Chapter 5
Servicing

5.1 Service Philosophy

After the VT52 has been installed and checked out at the customer’s site, it can be expected to operate error free for long periods of time. As with all electronic devices, however, the VT52 will occasionally require some corrective maintenance due to a failing or marginal component. When the terminal requires service, it is recommended that service be performed at the subassembly level; the fault should be isolated to a module or other major component and the faulty part replaced with a completely tested spare. If the failing part is a module, it should be returned to an authorized DIGITAL repair depot where facilities are available for efficiently locating the faulty component. After repair and complete testing, the module can be returned to Field Service stock.

With the exception of the Monitor/Power Supply module, all data and control circuitry in the VT52 depends upon the microprocessor for timing and/or data information. Without specially designed test equipment, field testing this circuitry to the component level is difficult and time consuming and is not recommended.

NOTE
The spares kit consists of a complete set of modules for the VT52.

5.2 Subassembly Level Troubleshooting

To find and repair the cause of a failing terminal, the serviceman must first identify the faulty subassembly and then replace it with a good spare part. Replacement procedures for all major subassemblies are described in Paragraph 5.3. To identify the faulty subassembly, the serviceman must analyze the symptoms and make a judgment as to which module or major component is at fault. Replacing the suspect subassembly will fix the problem or at least eliminate that subassembly as a possible cause of the problem.

5.2.1 Identifying the Faulty Subassembly

Identifying the faulty subassembly will be fairly easy in some cases and quite difficult in others. Failures to the Monitor/Power Supply module will usually result in an obvious display malfunction, i.e., loss of horizontal or vertical deflection, no display, etc.

In some cases, the serviceman will be able to decide which subassembly is failing by analyzing the symptoms displayed on the screen. In other cases he will have to make additional keyboard tests and/or voltage and waveform checks to determine the failing subassembly. To assist you in making this decision, timing waveforms and other test data are included in this chapter.

The PMK04 Off-Line Terminal Tester can be used to exercise serial terminals and assist the serviceman in isolating faults within the terminal or communications interface. (Refer to PMK04 manual).

5.2.2 Analyzing the Symptoms

To start with there are some basic checks the serviceman should make before disassembling the unit.

1. Is the unit turned on?
2. Is the unit plugged into the outlet?
3. Is the intensity level set too low?
4. Are the baud rate switches set to legal positions?
5. Are the send/receive baud rates of the terminal set to the receive/send baud rates of the host?

After determining that the terminal is properly set up for operation and still does not work, the unit should be placed off-line and some keyboard tests made to develop a list of symptoms and possible causes. The more symptoms the serviceman has to work with, the better the judgment that can be made as to which part is failing.

NOTE
The possible equipment malfunctions listed in Tables 5-1 and 5-2 are categorized by abnormal symptoms. Corresponding causes are listed in order of probability. Corrective actions are for the most part limited to the adjustment and/or substitution of major replaceable subassemblies.

1. After warm-up, is the cursor at HOME position and blinking?

   If not, go to step 2.

   If the cursor is at HOME position, the serviceman can assume that the CRT and its associated components and the Monitor/Power Supply module are all functioning normally. Further testing is required (Table 5-2).

2. Is there a raster on the screen when the intensity level is set for maximum brightness?*

   * On some units the raster is not visible even with the intensity control set for maximum brightness.

   If there is no raster, see Table 5-1.

   If there is a raster, the serviceman can assume that the MONITOR/PS module and the CRT and its associated components are alright. A possible exception is the video amplifier circuitry on the Monitor/PS module.

If further testing is required to locate the problem, the unit must be tipped back and the base removed. The base removal procedures are described in Paragraph 5.3.

Table 5-1 Power On Troubleshooting Procedure

Symptom	Possible Cause	Corrective Action
No cursor	Intensity control turned down	Adjust intensity control (Figure 5-1)
No cursor, raster missing	Blown fuse	Replace fuse (Figure 5-1)
No cursor, raster missing	No prime power	Restore source of prime power
WARNING There are high voltages present in the power supply and CRT. Make sure the terminal power line is unplugged and the CRT and capacitors are discharged as shown in Figure 5-2 before handling the power supply or CRT components. Make sure ground connection is made before discharging these points.
No cursor, raster missing (Cont)	Anode cap disconnected	Connect anode cap at power supply (Paragraph 5.3.3)
	Anode clip disconnected	Connect anode clip at CRT bell (Paragraph 5.3.3 and Figure 5-3)
	Defective monitor/power supply assembly	Replace monitor/power supply assembly
WARNING Use caution when handling the CRT to avoid shattering the tube. The CRT is an evacuated device which can implode when broken.
	Defective CRT	Replace CRT (Paragraph 5.3.5)
	Defective RUT module	Replace RUT module (Paragraph 5.3.2)
	Defective DP module	Replace DP module (Paragraph 5.3.2)
	Defective character generator module	Replace character generator module (Paragraph 5.3.2)
WARNING There are high voltages present in the power supply and CRT. Make sure the terminal power line is unplugged and the CRT and capacitors are discharged as shown in Figure 5-2 before handling the power supply or CRT components.
	Defective monitor/power supply assembly	Replace monitor/power supply assembly (Paragraph 5.3.3)
No cursor, raster present, arcing heard	Anode cap loose	Tighten cap (Figure 5-2)
	Anode clip loose	Tighten clip (Figure 5-3)
Cursor present, but display flickers	W7 jumper on RUT	Remove W7 jumper (60 Hz units) or insert jumper W7 (50 Hz units) (Figure 5-4)

Table 5-2 Operational Troubleshooting Procedure

Symptom	Possible Cause	Corrective Action
Unable to adjust focus; display remains fuzzy	Defective monitor/power supply board	Replace monitor/power supply board (Paragraph 5.3.3)
Entire raster too short (Figure 5-5)	Vertical size/linearity out of adjustment	Adjust vertical size/linearity control (Paragraph 5.4.1 or 5.4.3)
Unable to adjust raster or character width	Defective monitor/power supply board	Replace monitor/power supply (Paragraph 5.3.3)
Entire raster too narrow (Figure 5-6)	Horizontal size out of adjustment	Adjust horizontal size control (Paragraph 5.4.2)
Unable to adjust intensity/brightness	Defective monitor/power supply board	Replace monitor/power supply board (Paragraph 5.3.3)
	Defective CRT	Replace CRT (Paragraph 5.3.5)
Characters too short (Figure 5-7)	Vertical size out of alignment	Adjust vertical size control (Paragraph 5.4.1)
	Vertical linearity out of adjustment	Adjust vertical linearity control (Paragraph 5.4.3)
	Defective monitor/power supply board	Replace monitor/power supply board (Paragraph 5.3.3)
Illegal characters generated (Figure 5-8)	Defective character generator board	Replace character generator board (Paragraph 5.3.2)
	Defective RUT module	Replace RUT module (Paragraph 5.3.2)
Random characters generated or screen fills with random characters when first turned on (Figure 5-9)	RUT/Character ROM/PROM chips are incorrect	Check that ROM/PROM chips are correct (Figure 5-10 and Appendix A)
	Defective character generator module	Replace character generator module (Paragraph 5.3.2)
	Defective DP module	Replace DP module (Paragraph 5.3.2)
	Defective RUT module	Replace RUT module (Paragraph 5.3.2)
Figure 5-10 ROM Chip Location
Wrong character displayed when typed	Defective keyboard	Replace keyboard (Paragraph 5.3.4)
	Defective keyboard character generator	Replace character generator (Paragraph 5.3.2)
No key-clicks; cursor present	Defective RUT module	Replace defective module (Paragraph 5.3.2)
No key-click or cursor, raster present	Defective DP module	Replace defective module (Paragraph 5.3.2)
	Defective keyboard	Replace keyboard (Paragraph 5.3.4)
No key-clicks; characters displayed	Defective DP module	Replace defective module (Paragraph 5.3.2)
	Defective RUT module	Replace defective module (Paragraph 5.3.2)
No key-clicks; characters generated wrong but legal	Defective DP module	Replace DP module (Paragraph 5.3.2)
	Defective RUT module	Replace defective module (Paragraph 5.3.2)
	Defective keyboard	Replace keyboard (Paragraph 5.3.4)
	Defective keyboard character generator	Replace character generator (Paragraph 5.3.2)
Entire raster tilted (Figure 5-11)	Incorrect yoke position	Adjust yoke tilted on the neck of the CRT (Paragraph 5.4.1)
Raster too large; all characters enlarged and fuzzy (Figure 5-12)	Incorrect yoke position	Push yoke forward to the CRT bell as far as it will go. Tighten hold down screw.

5.3 Subassembly Replacement

The following paragraphs provide step-by-step procedures for swapping modules and major components in the VT52 terminal.

5.3.1 Removing the Base Assembly

To remove the base assembly refer to Figure 5-13 and perform the following steps.

1. Make certain the unit is not plugged into an electrical outlet and the power (ON/OFF) switch is OFF.
2. Place the unit upside down on a bench or table.
3. Unscrew the sliding cover (two screws) covering the line cord opening.
4. Remove the base front screen (three screws).
5. Remove the screws (as many as ten) holding the rear louver plate on (Figure 5-15).
6. Remove ground leads.
7. Slide the interface cable and its rubber grommet from the slot in the rear screen.
8. Unfasten the ground connection from the rear of the base screen (one screw).
9. Remove the base assembly from the shell by first turning the five, ¼ turn fasteners 90° counterclockwise. Notice that the five ¼ turn fasteners are not removed but merely turned, using a ¼ inch nut driver, 90° counterclockwise to loosen, and 90° clockwise to tighten.
10. Reverse steps 1 through 9 to reassemble.

To remove the base assembly from older units,* do steps 1, 2, 3, and 4. Then remove the base-securing screws with a ¼ inch nut driver and lift the base from the unit.

* Up to and including the following serial numbers: Westfield serial number 3657, Phoenix serial number 5400.

Figure 5-14 is a bottom view of the terminal with the base cover removed. J1 and J2 on the ROM, UART, and Timing (RUT) module connect directly to J1 and J2 on the Data Path, Memory, and Decoder (DP) module. The keyboard is cable connected to RUT connector J5. The Monitor signals are cable connected to DP-J8; the Power Supply is cable connected to DP-J9.

5.3.2 Replacing the ROM, UART and Timing (RUT) or Data Path, Memory and Decoder (DP) Module

The RUT module and the DP module must be removed and replaced as a unit; they should not be separated until they have been removed from the terminal. To replace the RUT/DP modules, refer to Figure 5-14 and do the following:

1. Remove the base assembly from the shell (Paragraph 5.3.1)
2. Unplug the EIA or 20 mA adapter card, the keyboard connector, and the two connectors to the power supply board.
3. Unscrew nine small plastic wing nuts.
4. Remove two 8-32 × ¼ inch aluminum standoffs.
5. Pick up the assembled board set from the unit and set aside. Separate the RUT module from the DP module being careful to save the small plastic clip holding the corners of the two boards.
6. A board set must be reassembled (including the small plastic clip) outside the shell and installed as a set. It is installed by sliding the front board slots under the three front ¼ turn fasteners and over the 8-32 studs.
7. The plastic wing nuts can be screwed on or pushed on with a smaller nut driver.
8. If the character generator is to be removed from the DP module, unplug the assembly. (The character generator is secured to the DP module by its connectors).

To replace the RUT and DP modules reverse steps 1-8.

The following steps outline the procedure to be followed when removing the RUT or DP module from earlier terminals (Westfield serial number 3657 and lower; Phoenix serial number 5400 and lower).

CAUTION
When replacing the RUT and DP modules, make sure that connectors are aligned properly before applying pressure. Observe that color-coded wires on both sides of the connectors match and, where indicated, pin 1 is located at the extreme left of the connector as shown in Figure 5-14. If the character generator has been removed, make sure the arrow on the assembly points to the rear of the unit.

5.3.3 Replacing the Monitor/Power Supply Module

WARNING
There are high voltages present in the power supply. Make sure that the terminal line cord is not plugged into a source of power when servicing, removing, or replacing power supply assemblies or components. Using a screwdriver or clip lead, discharge the filter capacitors by shorting the capacitors to the assembly chassis or any suitable ground. Make sure ground connection is made before discharging the capacitors.

Refer to Figure 5-15 and 5-16 and do the following:

1. Remove the base (Paragraph 5.3.1)
2. Remove the RUT and DP module set (Paragraph 5.3.2)
3. Remove screws (as many as ten) holding the heat sink (louver plate) plate on. Set heat sink aside.
4. Remove ground leads.
5. Disconnect 9-pin Mate-N-Lok connector.
6. Disconnect the socket from the neck of the CRT and the Faston connector on the black ground wire from the braided strap.
7. Disconnect the 6-pin Mate-N-Lok that connects the yoke to the Monitor/Power Supply board.
8. Disconnect the four wires from the two large capacitors. Note that the red wires go to the positive terminal.
9. Remove the two small plastic clips holding the left and right corners of the power supply board.
10. Remove one screw from the middle of the heat spreader. Note that there is a small plastic spacer between the power supply board and the shell. The spacer must be saved and reinstalled with the new board.
11. Remove the power supply assembly.
12. Install a new power supply assembly being careful that the two lower corners fit into the molded-in slots and that the plastic spacer is used between the power supply board and the shell.
13. The small plastic clips that hold the top corners are installed using the outer slot of the clip on the left corner and the inner slot on the right corner (right and left are defined when facing the rear of the upside down unit).

The following steps outline the procedure to be followed when removing the Monitor/Power Supply module from earlier VT52 terminals (Westfield serial number WFO 3657 and lower; Phoenix serial number PNO 5400 and lower).

1. Remove the Data Paths, Memory, and Decoders module (Paragraph 5.3.2).
2. Remove the 10 screws holding the heat spreader to the large heat sink.
3. Disconnect the yoke connector.
4. Disconnect the CRT socket connector.
5. Disconnect the anode connector from the CRT high voltage socket and then squeeze the side tabs on the plastic socket to remove it from the module.
6. Disconnect the power supply filter capacitors by unscrewing the four connectors.
7. Remove the module securing screw from the center of the heat sink.
8. Squeeze the tabs on the four plastic locking posts while exerting slight pressure on the etch side of the module.
9. Lift the module.

5.3.4 Replacing the Keyboard

Refer to Figure 5-17 and do the following:

1. Place the terminal upside down on a bench or table and remove the base assembly (Paragraph 5.3.1).
2. Remove the RUT and DP modules (Paragraph 5.3.2).
3. Remove the six hold-down screws from the keyboard module (Figure 5-17).
4. Lift the keyboard from the terminal.

5.3.5 Replacing the CRT Assembly

WARNING
There are high voltages present in the power supply and near the CRT. Make sure the terminal power line is unplugged and the CRT and capacitors shown in Figure 5-18 are discharged before handling the high voltage anode clip and the CRT. Use care when handling the CRT. The CRT is an evacuated device and could shatter if broken.

Refer to Figure 5-18 and do the following:

1. Remove the base (Paragraph 5.3.1).
2. Remove the RUT and DP board set (Paragraph 5.3.2).
3. Remove the keyboard assembly (Paragraph 5.3.4).
4. Unclip the connectors from the yoke assembly.
5. Remove one ground screw if it has the old arcing ECO or unclip one wire if it has the new ECO.
6. Remove the screws (as many as ten) holding the new CRT wire form.
7. Remove two plastic spacers before removing CRT assembly. Be sure to replace the spacers when CRT is reinstalled.
8. Lift the CRT assembly out.
9. Slide a new one in and reassemble the unit.

5.3.6 Replacing the Transformer Assembly

1. With the unit right-side up, remove the fixed copier cover by first removing the louvered heat sink plate (four screws) and then pushing the small plastic tab of the cover at the upper left-hand corner of the shell while lifting on the cover. Notice that the "snap-in" tabs are molded on and care should be taken when reinstalling the cover that they are securely fastened. The cover is securely on if, after installing it, you can lift the side of the unit off the table with your fingers under the serrated edge.
2. After removing the fixed copier cover, turn the unit on its back and remove the base (Paragraph 5.3.2) and the RUT and the DP modules.
3. Disconnect the cables leading to the power supply board.
4. Remove two 8-32 kep-nuts.
5. Slide out the old transformer assembly and replace it.
6. Reassemble the unit.

5.4 Display Alignments and Adjustments

Normally, except for the intensity level, no adjustments are required to the terminal after the initial manufacturing checkout and alignment of the unit. However, after replacing a CRT or deflection yoke, the display must be realigned and adjusted.

5.4.1 CRT Yoke Alignment

To align the CRT yoke, perform the following:

1. Remove both covers on the top of the terminal.
2. Loosen the Phillips head screw securing the yoke to the neck of the CRT (Figure 5-18). If the screw is inserted so that the head is not accessible (upside down), remove the RUT and DP modules as directed in Paragraph 5.3.2. Remove the Phillips head screw and insert it in the opposite direction so that the screw head will be accessible when the terminal is assembled. Reassemble the terminal but leave the top covers off.
3. Set up the terminal for local operation as described in Chapter 3 and type a screen full of characters.
4. Rotate the yoke until the display is straight. Do not center the display at this time.
5. Tighten the Phillips head screw until the yoke is snug.
6. Adjust the yoke tabs as necessary to center the display (Figure 5-18).
7. Replace the top covers.

5.4.2 Display Adjustments

All adjustments are located on the monitor/power supply board. Remove the rear louver plate assembly as described in Paragraph 5.3.3. Set the unit to the off-line mode of operation as described in Chapter 3 and fill the screen with characters before making any adjustments. Refer to Figure 5-19 for locations of the adjusting coil and potentiometers.

5.4.2.1 Height

Adjust the vertical-size potentiometer (R98) until the height of the display is 114 mm or 4.5 inches. This measurement is from the upper edge of the top character line to the lower edge of the bottom character line.

5.4.2.2 Width

Using a non-conductive hexagonal-tipped alignment tool, adjust the iron slug in coil T3 until the width of the display measures 218 mm or 8.6 inches.

5.4.2.3 Vertical Linearity

Adjust the vertical linearity potentiometer (R92) until the characters displayed on the top line are the same size as the characters displayed on the bottom line.

5.4.2.4 Focus

Adjust the focus potentiometer (R71) until characters in all sections of the screen are sharp and clear.

5.5 Voltage Checks

If further testing is required to locate the problem, the unit must be tipped back and the base removed. To scope the monitor waveforms, the unit must be placed upside down with the heat spreader removed. The base removal procedures are described in Paragraph 5.3. With the unit tipped back and the base removed, the Monitor/Power Supply connectors J1 and J2 are accessible for measuring the voltages listed in Table 5-3.

CAUTION
Use extreme care when measuring voltages on J2 of the Monitor/Power Supply module. The CRT anode connector carrying 11,000 V is mounted close to this connector.

If any voltages are incorrect, remove the heat spreader and proceed to Paragraphs 5.5.1, 5.5.2, 5.5.3, or 5.5.4. If the voltages are correct, check the voltages after distribution to other modules.

5.5.1 Testing the NO VOLTAGE Condition

5.5.2 Testing the +15 V Supply

5.5.4 Testing the -12 V Supply

5.6 Timing Waveforms

Figures 5-20 through 5-28 are waveforms of the VT52 timing signals. Included in each figure are the signal location and the oscilloscope setting for observing the waveform. The serviceman should not assume that because a signal is present at the indicated pin location it is not suspect. It may be missing at some other point in the logic due to a broken etch or connection.

For instance, B OSC A, the basic timing clock, may be observed at J1-22 as indicated in the figure but may be missing at the input to the VSR.

Figure 5-20 B OSC A	Figure 5-21 B OSC A and Shift Register Output
Figure 5-22 Time States TE and TF	Figure 5-23 Time States TE and TW
Figure 5-24 Time States TE and TG	Figure 5-25 Time States TE and TH
Figure 5-26 Time States TE and TJ	Figure 5-27 Sync L
Figure 5-28 Vert H

5.7 Monitor Waveforms

The signals illustrated in Figures 5-29 through 5-39 show the normal Monitor waveforms; the waveforms one would observe in a working VT52 terminal. If the serviceman suspects that the Monitor/Power Supply module is the cause of some failing condition, he can confirm or reject his suspicions by comparing the waveforms on the failing unit with the waveforms illustrated in the figures.

Although component level troubleshooting is not recommended when servicing most of the circuitry in the VT52, the Monitor/Power Supply module can in many cases be serviced to this level because it is not completely dependent on the microprocessor.

Figure 5-29 Test Point 1

Figure 5-30 Test Point 2

Figure 5-31 Test Point 3

Figure 5-32 Test Point 4

Figure 5-33 Test Point 5

Figure 5-34 Test Point 6

Figure 5-35 Test Point 7

Figure 5-36 Test Point 8

Figure 5-37 Test Point 9

Figure 5-38 Test Point 10

Figure 5-39 Test Point 11