Davoli Echo Music restoration

[saint gillis]

  Hello I'm restoring this nice tape echo chamber.
An inductor has fried, it is a inductor taped with two winding, you can see it at to top-righ corner of the schematic :
https://groupdiy.com/index.php?action=dlattach;topic=71223.0;attach=65526

The two windings are in a network with a 2n capacitor next to the EL95's anode. Is it an oscillator? Do any of you have an idea of the inductors' values? or what kind of value would work...

 

[analogguru]

Appears to be the bias-oscillator (from the schematic I assume that this unit has a permanent magnet for erase head).

 

[saint gillis]

Yes there is a round piece at the right of the playback head that seems to be a magnet.
I quite embarrassed because I don't know how to replace this inductor...

 

[analogguru]

I suspect it will be a custom-made transformer.  Can  you make a picture of this inductor/coils/transformer ?

 

[PRR]

I have a wild idea it is just an AM radio local oscillator coil. Function is similar (but AM LOs came in *many* variations). The fixed caps are much bigger than an AM radio's tuning cap, and just might put it down into ultrasonic zone.

Without much more autopsy, or a Davoli fan-club, there's no real way to know.

 

[saint gillis]

Here's a pic of a good working inductor from a website (not from my unit)
I'll try to estimate the number of turns on mine...

 

[saint gillis]

Here's the schematic of another version that is slightly different :
http://musiqueindustrie.free.fr/page50/page57/files/davoli5.jpg

 

[saint gillis]

Now my self looks like a plate of spaghettis..
I've resolders the pieces of wires that seemed broken, and with my cheap LC meter I measure 2mH for the big winding, and 200µH for the small one, I don't know if it makes sens now I'll try to count the turns...

 

[saint gillis]

So the wire seems to be 33 AWG (0.18mm).
The small inductor wire is 5100mm (5.1m) long.
The big inductor wire is 27000mm (27m) long.

The small inductor is wired in the inside so it's diameter is ~9mm.
The big inductor is wired at the outside, it's average diameter is ~12mm.

So the small one has 180 turns, and the big one has 716 turns.

It's all on a piece of plastic, so I suppose the relative permeability is 1µR.

So, according to this : https://www.eeweb.com/tools/coil-inductance
I find a theoretical 730µH for the small inductor and 16.5mH for the big inductor.

I don't know if it makes any sense...

 

[saint gillis]

So fyi, I used 680µH and 15mH inductors.
I get a 25kHz 240v peak oscillator.
And it work very well.