Polar Instruments T1500A User manual

Polar Instruments Ltd.
Garenne Park
St. Sampson
Guernsey
Channel Islands
GY2 4AF
ENGLAND
Fax: [phone]
MAN159–9604
T1500A
SERVICE
MANUAL

T1500A SERVICE MANUAL
WARRANTY
For a period of one year from its date of purchase new and undamaged
from Polar Instruments Ltd, POLAR INSTRUMENTS LTD or its authorized
distributors will, without charge, repair or replace at its option, this product if
found to be defective in materials or workmanship, and if returned to POLAR
INSTRUMENTS LTD or its authorized distributors transport prepaid. This
warranty is expressly conditioned upon the product having been used only
in normal usage and service in accordance with instructions of POLAR
INSTRUMENTS LTD and not having been altered in any way or subject to
misuse, negligence or damage, and not having been repaired or attempted to
be repaired by any other than POLAR INSTRUMENTS LTD or its authorized
distributors. EXCEPT FOR THE FOREGOING EXPRESS WARRANTY OF
REPAIR OR REPLACEMENT POLAR INSTRUMENTS LTD MAKES NO
WARRANTY OF ANY KIND, INCLUDING BUT NOT LIMITED TO, ANY
EXPRESS OR IMPLIED WARRANTY OF MERCHANTABILITY OR FITNESS
FOR ANY PARTICULAR PURPOSE, AND POLAR INSTRUMENTS LTD
SHALL NOT BE LIABLE FOR ANY DAMAGES, WHETHER DIRECT OR NOT
OR OTHERWISE, BEYOND REPAIR OR REPLACING THIS PRODUCT.
Copyright Polar Instruments Ltd. 1996

DECLARATIONS
ELECTROMAGNETIC COMPATIBILITY
European Community Directive Conformance Statement
This product is in conformity with the protection requirements of EC Council Directive
89/336/EEC on the approximation of the laws of the Member States relating to
electromagnetic compatibility.
A declaration of conformity with the requirements of the Directive has been signed by
POLAR INSTRUMENTS (UK) LTD
11 College Place
London Road
Southampton
England
SO1 2FE
This product satisfies EN50081-1:92 and EN 50082-1:92

T1500A SERVICE MANUAL
ii
SAFETY
WARNING The service instructions contained in this manual
are for use by qualified electronic service personnel only.
WARNING
The LIVE and NEUTRAL lines on this unit are BOTH fused.
When the unit is connected to its supply, the opening of covers or removal of panels is
likely to expose dangerous voltages.
GROUNDING
This unit must be earthed (grounded); do not operate the instrument with the safety
earth disconnected. Ensure the instrument is connected to an outlet with an effective
protective conductor terminal (earth). Do not negate this protective action by using an
extension cord without a protective conductor.
Note: This instrument is fitted with 3-wire grounding type plug designed to fit only into a
grounding type power outlet. If a special local plug must be fitted to the power cord
ensure this operation is performed by a skilled electronics technician and that the
protective ground connection is maintained. The plug that is cut off from the power cord
must be safely disposed of.
Power cord color codes are as follows:
Europe
brown live
blue neutral
green/yellow earth (ground)
United States
black live
white neutral
green ground
POWER SUPPLY
Check that the indicated line voltage setting corresponds with the local mains power
supply. See the rear panel for line voltage settings. Instruments with a serial number
prefixed with a letter (e.g. A1234) are configured for 90 – 110 Volts only.

SAFETY
iii
Changing the line voltage settings on this instrument must only be performed by a
skilled electronics technician. Instructions for changing the line voltage settings are
contained in Section 5.
OPERATION
This manual contains instructions and warnings which must be observed
by the user to ensure safe operation. Operating this instrument in ways
other than detailed in this manual may impair the protection provided by
the instrument and may result in the instrument becoming unsafe. Retain
these instructions for later use.
The unit is designed for use indoors in an electrical workshop environment at a stable
work station comprising a bench or similar work surface.
Use only the accessories (e.g. test probes and clips) provided by Polar Instruments.
The instrument must be maintained and repaired by a skilled electronics technician in
accordance with the manufacturer’s instructions.
If it is likely that the protection has been impaired the instrument must be made
inoperative, secured against unintended operation and referred to qualified service
personnel.
Protection may be impaired if, for example, the instrument:
•Shows signs of physical damage
•Fails to operate normally when the operating instructions are followed
•Has been stored for prolonged periods under unfavourable conditions
•Has been subjected to excessive transport stresses
•Has been exposed to rain or water or been subject to liquid spills
CAUTION
Electrical Isolation
Always disconnect the board under test from the local mains supply (including ground)
before using this instrument.
Static Sensitive Devices
This unit contains Static Sensitive Devices. Static discharge can damage some
electronic components. Care must be taken when handling these components. Observe
appropriate precautions to avoid damage.

T1500A SERVICE MANUAL
iv
SPECIFICATIONS
ASA Test Ranges Open Circuit Voltage Short Circuit Current
Junction 1V 500µA
Logic 10V 5mA
Low 10V 150mA
Med 20V 1mA
High 40V 1mA
Hi-Cap (1Hz fixed) 10V 150mA
Low-Cap (2kHz fixed) 10V 20µA
ASA Test Frequencies
Low 95Hz
Medium 500Hz
High 2kHz
Display Internal CRT
ENVIRONMENTAL OPERATING CONDITIONS
The instrument is designed for indoor use only under the following environmental
conditions:
Altitude Up to 2000m
Temperature +5°C to +40°C ambient
Relative humidity RH 80% maximum at 31°C — derate
linearly to 50% at 40°C
Mains borne transients As defined by Installation Category II
(Overvoltage Category II) in IEC664
Pollution Degree 2 (IEC664)
Power Requirements
230V ± 10%, 115V ± 10% or 100V ± 10% at 50/60Hz, 40VA.
Fuses
230V 160mA T
115V 315mA T
Channel A and B Output Fuses
160mA Fast

SPECIFICATIONS
v
Physical characteristics (excluding accessories)
Dimensions 300 mm (11.8 in.) wide
110 mm (4.4 in.) high
260 mm (10.3 in.) deep
Weight 1.5 kg (3.3 lb.)
SYMBOLS
REFER TO MANUAL These sockets are for connecting only Polar Instruments
probes and connectors for use as described in the
Operator Manual. To prevent damage to this product and
to ensure its safe use observe the specifications given in
this manual when connecting to terminals marked with
this symbol.
COM This terminal is internally connected to earth (ground).
ACCESSORIES
Standard Accessories
Probe set MMP159
Test clip set ACC110 (Red)
ACC111 (Black)
Operator manual MAN151

T1500A SERVICE MANUAL
vi
CONTENTS
DECLARATIONS............................................................................................................. i
ELECTROMAGNETIC COMPATIBILITY.......................................................................i
European Community Directive Conformance Statement..........................................i
SAFETY..........................................................................................................................ii
WARNING....................................................................................................................ii
GROUNDING ...............................................................................................................ii
POWER SUPPLY.........................................................................................................ii
OPERATION................................................................................................................iii
CAUTION.....................................................................................................................iii
Electrical Isolation.....................................................................................................iii
Static Sensitive Devices ...........................................................................................iii
SPECIFICATIONS..........................................................................................................iv
ENVIRONMENTAL OPERATING CONDITIONS ........................................................iv
Fuses........................................................................................................................iv
Channel A and B Output Fuses................................................................................iv
Physical characteristics (excluding accessories).......................................................v
SYMBOLS ....................................................................................................................v
ACCESSORIES............................................................................................................v
Standard Accessories................................................................................................v
SECTION 1 – PERFORMANCE CHECK .................................................................... 1-1
MAINTENANCE AND CALIBRATION OF INSTRUMENTS ..................................... 1-1
Performance Check............................................................................................... 1-1
Adjustment Procedure........................................................................................... 1-1
Recommendations for Routine Maintenance ........................................................1-1
PERFORMANCE CHECK ........................................................................................ 1-2
Procedure.............................................................................................................. 1-2
SECTION 2 – CALIBRATION...................................................................................... 2-1
CALIBRATION PROCEDURE.................................................................................. 2-1
Equipment required............................................................................................... 2-1
Calibration procedure............................................................................................ 2-1
SECTION 3 – CIRCUIT DESCRIPTION......................................................................3-1
T1500A CIRCUIT DESCRIPTION............................................................................3-1
Sine generator....................................................................................................... 3-1
Sine drivers............................................................................................................ 3-1
Switching...............................................................................................................3-2
Amplifier ................................................................................................................ 3-3

CONTENTS
vii
CRT....................................................................................................................... 3-3
Power supplies...................................................................................................... 3-3
High voltage supply............................................................................................ 3-3
Low voltage supplies.......................................................................................... 3-4
X and Y-plate drivers............................................................................................. 3-4
SECTION 4 – DISMANTLING INSTRUCTIONS ......................................................... 4-1
DISMANTLING THE T1500A ................................................................................... 4-1
SECTION 5 – LINE VOLTAGE AND FUSE CHANGING............................................ 5-1
LINE VOLTAGE SELECTION AND FUSE CHANGING........................................... 5-1
Changing the line voltage...................................................................................... 5-1
Fuse Replacement. ............................................................................................... 5-1
SECTION 6 – FAULT DIAGNOSIS ............................................................................. 6-1
TROUBLESHOOTING HINTS.................................................................................. 6-1
SECTION 7 – MAINTENANCE AND CLEANING........................................................ 7-1
Cleaning................................................................................................................ 7-1
Technical Support ................................................................................................. 7-1
Instrument repair................................................................................................... 7-1
SECTION 8 – REPLACEMENT PARTS...................................................................... 8-1
CRT BOARD PARTS................................................................................................ 8-2
MAIN BOARD/PANEL PARTS................................................................................. 8-2
MECHANICAL PARTS............................................................................................. 8-6
SECTION 9 – SCHEMATIC DIAGRAMS..................................................................... 9-1

1-1
SECTION 1 – PERFORMANCE CHECK
MAINTENANCE AND CALIBRATION OF INSTRUMENTS
For most of Polar Instruments’ products there are two maintenance procedures — the
Performance Check and the Adjustment Procedure. Some instruments may have a
single (combined) procedure.
Performance Check
The Performance Check (or Checkout Procedure) is used to verify the basic functions
of the instrument. This does not usually require the removal of instrument covers, but
may require the use of external test equipment. This procedure is not intended to verify
the calibration of the instrument.
Adjustment Procedure
The Adjustment Procedure (or Calibration Procedure) is used to check and, if
necessary, adjust the instrument’s calibration settings. Before carrying out the
Adjustment Procedure the instrument’s Performance Check (if applicable) should be
carried out, and any detected defects should be rectified. The Adjustment Procedure
and rectification of defects should only be carried out by qualified technician.
Recommendations for Routine Maintenance
Where a Performance Check is available for an instrument this may be used as
required to confirm the basic operation of the product.
To maintain the calibration of an instrument it is recommended that its
Calibration/Adjustment Procedure is carried out at intervals not exceeding 12 months.

T1500A SERVICE MANUAL
1-2
PERFORMANCE CHECK
Procedure
1. Connect the T1500A to a suitable source of power, turn on and allow
to warm up for 20 minutes.
2. Short A to COM and adjust the position controls to confirm their
operation. Set the trace for a “+” display. Check that the ends of the
traces can be seen — adjust trace intensity if needed. Check the
display is focused.
3. Check each range and frequency combination and confirm the display
maintains a cross. Some looping may be seen in LOCAP range.
4. Short B to COM instead of A. Check the display again for a cross —
check the “cross” display is maintained in all ranges.
5. If desired a further check on the ranges may be made by testing
components as follows:
RANGE TEST COMPONENT DISPLAY
HIGH 47KΩDiagonal Line
MED 22KΩDiagonal Line
LOW 56ΩDiagonal Line
LOGIC 2K2 Diagonal Line
JUNC 2K2 Diagonal Line
LOCAP 200pF Circle
HICAP 2,200µFCircle
Note: the quality of the component may affect the display seen — this
table uses commonly available parts to check that no major faults
exist in the product.

2-1
SECTION 2 – CALIBRATION
CALIBRATION PROCEDURE
WARNING: Hazardous voltages are exposed on the PCB when the cover
is removed with power applied and for up to five minutes after power is
removed. This procedure should only be performed by a technically
qualified person aware of the hazards and taking all reasonable care.
Equipment required
DVM
Insulated trimmer tool for adjusting preset potentiometers
Calibration procedure
1. Warm up the instrument for 20 minutes in LOGIC range and MED
frequency then remove the cover.
2. Measure the line voltage. If available use a variable transformer to set
it to the nominal line setting, e.g. 230V (or 115V).
3. Measure the voltage at R41 (4K7) and adjust R4 if necessary for 220V
at nominal line voltage.
If the line voltage is not at nominal value calculate the setting as
follows:
Voltage at R41 = 220V + (percent error from nominal line voltage x 20)
e.g., if the line voltage is 240V, this is 4.3% high,
∴Voltage at R41 = 307V
This optimises the range of the high voltage regulator.
4. Short A to COM
5. Adjust the intensity control to minimum. Adjust R241 (middle pot of the
three) on the CRT board for a very dim trace. Set the intensity to
normal viewing level.
6. Adjust R105 and R126 on the CRT board for a sharp, well focused
trace.

T1500A OPERATOR MANUAL
2-2
7. Adjust R27 to set the trace length of the horizontal trace so that both
ends are just visible.
8. Adjust R32 to align the trace with the horizontal graticule line.
9. Adjust the front panel Trace Rotation control to rotate the trace if
necessary to align with the horizontal graticule line.
10. Remove the short from A to COM and short B to COM.
11. Adjust R17 to set the trace length of the vertical trace so that both
ends are just visible.
12. Adjust R33 to align the vertical trace with the vertical graticule line —
adjust R19 to rotate it if necessary.
13. Refit the cover and retaining screws.

3-1
SECTION 3 – CIRCUIT DESCRIPTION
T1500A CIRCUIT DESCRIPTION
Sine generator
Refer to the T1500A Sine Generator schematic
Oscillator U10B runs at 2MHz and is controlled by crystal X1. This forms the master
clock which sets the frequency of all outputs and system timing. Binary counter U11
reduces the clock to the four source frequencies for the sine wave generator, 1MHz,
250kHz, 62.5kHz, and 488Hz.
Dual data selector U18 selects one of these frequencies and by its second “pole”
selects the display alternating clock. C9 supplies one cycle A then one cycle B while
C10 supplies two cycles A then two cycles B.
Synchronous counters U12 and U16 together form the address counter for Read Only
Memory (ROM) U13 which holds the sine wave in digital form. Waveform data bytes
are progressively clocked out to Digital to Analog Converter (DAC) U14 which produces
a differential current output at pins 2 and 4.
The magnitude of the current is set by R114, pin 14 being a virtual earth. U15B
converts the current to a voltage sine wave of 5V pk at pin 7. R201 and C66 form a
filter to remove transients from the converter at MEDIUM and HIGH frequencies and
C67 is added for LOW frequency by analogue switch U17A.
U15C converts the 5Vpk to 8Vpk to drive the output stages, while U15A reduces the
signal to 2.5V pk for internal use but also applies phase correction by C41 and C40
when switched in by JUNCTION range.
Sine drivers
Refer to the T1500A Sine Drivers schematic
Power op-amps U8 and U9 provide the drive for outputs B and A respectively for all but
MEDIUM, LOCAP and HIGH ranges. The op-amps’ gain is set by Rfof 20K and Riof
16K to amplify the 8Vpk reference to 10Vpk. Protection is provided by the 160mA fast-
blow fuses and the 11V Zeners which hold the output of the op-amp at a safe level until
the fuse opens. The 1R0 and 0.1uF snubber stabilises the feedback circuit.
The high voltage drivers are also op-amps, built with discreet transistors, with Riof 24K
and Rfof 162K amplifying the reference to just over 50V pk. The 3.3µF capacitor shifts
the DC level of the sine wave from about 60V to symmetrical about ground at the
output. D50 and D53 (D57, D54) balance the charge in the capacitor C35 (C36).
Protection is provided by varistor R98 (R99).

T1500A SERVICE MANUAL
3-2
Switching
Refer to the T1500A Switching schematic
Each range comprises a different combination of output voltage and source (series)
resistance. In addition, with the exception of the specialised ranges, a choice of three
frequencies can be made. LEDs confirm the button selection made and are interlocked
so that the specialised ranges which force the frequency also turn off the FREQUENCY
LEDs.
The voltages at the outputs are sensed by the Input Amplifiers (refer to T1500A
Amplifier schematic) whose Riresistors are selected by the switches. Note that there
are two Riresistors of 100K (R80, R81) and of 1M0 (R78, R79) and that the pair not
used are grounded so that they cannot affect the op-amps’ virtual earth inputs at “A
INP” and “B INP”.
In the remainder of this section we describe each range (Channel A components only).
Rifor the Input Amplifier (refer to T1500A Amplifier schematic) is R78 unless stated
otherwise.
For HIGH range the source resistor is a combination of R72, and the dividers R204,
R74 and R76.
The MEDIUM range selects the divider R76, R74, R204 to obtain 20Vpk at a source
resistance which is a combination of the divider and R71.
The LOW range uses the 10V source with resistors R54 and R55.
The LOGIC range uses the 10V source with R57.
The JUNCTION range obtains its 1V from divider R65, R60 which also provides the
source resistance. Note: this range uses R80, 100K, as Rifor the Input Amplifier.
The LOCAP range is a specialised range for low value capacitors and uses a unique
combination of resistance and frequency. Frequency is forced to the HIGH range by
pulling FREQA and FREQB lines low. Source resistance is R61 and R68 and voltage is
20V from the divider, however as the amplifier input is 1M0 the actual output is half this
at 10V.
The HICAP range is dedicated to large value electrolytic capacitors and uses the LOW
range resistors and voltage source but forces a very low frequency not available in any
other mode. This is performed by opening CKSEL from ground when S6 is pressed
allowing both FREQA and FREQB to go high.

CIRCUIT DESCRIPTION
3-3
Amplifier
Refer to the T1500A Amplifier schematic
The Input Amplifiers are comprised of U20C and U20D whose gain is controlled by
feedback resistors (Rf) selected by analog multiplexer U21. The gain is set so that the
peak voltage on any range will be 2.5V at XA or XB. Note that the input resistors (Ri)
for the op-amps are shown on the Switching schematic.
The outputs of these amplifiers drive the CRT X (horizontal) plates and are also
combined with the 2.5V reference sine wave, via op-amps U20A and U20B, to
generate the Y (vertical) signal. Phase correction is provided by C60 (C62) and is
increased in LOCAP range by the 330pF of C61 switched in by analog switch U17.
Analog switch U19 provides the alternating display of Channel A and Channel B on the
CRT by switching between them at the ALT_CLK rate. XDRIVE and YDRIVE are the
virtual earth inputs to the CRT drive op-amps (refer to the T1000A Power Supplies
schematic). R142 to R145 are the input resistors for the CRT drive op-amps; position
offset for channel A is supplied via R122 – R125. When Channel B is selected, to
prevent switching aberrations the Channel A source is held close to ground by diodes
D81, D82 for X and the “Z” switch element for Y.
CRT
Refer to the T1500A CRT schematic
The high voltage supply connects to the anode of D14, whose cathode in turn connects
to the CRT cathode. The CRT cathode is therefore 91V less negative than the high
voltage supply and this allows the grid to be biased from a potential divider between
cathode and supply. The divider consists of R108, R109 and potentiometer R241.
R240 connects to the CRT grid, the transistor of opto-coupler ISO1 is placed in parallel
with R108. By increasing the conduction of this transistor the grid can be driven more
negative and the CRT trace is dimmed. The range of this variation is set by R241 to
match the CRT characteristics.
Other bias voltages are obtained by dividers for control of focus and astigmatism. A
fast blanking path to the grid is provided by C55 but protection is provided by D69 for
start up and switch off transients.
Power supplies
Refer to the T1500A Power Supplies schematic
High voltage supply
All supply voltages are obtained from the mains transformer. Diode capacitor
arrangements supply the regulators for the low voltage and +180V supply but the high
voltage is generated by a multiplier from the 150 volt winding producing about –1500V,
depending on line voltage.

T1500A SERVICE MANUAL
3-4
When the high voltage regulator is operating, the supply to the CRT is at about –1400V
so the +ve side of the –1500V multiplier output is at +100V and the return to earth is
through Q6. Op-amp U4 compares the –1400V to +15V and adjusts the conduction of
Q6 to maintain this. For example, if the supply dropped from –1400V to –1450V then
the op-amp input would go –ve and its output –ve, turning off Q6. The volt drop across
Q6 would rise and the multiplier output would lift the high voltage output back to –
1400V.
Diode D4 and varistor R3 limit the voltage at Q6 collector to within safe limits during
start-up or fault conditions. C27 increases loop gain at 50Hz to reduce ripple on the
supply.
Low voltage supplies
Low voltage supplies are simple series pass regulated, but note that the CMOS
switches are fed from a current limited supply VEE and VDD
The +180V supply also uses a series pass regulator but R1 is needed to limit the
maximum volt drop at switch on or fault condition.
X and Y-plate drivers
The X-plate driver consists of gain setting op-amp U5B into whose virtual earth is
added an offset to centre the display on the CRT screen. U5A inverts the output of U5B
to generate a true push-pull signal for the CRT plate driving transistors.
The Y-plate driver is similar. To obtain trace alignment in the vertical axis a little Y
signal is fed into the X amp and similarly to align the horizontal a little X signal is fed to
Y. The pots are connected across the push pull so each signal is available in phase at
one end of the pot to zero at centre and out of phase at the other end.
R51 and R30 set the current into the LED of opto-coupler ISO1 (refer to T1500A CRT
schematic) to set the intensity while U7A and B produce pulses at the edges of the
Alternate clock to create blanking pulses to the CRT.

4-1
SECTION 4 – DISMANTLING INSTRUCTIONS
DISMANTLING THE T1500A
•Ensure the power cord is removed and the unit has been turned off for
at least five minutes.
•Remove the CRT base and then the four screws holding the shield to
the front panel. Remove the CRT and graticule and gasket.
•Remove the two screws retaining the power inlet and lift it off its lugs.
•Remove the three wires from the probe sockets.
•Remove the two screws and lift the intensity pot board away from the
front moulding.
•Remove the screws retaining the panel and main boards to the case.
•Pull off the power button and hold the switch in while lifting the rear of
the board to release it from the case. Lift the boards out of the case.

5-1
SECTION 5 – LINE VOLTAGE AND FUSE CHANGING
WARNING: There are hazardous voltages inside the instrument when
connected to its power supply. REMOVE THE POWER CORD before
touching any part of the line input circuit. Note that high voltages may
continue to be present for 5 minutes after power is removed until internal
capacitors discharge.
LINE VOLTAGE SELECTION AND FUSE CHANGING
Note: When replacing fuses always use the type and rating stated in SPECIFICATIONS.
Changing the line voltage
•To change the line voltage disconnect the power cord and wait five
minutes for internal voltages to discharge if the unit has just been
running.
•Remove the two screws which retain the cover and lift the rear of the
cover then remove it completely.
•Remove the line select link(s) with pliers from their sockets adjacent to
the power transformer. Replace one link for 240V or two for 120V in
the marked locations. The links are made of 1mm diameter wire if
replacements are needed.
•Remove the two line fuses F1 and F4 and replace with the correct
values listed in “SPECIFICATIONS”.
•Obliterate the marking on the rear label and clearly mark the new
setting.
•Refit cover and retaining screws.
•Connect to a suitable supply and confirm operation.
Fuse Replacement.
•The line fuses are located near the power inlet connector. Replace
only with parts of the value listed in “SPECIFICATIONS”. Note that
Neutral and Live are both fused.
•The Output Fuses for LOW, LOGIC, JUNCTION, and HICAP ranges
are located alongside the CRT. One fuse is supplied for A channel
and one for Channel B.
See “SPECIFICATIONS” for the value of fuses.
•Note: See Section 8 – REPLACEMENT PARTS for information on the
fuse wired into the CRT heater circuit.