Repairing a Kortek/Sharp Image KTM-F Arcade Monitor (STRS5741 VR Pinout) - Page 1
Forum section: electronics / projects

author: WolfgangFox
author_rank: Contributor
timestamp: January 03, 2025, 02:06:32 am
content: Hey all,I am repairing an arcade monitor and I need to know the pinout of the voltage regulator on the chassis/board. It is an STRS5741 VR, and the only datasheet I can find is a schematic of it ("https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcQcu4Ov7xwGrYZMCPpDMIVgtzjFFZOGxl-hAA&s") ("https://datasheet4u.com/pdfhtml/0558/609592/page-000001.png") (Hard to find a high quality pic of the datasheet)I'm trying to troubleshoot a no b+ voltage issue, and having this pinout would be incredibly helpful. It looks like either pin 1, pin 9, or pin 2 would be the output (not sure what voltage that would be) and either one of the other pins is another voltage output? If someone could help me answer this question of the pinout, I'd appreciate it.I do have a schematic of the monitor board/chassis showing the connections to the VR if that would help.Edit: I found this also, on a mexican site? ("https://www.carrod.mx/cdn/shop/products/2_93bdfdba-703c-4abe-8243-1f43d4d43353_1024x1024.png?v=1658775512")

author: bostonman
author_rank: Super Contributor
timestamp: January 03, 2025, 02:33:38 pm
content: If you just need the pin out, then just measure the pins to other points on the board. If the schematic you sent is correct, then the emitter is ground, so you know one pin. From there you can figure out which one is the collector by measuring the other two pins in diode mode; or measure the pins to other components in the circuit.

author: WolfgangFox
author_rank: Contributor
timestamp: January 03, 2025, 02:50:43 pm
content: Yes, I thought of that, but I was trying to get the voltage references (or am I looking at this wrong?) So, I did find the chassis ground pin (2), but I get voltages on pin 1, and 9 I think it is, so which is input, output, and the other pins may have a different voltage reference on it? I only see a 12v reference on the schematic, and the B+ which I'm trying to troubleshoot should be around ~130v.Here is the schematic snippet from the monitor itself and the voltage regulator use on the schematic:The B+ measuring point is the cathode of D605, and I get ZERO volts there, so the VR could be a fault, but I don't even know how to fully test it to ensure it's putting out the proper voltages? When I measure at the C606 filter cap, I do get around 160ish but just wanted to make sure the VR was doing it's job.

author: bostonman
author_rank: Super Contributor
timestamp: January 03, 2025, 03:07:42 pm
content: I read that wrong. The "Mexican" schematic shows you circled a transistor, so I thought you were looking at a three-pin device and trying to figure out the base, collector, and emitter.Now I understand, the regulator is six or more pins.Without the datasheet, you can only "shotgun" and trace. We didn't have schematics for most monitors when I worked in the field, so that's how we did it. As for tracing, I had to figure out a multi-layer board with blind vias, so I would measure, draw, measure, draw, etc... and that's how I figured out pin outputs for a particular section of the PCB.

author: WolfgangFox
author_rank: Contributor
timestamp: January 03, 2025, 06:13:00 pm
content: Yeah, that is definitely what I am faced with here.The mexican site does have the transistor "section" circled. I am used to having the voltage in on a certain pin, voltage out, on a certain pin. Etc. There is even conflicting information on what the voltage regulator should be outputting (I'm thinking, once again, around 133ish volts).Any ideas from anyone?

author: floobydust
author_rank: Super Contributor
timestamp: January 03, 2025, 06:19:07 pm
content: STR-S5741 is only 5 transistors inside.If you can (carefully) measure each pin voltage and post here.Some junctions inside the STRS you can check (making sure C606 330uF is discharged) with diode-test across pin 1 (C) pin 2 (E) pin 3 (B) of the switch transistor.It seems start-up is from base-drive from R604/R605 which nudge the switch on to start it oscillating. The electrolytics are critical for it to work, so C609 33uF, C610 10uF must not have dried out.It won't start up if there is an overload or short on the secondary side, like HOT shorted.

author: WolfgangFox
author_rank: Contributor
timestamp: January 03, 2025, 11:09:01 pm
content: I'll give you the backstory.Monitor came to me with the complaint of a "burning smell". Well the plastic protector covering the neck had melted and dripped searing hot plastic onto the chassis destroying several wires. The back of the neck board was completely and totally black, covered in soot, as well as inside the neck board socket that fits onto the tube.I tested the tube, and it tested fine with my B&K 467.I repaired the wiring, cleaned the neck board, replaced the socket, and flyback and turned it on to test.It worked, but the B+ was 133, and I thought it was supposed to be in the 120s.I re-capped the entire power supply section (you see in the schematic above) turned it on and NOTHING.When looking at the board, I accidentally put in C610 (I think could have been C609) backwards when I tested it.Re-installed the Cap, and also changed out C909. Still 0 volts on the B+ test point (cathode of D605).Changed the VR (this is a highly counterfeited part, so hard to get) and same issue, 0 on the B+.Ran wires from the filter cap (c606) and the bridge (D601) both were putting out 170volts.Tested the VR and only get 170volts on pin 1, nothing on 9 which I thought was the output.I originally thought it was the VR, they're about $10 a piece so I thought I'd test them and they are not shorted, but not putting out the voltage. So I thought I'd learn more about the VR and the pinout, and you see the datasheets I do. No other references on the net or the arcade forums about this, so it seems like I'm on my own.It seems like the VR is getting an "ok" supply voltage, but not outputting anything. I am more familar with the VRs that do everything themselves, and this one has quite the schematic behind it, as well as about 11 components connected to the other pins for whatever reason.

author: WolfgangFox
author_rank: Contributor
timestamp: January 03, 2025, 11:10:26 pm
content: Quote from: WolfgangFox on January 03, 2025, 11:09:01 pmSo I'd like to still figure out the pinout. I think I'm going to start testing the components in between C606 and D605 (namely I guess the transformer T601) and see if there are any discrepancies per the schematic unless anyone else has any brighter ideas.

author: WolfgangFox
author_rank: Contributor
timestamp: January 04, 2025, 05:33:38 pm
content: Quote from: floobydust on January 03, 2025, 06:19:07 pmI will do that. This monitor also has an obvious chassis vs earth ground seperation. When I test things in the power supply, I cannot use earth ground, it gives no reading. The easiest place to pick up chassis ground without modifying anything is pin 2 of the VR.