INT (JKT)   INLAND POWER AMPLIFIERS 

Introduction

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:
   
BAY 1
BAY 2
BAY 3
RA QUICK MOTION
CASS TURNTABLE
SPARE POWER AMP
 
RA SLOW MOTION
DEC QUICK MOTION
   
DEC SLOW MOTION

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.  

Description

Note:  The notes below can also be applied to the JKT which uses indentical power amplifiers. Power amplifiers can be inter-changed between the INT and JKT with no problems.

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).
 

Fault-finding

Although the amplifiers are current-limited at 16A, they can blow-up sometimes!

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.

Firstly

Before changing an amplifier, first check that the green RESET button (near the emergency stop switch on the control desk) is NOT active (i.e.lit). If it is, pushing this will reset the relays in the CLAMP LOGIC tray and return 40V to the power amplifiers. If still no current is seen then the problem is almost certainly with a power amplifier. n.b. The OIL PUMP must be running before it is possible to move the telescope in RA in either QM or SM.

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.
 

Changing a power amplifier


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.
 

Repairing a power amplifier


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.

Testing the power transistors

Power transistors can be tested with the Levell TM12 transistor tester to measure Vbe. To check Hfe, its better to use the Hfe-tester (made by me (rjp) from an Elektor kit ). This tester is capable of driving the transistor Ic with 10A max. I've had transistors in the past which look ok on the Levell TM12 tester, but fail with high current.

INT POWER AMP TAG-STRIP WIRING IN CLIP-CENTRE

 
RA QUICK MOTION
TAG STRIP NUMBER CABLE IDENTIFER COMMENTS
555P4  PSU +40V SUPPLY
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

 
RA SLOW MOTION
TAG STRIP NUMBER CABLE IDENTIFER COMMENTS
561Q4  PSU +40V SUPPLY
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

 
DEC QUICK MOTION
TAG STRIP NUMBER CABLE IDENTIFER COMMENTS
576R0 PSU +40V SUPPLY
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

 
DEC SLOW MOTION
TAG STRIP NUMBER CABLE IDENTIFER COMMENTS
573R4  PSU +40V SUPPLY
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

 
CASS TURNTABLE
TAG STRIP NUMBER CABLE IDENTIFER COMMENTS
564Q4  PSU +40V SUPPLY
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

 

POWER AMP TAG STRIP CONNECTIONS    (SAME FOR JKT)

 
          PA CONNECTIONS                  EXTERNAL CONNECTIONS
                            __________
         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)
                            |    o    |            |
            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    |
          |                 |    o    |
          |                 |    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)
          |                 |    o    |
          |    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    |
                            |_________|


Original document:  Renee J. Pit, 17-Nov-1995
HTML'ed:  Sept 2002  ejm
Last modified: Renee J. Pit, 14-April-2011