Memotech MTX Power Problem

Power Transistor Q4, TIP 2955, Failure

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 VERY HOT

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 Operator's Manual

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.