Wavetek 25XT problems - Page 1
Forum section: electronics / testgear

author: LoneTech
author_rank: Contributor
timestamp: February 01, 2018, 08:44:23 pm
content: Hi everyone, I have 5 of these units..well actually 4 because one of them has a burnt trace under the control knob,and i've decided to dub that one a parts unit.anywho, does anyone have a schematic for this unit.or have any idea of what each chip on the main board does as far as the functions go..I don't believe any of the problems on the 4 units is on the daughter board as i can swap it out with a known good one and the results for each unit is the same.problems for each unit vary ,,so i'll focus on just one for nowDC 200 mV scale, has a residual reading of -30.5 mV and the 2V scale has -.030V and the 20V scale -0.02V with no leads hooked to the meter..if i place say a 220 ohm resistor across the meter leads the reading goes to 0v..so I'm assuming there is a open resistor or trace ..but no idea where to start looking..all transistors check good, all IC's have been changed, even the UM7108F with a new one. no change in readingon another note i also noticed that if i feed the meter a 190mV..it actually comes up with the right reading..same for all other scales..very weird.edit:I forgot to mention that i have a perfectly working 25XT that does not behave this way..I know that for some meters a fluctuation in the lower scales are normal..but that isn't the case for this meter

author: retiredcaps
author_rank: Super Contributor
timestamp: February 01, 2018, 11:02:04 pm
content: On DCV, if you measure a 9V battery and 1.5V AA cell, does the meter show similar readings to a known good working one?For the slight negative offset, I bought an used/abused Fluke 179V that would show -0.02 ohms with the leads shorted. I cleaned the entire pcb twice with IPA and that fixed the problem.

author: LoneTech
author_rank: Contributor
timestamp: February 02, 2018, 12:59:04 am
content: Hi, Thanks for the reply..I couldn't find any new battery's..so this is the results using used battery's for the test..good unit 200mV scale no leads attached = 00.0 mVFaulty unit 200mV scale no leads attached = -25.3 mVgood unit 2V scale no leads attached = .000 VFaulty unit 2V scale no leads attached = -.026 Vgood unit 20V scale no leads attached = 0.00 VFaulty unit 20V scale no leads attached = -0.02 Vgood unit (1.5V AA) 2V scale = 1.367Vfaulty unit (same battery) 2V scale = 1.355Vgood unit (9V) 20V scale = 8.85Vfaulty unit (same battery) 20V scale = 8.88Vas you can see even tho the faulty meter doesn't settle back to 0 v with nothing attached it still manages to get approximately the same results as the good unit..very odd behavior

author: LoneTech
author_rank: Contributor
timestamp: February 02, 2018, 01:57:38 am
content: I made a voltage chart from the good unit,,oddly enough all of the voltages on the faulty one pretty much match this chart with only very minor differences..like maybe +/- 0.10v difference

author: retiredcaps
author_rank: Super Contributor
timestamp: February 02, 2018, 07:42:28 am
content: So the readings look okay. Just trying cleaning the pcb with IPA and see if the negative offset goes away. If not, maybe the DCV pots needs a little

author: LoneTech
author_rank: Contributor
timestamp: February 02, 2018, 11:40:05 pm
content: took the units apart, cleaned both sides thoroughly, put them all back together againand same issues on all of them.starting to wonder if got a bad batch one or IC's..they all came from aliexpress.tomorrow i'll try swapping parts from one to the other and see if the issues swap with them

author: ModemHead
author_rank: Frequent Contributor
timestamp: February 03, 2018, 04:26:39 am
content: Quote from: LoneTech on February 02, 2018, 12:59:04 amDoes the troublesome meter read zero with the inputs shorted? If so, you're chasing a really small problem.With the leads open, it only takes 2.5nA to develop 25mV across 10Meg. Probably a tiny leakage current coming through contamination on the PCB surface or between IC leads. Usually an IPA bath (or two) is the answer. Sometimes the path is sneaky though, like across two wafers of a wafer switch that should be isolated.The end result is like a very high-impedance voltage source permanently connected across the meter input. When you connect a much lower-impedance source, like a battery you want to measure, it swamps the weaker source and the meter reads as expected anyway.An interesting experiment might be to connect the troublesome unit in parallel with a good unit, set both on 200mVDC. Do they both read about the same? Maybe half the open-circuit reading?

author: LoneTech
author_rank: Contributor
timestamp: February 03, 2018, 04:49:07 am
content: Hi, yes the meter goes to zero when the leads are shorted,, i also noted that the residual voltage changes from time to time..like now it's reading -32 mV ,, tomorrow when i turn it on it may have a totally different reading.I'll try the 2 meters together thing tomorrowQuestion: I use NC-559-ASM solder flux to install all SMD IC's..surely this isn't causing the issue is it?and I use Acetone to clean PCB's of the flux

author: LoneTech
author_rank: Contributor
timestamp: February 03, 2018, 10:41:13 pm
content: well i hooked 2 meters together and the reading transferred to the other meter...I removed the lcd display again and cleaned the contacts on the PCB as well as the connector strip..that seemed to solved the problem.It may have been for nothing tho..the cap range doesn't work on this particular unit..on the daughter board there are 2 rows of pins,,one with less pins that the other side..on the shortest pin side the pin closest to the LCD has a triangle waveform(more or less). The good meter has a totally different waveform that pulses.I traced this pin back to the UM7108F IC so it would appear that the UM7108F is defective..so much for that meter :-)If the UM7108F didn't cost ~$25 each total. I'd order a few of em to fix this one..but hey who knows maybe the price will dropthanks everyone for the input on this subject it helped