Fitting a different arm on your Lenco can be a problem, here's some hints for getting it right. 

First let's agree that a pivoted arm needs setting up with some care to ensure best performance with least distortion and no record damage (worth hoping).  Fortunately clever people who know much more than me have been working on this for a long time.  Today the accepted wisdom states that the arc described by the stylus across the disc should allow the stylus to be at a tangent to the groove in 2 places.  Unfortunately, you guessed it, there are several conflicting theories about where these 2 places are.

I recommend that anyone about to change/setup an arm should read about the various theories relating to this on the net, and decide which you prefer.  Then get the relevant alignment protractor and learn how to use it.  Here's a great place to start click.

Here's a site with loads of turntable setup info that unfortunately stopped working recently, but may be worth a click to see if it's back in business. click

L75, L78, or GL75, GL78 - the most popular and readily available heavy platter Lencos.

The L75 and L78 decks use the same arm with the 78 having an autolift at end of side.  They should be no problem to remove for most folks who like to mess with record players, however if you are not mechanically minded or wary of the task - don't try it, get help from someone who can. 

The platter should be removed to allow access to the cable connecting terminals, which should be under a clear plastic cover.  Desolder or cut the thin wires from the terminal, you may also decide to remove the thicker output cables (the ones that go to your preamp) and do away with the terminal block altogether.  It's a good idea to replace the fixing screw to use later as a common earth point.

The arm counterweight and any removable bits like the anti skate fixings/weight should be removed.  Remove the headshell. 

Arrange the deck safely so you can access the underside and remove the plinth base.  Pull the disconnected arm wires through the hole and ensure all cables connected to, or coming out of, the arm are attached only to the arm.

Back with the deck right way up, use a small flat blade screwdriver to release the 2 grub screws in the arm collar.  Support the arm as they loosen or it will drop, not fatal but it will make you jump!  Once the arm feels loose in the collar pull up slowly and it should come right out easily with the cables trailing.  Some pics of the process here.

If you measure from the centre spindle to the centre of the collar you should get a dimension of 210mm, this is the fixing distance for a standard 9" (228/9mm) effective length pickup arm.  It allows for 18mm stylus overhang.  Several good arms are available with these same specs, VinylEngine has a useful list of arms.  If you have an unknown arm ........

These 3 dimensions - effective length, spindle to pivot, and overhang are usually supplied by the arm manufacturer, but sometimes they only quote the effective length, and sometimes you know nothing!  The calculations that tie all these dimensions together also take into account the offset angle of the headshell.  They are beyond me, so I refer to the several lists that are on the net to find out what the spindle to pivot distance of mystery arms should be. 

Just measure from the pivot centre to a point halfway along the headshell cartridge mounting slots, add 9.5mm (an average stylus to mounting holes distance).  This is the effective length of the arm.  Find this dimension in the list and read off the spindle to pivot dimension.  Measure the offset angle of the headshell also because it can vary with different length arms.  The lists show the different dimensions for various offset angles.

One arm that many folks like is the Rega 250, like all Rega arms the manufacturer quotes 237mm effective length and 222mm spindle to pivot.  Word on the net is that 219.5mm is a better spindle to pivot distance to use.

Another good link for arm setup click.

What follows can be dangerous - do it at your own risk.

If you're lucky the arm you have is 9" effective length, fits perfectly in the 1" (75/78) Lenco collar, and will adjust down enough to get correct vta with a standard mat on the platter.  Tall order? Yes - but Linn arms like the LVV and LVX do!

OK it's not a Linn, and it's not 9", and you don't want to cut the corner off the deck, and you're prepared to use 2 mats, or cartridge spacers to get correct vta.

Let's say you got a Rega and you want to set it at 219.5mm.  Grab a piece of 2x1 about 18" long, and drill an 8mm hole near one end. Measure 219.5mm from the centre of this hole and mark and drill another hole that your trusty pencil will fit in.  Use this device to draw an arc across the corner of the deck.  Check the measurement and adjust if necessary.  Draw a line parallel to the right edge of the deck that passes thro' the centre of the Lenco collar.  Where these 2 lines cross is where you need to drill the hole for the Rega pillar.  Obviously the Lenco collar has to be removed, and it pays to clamp a piece of mdf under the position where you need to drill.  Use a hole saw/cutter to cut the hole, the metal is quite tough,  take your time.  The hole will probably break into the Lenco hole but the mdf will help to stabilise the cutter. 

Feb 2006 Thorens TP13/TP13A arms are close at 230mm eff length.  Here's one I tried earlier.

This is a light platter GL72 deck plate corner with holes at 200, 210 and 220mm from the centre spindle.  The 200mm (left) setting works for the Connoisseur SAU2 arm and several S shaped arms from Japanese 80's decks, the 210mm setting (original) could be used for Linn arms with some drilling, and the 220mm setting (small pilot hole on the right) could be opened up for Regas.  If the holes were formed into a long slot, the deck would be very flexible for mounting different arms and fine tuning the pivot to spindle distance.