Mission Impossible - I think so - Old italian Davoli amp preamp problem

[kagliostro]

A friend has an old italian amp a Davoli Show V

it is a transistor + tube amp

he has a problem in the preamp (transistor) section

the problem is located in a potted module (a metal box + resin)

that is near impossible to open without damage further the content

The module name is AUTOTONE (see the attached schematic)

and till now to find a schematic was impossible

There is someone who has an idea about the schematic of this module ??

Thanks

K

[PRR]

You don't need that module to "get sound".

You should have midrange, bass, treble, and volume, signal out to 60W power amp.

There is the Reverb section, and I do not understand the switching.

If you put a line-level signal in at Effects, put the lower-left switch at Bianco side, put the top-right switch to the right, you should have a mix of dry/reverb available at the volume control and power amplifier.

However there ought to be a way to have all the tone controls AND the reverb on the same signal. The switching is confusing.

The "Autotone" may do something more, but the lower-left switch allows you to bypass it, so it should not be essential.

[kagliostro]

Ciao PRR

Thanks for the attention

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EDIT: The switch is a DPDT are two SPDT, so both the section aren't acted at the same time

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Someone advanced the hypothesis that the AUTOTONE acts as a Vibrato or echo circuit, but I can't confirm

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I'm really in trouble - if the switch confuses you, imagine me  

EDIT: I forgot to say that the switch in the front panel is labeled

left:     Guitar Jazz Only

right:   Guitar Pitches or bass Instruments

this give me the idea that the Autotone has nothing to do with Vibrato or Echo

EDIT: The other switch (that acts the Autotone insertion) is incorporated to the pot

Franco

[Willabe]

I think it's a tone shaping filter.   


            Brad     

[PRR]

What works? What does not work?

Do you have roughly the right DC voltages? They are all very variable, but Collectors should be in the 10V-25V range except TR4 should be 40V-50V and TR5 1V-2V. If way outside these ranges (like 2V or 80V) then fix that stage. Coupling caps from one stage to the next may be leaking DC and upsetting the DC voltages.

If you go into the Normal input, adjust Volume, and flip that switch one way then the other, does it play as a guitar amplifier with Mid Treb and Bass controls? When the upper-right switch is in its Left position, it should play.

Take the Bianco and Verde wires off the switch (make notes/photo!) and set them aside. Connect the wire from Effects jack (was on the Bianco pin) and connect to the third pin of the switch (so this pole of the switch does nothing). Now signal runs TR3 TR5 TR4, to spring, TR6 to Reverb knob; also direct to the other side of reverb knob. Now the switch selects whether signal runs through TR1 TR2 (and tone knobs) or direct to power amp. You probably need TR1 TR2 for guitar; the other way when you have a strong signal which already has tone-control and you just want reverb.

[kagliostro]

Thanks for the indication PRR

I'll investigate further

Franco

p.s.: I corrected some wrong info I give in the previous post (SORRY)

[kagliostro]

Saturday I was in Marzaglia for the September edition of the biggest Italian Radio Amateur Flea Market

there I've find a guy I've known in the same place a pair of years ago

today I received from him the schematic of the Autotone Module

now the only question is about L3 that has an unknown value

and the 4p34 diodes

K

[LHPcope]

What an interesting circuit!  L3 and the 3.3pf cap form a tank circuit that resonates at some frequency forming the load for that transistor.  Not only do you need to know the inductance of L3, you need to know the DCR as well in order to know the Q of the tank circuit.  One possible course of action would be to breadboard this thing up and start experimenting with inductors, or better, first feed it to Pspice.  Keep in mind that depending on the physical makeup of L3 there will be some additional distributed capacitance which will figure into the resonant frequency of the tank.

Have you ever heard one of these amps in action to know what this autotone circuit is supposed to sound like?  It seems, like among other things, there is a 2nd order resonant peak in the frequency response.  

Also, the 4p34 diodes form a clipper, no?  Is that 500k pot an external control or is it a trimmer that gets set before the epoxy is applied?

[kagliostro]

No, sorry, I didn't hear that amp

the guy who asked me the schematic and to whom I sent the schematic found

told me he'll try to dismount the original autotone circuit (removing the resin that covers it)

and to measure the inductor

He isn't trying to build a clone or to replicate part of the circuit, he only want to repair an amp

K