23 Apr
2009
23 Apr
'09
11:21 a.m.
On Thu, 2009-04-23 at 22:25 +1200, Mike van Bokhoven wrote:
Good idea - my description there did go a bit awry; I meant to write
something along the lines of 'if the transistor isn't obviously
faulty...'. Another thing I'll have to remember how to do, I haven't
tested a transistor since maybe 1990. Oh well. I'll try to refresh my
memory tomorrow. Don't forget that an interest in classic computers and
an ability to puzzle through simple schematics doesn't necessarily mean
someone knows one end of a transistor from the other!
The most basic test is to remembner that a transistor has 2 diode
junctions, one between base and emitter, the other between base and
collector (no, you _can't_ make a transistor from 2 diodes!). So you can
test each of those junctions as a diode using either an analogue
ohmmeter, or more likely these days the diode-test range of a DMM.
I'll assume you're using the latter. Desolder the transistor form the
circuit and connect the meter, on the diode test range, between the base
and emitter. Then try it again with the proes the other way round. One
way should read about 0.7V (forward biased Si junction), the other should
be 'overrange'. Then do the same tests with the meter conencted both ways
between the base and collector terminals of the transistor. Again, one
way should be 0.7V, the other way 'overrange'.
If either junction measures 'overrange' both ways round, it's open. If it
reads 0V (or 0.7V) both ways round it's shorted.
This will actually find a lot of defective transistors. Most failures end
up with one of the junctions open-circuit.
If a transistor passed this test, you need more complex equipment to test
it. Basically you bpass a small current trhough the base-emitter juction
and see how it affects the collector current -- in other words you use
the transisorr as am amplifier. Real enthuisats have a thing called a
'curve tracer' which does this automatically and plots one of a range of
voltage-current or current-current traces on a CRT. But I find my Tekky
575 (such an instrument) rarely gets used when I'm faultfinding, the
defective transsitor can be found with simpler tests.
3 - How
can I check? Could I pull the 5.6K resistor and the suspect
transistor, replace the resistor with a 5K pot set to 5k initially, and
wind the pot down a little, see if the picture comes back in some form?
Good idea, or misguided?
You could test the transistor, or even replace it.
Gordon
Also, replacing the transistor was something I intentionally left out of
my earlier mail - the part number is 60304e, both on the schematic and
the component. Google doesn't know what to do with that, and neither do
I. Hopefully there will be a close-enough alternative. Does it sound
familiar to anyone?
I don;t recognise that sort of number at all. US bipolar transistors tend
to have numbers starting 2N, European ones are things like 'BCxxx or
BFxxx' and Japanese ones start 2SA..2SD (often the '2S' is omitted, but
you still start with a letter).
My suggestions is to look at the maximum voltage in this part of the
circuit (is it just 12V'? and to pick a small-signal transistor of the
right polarity (NPN or PNP). The well-known 2N3904 (NPN) and 2N3906 (PNP)
would proalby do.
-tony