[Phil Allison[_4_]]

Don Pearce wrote:


Phil Allison


I do keep a Simpson 260 on my bench and I use it often.


** I have a similar Japanese made multimeter that I use occasionally.

Taut band, 20kohms/V, 100kHz response plus dBm scale.

The ohms x 1 range will reliably forward bias semiconductor
junctions while in circuit.



For in-circuit testing I find it useful to have an ohms range that is
of low enough voltage that it can't forward bias a junction. I have a
separate range that does provide forward bias, and reads out a voltage
rather than a resistance.


** The diode check range ?

Limited current ( typ under 1mA) makes them near useless for in circuit and other work.


** FYI:

If you connect say a 100 ohm pot wired as a rheostat to the ohms x 1 range of an analogue multimeter, plus monitor the voltage across the probes with another meter ( say a DMM) you can arrive at DC voltages corresponding to various ohm readings.

Then you can carefully mark another scale just above the ohms one- in my case up to about 2.2 volts. Checks Ge and Si diodes, BJTS & Darlingtons and LEDs except with blue or white ones.

----------------------

When repairing an amplifier suspected of having blown BJT outputs & drivers, plus various small signal types and diodes in the SOA limiter - I found a meter that reliably forward biases junctions is needed to find the bad ones.

BJTs can fail with junctions shorted or open, plus C-E shorts - an analogue meter gives unambiguous readings with these conditions while a DMM gives mostly ambiguous ones, when testing in circuit.

Even if the DMM has a "diode check" range, the current is limited to under 1mA.
Either this is insufficient to bias the junction/s or a low value resistor in parallel the draws all the current.

Darlington power transistors read like single BJTs devices with "diode check" due to inbuilt resistors bridging B-E junctions.

High voltage BJTS ( eg horizontal defection ) have 30 to 100ohm resistors across the B-E junction internally.

Early Phase Linear models had output BJTS with inbuilt 70 ohm B-E resistors as well as 10 ohms across parallel sets on the PCB.


..... Phil