Radio Shack Digital Multimeter Cat # 22-811

[CapSnap123]

I'm trying to fix a digital multimeter. It was working until I blew the fuse (actually the "buzz" for continuity wasn't working). I've now replaced the fuse but it's acting strangely. When I turn the dial to V, nothing happens. But when I turn to anything else, it makes a continuous, very fast clicking sound - like the piezo buzzer is trying to do something. And at each position of the dial (other than V) there are brief flashes of life on the LCD. It doesn't seem to matter if the probes are connected. I wasn't sure where to start, so I've tested most of the diodes and cleaned the zebra connector on the LCD with alcohol (without removing it). But no change. Any suggestions?

[stj]

pictures are a usefull start

[CapSnap123]

Here are some images.

[stj]

check all the transistors for shorts - they may be used as protection devices.
and clean the switch tracks and see if they are burned - i'v seen that before after an over-current situation.
put silicone grease on them afterwards - it prevents arcing.

[harp]

"it makes a continuous, very fast clicking sound".................. if is about 3 or 4 times per seconds, that may be represent a refresh rate of reading, and I guess some short maybe, if LCD readings go off simultaneously with clicking... Check whats going on processor supply voltages, is stable, or follow clicking?

[CapSnap123]

check all the transistors for shorts

Is it possible to do so in-circuit with just an analog VOM? This is my only DMM.

[stj]

yes, just check resistance between the pins.
do you have a manual?
radio shack stuff often had schematics in the manual

[CapSnap123]

I have the manual but it doesn't have a schematic and I haven't been able to find one online either.

[CapSnap123]

I checked the resistance of all the transistors (everything marked Q with 3 pins), but I'm not sure how to interpret the results. Some only had a low resistance (around 6 ohms) between 2 of the pins. A bunch had low resistance between multiple pins, but couldn't that be due to parallel components?

[stj]

it could be other components but it still sounds suspect.
a good transistor looks like 2 diodes on a multimeter,