It sounds like excessive current draw on the power supply is causing it to shut down. The behavior is typical of an overdraw on a switching power supply. There is a very large inrush current when the 18v is first applied to to the telescope due to a large electrolytic capacitor on the control panel board directly across the power connector pins. I suggest that you connect the supply to the telescope 1st, then plug in the power supply. This will charge that large capacitor on the telescope. Then turn on the scope last.
Regarding capacitor replacements,
I would recommend that you use 10uf 50v electrolytic capacitors with a 105 C rating. Tantalum's are risky when used on the main power supply line.
There are 5 tantalum capacitors that are directly fed +18V from the external power supply, one on each PC board inside the scope and
one in the handset. Even if these capacitors look ok they can fail from age and there is a tendency for these to fail in a "short" mode that can cause
Each board has an LM7805 voltage regulator and this capacitor is between pin 1 (input side) of the regulator to ground.
The tantalum on the output side of the regulator does NOT need to be replaced. It only sees the regulated 5v output voltage.
The OP's pic of the main board, C3 has a burn spot. It is failed.
All these boards are double sided and you must be extremely careful when desoldering to not damage the traces on either side of the board and when
installing the new components to be sure that solder has flowed through from the solder side to the component side and that traces on both sides are connected to the capacitor leg.
On the DEC and RA boards, it is extremely easy to ruin the double sided traces changing the tantalum's. The traces are very very thin. One option is to smash the old tantalum with pliers and then solder the new capacitor to the old legs.
There are several checks you can make if the board is not running correctly after you've worked on it. If the board is not right you will get runaway motors.
Also, be very careful of the connections to the photo diodes and leds. These are very delicate.
I'd remove the entire motor assembly to work on these then re-install the motor assembly and be sure to put the assembly back exactly where the screws were before. (mark them).
1) The transistor looking device next to the capacitor is actually a 7805 voltage regulator.
3) With the wiring disconnected from J1 (six pin connector), check with an ohmmeter from the connector pin 4 (+18v) to pin 1 of the regulator to be sure there is continuity. The trace runs on the component side from the connector pin to the + leg of the capacitor and then to the regulator pin 1. From there the trace makes it's way to pin 3 of the IC. Removing the old cap, it is easy to damage this trace.
4) Check continuity from pin 4 of J1 to pin 3 of U1 (the IC on the board). This supplies +18v to the IC.
When power is applied to this board, there should be +5 volts coming out of the regulator. This can be checked at the resistor R3 which is next to the mounting screw. The end of the resistor sticking up in the air should read +5v. This voltage supplies the IR LEDs
Note that although the circuit boards are the same, the RA motor board also has wires going to the hall effect transistor that are not there on the Dec board.
On J1, the RA board will not have a wire at pin 6, where on the DEC board the connector will not have a wire at pin 3.
On the control panel circuit board, removing the metal panel can be difficult. On the power connector, after removing the metal nut and washer there is a plastic insulator that looks like a shoulder washer. That plastic piece is actually 2 pieces. It is threaded. The top threaded piece holds down a thin wedged plastic washer that keeps the connector barrel from touching the panel. I used a small dental pic to turn this small bit of plastic.
For schematics and lots of helpful information I refer you to the site
Clear skies !!
Edited by MikeBY, 06 June 2021 - 01:39 PM.