First of all: a copy of this document is normally kept in the INT clip centre, tucked in the back of BAY3.
The INT has 6 power amplifier trays, these are located in the CLIP
centre as follows:
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They are not marked as such on the front panel, because they are interchangeable. There is, however a small label in the the back of the BAY's on the brackets which support the amplifiers.
n.b. BAY4 which used to contain the Platform Drive and the Cass TT power amplifiers has been removed in order to give some more space in the CLIP centre. The platform drive; something which was left over from when the INT was at Herstmonceux, has never been used here. The cables and equipment were removed many years ago.
The Cass TT amplifier was moved into BAY2 which had room for an extra
RA-SM amplifier. This RA-SM amplifier has never been implemented. I've
connected the Cass TT to that power supply and re-labelled it.
The power amplifiers are driven by a DC demand signal coming from the pre-amps mounted in a card crate in BAY3. This demand signal is fed into a hybrid DC servo amplifier module manufactured by INLAND. This is mounted vertically in a socket between two heatsinks. The hybrid module drives an H-bridge with 12 power transistors; 3 in each leg, the DC-servo motor effectively sits in the middle of the H-bridge.. The amplifiers are powered by 3 phase supply in the bottom of each bay delivering 40V DC. A neon light is lit if one of the fuses has blown in the power supply.
All the power amplifiers have an overload indicator on the control desk.
These indicators are lit by a relay contact which is called by 2 thermal
switches mounted on the heatsink of the amplifier. In the case of the RA
QM and SM amplifiers, the relay is also called by a contact when the oil-pump
is NOT running. At the same time, a red indicator lamp illuminates on the
front panel of the amplifier when this relay is called. The relay also
switches off the 40V DC supply to the amplifier. There is also another
red indicator lamp which is lit if the 20A fuse blows. (see also Drawing:
ED142).
The first sign of trouble is usually that an either the DEC or RA motion (can be QM or SM) refuses to move when the manual RATE demand control is turned fully clockwise and a +/- DIRECTION button pushed in Engineering Mode. Seeing NO current being drawn on the meters is also a good indicator that a power amplifier has blown.
In case of trouble with one the amplifiers, the first thing to check is if the cooling fan is running then check the fuses. Only try to replace the fuse once. If it blows again, change the amplifier tray for the spare one.
IMPORTANT: Never replace the fuse with one of a higher rating
If the fuse keeps blowing, this usually indicates there is one or more shorted transistor(s) in the H-bridge or the hybrid module is faulty. If this is the case, the amplifier needs to be disassembled and carefull testing of ALL the (12) power transistors needs carrying out. Replaced transistors need to be matched in each leg of the H-bridge.
Also check for a burning smell and black spots on the hybrid amplifier
module. If this is the case change the faulty amplifier tray for the spare
one.
CAUTION
Before any replacement is done, first switch OFF the "Power
amplifiers"
breaker marked: CB18 (POWER AMPLIFIERS) in BAY10 and the breaker
marked: MCB10 (FERRANTI, PA FAN, CLIP CENTRE) in BAY12. This kills
the 40V DC supply and the 240V AC for the fans. check this!!
To replace the amplifier you have to disconnect all the Lucar crimps from the tag-strip. Then remove the earth-lead form the cabinet. Now you can remove the chassis by sliding it out from the front of the cabinet. Slide in the spare amplifier and reconnect the Lucar crimps. To facilitate this operation I've made a wiring diagram for each amplifier. Do not forget to reconnect the earth-lead!
Switch on the breakers and check if the fan is running. If the green
power indicator is not on, then either the bulb is blown or you have to
press the green reset button in the control room. Drive the telescope first
in manual mode watching the current meters. The motor current should be
balanced in both +/- directions of movement. If there is LARGE error, suspect
the telescope being OUT OF BALANCE and call for mechanical support.
CAUTION
Never replace the INLAND hybrid module without all the power
transistors having first been tested.
One faulty power transistor
can blow up the hybrid again and these are expensive (£750 a piece!)
to replace, apart from the fact that we are running low on spare
modules as they are not made anymore, so we have to rely on the JKT if
needed.
In case you need to repair an amplifier chassis in the workshop, there
is a document written by Mary-Ann Hackman from INLAND motors explaining
the whole procedure of testing and repairing. This document is kept
in the top drawer of the Equipment filing cabinet in the INT electronics
workshop. The folder is labeled: AMPLIFIERS DC-SERVO SYSTEM.
There is also a box with test-leads, spares, a test-box for the hybrid amp and a load-resistor, kept on the shelf in the INT workshop. Spare selected power transistors are kept in the store-room in component drawer # C34. The transistors have been selected based on their base-emitter voltage (Vbe) which is written on the transistor. The better these transistors are matched in the H-bridge the longer they will last. Always use transistors with the same value in one leg of the H-bridge.
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TAG STRIP NUMBER | CABLE IDENTIFER | COMMENTS |
3 | 555P4 | PSU +40V SUPPLY |
7 | 557P4 | PSU 0V return |
10 | 1817Q0 | - |
11 | 641R0 ( thin green wire) | To Current meter |
13 | 643R7 (thin white wire) | To Current meter |
14 | 152E4/152E4/634N0 | Motor output - NB: on top: thin red wire |
15 | 153E4/153E4 | Motor output + |
21 | screened grey cable (blue wire) | interlock (oil pump) |
24 | 1550Q1 | Fan mains neutral |
25 | 1509P2 | Fan mains live |
27 | screened grey cable (red wire) | interlock (oil pump) |
28 | thin violet wire | PA overload indicator (control desk) |
29 | screened grey cable (blue) | Input signal - return |
30 | screened grey cable (red) | Input signal + from preamp |
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TAG STRIP NUMBER | CABLE IDENTIFER | COMMENTS |
3 | 561Q4 | PSU +40V SUPPLY |
7 | 563Q4 | PSU 0V return |
10 | 1817Q0 | - |
11 | 641R0 ( thin green wire) | To Current meter |
13 | 643N7 (thin white wire) | To Current meter |
14 | 83E4/83E4/634N0 | Motor output - NB: on top: thin red wire |
15 | 84E4/84E4 | Motor output + |
21 | screened grey cable (blue wire) | interlock (oil pump) |
24 | 1550P1 | Fan mains neutral |
25 | 1509P2 | Fan mains live |
27 | screened grey cable (red wire) | interlock (oil pump) |
28 | thin violet wire | PA overload indicator (control desk) |
29 | screened grey cable (blue) | Input signal - return |
30 | screened grey cable (red) | Input signal + from preamp |
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TAG STRIP NUMBER | CABLE IDENTIFER | COMMENTS |
3 | 576R0 | PSU +40V SUPPLY |
7 | 5785R0 | PSU 0V return |
10 | 1817Q0 | - |
11 | 641R0 ( thin green wire) | To Current meter |
13 | 643N7 (thin white wire) | To Current meter |
14 | 217D4/217D4/634N0 | Motor output - NB: on top: thin red wire |
15 | 218D4/218D4 | Motor output + |
21 | Link cable | Linked to 27 |
24 | 1509P2 | Fan mains neutral |
25 | 1550 | Fan mains live |
27 | Link cable | Linked to 21 |
28 | thin violet wire | PA overload indicator (control desk) |
29 | screened grey cable (blue) | Input signal - return |
30 | screened grey cable (red) | Input signal + from preamp |
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TAG STRIP NUMBER | CABLE IDENTIFER | COMMENTS |
3 | 573R4 | PSU +40V SUPPLY |
7 | 575R4 | PSU 0V return |
10 | 1817Q0 | - |
11 | 641R0 ( thin green wire) | To Current meter |
13 | 643R7 (thin white wire) | To Current meter |
14 | 97D4/97D4/634N0 | Motor output - NB: on top: thin red wire |
15 | 98D4/98D4 | Motor output + |
21 | Link cable | Linked to 27 |
24 | 1509P2 | Fan mains neutral |
25 | 1550 | Fan mains live |
27 | Link cable | Linked to 21 |
28 | thin violet wire | PA overload indicator (control desk) |
29 | screened grey cable (yellow) | Input signal - return |
30 | screened grey cable (red) | Input signal + from preamp |
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TAG STRIP NUMBER | CABLE IDENTIFER | COMMENTS |
3 | 564Q4 | PSU +40V SUPPLY |
7 | 566Q4 | PSU 0V return |
10 | 1817Q0 | - |
11 | 641R0 ( thin green wire) | To Current meter |
13 | 643N7 (thin white wire) | To Current meter |
14 | 336C4/336C4/red | Motor output - NB: on top: thin red wire |
15 | 347C4/347C4 | Motor output + |
21 | Link cable | Linked to 27 |
24 | 1509P2 | Fan mains neutral |
25 | 1519P3 | Fan mains live |
27 | Link cable | Linked to 21 |
28 | thin violet wire | PA overload indicator (control desk) |
29 | screened grey cable (blue) | Input signal - return |
30 | screened grey cable (red) | Input signal + from preamp |
PA CONNECTIONS EXTERNAL CONNECTIONS
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coax-input ====O---|30 o |--------- + input from preamp
\---|29 o |--------- - input from preamp
641---------|28 o |--------- overload ind. control room
639---------|27 o |------------| oil interlock (RA only)
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fan 656---------|25 o |--240V L | Link between 27 and 21
fan 655---------|24 o |--240V N | needed for DEC and CASS-TT
| o | | AND JKT
| o | |
+-----------------|21 o |------------| oil interlock (RA only)
| | o |
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| | o |
| | o |
| | o |
| 645---------|15 o |--------- servo motor +
| 644---------|14 o |--------- servo motor -
| 1002---------|13 o |--------- ammeter (eng rack)
| 1002---------|12 o |--------- ammeter (eng rack)
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| 1009---------|10 o |
+-----------------|9 o |
638---------| o |
1003---------| o 7|--------- 0V from PSU
1000---------| o |
1000---------|5 o |
1000---------| o |
636---------| o 3|--------- 40V from PSU
| o |
|1 o |
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