Bias Mechanism on Garrard SP25 Mark111

[Fiddlersthree]

Hello everybody, I am new to the forum. I have been trying to resurrect my 50-year-old Garrard SP Mark 111 and have found several golden nuggets of information here. In a previous thread Dr Bulgin raises the question of the positioning of the V-shaped bias spring under the little black cover on the Garrard SP 25 ( I am not sure whether his model was Mk3 or Mk4). One arm of this spring is connected to the little black plastic bias lever. Can anyone explain what happens to the other arm of the spring on the mk3 version. Does it just rest against the tone arm? Or should it be fastened in some way? The end of this arm of my spring is bent down at an angle of 90 degrees.

[indigo.girl]

https://www.vintage-radio.net/forum/...ad.php?t=64741

Hi - can you post a couple of photos please. I have recently stripped down a SP25 and am looking back through a few of my photos to see if I recorded it.

[Fiddlersthree]

Hi Indigo.Girl. Thanks for the offer of help. My own photographic possibilities are very limited. The best I can do is to refer you to a photograph of a replacement unit posted on ebay.

http://picclick.co.uk/Replacement-bi...610713077.html

It is the loose end of spring sticking out of the right side of the black cover that is the puzzle. Is this attached somewhere, or does it simply lie against the tone arm? The bent end looks as though it should be lodged inside or against something. The black cover screws onto the deck behind the tone arm.

[GP49000]

It hooks to a part of the tonearm mechanism.

It has been a long time since I tried to put back one of those. I recall it as being quite fiddly, and a tiny black nylon piece clipped to one of the levers on the tonearm mechanism, providing a hole into which the antiskate spring was to hook. That piece MAY be #156 in the exploded views in the service manual for the SP25 Mk VI, attached. The spring was supplied as part of the plastic antiskate cover but was also catalogued separately for the SP25 Mk VI.

Note that at tracking weights around 2 grams, which is the minimum for these units, antiskate may be detrimental. The pickup arm system has a constant drag, between the Pickup Lever and the Friction Link, that limits how light these can track, even as improvments were made to this part of the basic Autoslim mechanism since its introduction in 1960. Adding antiskate can make things worse.

[GP49000]

ERRATUM: In the above reply, I wrote, " That piece MAY be #156" but that was in error. I should have written, "That piece MAY be #155."

The Pickup Lever and Friction Link referred to are the two parts at the far bottom of the graphic.

[Edward Huggins]

Of the many Garrard versions of the SP25, I thought only the Decca "Deccadec" version with its Deram cartridge, could track as low as 2 grams. And this had no anti-skate feature.

[Fiddlersthree]

Many thanks GP49000. I dld notice a tiny attachment to the pickup lever when I first tried dismantling the auto mechanism, but at some point it got detached and I can no longer find it. Your comments about antiskate being detrimental is useful. Perhaps I shouldn't be too despondent about not being able to reconnect it.

[GP49000]

Originally Posted by Fiddlersthree

Many thanks GP49000. I dld notice a tiny attachment to the pickup lever when I first tried dismantling the auto mechanism, but at some point it got detached and I can no longer find it. Your comments about antiskate being detrimental is useful. Perhaps I shouldn't be too despondent about not being able to reconnect it.

Yes, it did tend to fall off and it was so tiny, it was difficult to find when it did! I recall it was also rather difficult to put back. Thankfully I didn't have to do more than a couple of them over many years, as after the first one I tried to leave the antiskating alone on that chassis. I still own an SP25 Mk V, and its antiskate has never been removed or serviced. Hopefully it won't have to be, within its lifetime or mine!

Originally Posted by Edward Huggins

Of the many Garrard versions of the SP25, I thought only the Decca "Deccadec" version with its Deram cartridge, could track as low as 2 grams. And this had no anti-skate feature.

The Deccadec version was mechanically identical in the pickup arm area, and if in good repair, any of the SP25s could do 2 grams, as could the record changer variants of the Autoslim chassis with the Delrin® antifriction insert for the auto trip slide, and an adjustable counterweight on the tonearm (if equipped with a cartridge that could track that low, the "60" series and the 3500/SLx); also the single-play 125SB and 35SB.

What made the Deccadec and Deram special was its low-tracking weight capability in a ceramic cartridge. It is a shame that since the cessation of Decca Special Products, the only aftermarket styli for it are so poor that its original performance cannot be achieved any more.

[Fiddlersthree]

Eureka! I found the small plastic clip at the bottom of the chassis box and have successfully reconnected things. Thanks to everybody on this site for their help. Have learnt a few things about anti-skate on the way.

[GP49000]

Very good! It IS tiny, isn't it? When setting the antiskate, which is approximate in any case because the skating force varies with lateral tracking error in a pivoted arm, one can use a grooveless record and set so that the arm does not slide inward when set down. But most of us don't have one. If you have a record where the modulated area ends farther out, so the "dead wax" leadout area is wider, you can set the arm down there, using the cueing lever, watching very carefully to see what it does when you "hit" between the engraved leadout groove. This is tedious, as once the stylus drops into the leadout groove, the auto trip will initiate the tonearm return and you'll have to repeat the process over and over. Use the minimum antiskate setting to achieve no inward drift. For optimum tracking you should also have serviced the auto trip so it is operating perfectly freely and dry...without any lubricant*. The setting with a grooveless record is often said to be slightly less than in a groove, but skating is generated by friction and friction is theoretically dependent only on coefficient of friction and the tracking weight (secondary school physics!). Also, theoretically antiskate should still allow the arm to move inward very slowly, at about the tracking rate of modulated grooves. Skating force also varies with groove modulation so any antiskate setting will be a further approximation anyway.

* Sticky post on auto trip servicing on Garrard Autoslim-chassis, with diagram:

https://www.vintage-radio.net/forum/...d.php?t=112708

[Fiddlersthree]

Many thanks, GP49000, for sharing this little trick for setting the anti-skate without a trackless record, I'll give it a go. On another point, is the scale on the tracking weight dial supposed to correspond to grams. If I set this at 2, do I get about 2 grams of tracking weight? I would have thought the amount of wear on the spring would be a critical determinant here.

[GP49000]

Yes, that is correct. There is really not much weakening on the spring due to tension over age, though. Typically there is about a 10% error among various samples of that mechanism unless the spring has been stretched accidentally during repair. Don't ask how I know.

Garrard provided an adjustment screw on one anchor of the tracking weight spring on most of its newer dynamically balanced tonearms beginning with Lab.80 (AT6-AT60-SP25 Mk I/II, A70-70 Mk II don't have it). It is not even identified as such in the Service Manuals and only provides slight adjustment. The one on the SL65B and SP25 Mk III-IV is on the front of the tonearm pivot housing; it moves the front anchor of the spring. But if you unscrew it too far it will fall off. I don't bother and don't suggest anybody do it unless you want to get into putting it back which is no picnic; the easiest way is to disconnect the back end of the spring first, then put back the adjuster and its screw, and then re-hook the back end.

[Fiddlersthree]

Thanks again GP49000 for this information. I can see the adjustment screw on the tonearm pivot house. I don't see any simple way to check the calibration of the tracking weight mechanism and, as the turntable seems to be playing quite nicely, I think I'll leave this well alone. I've certainly not forced the spring so I think it should be OK.

[Edward Huggins]

Even if the calibration is not particularly accurate (and it isn't on these) you can always weigh the tone arm to ensure an accurate 2.5/3 grams tracking weight.