18
Localizing the Problem:
The key to successful troubleshooting is to localize the problem as much as possible before trying to
pin the problem down to a specific component. Certain questions should be asked such as "Does
the problem occur on all ranges or on a specific range only?” The power supplies are also one of
the first things that should be checked.
As it is not possible to anticipate all failure modes of the 4021, servicing personnel should become
familiar with this section to gain a complete understanding of the internal workings of the Model
4021.
Component Replacement
If the problem has been identified as a faulty component, the accuracy of the 4021 can be
maintained only if the following precautions are taken:
a)
Use only the specified component or its exact equivalent. Spare parts can be ordered
from your nearest Valhalla Scientific Service Center or directly from the factory by
referring to the Valhalla Stock Number listed in the Parts Lists section at the back of this
manual.
b)
Use only 63/37 grade rosin core electronic grade solder with a 50W or lower maximum
power soldering iron.
c)
When soldering, heat the terminal of the component, not the solder. Apply solder
smoothly and evenly. Do not move the component until the solder has cooled. Bad
solder joints can cause additional problems!
d)
Static sensitive parts require special handling procedures. Always treat an unknown part
as if it were static sensitive.
Circuit Description
The circuit descriptions which follow are referenced to Figures 2, 3, 4 and the schematic
diagrams in Chapter 11. In the following descriptions, references to integrated circuits are given
in the form "IC2-1", which refers to Integrated Circuit 2, pin 1.
Power Supply
The 4021 operates from 4 supplies: +5V, +5Vb, -5V and -10V. The +5V supply is rectified by
D1 and D2 and regulated by IC1 in AC units. The +5V supply is provided directly by the
batteries in DC units. The other power supplies are identical in either AC or DC units.
The -10V supply is the result of a DC-to-DC converter composed of Q1, Q2, T1, D5, D6 and
associated components.
