Linn Axis Power Supply:No Slowdown

telecine

Member
2010-09-28 8:05 pm
Hello to all, first post, great Forum!

I'm using a Linn Axis power supply in a remote enclosure to drive my LP12.

The Axis supply was a bit of a "stretcher case" when I first got it, but several BUX85's and op amps later, the supply is now running at both speeds.

The output transistors are getting pretty hot though, and I noticed that the reduction of the motor volts, once up to speed, isn't happening. I still have 76v (?) to both windings with no variation.

Anyone able to point me in the right direction?

This is a swine of a circuit to get going, but I'm so close to success now, maybe someone had already been here?

I have a copy of the circuit diagram.

Thanks in advance.
 

telecine

Member
2010-09-28 8:05 pm
Of course when I said "No Slowdown" in the thread title, what I really meant is that the PSU never makes it into "quiet mode" to offer less volts to the motor.

Mine is running permanently at full voltage, leading to the hot BUX 85's in the MDA section.

I think I have at least narrowed the problem down to the last two op amps in the U5 chain. U5 pin 1 is seemingly stuck at about 12v. I suspect someone has "been there" before me as some soldering is evident in this area. I'm a little suspicious of c19 in particular. What should the value of this electrolytic be? Also of c17?

I have the circuit diagram, but no components values are included. Does anyone have the accompanying sheet with component values?

Any help very much appreciated at this point as I've hit the "brick wall"!

I'd rather fault find logically than shotgun components at this stage, as I'd like to learn something from this ordeal.
 

telecine

Member
2010-09-28 8:05 pm
Well things aren't quite what they seemed. The Volts reduction does work, but only at 45rpm.

The problem lies around U7.

If I short pins 1 and 2 together momentarily (to simulate U7 13,1,2 closing), the volts drop to 28 at the motor (33rpm), and remain there. Surprisingly the voltage at the control pin 13 also rises to 13v which is where I would have expected it to be in "quiet mode" all along!

I have run the circuit without U7, the logic coming from the bistable is fine (12 or 0), as soon as U4 is added into the mix, pin 13 sicks at around 3 volts, until 1 and 2 are shorted togaether again, then every voltage isas it should be.

Cold checks looking back up the lines to the bistable reveal nothing amiss.

It's almost as if the chip is "leaky" (contact resistance?)

Are all 4066 created equal?

4066B doesn't work at all in this circuit position, and CD4066BE gives the results outlined above.

I'm missing something obvious, but can't see the wood for the trees I think.

Any advice very much appreciated.