Tektronix "TekMeter" (THM560, THM565, THM571) won't power up - Page 1
Forum section: electronics / repair

author: JWag
author_rank: Contributor
timestamp: October 20, 2018, 10:21:04 pm
content: Forum user jal1234 sent me two of these units. One is marked MAC ET 2020 on the front, but labeled "THM571" on the back nameplate. It is substantially similar to the THM560 and THM565 that I think this troubleshooting will apply to all.I'm making various measurements and comparing them to a functioning THM565. There is no schematic or service manual available from Tektronix.Symptoms:Attach power (9V, current-limited to 500mA). Device draws about 120mA. On a functioning unit, it's less than 10mA after a few seconds.Press "power" button, current rises to 220mA, no displayPress "power" button again, current drop to ~180mA, but only while button is held. Otherwise it's 220mATroubleshooting:U606 is a linear regulator (LP2951), putting out 5V on pin 1 as expected.U605 is a switching regulator (LT1174). As soon as power is applied, 2.5V appears at the SHUTDOWN pin 8, which is enough to turn on the device. This should not be. The SHUTDOWN signal should remain at 0V and rise to 5V only when the power button is pressed. Pressing the power button lifts this signal to 5V, so the logic behind this signal still seems to be functioning.The +5 rail created by U605 only rises to ~2V.U604 is also an LT1174, configured to supply -5V. It also receives ~2.5V on its SHUTDOWN pin and attempts to start.The -5V rail created by U604 rises to +0.2V!I suspect a problem with U604 that is putting out the erroneous positive SHUTDOWN signal. Will be looking into that next.

author: JWag
author_rank: Contributor
timestamp: October 21, 2018, 10:22:04 pm
content: I was wrong.2.5V remained on the U605's SHUTDOWN pin even after removing U604. It turns out these two devices' SHUTDOWN pins are actually separated by a PNP transistor, not shorted together as I'd assumed.After removing U605, the 2.5V disappeared. The SHUTDOWN pin behaved normally, staying low until the power button was pressed, which brought it up to 5V.I spent a little time reverse-engineering the power-on circuit. U600 us a 74HCT27 triple 3-input NOR. The power button ties into pins 1,2 and 13, with a 20k pullup to the 5V Standby rail. Pressing the power button shorts these pins to ground. Both of the other gates seem to be involved in controlling the SHUTDOWN pins of U604 and U605 also. I did not trace out all of the paths.U600.6 is connected directly to U605.8, and also the emitter of a PNP transistor. The base of this transistor is grounded through a 100k resistor from rpack RP601, and the collector is connected to U604.8.Replacing both U604 and U605 from a working unit fixed the issue. Now both supplies come up and the device powers up and down as normal.The electrolytics in this unit seem to be leaking also; this may have created a low enough impedance to overload the LTC1174s and kill them, maybe?

author: JWag
author_rank: Contributor
timestamp: October 21, 2018, 10:31:59 pm
content: Attached is an image of the power section of a different (newer) THM565.The ugly hack shorting U604's pins 6 and 7 together is apparently a factory bodge to increase the current output of U604. There is a trace connecting pin 7 to pin 4, which would set a lower limit, but this trace has been cut an an ugly splodge of solder added to short together pins 6 and 7. From the datasheet:Quote from: LTC1174 datasheet

author: mrlepipi
author_rank: Newbie
timestamp: February 25, 2019, 03:35:10 am
content: Hi JWag! Pips from New Zealand here, I found a THM565 on the bottom drawer of my old managers office, and I would like to fix it, could you please guide me on what to check? I've tried the battery pack and when pressing the ON button I can clearly hear a beep and a relay contacting. but no image on the screen, I have the original manual but doesn't say much more that take the battery pack out and plug it in again....I'm very interested in fixing it, because it will help me in my day to day work...I have some electronics tools and can test some things. I would appreciate your guidance.Thank youPips

author: mostorer
author_rank: Contributor
timestamp: April 07, 2020, 10:33:23 am
content: Hi JWag.I have a THM550 and in last days started doing strange thing with the LCD. it perfectly works but the LCD contrast. It started with screen disappearing for a few time, then reappear and now most of the time it is not readable at all.Starting from the LCD display datasheet (DMF682AN) I found that on pin3 it should have around -8V, but when I measure it I found something that is between -3V and -5V, so I suppose is the part of the circuit generating the negative voltage.Following your posts, I checked the pin 8 (shutdown) of both dc-dc converters (LTC1174) and that is ok (note: on my U604 no bridge on pins 6 and 7. Pin 7 is connected to Gnd).Anakyzing the strange behaviour I would suppose a problem to electrolytics more than to LTC1174, but I have no reference test points and values around the ICs to be checked.Pin 3 of the LCD is seems to be connected to pin 1 of U607 (MC34002an OpAmp), but I'm a little bit confused about how from U607 go back to U604 and U605.In my understanding output from U604 is applied to pin 4 of U607 (Vee - in my case I have -6V, while pin 7 Vcc is +5V), but if the pin 8 of the OpAmp is the ouput, where do the signals to its inputs come from ? ... also, if the ouput should be around -8V, Vee -6V is really too little.Do you have some more info on voltages around U604, U605 and U607 ? Can you please take some measures ?What about electrolytics values ?Thank you in advance for any help.

author: mostorer
author_rank: Contributor
timestamp: April 09, 2020, 07:05:30 am
content: Spent a night over the THM to make it working again. My first idea was to provide -8V to Vee pin of the LCD through a brand new dc-dc interter (TMA 0512s with a resistor divider), but in this way no more contrast adjust.Looking again and again I found that pin 3 of U607 is connected to an IC within the signal acquisition board. That is a MAX529 (serial DAC) that is used to define the signal that, amplified from U607, regulates the LCD contrast.MAX529 Vee should be -5V but it was something continuously changing from -1V and -3V.Ok, let's use the new inverter to put the -5V on IC Vee. I don't want to cut the track, so try to add the -5V in parallel directly on IC pin. Seems to work but, after a while, started again as before.I then decided to check where the Vee come from. Found !!! it's the ouput stage of the U604. To be precise, the electrolytic highligted in the attached picture (left lower side). It is a 150uf 10v low ESR.At its ends I should find -5V, but it is not so. Let me try to desolder it and check, as the so variable voltage is a typical behaviour of a broken electrolytic.As soon as I put the soldering iron tip on the pcb, I felt a typical smell and found electrolyte liquid around the component.Replaced it with a new capacitor, a 220uf 15V (I have not spare low ESR 150uf capacitors), but it should not be a so crytical component at that stage of the inverter so, let's try.Go back to measure output voltage and ... well done! I have -4,8V on MAX529 Vee.Magically the LCD screen started to work, but the better thing is that also contrast regulation is working like a charm.Spent an hour more to test all functionalities and to be sure that the repair is stable, then close it.I wanted to share with all my experience, as someone else could find it useful to make his/her THM revive.mostorer

author: Ypsi
author_rank: Contributor
timestamp: January 09, 2021, 06:47:44 pm
content: Hi friends.I bought this nice tool last week from E-bay. Status of device was unknown. After first sight looks all ok, but problem occur on multimeter AC ranges. With shorted input multimeter shows cca 4.3-4.4V AC, other ranges with huge deviation..I had some experience with this problem by the repair Tek THS, witch has the same principe of measuring AC voltages /AD736 as TrueRMS to DC and MAX139/, that i seek problem around AD.. Whoa, i founded a small-small drops next on aluminium capacitors, conected to the AD736. Capacity few nanofarads, ESR more than 1kohm.Desoldered, PCB cleaned from electrolytics residuents, new capacitors putted on place. Standart procedure.Now are all AC ranges Ok, autorange function without any problems.I'm glad that my collection of Tek's handheld is again greater

author: JWag
author_rank: Contributor
timestamp: September 22, 2025, 12:17:46 am
content: Long time no post!I dusted off my THMs to either fix them (and sell them) or just sell them as broken.I pulled off all the inductors and caps and LTC1174s so I could trace out most of the power supply with a meter. See attached. Apologies for my poor handwriting.I added reference designators where I could find them. The split between "analog" and "digital" on the +5V side is an educated guess. The digital side went to some of the logic ICs on the main board and the analog side went out to the measurement board so it seemed like a good call.The big Coiltronics dual inductors kind of mystify me. They're in parallel for each LTC1174 but wound on the same ferrite core, so I don't know whether to treat them as serial, parallel, or what. It's beyond my knowledge level. A cheapo component tester measured them as 100uH in parallel, FWIW.I did not draw the shutdown logic or the low battery circuits. A previous attempt to trace out the shutdown circuit to the power button resulted in confusion at some three input NOR gates.

author: TERRA Operative
author_rank: Super Contributor
timestamp: September 22, 2025, 09:39:01 am
content: I have two of these, both needed a full set of replacement capacitors, and one needed a new vreg. The usual story.Now I just need to source a replacement LCD for one that doesn't cost a billion dollars....I'm also working on a serial adapter, and also a USB-serial adapter. The USB-Serial adapter currently works for calibrating and serial communications, but I need to figure out how to load firmware via USB as the original firmware loader software only works with a real serial port, and you need to load the firmware in to reset the unit once you've done 10 calibrations, because reasons....I plan to run a firmware update via serial and sniff the serial data to replicate that in a Python script.I have uploaded the latest firmware/software, Firmware Loader schematics, etc to tekwiki a while ago too.https://w140.com/tekwiki/wiki/THM500