WARNING! Repairs to monitors should
only be attempted by persons who are fully aware of working safely with high
voltages. The Hantarex monitor like all
monitors produce very high, lethal voltages up to 28,000 volts. There are mains
voltages on the degaussing circuit and 130V DC on the chassis. When working,
don't go in with both hands as this can make a circuit up the arms and across
the chest and heart! Keep one hand away. If testing on a bench, an ISOLATED
supply MUST be used. We can not be held responsible for errors in this document
or for injury or death resulting from use of this information. If in doubt,
leave it alone!
Introduction. The following information is related to the Hanterex MTC9000 and in some cases the MTC9110 which is very similar. It wont show you how to fix all faults, but may help in finding common problems associated with this chassis. Please do not contact us with individual problems, refer to this document or to other sources available on the WWW. Please report any errors to swallow@swallowam.co.uk
The MTC9000 is mechanically and electrically interchangeable with earlier Hantarex monitors MTC 900 and MTC 900E with respect to input signal, input power, deflection connections and fixing points. The CRT socket may need to be changed.
The MTC9000 features a very sharp and colourful display that is that is not found on other monitors. The MTC 9000 uses a RCA picture tube, a main feature which can be seen by looking at the neck. The purity and convergence ring assembly that is normally found just behind the yoke has been replaced by a non-adjustable ferrite sleeve. Instead of using a set of six magnetic rings to manually converge the picture tube, this CRT is converged by a computer during assembly. After adjustment, glue is injected between the yoke and the picture tube to hold the yoke in place. Once applied, the yoke cannot be removed! It becomes an integral part of the picture tube. The ferrite sleeve is then selectively magnetised by computer controlled electromagnets so that the red, green and blue beams from the electron gun are precisely aligned. This replaces the purity and convergence ring assembly, and eliminates the possibility of ending up with a poorly converged monitor.
What is the degaussing circuit for? If the screen becomes magnetised, the colours on the screen become impure. Patches of blue will become red for example. When the monitor is powered up, a diminishing voltage is applied to the large coil wrapped round the front of the tube which demagnetises the tube.
What does LOPT stand for? Line output transformer. This produces all the voltages required for the game including the 25.000V.
What does EHT stand for? Extra High Tension. That is the 25.000 Volts.
How do I remove the EHT cap safely? You need to discharge any high voltage left stored in the tube. This can still hold a charge even after a few weeks of being switched off. You need to short the EHT cap to ground. This is best done with a long well insulated screw driver and a lead with crocodile clips at each end. Clip one end to the earth braid running round the front of the tube, and the other to the metal shaft of the screw driver. Now with one hand only, slide the end of the screwdriver under the EHT cap until you reach the metal prong. Any EHT will discharge with a "crack" to ground. Hold it there for a few seconds to be sure the tube has been fully discharged and keep it there while you remove the EHT cap. Repeat the procedure before replacing the cap as a charge can build back up in the tube even when disconnected.
How do I set the "Screen" control on the LOPT? A quick way is to switch the monitor on and set the brightness control mounted on the bottom left hand side of the chassis half way. Disconnect the video input plug and turn the screen control fully anti-clock wise. Look closely at the front of the screen (use a mirror if you can't reach round) then turn the screen control up until a white screen just shows. Now back off until it just disappears and the screen is black. The screen voltage is now correct. Reconnect the video and use the brightness control to obtain a clear picture. The Focus control can now be adjusted until a sharp picture is displayed.
Which monitor do I have? The following is a description of the main differences between the various chassis I have come across working with videos in the UK. Hopefully this will help you determine which monitor type you have. Read through the list and check all the items mentioned. They are in order, with the oldest ones first. We can supply manuals and most parts for all the monitors listed below.
MTC90. Very nasty! The chassis is divided into two halves with a large gap between them. The side with the LOPT has four removable boards and the other side has three removable boards. This is an old chassis and if you still have one working, you have done well.
MTC900. This has two main boards, one about half the size of the other. The main feature is the Brightness, Contrast and Degaussing controls mounted on a black plastic bracket. This are on a long fly lead and is either mounted on the chassis or remotely in the cabinet. This plugs into the smaller board on the left. There is also the main power supply board mounted vertically on the chassis side to the left. The scan coil plug is on the far right hand side.
There are two versions of this chassis. To determine which type you have, you need to look at IC3 mounted on a heat sink just behind the six set up controls on the main board. If you have a TDA1470 (pins on both sides) mounted on a "U" shaped heat sink you have version 1 and if you have a TDA 2653A (pins staggered on one side only) mounted on a "L" shaped heat sink you have version 2. (This is also used on the MTC900E, look at other features too). The manual for version two only reflects this in the circuit diagram, and not in the pictures or on the cover.
MTC900E. This chassis has two main boards of equal size. The left hand board contains the power supply, video and power connectors with the brightness and a triple gang contrast control next to it.. The right hand board has the single scan coil connector and IC2 (TDA 2593) is mounted left to right. There are also two versions of this chassis. The USA and European. The European version has two green connectors for the power and video. The USA version has the power lead hanging from the back with a two pin Molex connector and the video uses a six way connector for positive video but also has a three way connector behind this one for negative sync signals. They are the same as the Electrohome G-07 connectors which I assume are commonly used in the USA.
MTC9000. This is the chassis covered by repair guide article. It will run 14", 16" and 20" tubes. Looking from the back, to the left just behind the video input is a slide switch. There is the small removable control PCB in the centre containing 6 controls. The large heat sink round the LOPT has TR15 (BU508A), IC1 (TDA1670A), TR17 (BDX53A) mounted on the left and the large power resistor on the right.
MTC9110. This is the same as the MTC9000 chassis (above) with one major addition. A large fan mounted over the LOPT. Because of the additional power that is required to drive the 25" CRT, a fan has been mounted on the heat sink that supports the voltage regulator and the horizontal output transistor. A temperature sensor has also been mounted on the heat sink. Should the fan fail, the sensor will detect the rise in temperature and shut down the monitor.
Polo. This is a "Free Voltage" chassis which can use a
supply from 180V to 264V 50Hz. Europe or 80V to 130V 60Hz. USA. It will run any
tube from 10" to 33". Looking from the back, it is easily identified
by the large power supply cage on the left hand side.
Repairing the MTC9000 Chassis. Don't overlook the obvious. You could spend hours trying to find some curious fault when the obvious is staring straight at you. You don't need lots of expensive test equipment, just common sense and observation. Make sure all plugs are firmly seated and that the black earth wire to the neck board from the tube braid is always connected to the monitor.
1. Dry joints. Check the entire chassis for dry joints and cracked or burnt tracks on the PCB especially around the LOPT, power plugs and large components.
2. LOPT. After you have checked all the fuses, listen for the static build up when you switch on. If you don't hear this, is the large power resistor mounted to the side getting very hot? The high-wattage resistor (R105 330 ohms, 30 watts) that is connected in parallel with the power regulator transistor suffers when the Over Current Protection circuit (OCP) shuts down the regulator circuit due to a short. Current from the unregulated power supply passes through R105 to ground. This 30 watt resistor is now dissipates over 60 watts of power. That is why it burns and gets very hot. It should only get warm. If there is no static and the 330 ohm resistor gets extremely hot then there is a good chance the LOPT or the horizontal output transistor [section 5] has shorted out. Check for leakage between pins 1, 3, 9 of the LOPT and pin 5. Swelling or cracks on the side of the LOPT will confirm this. Hantarex does not manufacture their own high-voltage units and, unfortunately, they got a bad batch of units from the manufacturer. If replacing the LOPT, check R81 (4.7 ohm 9W) as this could also fail. There is a side-effect to the operation of the OCP circuit that can be somewhat confusing if you don't know what to look for. In just about every other type of monitor, this failure will blow the fuse but in the Hantarex MTC 9000, it activates the OCP. The fuse will not blow.
3. Check the 130V. The B+ voltage is best measured from black wire
link in the scan coil plug. The scan coils are the ones round the neck of the
tube. Place the red meter probe on the black link and the black probe to metal
chassis. Alternatively use test point 10 (TP10). This should read 130V DC and
should not vary by more than a few volts. If this is well below the 130V, three
things could be at fault.
A. C34, a 22MFD 160V capacitor. 2. C53, a 470MFD capacitor. You
will have B+ at 60-70Volts.
B. TR20, the TIPL762 (BU508) power transistor mounted on the chassis by
the 330 ohm resistor is open circuit. If you want to check it with a meter,
remove it first.
C. Check R110, a 33K ohm resistor at the top right corner. Lift one end
and measure it. If it needs replacing, use a 1/2 watt or higher resistor in
place of the 1/4 watt one fitted.
D. If the B+ voltage is to high and you have a wavy picture, TR20 may be
short circuit. Another cause of wavy screens is C53.
E. Another reason for a missing B+ 130V is the activation of the X-Ray
protection circuit. A reference voltage is produced from the LOPT and is fed
via a resistive divider to pin 8 of IC2 (TDA 2595). The voltage here is
compared with an accurate reference voltage and if the EHT exceeds 28KV, the
voltage at pin 8 operates a trigger circuit which inhibits the oscillator and
shuts the monitor down. This can be noticeable when the monitor comes on
briefly then shuts down. Temporally disable this circuit by lifting one end of
D10. Now apply power to the monitor. If the monitor now gives you a picture,
the X-ray circuit is activating. The problem could be in the voltage regulator
circuit. Never leave a monitor running with the X-ray circuit disabled. Check
C36, C37, C40 and C41.
4. Vertical Output. Check that TR15 (BU508A) has not gone open or short circuit by measuring it out of circuit. If you have no vertical sync. Check the value try replacing C18 (10MFD 25V).
5. Horizontal output. The horizontal output transistor TR15 (BU508A) is the hardest working transistor in the monitor. All the current for the horizontal deflection coil in the yoke and the flyback transformer passes through the this transistor. If this has shorted you will have a dead monitor.
If you have a horizontal white line across the centre of the screen,
then IC1 (TDA 1670A) could be at fault. Equivalents for IC1 are TDA 1670 and
TDA 1675. If it blows R94 (3.9 ohms) will burn out as well. If this resistor is
replaced, mount it away from the PCB as it can burn the board. The supply for
IC1 is 26V DC and appears on pin 2 via D5 (IN4004). (TP13). Is the adjustment
board at the back fitted correctly?
If you have limited control of the horizontal hold after 5-10 minutes, check
C38 (1MFD 63V), D5 (IN4004) and D4 (IN4148). If the problem is intermittent,
check C9 (100MFD 35V).
6. Other things to check. ZD1, a 12 Zenner diode if short circuit,
will stop the 12V the supply for the TDA 2595 oscillator.
If the supply fuse blows when you switch on, one or more of the rectifier
diodes could be short circuit. D19-D22. Any 400V 1.5A diode will do, (BYV96C or
IN4007).
If the brightness varies with the image and the 130V B+ is correct, check that
R90 (270K) is not open circuit.
Again if the 130V B+ is correct and you have white fly back lines across the
picture, check components associated with TR18 (BC639).
If you have a plain white screen, check the voltage across C205 on the neck
board. This should read around 190V DC. If not check R126 (47 ohm) and D15 (BYD
33G).
Insufficient width problems and picture varying in size can be caused by
C42.
Shimmering at the edges of the picture can be caused by C57 (1000MFD 35V). If
there is whistling as well check C34 (22MFD 160V).
R122 glows red hot. Lin coil open circuit.
Ticking noise with any supply voltage up to 128V. LOPT.
Exceptional width when all else fails. LOPT.
We can supply most parts for Hanterex monitor chassis including a cap kit.