davemascera
Active member
- Joined
- Mar 22, 2017
- Messages
- 42
I've been using this machine for 2 years now, not really any serious problems to speak of besides a rumble in certain channel cards that was easily solved..
At some point, we noticed the lights on the meters dimming and checked it out... the power cable had begun corroding at the contact point, so we cleaned it out, filed the contacts, etc, and the machine performed just fine for a while.
Recently, while recording, we noticed a 'fuzz' on recently recorded tracks, which would resolve itself after a while or after a machine reset sometimes as well. In addition, the lights (a 6.3 vac line straight from the transformer) began dimming and flickering, probably worse than they ever had. We checked the power cable and it seemed pretty clean, and measured no resistance between the wall plug and transformer input.
I started taking measurements on the output of the filter caps and power transistors for each line and didn't notice anything fishy, except for a 30v line with 2.5% ripple and that the -40V rail was operating at 2V less in difference from zero than the positive side when the machine was taped up and at idle (-38V, as opposed to the +40V on the other side, simultaneously), and would dip significantly more than the positive rail when the machine was set to fast-forward and rewind. The dips would coincide with the lights dimming quite perfectly.
So we decided to measure the current on the negative and positive 40V rails drawn by the machine. What we found was that the positive rail drew approximately 120 ma at idle (tape loaded up and reels providing tension), but the negative rail was drawing 4200 milliamps. This result doesn't seem correct to me, but I really have no reason to believe it was incorrect after further tests (outside of my gut thinking that a machine shouldn't need 4.2 amps to idle).
Prior to taping up, the positive and negative rails would sit at appr 120 ma each, and during a fast forward or rewind, the positive would jump to a few amps, and the negative would jolt up to a significant few more than that. I melted an alligator clip testing this.
There are 3 -40V lines going to the motor assembly. The first line goes to the middle, which seems to operate fast forward and rewind operations, and the other two lines go to the left and right reels and seem to provide tension.
I was hoping that i'd find a specific side that was leaking current and dragging down the supply, but when I checked current drawn by the negative line on either side, they were roughly equal, and disconnecting either side prevented the light flicker/dimming.
This leads me to believe that either the current currently being drawn by the negative rail is either perfectly intentional, or there is some sort of crazy incorrect voltage provided to the base of a large transistor behind the reel motors. So I'm stuck.
Anyone have any ideas for me?
At some point, we noticed the lights on the meters dimming and checked it out... the power cable had begun corroding at the contact point, so we cleaned it out, filed the contacts, etc, and the machine performed just fine for a while.
Recently, while recording, we noticed a 'fuzz' on recently recorded tracks, which would resolve itself after a while or after a machine reset sometimes as well. In addition, the lights (a 6.3 vac line straight from the transformer) began dimming and flickering, probably worse than they ever had. We checked the power cable and it seemed pretty clean, and measured no resistance between the wall plug and transformer input.
I started taking measurements on the output of the filter caps and power transistors for each line and didn't notice anything fishy, except for a 30v line with 2.5% ripple and that the -40V rail was operating at 2V less in difference from zero than the positive side when the machine was taped up and at idle (-38V, as opposed to the +40V on the other side, simultaneously), and would dip significantly more than the positive rail when the machine was set to fast-forward and rewind. The dips would coincide with the lights dimming quite perfectly.
So we decided to measure the current on the negative and positive 40V rails drawn by the machine. What we found was that the positive rail drew approximately 120 ma at idle (tape loaded up and reels providing tension), but the negative rail was drawing 4200 milliamps. This result doesn't seem correct to me, but I really have no reason to believe it was incorrect after further tests (outside of my gut thinking that a machine shouldn't need 4.2 amps to idle).
Prior to taping up, the positive and negative rails would sit at appr 120 ma each, and during a fast forward or rewind, the positive would jump to a few amps, and the negative would jolt up to a significant few more than that. I melted an alligator clip testing this.
There are 3 -40V lines going to the motor assembly. The first line goes to the middle, which seems to operate fast forward and rewind operations, and the other two lines go to the left and right reels and seem to provide tension.
I was hoping that i'd find a specific side that was leaking current and dragging down the supply, but when I checked current drawn by the negative line on either side, they were roughly equal, and disconnecting either side prevented the light flicker/dimming.
This leads me to believe that either the current currently being drawn by the negative rail is either perfectly intentional, or there is some sort of crazy incorrect voltage provided to the base of a large transistor behind the reel motors. So I'm stuck.
Anyone have any ideas for me?
