DVM circuits

[barry wilkins]

Unforunately I damaged one of my DVM,s when working on some power supplies
measuring some high voltages ,because there was some rf on the dc I was measuring.
It still works but is noisey and it will not measure diodes, I would like to fix it if possible.
Does any one have any circuits for DVM,s

thanks

Barry Wilkins

[redwire]

Re: DVM circuits

There's lots of DVM schematics out there, if you have a make/model or pictures of the board it can help figure out the DVM chip used and what parts might be damaged.

Usually a low cost DVM overloaded (by voltage) will damage its input protection clamp diodes, so you always get a low reading on ohms or diode test, when no leads are connected. Just speaking generally.

[barry wilkins]

Re: DVM circuits

Hi attached some pictures cant see any makers name on it .Fixed the noise problem by cleaning the switch ,just seems a problem trying to measures diodes ,very low voltage applied to the device .
There are two op amps ,dual and a quad .
Could it be a problem with the LM324 ?
Bought a new one from Amazon that had 1000 V on it ,was rubbish didnt have any sockets fitted for the transistors, so it returned it.
Got a different one which is better.

Barry

[stj]

Re: DVM circuits

Bought a new one from Amazon that had 1000 V on it ,was rubbish didnt have any sockets fitted for the transistors, so it returned it.

Barry

lol
there is a running joke that any meter with a transistor (HFE) socket on the front is junk aimed at hobbyists.

[redwire]

Re: DVM circuits

The PC board is "MX-480D-B" but no hits.
I don't see the usual two-transistor clamp or a PTC thermistor. Low cost DMM's sometimes have missing protection circuits and they easily die.
I would (using another multimeter) test the six black diodes. Otherwise, all that remain are the LM324 and the big IC.

hFE socket is also supported by the ANENG/Dreamtech chips, is in DT830 and even Harbor Freight freebies. I don't mind it.

[R_J]

Re: DVM circuits

It is likely a Maxcom MX480 multimeter, or copy. There is a blue PTC1 located close to the board number

[zx8401]

Re: DVM circuits

Looks like a dt890 to me, there are a few versions A B C D E F.

[barry wilkins]

Re: DVM circuits

Hi thanks to all of you for the info ,I will look at it in a few days .
Not too bothered about fixing it as I have quite a few other meters
1 I keep my car 1 in my tool box 1 in my caravan, 3 in my little workshop , a TEK scope and a Hantek scopemeter.

regards

Barry

[barry wilkins]

Re: DVM circuits

I could see there was something wrong around the TL 062 opamp ,so I removed it and replaced it with an 072 ,replaced the diodes and capcitors at the same time. A bit more investigation found R 36 was O/C.

Barry