Reverb recovery stage with AC125 transistor in Philicorda GM751 organ - problems
- Thread starter bram9206
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Heat gun and spray cold can help coax intermittent faults into revealing themselves.
JR
JR
Nele, any idea where that fuse of yours was located? I only have 3 fuses here at the secondaries of the PT. Just cleaned all contacts, re-installed the three fuses, but nu results...
I also linked out the SK10 switch with a clip wire, but no results either. This proves the problem is not in the switch.
The PCB is a very old one (from 1963) and it doesn't have a coating on the traces, so I can go over those traces with a solder iron and solder and leave a thin fresh solder trace behind. I already did this where the switch is mounted to the PCB and with most other traces (that are related to the reverb recovery) on the power amp board, but so far no results... I also reheated most solder joints, but no results either... Will continue tomorrow.
I touched the trace from the AC125 collector to the 16uF C677 cap and the organ was still on. So I acidentally made a short to ground. And guess what... the reverb kicked in again. But, of course... disappeared again a minute later. So it seems like there are several points where I can do the short-trick-thing to get back the reverb (for a while).
Any one any idea? Seems to be a very hard one...
I also linked out the SK10 switch with a clip wire, but no results either. This proves the problem is not in the switch.
The PCB is a very old one (from 1963) and it doesn't have a coating on the traces, so I can go over those traces with a solder iron and solder and leave a thin fresh solder trace behind. I already did this where the switch is mounted to the PCB and with most other traces (that are related to the reverb recovery) on the power amp board, but so far no results... I also reheated most solder joints, but no results either... Will continue tomorrow.
I touched the trace from the AC125 collector to the 16uF C677 cap and the organ was still on. So I acidentally made a short to ground. And guess what... the reverb kicked in again. But, of course... disappeared again a minute later. So it seems like there are several points where I can do the short-trick-thing to get back the reverb (for a while).
Any one any idea? Seems to be a very hard one...
Internal fuses are typically required by UL to prevent fires in case of semiconductor short circuit failures. So by definition such internals fuses will be associated with power supplies and/or power devices.
JR
JR
Great news, I located and fixed the problem!
I re-checked all solder joints and PCB traces without any result. So, getting tired of this seemingly unfixable reverb issue, I decided to re-check every wire that has something to do with the reverb recovery. I touched and moved them before, but now I really took the time to move and bend every single inch of each and every wire, taking a least 5 or more seconds per inch. First few wires passed the test, but then I began testing the wire that goes from the reverb tank output to the recovery PCB board. I was already gently bending and moving a certain point of that wire for more than 10 seconds, really where one would conclude "ok, nothing happens, this piece looks to be ok", when all of a sudden I heard some crackling! So I started moving that piece more and more, tapping the reverb tank, when all of a sudden I could hear the reverb break through once in a while. When I held the wire in a certain position, the reverb was working, but when I let it go, reverb disappeared.
Long story just to tell you guys I simply replaced that wire (as well as the input wire, as both wires are very thin and fragile) and the reverb is back in full glory!
Still one thing I would like to ask: how come that when I did this short-thing (touching that point on the PCB with a clip wire to ground) the reverb always kicked in again? Eventually it always disappeared again, but that short-thing could always bring it back too. So I would like to know how that was possible.
In short: problem solved! Thanks to everyone for the great help!
I re-checked all solder joints and PCB traces without any result. So, getting tired of this seemingly unfixable reverb issue, I decided to re-check every wire that has something to do with the reverb recovery. I touched and moved them before, but now I really took the time to move and bend every single inch of each and every wire, taking a least 5 or more seconds per inch. First few wires passed the test, but then I began testing the wire that goes from the reverb tank output to the recovery PCB board. I was already gently bending and moving a certain point of that wire for more than 10 seconds, really where one would conclude "ok, nothing happens, this piece looks to be ok", when all of a sudden I heard some crackling! So I started moving that piece more and more, tapping the reverb tank, when all of a sudden I could hear the reverb break through once in a while. When I held the wire in a certain position, the reverb was working, but when I let it go, reverb disappeared.
Long story just to tell you guys I simply replaced that wire (as well as the input wire, as both wires are very thin and fragile) and the reverb is back in full glory!
Still one thing I would like to ask: how come that when I did this short-thing (touching that point on the PCB with a clip wire to ground) the reverb always kicked in again? Eventually it always disappeared again, but that short-thing could always bring it back too. So I would like to know how that was possible.
In short: problem solved! Thanks to everyone for the great help!
If a connection is oxidized (if you look inside that wire they are probably frayed and nasty), a voltage spike can punch through the oxide layer and will conduct like normal until it oxidizes again.
Notice how it's always the last thing you checked. ;D
JR
Notice how it's always the last thing you checked. ;D
JR
