WAVETEK 27xt DMM - problem

[Naturalizt]

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,

[Kleinstein]

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.

[retiredcaps]

A similar case with a fried 27XT,

http://www.antiqueradios.com/forums/viewtopic.php?f=8&t=166996

As 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.

[Naturalizt]

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!

[Naturalizt]

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 link

https://www.ebay-kleinanzeigen.de/s-anzeige/wavetek-27xt-digital-ammeter-defect/741183490-168-8047

Many greetings from Germany.

[LoneTech]

It's probably too late to ask this but here goes...Do ya still have this defective unit?