Repairing Linn Axis Motor Drive Amplifier

[John Caswell]

Hi all,
As i said if I managed to fix the Axis motor board I would post accordingly, so here goes.
FIRST A WARNING, the motor board is connected directly to the mains supply so will need VERY careful handling if you are not to give yourself some unpleasant surprises.
Remember electricity will not give you a second chance and until the fuse blows Battersea Power Station or its equivalent is sitting at the other end of the cable. Also any test equipment that is connected to this circuit will be "live", so earthed equipment (scopes etc) MUST either have the earth wire lifted from the plugtop or not used at all. If you have any doubts about your capabilities do not work on this board.
There is a way around it and that is to use an isolating transormer and then you can earth the secondary but it is somewhat a costly exercise.
Also remember the "one hand in the trouser/skirt pocket" rule when using test equipment to measure voltages etc
So..........
My unit would not turn the motor, so first check was voltages on the bridge rectifier, about 320V DC, correct, and also across C5 about 12-13VDC, correct, so I initially suspected the two main coupling capacitors C8/C9, which were removed, measured (both read 33µF, correct value) and substituted for good measure. No improvement of course.
I then turned my attention to the other other electrolytic capacitors C6,C11,C16,and C17. These are all 22µF/63V capacitors and they all measured very low capacity generally 5-6µF. Replacement appeared to effect a total cure, however I was somewhat puzzled when I looked at the sine wave feeding the motor to see it drop in level and the motor stop rotating.
I then remembered Linn's penchant for reducing motor volts so it just turns the platter, the motor relying on the inertia of the platter to keep the motor rotating and vice versa. Once assembled totally all worked fine, with speed being fractionally fast to overcome stylus drag etc, however the speed adjust controls are available through the base of the case if required.
I reckon that the electrolytic capacitors dried out over the years 'til they reached the point of no return and I think that the best solution is to replace them all with new.
Hope this helps you all out there but if you have any further problems send me a pm with phone nr etc and I will try to assist

John Caswell

[tkwou]

Glad to see you got things up and running. I learned to fix the Axis power supply the hard way, without a schematic. I did get it running but I then caused major damage to the board because I didn't know that it is designed to power down after 7-8 seconds so I kept trying figure out what was wrong. The ensuing damage required me to change 4 of 7 chips to restore normal function. Lesson learned. Do your homework and get a schematic.

[John Caswell]

Hi Tkwou,
Normally, like yourself I dive in, but knowing what Linn electronics seem to be like I just thought I would ask for any info, which of course was forthcoming, for which I am extremely grateful.
Hopefully it will help me to help others in dire straits

Regards

John

[bubbamc119]

Good stuff. I have my caps on the way and will let you guys know how it goes.

[PeterMoreton]

My Linn axis had died, as they all seem to, and my findings are similar to previous postings; replacement of all electrolytics effects a total cure. I have used slightly higher voltage caps where PCB space allows, since several of the caps are run within a few percent of the rated voltage. Higher voltage parts ought to last a bit longer. Note also that the on/off switch does not actually power down the PSU, so if you leave the turntable plugged in, then it is using up the rated lifetime of the electrolytics.

My Axis PCB showed signs of a failed repair attempt; the two 33uf 350v caps and the output transistors had been replaced recently (June 2006 date codes on the O/P devices). The replacement output transistors (BUX85's) were directly attached to the heatsink, without any TO220 insulator, which resulted in the transistor tab = collector = 320V being connected to earth. Hence, the fuse blew whenever powered on. I fitted TO220 insulators, and that problem went away, but I'd like to ask if anyone knows what the original output device is, and are they fitted with insulating spacers from the factory?

Finally, since the bridge rectifier would have been stressed by the peak current drawn when the fuse blew, I swapped the bridge. For information, it's an International Rectifier 1KAB100E part costing tuppence.

[tkwou]

Hi Peter

The Axis uses BUX85 originally but they are the insulated version which is essential as the heatsink is grounded.

The Axis high voltage power supply section which is identical to the Valhalla's is pretty basic and is always on when the units are plugged in. Still they seem to last quite a long time. Mine was plugged in for 18 years before it finally failed.

[PeterMoreton]

Thanks TKWOU. In that case lets re-iterate a word of warning to people who might replace the BUX85 o/p transistors in a LINN AXIS: most BUX85's have the metal tab connected to the collector, and will therefore attempt to 'liven' the grounded heatsink. When servicing this pcb, I would always insert an insulating washer between the BUX85 and the heatsink.

[multi-volti]

What a nice find.

Thank you Tim for the Axis email.

I found a Valhalla schematic also on another site for comparison.

I'm going from memory here, so I could have my stories crossed and am describing another TT, but I think the following really happened to me with my (healthy) Axis. I did not change any names to protect the innocent or ignorant.

My Axis stopped turning one day, and after messing with it for a while, at night, noticed a faint glow & whisps of smoke periodically. There was an obviously underrated resistor that had cracked in half (is underrated an understatement?) I can' t remember but I think it was some film type, not a wirewound.

If I poked the cracked resistor so the two halves touched, it would actually run, with a colorful orange glow at the fracture point.

I either couldn't figure out an easy way to get the PCB out or was too lazy, or both, so I unsoldered each half resistor from the top side. There were two resistors. I think only one was cracked, but I'm getting into unreliable teritory here.

I replaced them with IIRC 5W metal oxide ones, soldered from the top, with a little space off the PCB. I think the originals looked like 1-2 W but can't remember. I considered WW as being more sturdy, but figured there was some reason they weren't used origiinally (maybe too high a value for WW).

Up & running again.

Now, having been enlightened this AM by the Axis schematic, I have another challenge.

Late 80's, early 90's or so, a friend in Aurora Illinois suffered a seriously flooded basement. He was out of town & his wife thoughtfully tried to haul his audio & PC gear upstairs. She wrestled a big Sola power conditioned part way up the stairs and dropped it, breaking her foot. Her rescue efforts ceased at this point, and his Linn Axis (dutifully powered up as they are), eventually dogpaddled around the basement, leashed by its power cord.

It wasn't as bad as Katrina, but he said the water was less than sanitary. He collected insurance and one day a box showed up o nmy doorstep via UPS with a note to keep it - if anyone he knew could revive it it was me. This Axis schematic was a, uh, Tim-send, shall we say.

I feel so much more enlightened now that I am considering what can be done to improve the working one. Now at least I know what I'm dealing with on the other one. The MDF base went to you know where in a handbag.

I'm contemplating ideas for an alternative base/plinth for the bloated/split one.

To mod my good one, I'm contemplating two options from the start: A better regulator for the +13.5 VDC source and possibly and external regulated +320 VDC source. I'll probably do this with the 'floater' Axis because the only VISIBLE damage was some charring at the AC input where the water must have had sufficient conductivity due to uh, 'dissolved solids' (this is a family website, after all). I haven't looked closely yet at the frequency source but that might be another possibility.

So, it's not hopeless, and it's just a project after all. I don't think anyone here would discourage me from trying to revive it (with all due caution - I'm experienced in power electronics).

Any recommendations for improvements on the working Axis (short of replacing it) ?

Thank you

Murray

[tkwou]

Hi multi-volt

I have actually built a regulated external power supply for the Axis using LM317 for both the 13.5V and the 320V. The high voltage regulator uses high voltage transistors to regulate most of the voltage drop and the LM317 does the fine tuning. Don't make the mistake I made though. Make sure you get a big enough transformer for the HV. That Axis board sinks a lot more power than I realized. I had a 15VA transformer kicking around and it was no where close to keeping up. I am in the process of ordering another transformer now. The problem is I already installed everything into a case and now I have limited the size of the transformer I can use.

[tkwou]

Well I did it again. This is a habit I've got break. I had my spare Axis power supply jury rigged to run my LP-12 while I working on the new power supply and in the process of a moving things around a wire jiggled loose and momentarily shorted one of the voltage doubling caps in the power supply to ground. There was a quick spark and the fuse blew on the power supply. I disconnected everything cursing my stupidity and laziness for not disconnecting everything before I moved it. Well I hoped the damage was limited but from past experience knew that it this was unlikely. Sure enough it wasn't. So two flips-flops, one quad switch , two quad opamps and a decoupling cap later the board is finally working again. So just a word of warning to you guys working on the Axis power supply. It is not tolerant of mistakes.