Mechanical dismantling of Rohde & Schwarz Sig Gen
 Outer covers: The top and bottom covers are secured by semi-captive M4 screws. Remove these completely to avoid damaging the bench surface.
 Side EMI strips: These have phosphor bronze fingers which engage with the top and bottom panels and can be removed by lifting the end fingers to access M4 screws.

 Rear panel: The rear panel carries the power supply circuit board. Despite the assembly connected with copper coax leads appearing to be an integral part of the rear panel, all you need to do is remove the two M3 screws whose heads are visible in the heatsink, thus leaving the module in place.

Next remove the four M4 screws holding the panel to the sides of the equipment.

The next issue is the mains on/off switch which connects to its push button via a metal rod. You'll note a tube carrying a cut pushed onto the switch operating spindle which can be detached by inserting a flat screwdriver blade into the gap and gently twisting the blade.

There are a pair of flat cables needing to be unplugged from the circuit board. these can be levered off by using a flat bladed screwdiver to detach the headers from the 24 and 16 way DIL sockets. Next is a small cable to the RF assembly which can be unplugged.

The next task is to note the two circuit boards at the side of the screened box. These plug into a small motherboard at the base of the equipment and can be removed using their end levers. In the past with other equipments this type of lever can snap due to ageing of the plastic so take care to use minimal force. The outer of the two board connects to the rear panel via a flat cable using DIL headers. It's much easier to pull the board out slightly to lever these headers from their sockets.

At this point the rear panel (which is pretty heavy because of the toroidal mains transformer) can be lifted awayfrom the chassis.

 Reassembly of Rear Panel

 Once the power supply board has been detached from the rear panel here are some details of putting it back together.

 Because of flexibility in the transformer leads the PSU pcb need not be detached from these wires.

Fit the circuit board to the rear panel using the three screws not connected with the heatsink, but dont tighten these fully until the heatsink is exactly in place with respect to the various cut-outs and TO-3 sockets. Secure the black heatsink using the two screws at the bottom corners allowing enough movement to align it.

Fit the first TO-3 device bearing in mind the location of the uA7915. To fit the TO-3 devices it's essential to ensure the three device pins locate in their sockets. Offer up the insulating material and the metal plate with the single pin and click into its socket. Offer up the TO-3 device and you should be able to feel the two pins locking in place.The TO-3 device should be perfectly aligned with the plate once its plugged in. If not you've probably got only one TO-3 pin in its socket. Once aligned the blue insulators can now be plugged into place, although even a tiny mis-alignment may not permit the insulators from pressing fully in, so use the securing screws to complete the insulator fit by carefully tightening them. The blue insulators should locate snuggly as the screws are nearly tight. Repeat for the remaing three devices. Once everything looks OK, tighten all the screws but don't overtighten the TO-3 screws because you might crack a blue insulator.

The two stud diodes are insulated from the heatsink by mica washers and are a bit tricky to fit because of the clearances of the wires at the solder tags. One of the diode stud flats should be adjacent to the solder tab allowing the tab to be bent away from the chassis once the diodes are secured.

In my case I needed to fit a circular mica washer then two washers between the diode body and heatsink to give sufficient clearance between the connecting wires and the aluminium panel. With these fitted, hold the diode in place against the heatsink and fit the small white plastic ring in the heatsink hole. This is fitted to prevent the diode thread from accidentally touching the heatsink. Fit the second mica washer followed by a plain washer, crinkle washer and finally the 10mm nut. Check the alignment between the diode flat and the solder tab to allow the latter to be bent away from the aluminium panel then tighten the nut. Repeat for the second diode and connect the black wire to the tab on the pcb.

Confirm the diodes are not shorting to the panel and check the TO-3 devices are plugged into their sockets using a diode-test meter to see the base-emitter-collector junctions for the 2N3055s.

 
 

 Return to the overhaul page...