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CB24 panel stopped working - hot diodes + resistors...

My SCS300B-24 with a CB24 control board has been working perfectly for a few years now, until they completely stopped responding to all the remotes a few weeks back.

It's taken me until today to have a looksee...

Inside the unit, the power light is on and attempting to re-ascribe the remote control to the board (my first test) found the 'connect' LED flickering a response, but no 'relay clicks' to acknowledge success. And it didn't work.

So I had a closer look at the board and found the vertical column of resistors and diodes in the top-right corner of the board to be very hot - far too hot to touch. I had a little fiddle with the components there and a relay began rapidly clicking when I slightly pushed the red LED adjacent to the top resistors. For a few minutes the board came back to life and operated the gates again.

Ok, this obviously suggest a bad connection/ dry joint/ overheated connection of some sort, so I will be pulling the board forward to have a gander behind it soon.

However, any idea what's leading to all these resistors - and especially the diodes - to be so VERY hot? The power transistors along the board's bottom edge - on the heatsinks - were also pretty warm, but these diodes were sizzling. I also notice that the PCB around these resistors and diodes has been heat discoloured (see photo). And they seemed to be hot all of the time, even tho' the gates haven't actually been operated for weeks.

Any ideas why, please? Best to replace them while I'm at it? The resistors appear to be 330ohm and 2W, but I'm not sure of the diode codes for Z20 and Z22. Can anyone help please?

Would it be worth while fitting higher wattage resistors?

Many thanks.

Attached Files Image(s)
My CB24 has just started exhibiting the same problem of high temperature resistors/diodes and system degradation. I notice that the transformer/batteries are delivering 34V rather than the 24V expected. Did you sort your system out? Advice would be appreciated!

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