Transforming a Leslie 710 into a 770

This article describes the conversion of a 710 Rotosonic Leslie into a 770 model. The modification involves:

    • Making a new bass rotor drum
    • Making a bass rotor upper bearing support strap
    • Moving the bass rotor lower bearing position
    • Making and fitting a new shelf
    • modifying the lower motor to operate the other way up (to make space for a future valve amplifier)
    • Making an electronic crossover
    • Repairing and refinishing the case


The Leslie 710

Stripped Carcase

The 710 is a Rotosonic Leslie whereby the lower rotor houses a spinning elliptical speaker. The upper rotor is a conventional treble horn. Two static elliptical speakers and a 15 inch bass driver complete the line up. The amplification is provided by two 50 W transistor amplifiers. Rotosonic Leslies are not considered as desirable as those with a rotating drum beneath the 15 inch bass driver. I bought mine from Ebay as 'spares or repair' for about £70. Apart from it "beat-up" state, the only fault I have found is an open circuit voice coil on the rotating speaker. I suspect this is a common fault in these models; this speaker should not receive low frequencies but there is no internal filter to protect it and connecting it to the wrong organ will direct damaging low frequencies to the small rotating speaker.


The image shows the part-stripped carcase. The bottom shelf has yet to come out.




New Bass Rotor

I designed the rotor from photographs of actual wooden rotors along with the outside dimensions given in an Ebay advert. This is the second one I made. The first was too heavy and I was unable to balance it adequately. This is made from HDF, 6 mm for the top and 10mm MDF for the sides. The curved deflector was laminated from 3-layers of 1mm hardwood, ash. I made templates for the top, bottom and curve of the sides and routed the components on a pin router.

I solved the balance problem by calculation, using a spreadsheet to perform a finite element analysis to give dynamic balance. The dimensions of the centre bracing fillet was adjusted to give balance at the bottom and a small weight added for the top balance. The spreadsheet indicated its mass and position and a static balance test on the finished rotor confirmed it. This second attempt rotor runs at high speed with barely detectable vibration.



Carcase Modification

Carcase with repositioned bearing

The bottom bearing had to be moved to provide the correct positioning of the bass rotor. A section was routed out and rebated then a section of 19 mm plywood was let in and glued and screwed, then the new bearing recess was cut with the router

The shelf was not completely removed but routed out using the inside surface as a guide for the router, leaving the outer edge. A rebate was cut to provide a large  glueing area and new shelf was made from 19 mm plywood.New Shelf






Motor Modifications

I wanted to mount the motor above the lower shelf, a change from the 710 design, so that space beneath would be freed up for a valve amplifier. Apart from the mounting arrangements, the only motor changes necessary were to the low speed motor where the existing rotor spring had to be remove and a new, stronger one fitted to the opposite end to the armature. This spring causes the low speed motor to disengage when the power is removed. I obtained it from the Tonewheel General Hospital in the USA Great service! While I had the motors apart, I oiled the bearing felts. I used tonewhell oil applied with a tiny plastic pipette as used in fingernail artwork.

Leslie shelf 

This is a trial with the first rotor. It seemed OK at low speed but was hopelessly unbalanced at high speed, despite having been balanced statically on a pair of knife edges (rules). The top bracket shown here also proved inadequate. I experimented with various weights on both the top and bottom flanges but was unable to improve the balance sufficiently. In industry, or even the tyre centre, instrumentation would be employed to calculate the required weights but my own resources would not accommodate that solution.





completed_pcbHere is the completed crossover PCB. It is constructed more or less to the original circuit diagram of the 760/770 crossover board. Before building,I redrew the circuit in LTspice and checked its behaviour. This also provided a means of generating the netlist. After making the board, I tested it by injecting a swept sine wave signal, 20 hz to 20 khz into the input, whilst monitoring the output using the free-wareHOLMImpulse . This programme is designed for measuring the impulse response of a room but proved excellent in this role too.

The circuit diagram is shown below. This is the output from LTspice. Note that parallel capacitors are used in two locations to make no longer available values of 33nF. Well I actually settled for 32nF. The simulation showed very little difference due to the error.







Frequency Response

Here are the results of the LTspice simulation and the actual measurements. Their similarity is remarkably good so I guess I got the circuit right. The crossover frequency is slightly lower than advertised. It is about 750 hz whereas the usually quoted figure is 800 hz. However, you can see from the simulation that if the treble gain was dropped by 2dB, the crossover frequency (or at least the point at which these lines crossed) would increase to about 800hz.





Here is the crossover installed on the amplifier plate.

I neglected to fit a series resistor in the positive supply rail. It should have been on the PCB but I realised too late. I first powered up the speaker without the resistor and it made a popping noise about about once per second, so called "motor-boating". Fitting the series resistor solved the problem, although it's a bodge job.



Top Bearing Bracket

This is the second incarnation of the top bracket, made from 1.2mm mild steel sheet formed into a channel. The first version was not stiff enough, allowing the bearing to wobble. Here it is, painted black and fitted.




I fitted a shroud  to the bottom rotor for good measure.



Carcase Repairs and Finishing

This was the biggest job of all and, despite good intentions, is still not as  good as I would wish.

The carcase had been re-finished before; lots of filler was evident. This was removed with paint stripper followed by many hours of hand sanding. This removed many of the scratches and small dings leaving just the more serious damage to be repaired.

I bought a selection of cherry veneer offcuts and set about repairing some of the damage. Below you can see a top corner. I cut off the splintered corner and made a new piece from sycamore, keeping the mitre as before. You can also see a piece of veneer that was replaced. All of the front corners were repaired in some way or other and numerous deep scars to the vertical rails either side of the front were repaired with inserts. Pock mark damage to the veneer was repaired using a punch to cut both the damaged area and the insert.


A light satin was applied to soften the contrast between old wood and new, though this will ( I hope) fade with exposure to light.

Then the box was French polished. Here are some shots of the finished job.





Pin It