problem, which I am unable to solve.Suddenly, switching off a load, the left channel of this supply died.As OP experienced, R34, the emitter resistor of the MJE350, as well as the MJE350 are blown up and continue to blow whenever replaced.However, the mentioned resistors in the overvoltage protection, as well as the Zener diode (68V) are still OK.checked symptoms: unreg LED on and output voltage is some 20 Volts. Connecting a 680 Ohm resistor over the output very shortly seems to light the display more or less. display stuff on the LH side is dead,except from the unreg led to be lit (a bit faint)Done so far: exchanged the main output FET with the other channel, proving it is still goodused the left dead channels logic on the front panel conneted to the RH channel, it is still good.Put a power supply on the rectifier diodes of the small voltages sources and the voltage regulators for + and - 5 Volts are good, it is also capable lf making 15 Volts.I also tried to find any differences between the left and right channel using the huntron tracker no seeing any after MJE350 and its emitter resistor replacement.Took out some 5 transistors to check they check good.changed out the LM318 opamp (by TL082 which should be alike)It looks whatever I do, the emitter resistor of the MJE350 and the MJE350 die in a split second after power on.There is no visible damage and it seems I am stuck.Are there more people having this specific problem (and hopefully a solution) with this specific supply?

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: March 14, 2025, 04:18:18 pm
content: Hello gslikker,Weird situation.Vds of Q9 should not exceed 1V.If the MJE350 (300V, 500mA) fries, it is likely because of overcurrent.R34 = 22 Ohms, so 500 mA are reached at 11 VDC.MJE350SOAshows no curve above 1 ADC, so let's take 1A, which leads to Vr34 max = 22 VDCWhat surprises me is a bit is that the OPTO IC6 only indicates Magamp failure above49V68V (D22 Zener) which then blocks the primary switcher.There must be an issue around the magamp or the auxiliary negative supply on C32At first unsolder the magamp (MA1), the DC-DC transformer (T1) and L3 andringthem.I would then change some additional parts unconditionnally, even if they check well with the tracker.For about 20 bucks you'd be able to source all that stuff.Aux negative supply :R12D11C11R26C32R37D25 (if populated)Magamp control :D34R29D15D14Q5R16R15D12Magamp control loss circuit :D22 (68V Zener according to part list)R27R28IC5About 10 years ago, it was possible to contact AIM-TTI and they provided extenisve guidelines to repair their gear.But since 2 or 3 years, they are completely silent.Check also the '+15 V' power supply at pin 4 of IC5. Take care : the associated GND is 'P0V' on the primay side.If there's an issue, replace IC3 and the parts upstream that produce '+ V'.Quote from: gslikker on March 14, 2025, 02:27:05 pmI could not find the LM318 in the SM. Which part ID is it ?Maybe the GATE signal goes bersek and fools the whole secondary side. Consider replacing IC7 (LM324) and D3 + D4 + D5 on the control board.Update #1 : D22 is actually a 68V Zener; '+15 V' power rail check ; question about LM318Update #2 : GATE signal check

author: gslikker
author_rank: Newbie
timestamp: March 19, 2025, 09:34:13 pm
content: Thank you for your time taken to help me!My mistake, it was the LM358 IC4 on the output card I replaced using a TL082, (not A LM318) I had available. I could have made a mistake, there, so I have to check whether it is suitable, at all..The 68V zener diode I already tested.The control board DOES work when the LH channel connections are connected to the RH side output board, therefore I am not suspecting anything on the control/display board as it is capable of handling the RH output board.I will dig in further when I have time available, also to teach myself how the thing actually works (have done a too sloppy way of RTFM, I really need to try understand it better...)