3.2.6 FEATURES OF THE DEVICE CONTROL UNIT OF THE VM. REPAIR METHOD OF UU

The control unit VM (hereinafter CU) performs the following tasks:

• Analysis of clock pulses from a computer and determination of the required mode of operation,
• Setting the operating frequencies of master oscillators for frame and line sweeps and linking them to clock pulses,
• Reception of signals for correction of raster parameters in accordance with the set mode,
• Signal processing from other nodes to protect the CRT and IP in emergency situations,
• Providing the operator with access to a set of tweaks on the front panel of the VM.

Diagnostics of control devices with the use of MPs are carried out by techniques adopted in microprocessor technology, namely, by measuring the logical levels of signals using an oscilloscope and observing the expected response to a change in control signals.

At the first stage, the supply voltage (in most cases +5 V) and the presence of the clock frequency, as well as its correspondence to the frequency of the quartz resonator, are checked. The clock frequency control is carried out with an oscilloscope on one of the resonator leads, while the generation can fail, then try to observe the signal on the other lead or plug a capacitor into the probe circuit

with a capacity of 20-100 pF. The frequency is determined by measuring the period of the signal on the oscilloscope screen and then calculating it (F = 1 / T); however, this does not require greater accuracy, but you need to make sure that it is close to the frequency of the resonator. The frequency mismatch or absence of generation indicates a possible defect in the resonator (this is checked by replacing it) or the MP itself.

Then, to make sure that there are no reasons interfering with the operation of the MP, check the state of the RESET signal. Usually the active level of this signal is low, for its formation they use a simple RC-circuit, sometimes a transistor. The presence of a high level at the output indicates the operational status of the MP.

Figure 58 - Typical control unit diagram

Further, if there is a circuit diagram of the VM, they control the most important for its operation signals at the terminals of the MP: input (from control buttons, clock signals, protection signals) and control signals (going to actuators in other nodes). Since most of the applied MPs are made using CMOS technology and have a supply voltage of +5 V, the high level voltage is close to it and is 4.5 - 5V. Intermediate levels of the observed signals on any conclusion indicate a defect in the MP or in the circuits connected to it.