WAVETEK 27xt DMM - problem  - Page 1
Forum section: electronics / testgear

author: Naturalizt
author_rank: Newbie
timestamp: October 05, 2017, 01:01:36 pm
content: Dear All,Today I wanted to test an electronic circuit for building my own Remote control airplane ignition system. The circuit was taken out of an electronic lighter and presumably was a high voltage inverter. I connected the output leads to the DMM for 1000DC/750AC scale, and turn on the ignition circuit for a very short time (Approx. 0.2sec). I heard some buzz, probably from the piezo. Then, immediately,, I didn´t measure any further.Observations and problems:- The display works as usual.- Apparently there PCB and the components are seemingly intact. I don´t see any burnt or smelling part.- I noticed that the DMM displays a low battery icon. it does not disappear by changing the battery.- It measures nearly half of the resistance. I had a 100 ohm resistance and DMM displays only 1.3 ohm.- The measured DC voltage is also reduced. Given 10V DC displays only 7V dc (at 20V scale).- The AC Voltage (230V) appears only 20 (at 750 AC scale)- As usual, one can hear the beep sounds while turning on and off.- As usual, the battery polarity can be measured accurately. (+ or - terminals)- As usual, the scale or measurement units are displayed while turning the selector dial..(dc, ac, H, Ohm, F, TTL, Cmos, ...)- The display disappears or becomes very dim while resistance measurement at 2000Mohm- There is a reading of -27.9 ohm at 200ohm scale while no measurement lead is connected.- At continuity measurement there is .377V and when the leads are connected together. The display shows 0 and beeps. Beeps sign appears and low battery sign disappears. Diode sign stays at its position.I have not tested the capacitance yet. If you have any ideas what can be damaged or any suggestions are welcomed.Many thanks and best regards from Germany,

author: Kleinstein
author_rank: Super Contributor
timestamp: October 05, 2017, 02:42:04 pm
content: The ignition usually works with more than 1000 V more like 10 kV.I would expect some of the input circuitry to be damaged. This could be MOVs that now turn on early. It's also possible that some resistors / semiconductors are damages. One could check if the input resistance is still at the usual 10 MOhms.

author: retiredcaps
author_rank: Super Contributor
timestamp: October 05, 2017, 06:35:44 pm
content: A similar case with a fried 27XT,http://www.antiqueradios.com/forums/viewtopic.php?f=8&t=166996As Kleinstein wrote, check the MOVs, they should measure infinite resistance with another meter.In the picture at antique radios, it looks like there is a spark gap near the v/ohm jack and possibly a PTC above the 10A fuse? That spark gap should measure infinite resistance and the PTC, I'm guessing, around 1.0k ohm.

author: Naturalizt
author_rank: Newbie
timestamp: October 06, 2017, 08:23:25 am
content: Hello everybody,I don´t possess a second DMM to test various active and passive components. I hope to tackle this issue in a couple of days.in the meanwhile I tried to search a schematic diagram of Wavetek 27xt. However was a difficult job as there is only Wavetek 27XL (?) with U10 TSC7106CKW chip. Any suggestion will be welcomed!

author: Naturalizt
author_rank: Newbie
timestamp: October 22, 2017, 03:10:01 pm
content: In the meanwhile, I have bought another DMM. Tried to find out the problem, replaced the HEF4053BT mux/demux IC, but in vain. Due to lack of time and other projects, this chapter is closed!If someone wants to have this DMM for spare parts or other purposes. Here is the linkhttps://www.ebay-kleinanzeigen.de/s-anzeige/wavetek-27xt-digital-ammeter-defect/741183490-168-8047Many greetings from Germany.

author: LoneTech
author_rank: Contributor
timestamp: February 01, 2018, 08:25:14 pm
content: It's probably too late to ask this but here goes...Do ya still have this defective unit?