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Old 4th February 2014, 02:55 PM   #1
amptech is offline amptech  Scotland
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Default Yamaha 100-b212

This old 70's guitar combo amplifier G100-b212 was in a rough state plus there was signs of rough re-work to it's pcb's...so looking at things for a bit I noticed tired looking psu cap and speaker coupling cap, well, it's 40yrs plus so it's a bit rough looking. The fault is low o/p with hum in sound..time to sick the kettle on and warm up the soldering iron...

1, out with the old looking psu capacitor 2200uf 100v and 2200uf 80v speaker cap yes it's 85v dc pos rail design amplifier..so checking these old transistors within it's amp stage(2) o/p's ok..drivers ok..it also uses fet input tail pair with o/p wave trim adjustment..
So after a few coffee's and some hunting for the schematic not much luck there so tracing things out a little well,sometimes we have to do this old school way
now,I did get the circuit but, by then I'd found the fault as it may...someone had it's fet's 2sk30a in the wrong way with only the gate pin correct..plus in it's feed back path the 1uf 160v coupling cap that links to 10k then ground was looking ready to pop open due to slight looking rounded bottom after removing the old glue to check it. A fresh one in plus some pcb re-dressing and all it ready working once more

pm if looking for schematic...
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Old 4th February 2014, 02:58 PM   #2
amptech is offline amptech  Scotland
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Also these old design's more or less used speaker coupling..this one had 82 volts sitting on a 80v cap!! no wonder it was humming....quick upgrade to 4700uf at 100v much better..
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Old 4th February 2014, 04:02 PM   #3
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Interesting (mis)arrangement of the 2SK30a. Many have said here that signal JFETs can be quite reversible with little operational change in either direction. Strange but true for some types at least. Without seeing the schematic nor having looked in one of these for many years, I think the 30a would be a later replacement type that is still available NOS.
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Old 25th March 2014, 07:43 PM   #4
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Default Schematic

Do you have a schematic for this amp? If so, may I obtain a copy?
Also, do you have a component ID/reference for the protection diode which attaches to the ledge of the heat sink? Mine has a lead which is broken and barely solderable.
Any help-appreciated.

Please notify me if you are contacted by a third party regarding this correspondence. Thank you.
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