I received this tip some time ago from Herbert Kroiher, but never had time to add it to my web page.
“I have bought a later model of the 303b (without FP-sync) in ebay few weeks ago. The flash doesn’t fire reliable. When the flash fires it happens when the second curtain has finished his travel => much too late. This defect happens at all shutter speeds except “B”. At “B” the flash fires when the first curtain has travelled to the end position. But then the flash fires again an again because the electrical contactor doesn’t come free.
I have not found any advices in the internet about this defect.
I have studied the service manual. The flash contactors are located in the very inner of the camera body. These contactors are not accessible only by removing the top cover or the bottom plate. You have to disassemble nearly the complete camera to reach these contactors (bad design).
I have sent the camera to a repair shop. The repair costs are about 180,- Euros, so I ordered the camera back unrepaired.
Then I tried a relative simply attempt. I removed the bottom plate and the battery housing. Under the battery housing is a hole of about 5 millimeters, see page 14 of the repair manual (appendix of this mail). Here you can see part 2113 (cross slotted nut, made of brass). This nut tensions a friction brake. A lever above the brake triggers the X-contactor. I compared the position of this nut with another SRT. The nut was tightened too much.
The service manual says: “Next tighten the brake spring adjusting nut until the nut may be located in the same plane with the bound stop shaft.”
I used a little home made wrench and turned the nut counterclockwise two and a half revolutions. The nut can only turned in quarter steps, because there is not enough room to manoeuvre. Be careful. One of the shutter curtain spools is very close.
After that the x-contact defect was fixed.”