My third P53

[Geaney]

When I scope the grid I have nice waveforms for all bands but zilch when I select MW.   Yes all logic points to coil/trimmer/contacts.   So far I can't nail it down.   

Confession time on the SW problem - that turned out to be me!   The noise turned out to be interference from a Powerline adapter.   Major egg on face!   At least I can now focus on the MW LO problem.   I will put on the silly hat and sit in the corner for the required amount of time....... 

Cheers,   John

[Geaney]

Finally nailed it down (MW LO problem) to a faulty wafer contact though I'm not sure if I can fix it.   If one contact is not bearing down properly on it's corresponding contact I doubt that me putting some pressure on it will make much of an impact.   At least I finally have cause for the effect!

Cheers,  John

[Terry]

Presumably the moving contact moves away from the fixed contact at some point. Assuming that it is contact pressure, rather than oxidation, that is causing the fault, can you push the offending contact beyond its normal limit when the moving contact is out of the way? This might help increase the contact pressure sufficiently.

On the other hand, might the answer be to fix an arm on the switch shaft so that it presses on a small switch to by-pass the contact? Can you identify exactly which contact is the cause of the problem? Obviously stray capacity could make this option a no-no but it might be worth a try.

[boater sam]

If all else fails try drilling out the rivet on the contact, sort it and re-fix with a tiny bolt and nut. I salvage contacts from old wafers, if you need any let me know. I would not have a wafer complete for this set, it is a complicated beast.

[Geaney]

Presumably the moving contact moves away from the fixed contact at some point. Assuming that it is contact pressure, rather than oxidation, that is causing the fault, can you push the offending contact beyond its normal limit when the moving contact is out of the way? This might help increase the contact pressure sufficiently.

On the other hand, might the answer be to fix an arm on the switch shaft so that it presses on a small switch to by-pass the contact? Can you identify exactly which contact is the cause of the problem? Obviously stray capacity could make this option a no-no but it might be worth a try.

Hi Terry, I finally tracked it down to switch S28 in the centre of the three wafer assemblies.  I unsoldered various wires and components so I could access the contact and I lifted the contact up close to where it is riveted and then pressed down on the other end of it.  When I released the 'rivet' end it then had more downforce and so far it seems to be holding up.  Time will tell if the contract metal is fatigued or if it will retain the increased pressure.   I'll confess it felt like I was repairing a watch, not a radio!

Cheers,   John
   

[Geaney]

If all else fails try drilling out the rivet on the contact, sort it and re-fix with a tiny bolt and nut. I salvage contacts from old wafers, if you need any let me know. I would not have a wafer complete for this set, it is a complicated beast.

When I finally located the offending contact it became clear that any repair that involved removing the contact would also involve completely dismantling the whole wafer switch assembly.   I'll be honest, that level of open heart surgery was a bit daunting to contemplate.   I'm hopeful that by having 're-bent' the contact such that there is effective contact once more will last.   I've been giving it a good stress test for the last few days and so far its looking solid.

Cheers,  John

[Geaney]

Well I hope this restore is now completed.   I still have some minor lacquer touch-up to do, and I need a knob, the rest is done.   Its been playing away for the last few days without any problems.  The project also gave me a chance to dust off the tuneable MW serial I built a while back - still impresses me when you pick up one station with the aerial pointing one direction and a different one when you move it 90 degrees!

Cheers,    John

[boater sam]

Looking good John.
Those knobs, are they similar to the larger knobs on Cossor Melody Maker 500/501 ? If so they should be easy to find.
My woodturning mate has spun a few of those for me in the past.