Mastech 9803R Repair Help

[larrybud]

I purchased a Mastech 9803R benchtop meter off ebay for "parts or repair" and everything works on it except two odd issues, which I suspect are related:

• Capacitance reads half the expected value
• The voltage drop when testing a diode is twice the expected value

I opened the unit up, checked for the common culprits (bulging caps, cooked resistors, cold solder joints) but didn't notice anything.

First, does anybody happened to have a schematic of this thing, and if not, any advice on what to look for?

Also, this uses the NJU9210 chipset.  I found the datasheet for that and adjusted the 1v reference pot, and the DC/AC. While I don't have a super accurate reference voltage, I'd say I'm close enough where the diode drop and capacitance values should be within a few percent. Starting to wonder if the chip is faulty.

[sean0118]

Have you checked that the DC volt mode works on all ranges? Could be one of the relays / reed switches or resistor network has died?

I suspect you're right and the two issues are related. If it's charging the capacitor with a fixed current and has a false reading of double the voltage the capacitance will be half expected. I = C(dV/dt)

[MathWizard]

If you have the time, I'd try and find the sections on the DMM, the parts and pins associated with the diode/cap mode. I would do it in something like LTSpice, and then it's easy to see to sometimes if removing 1 resistor or other part, or shorting some cap or diode, etc, could double or halve some current or voltage.

If you're lucky, the problem is outside the main chip. Here's the datasheet for the IC, there's probably schematics out there for full DMM's with the IC or something similar. Or might be bigger datasheets with more design guides.


I think a lot of diode modes work like resistance modes, by sending out (or sinking) some reference current, like 2mA, and then measuring some voltage drop. And they have some parallel resistance with the current source, to clamp it at 4V or whatever, when the probes are open circuit.

If you have another DMM, measure the output I, V, or you could make some circuit (if you have the parts and some know how), to get a rough idea of the current and voltage from the DMM.

[larrybud]

I did a little more investigation and started checking every cap on the board.

There are two of these caps (see attachment). One reads "103JG" and measures 10nf (in circuit).

The attached reads "103JU" and I get no reading with my DMM or ESR (overflow) in circuit, so I pulled it out, and I do get a 10nf reading.  Odd that it reads OPEN when installed?