Scott75s1974
Well-Known Member
Hi, I’m rebuilding my Grandfather’s Scott R75S from 1974. I’ve had it for about 30 years. Used it intermittently up until about 8 years ago.
***I’ve read a lot of posts but haven’t found any general newbie refurb FAQs. So if I’ve missed that, please direct me there.***
It’s symptoms at last use were:
I’ve looked at all the boards. I can’t see any obvious burnt or blown parts. There are some braided looking high wattage resistors on the driver boards. One looks darker. But check’s ok.
So, I’d welcome advice about how to proceed. It’s a very robust design with removable daughter boards. I have unsolder and replace skills, but not much knowledge of amp circuits. I also have an oscilloscope and a signal generator. (But those are much older and prob need recapped too!).
Here’s my plan:
-Remove the boards.
-Clean all pots and switch banks.
-Clean/deox all the contact pins for the daughterboards.
-Replace the caps in the PS. Measure that for proper output.
-Replace the caps on the regulator board. Reinsert. Measure that for proper output.
-Replace caps on the other boards.
-Resolder any suspicious looking spots (they all look very good...it is really a finely built piece of equipment).
All pretty straightforward.
Should I remove, check and reseat the big T03 transistors? My assumption is that these need mica insulators and non-conductive grease. (I have some arctic silver which I assume I cannot use.)
Are there any other component checks I should do on the boards? I don’t really know much about how to check components in situ. I have a transistor checker with probes. Do these work on a board?
I’ve read there are certain transistors/diodes that are likely fails. Is there any guide to those somewhere?
Any other obvious thing I should do?
Thanks for any help. And again, my apologies if I'm missing a FAQ.
***I’ve read a lot of posts but haven’t found any general newbie refurb FAQs. So if I’ve missed that, please direct me there.***
It’s symptoms at last use were:
-It was always pretty muddy sounding. Not much high range, maybe not a lot of bass.
-A build up hiss at power-off in both speakers, kind of like a waterfall for a second.
-Some combination of speaker switch channels didn’t work (schematics suggest this can only be a dirty switch. No electronics involved on that board.)
-Cracking and popping on the volume and tone controls
-The last time I used it, one side seemed to be going out. I haven’t plugged it in since. I don’t want to toast anything.
-A build up hiss at power-off in both speakers, kind of like a waterfall for a second.
-Some combination of speaker switch channels didn’t work (schematics suggest this can only be a dirty switch. No electronics involved on that board.)
-Cracking and popping on the volume and tone controls
-The last time I used it, one side seemed to be going out. I haven’t plugged it in since. I don’t want to toast anything.
I’ve looked at all the boards. I can’t see any obvious burnt or blown parts. There are some braided looking high wattage resistors on the driver boards. One looks darker. But check’s ok.
So, I’d welcome advice about how to proceed. It’s a very robust design with removable daughter boards. I have unsolder and replace skills, but not much knowledge of amp circuits. I also have an oscilloscope and a signal generator. (But those are much older and prob need recapped too!).
Here’s my plan:
-Remove the boards.
-Clean all pots and switch banks.
-Clean/deox all the contact pins for the daughterboards.
-Replace the caps in the PS. Measure that for proper output.
-Replace the caps on the regulator board. Reinsert. Measure that for proper output.
-Replace caps on the other boards.
-Resolder any suspicious looking spots (they all look very good...it is really a finely built piece of equipment).
All pretty straightforward.
Should I remove, check and reseat the big T03 transistors? My assumption is that these need mica insulators and non-conductive grease. (I have some arctic silver which I assume I cannot use.)
Are there any other component checks I should do on the boards? I don’t really know much about how to check components in situ. I have a transistor checker with probes. Do these work on a board?
I’ve read there are certain transistors/diodes that are likely fails. Is there any guide to those somewhere?
Any other obvious thing I should do?
Thanks for any help. And again, my apologies if I'm missing a FAQ.