125SB Tonearm Resting Position

the jewel in the crown
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turnitdown
Great Britain
Posts: 3
Joined: 08 Sep 2019 17:18

125SB Tonearm Resting Position

Post by turnitdown » 08 Sep 2019 17:29

Hi,

I bought a 125SB from ebay a couple of years ago and it didn't arrive in a great state and nor have I ever bothered to fix the issues before now. It has the auto play issue that I've seen 100 other posts about as well, but the first one I really want to try and fix is that the tonearm doesn't sit in it's rest. When I try to move it towards the rest there is resistance that tells me it needs to be adjusted.

I looked in the manual and have turned the screw that is supposed to move the arm outwards but if anything it made the problem worse and now to get it to sit in the rest requires considerable force.

I've uploaded a video that shows the problem: https://youtu.be/Y2bIDhJeAkE

Look at the way that it springs out when it is unclipped. You can also hear that the arm is hitting something that stops it from sitting in the rest.

I won't clip it back in again for fear of making the problem worse but has anyone got any tips on how this might be fixed? I've looked in the manual and service manual but couldn't see anything about this issue.

Thanks in advance!

DSJR
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Re: 125SB Tonearm Resting Position

Post by DSJR » 08 Sep 2019 18:59

There's so much plastic around the tonearm bearings I'd ask you to PLEASE be careful for fear of breakages...

Can I ask you to have a good gander at the autoslim service manuals upon which this deck is descended? I know there are differences but the underpinnings shouldn't be hugely different. I used to use the old SL65 as master manual, but the SP25 III is the better one for this deck as belt drive apart, the mech works very similarly.

One thing to check before taking stuff apart - the bias correction assembly (anti-skate). I I can't describe it in engineering terms but it used to be a shallow light spring with two 'arms' and the correction was done by the tonearm gently compressing one of the arms against the other as the arm moved across, the spring kind of moving with it with the 'other' arm secured in the plastic housing. The settings for this adjusted this spring as to to how much force was given to the arm bearing bracket as it pivoted in use. I'm so sorry to describe this so badly but if you study the SP25 service manuals from the mk III onwards you may get an inkling of what I'm desperately trying to describe. IF this sporing is mis set somehow I can quite easily visualise it doing nasty things to the tonearm bearing bracket it locates into.

I hope the above bulls**t description helps a bit. The arm is usually almost disconnected from the mechanism when in 'play' mode. there is a 'friction link' which all autoslims use (slightly different less intrusive shape in decks from SL65/SP25 III era onwards) and this shouldn't be greasy or binding to the tonearm's undercarriage. if lube is needed, a tiny drop of light oil is enough I think.

turnitdown
Great Britain
Posts: 3
Joined: 08 Sep 2019 17:18

Re: 125SB Tonearm Resting Position

Post by turnitdown » 08 Sep 2019 19:50

DSJR wrote:
08 Sep 2019 18:59
There's so much plastic around the tonearm bearings I'd ask you to PLEASE be careful for fear of breakages...

Can I ask you to have a good gander at the autoslim service manuals upon which this deck is descended? I know there are differences but the underpinnings shouldn't be hugely different. I used to use the old SL65 as master manual, but the SP25 III is the better one for this deck as belt drive apart, the mech works very similarly.

One thing to check before taking stuff apart - the bias correction assembly (anti-skate). I I can't describe it in engineering terms but it used to be a shallow light spring with two 'arms' and the correction was done by the tonearm gently compressing one of the arms against the other as the arm moved across, the spring kind of moving with it with the 'other' arm secured in the plastic housing. The settings for this adjusted this spring as to to how much force was given to the arm bearing bracket as it pivoted in use. I'm so sorry to describe this so badly but if you study the SP25 service manuals from the mk III onwards you may get an inkling of what I'm desperately trying to describe. IF this sporing is mis set somehow I can quite easily visualise it doing nasty things to the tonearm bearing bracket it locates into.

I hope the above bulls**t description helps a bit. The arm is usually almost disconnected from the mechanism when in 'play' mode. there is a 'friction link' which all autoslims use (slightly different less intrusive shape in decks from SL65/SP25 III era onwards) and this shouldn't be greasy or binding to the tonearm's undercarriage. if lube is needed, a tiny drop of light oil is enough I think.
Thank you, that's given me something to go off.

I've just tried to have a look at the assembly but it doesn't seem like I can do this from the top as the repeat lever prevents the panel from being taken off (see photo).
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Apologies for what might be a silly question but I'm very new to this and cautious that I may break something.

turnitdown
Great Britain
Posts: 3
Joined: 08 Sep 2019 17:18

Re: 125SB Tonearm Resting Position

Post by turnitdown » 09 Sep 2019 22:10

So I managed to get the unit apart this evening and found what is causing the arm to be restricted and the lower tonearm assembly is hitting the silver pole before it can travel to its rest.
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From the manual it looks like the silver pole is part of the lower casting assembly and doesn't move. From what I can tell it looks like the lower tonearm assembly isn't positioned right but I couldn't see anything that might let me adjust this. It seems well greased and not at all stuck.

Again, any assistance would be appreciated!

EDIT: From another read through of the service manual I believe what is restricting the tonearm is the pickup cam pivot pin?

DSJR
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Re: 125SB Tonearm Resting Position

Post by DSJR » 10 Sep 2019 17:09

The vaguely fork shaped friction lever which, when engaged, pulls and pushes the arm around, is caked in grease, which I believe should be totally cleaned off and used either totally dry, or with a fine finger smear of light oil on each side. this friction lever should pivot freely - it's connected to the main mech plate by i think, the C clip shown in your pictures. The curved slot in the mech plate which runs on the guide as pictured MUST be greased lightly as this is the main load bearing part of the mechanism and wear or looseness here (in the riveted 'anchor') causes the arm to 'dance' up and down half way through the cycle as the mech plate eventually begins to move back to rest position (you need to watch it underneath).

The arm horizontal bearings either locate or not. The bracket is vaguely an elongated straight-side C shape with a fixed tapered 'stud' pointing upwards into a sleeve in the main casting (I think there's a ball inside it locates on) and if memory serves, the top stud is attached to the main tonearm cradle fixing screw, accessible if the arm 'wand' assembly is removed first. On top in the bottom of the upper sleeve, there's a spring and ball in the casting this screw-down stud acts on to give some form of loading (all the Autoslims are the same here). I'd not try to remove this as there's an elongated wire 'spring' that'll fly off and be difficult to re-fit unless you know these decks. Poster A70BBen here once put up a pic of the similar '50' arrangement for this safety wire 'spring' but you'll have to trawl back a couple of years or more to find it and it's a pain to locate while the arm cradle is refitted - especially for now older fingers like mine.

Does this help a touch more so far?

HiFi Choice in 1976 tested the 125SB and their sample had phenomenally low tonearm bearing friction horizontally of 40mg (the competing BSR BDS 80 was 150mg in comparison). With all greases removed from the tonearm linkages, I suspect many of these later models could get close, but today and with grease in the top and bottom tonearm bearing sleeves as supplied in many of these decks, I'm not sure. The auto trip parts looked unchanged from the late 60's models but I could be wrong.

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