Tenma 72-450 AC Millivolt Repair help? (GW Instek GVT-417B also?)

[Mixographer]

Hi All,

I have a new to me Tenma 72-450 AC Millivoltmeter (same as, or similar to a GW Instek GVT-417B) that needs help. When I turn it on, the meter bounces to mid-level and back over about half a second, and then goes to full deflection. The range switch has no effect on this behavior. Leads connected or not, has no effect. Looking inside, everything looks good visually. It is really clean and all the caps look good, again visually. The relay works correctly as far as I can tell.

Does anyone have a schematic or service manual for either these units. The manual list the only troubleshooting is to put in a new input fuse. There are many test-points on the board, but without a service manual, I don't know what I'm looking for. Has anyone seen a similar issue or repaired one of these?

Thanks!

Jimmy

[Mixographer]

This may also be a copy of a Leader 181 AC Millivoltmeter.