SMPS TTi EX4210R Power Supply Running Very Warm Under No Load (With Schematic!) - Page 1
Forum section: electronics / repair

author: Jigglypuff
author_rank: Newbie
timestamp: June 14, 2025, 02:13:29 am
content: Hello! Casual hobbyist here. I've got a new-to-me TTi EX4210R power supply, bought under the auspice that the output will periodically turn off. I figured it was overheating for some reason and thought it'd be a relatively easy repair, but I'm stumped as I'm quite unfamiliar with switching power supplies. Can anyone please provide some pointers?Debugging done so far:- At most I've only loaded the PSU at 12V and 4A, and so far it actually hasn't tripped the overtemperature protection yet.- I still opted to open the unit up and have noticed the power FETS and snubber resistor (?) is quite hoteven with no load. There is an "Output On" switch on this model and it's still getting hot when output is off. Using a handheld thermocouple, I'm seeing the temperatures in the image below.- I did some online searching and found a very similardiscussionregarding another TTi PSU. Visual inspection of the analogous capacitor (C101 in my case, 470 pF) indicated evident browning.- C101 was replaced but so far R120 is still getting very hot with no load.And that brings me to this post. I'll keep digging around but seems like it really shouldn't be getting this hot when the output is off? If it helps any, TTi was kind enough to provide the full service manual, of which I've attached to this post.

author: Phoenix
author_rank: Frequent Contributor
timestamp: June 14, 2025, 03:25:38 am
content: That snubber is going to get warm even when not loaded. It may not be the cause of your shut-down. In any case you can check the integrity of the diode and resistor, you may need to take them out of circuit first.

author: Jigglypuff
author_rank: Newbie
timestamp: June 14, 2025, 04:10:42 am
content: Thanks! Removed R120 and it reads 102 ohms.D8 actually isn't populated and per the "*" note in the schematic that is intentional.D4 and D5 (and D2 and D3) are tucked underneath the heatsink and I'll have to spend a little more time disassembling things to get to them. Measuring them in the installed state, there ~0.4V of dropout and ~200kohm resistance.Before I pop the heatsink off, I'll try putting an o'scope on the gates and see if anything is happening there.

author: xavier60
author_rank: Super Contributor
timestamp: June 14, 2025, 11:26:58 am
content: Quote from: Jigglypuff on June 14, 2025, 04:10:42 amEven before you do that, tell us that you will be using an isolation transformer to power the PSU.

author: Jigglypuff
author_rank: Newbie
timestamp: June 14, 2025, 05:38:15 pm
content: On a scale of 1-10, how critical is an isolation transformer? Best I currently have is a variac which I don't believe is specifically designed with isolation... I'll probe around to verify.

author: xavier60
author_rank: Super Contributor
timestamp: June 14, 2025, 09:47:34 pm
content: Quote from: Jigglypuff on June 14, 2025, 05:38:15 pmIt's absolutely necessary to use an isolation transformer while using a typical grounded oscilloscope on the live side of a Switch Mode PSU.Otherwise, connecting the probe's ground clip will cause destructive mains current to short-circuit through the oscilloscope and also create a shock hazard.

author: Jigglypuff
author_rank: Newbie
timestamp: June 15, 2025, 12:16:21 am
content: Gotcha. Well, thank you very much for the incredibly helpful advice and helping prevent me from making a potentially catastrophic mistake!My variac is indeed not isolated so I'll have to track one down somewhere. I'll hold off on any o'scope tests for now.

author: Jigglypuff
author_rank: Newbie
timestamp: June 15, 2025, 08:38:42 pm
content: Update: I think (hope) this might just be a case of a dying case fan! I've been trying to recreate the overheat issue and had the fan disconnected for the bulk of all my testing. The one time I remember it being plugged in, the fan was slow to turn on. I figured it was just PWM-modulated, based on temperature, and normal.I was mulling the need to buy an isolation transformer and was still fairly confident everything was still just a temperature issue since the power output is otherwise normal. I pulled the fan, connected it directly to a 24V source, and noticed that it just barely moved any air. When powered off, it very quickly stopped, and when moving by hand it just didn't feel as smooth as some other fans I had on hand.Since I didn't have any 24V fans on hand, I opted to implement a two-fan mod with two 12V fans in series. It's perhaps not the prettiest, but temperatures measure much cooler! With an open case the resistor is ~155°F and heatsink is now ~90°F. I seldom use this much power but fingers crossed it doesn't overheat in the future. I'll update if it does!