The Memotech MTX Series
Power Transistor Q4, TIP
This page provides details of a problem that can occur in the
power regulation circuit on the MTX computer motherboard. It is
assumed that the MTX AC Power Supply is working, the red power
light is on and the PSU low voltage AC outputs have been checked
and are within their expected ranges.
My MTX PSU page
shows details of the MTX computer power supply and regulator
circuits, including the expected PSU AC voltage outputs.
System Description : Any MTX Computer System Board
Problem Description : MTX not working and the large
wire-wound resistor on the computer board is running
This problem has been seen on at least two faulty MTX
computers, in both cases, the owner originally thought that
resistor R60 was running very very hot. A look at
the MTX power regulation schematic shows that R60
is associated with the -5VDC supply, which is only used by the
Video RAM. If this resistor was running hot, this would suggest
a problem with one or more of the VRAMs. However, in both cases,
the hot resistor was actually R62.
MTX Power Regulation Circuit from the MTX
Although the schematic has a couple of errors on
it (the correct component values are described on my
MTX PSU page), it
correctly shows the arrangement of R60 and R62.
Erroneously identifying the hot resistor as
R60 is understandable - the component ID on the
computer board for the wire-wound resistor (R62)
is directly below the component, making it hard to see with the
resistor in place. Immediately above the resistor, there is a "R60"
label which relates to the small film resistor to the right of
R62, below the video board output connector that
is used in the -5V circuit.
R62 is a 10 ohm, wire wound resistor, as shown
on the schematic, it is rated at 2.5W - much higher power than
the other resistors on the computer board that are rated at
0.25W. As described on my
MTX PSU page, in normal circumstances, with no faults in the
+5VDC power circuit, there will be a voltage drop across
R62 of about 0.65-0.7VDC, turning on Q4, the TIP
2955 Power Transistor used to augment the available
power (~1A) from the 7805 voltage regulator. The
Power Transistor is capable of supplying up to ~15A, so the
computer is protected against over current in the event of a
fault by the 3.15A fuse.
If R62 has a higher than normal temperature,
there is more current flowing through it than normal and you
should be able to measure an appreciably higher voltage across
it than the usual 0.65-0.7V. This is indicative of the TIP
2955 having failed and the 7805 regulator
trying to deliver more power than it is rated for.
|Overview of the MTX computer board.
The voltage regulators are located under the
large capacitor at the top of the board and R60 &
R62 between the ROM chips and the Video board at the
bottom of the board.
|Mark Kinsey posted this helpful photo on the
Memotech forum, Memorum.
It identifies the
locations of R62 and R60 on the MTX computer board,
close to the Video board.
|The upper edge of the MTX computer board,
adjacent to the UHF modulator with C56 removed.
The positions of the +12VDC regulator (7812), the
+5VDC regulator (LM7805) and the power transistor
(TIP 2955) can be clearly seen.