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Sansui AU-666 refurbishment / revisie Sansui AU 666 picture

Deze Sansui AU666 versterker is een zwaar gebouwd apparaat van goede bouwkwaliteit, maar het beschikbare schema is een ander verhaal, op zijn zachtst gezegd klopt het niet helemaal.

Modificaties zijn gedaan naar eigen goeddunken, onder andere:

De "crowbar" luidsprekerbeveiligings schakeling die de versterker moet kortsluiten indien hij defect raakt is vervangen door een eigenbouw schakeling met een relais. Het relais is een professioneel type dat ook in de luchtvaart wordt gebruikt en nieuw ca 140 euro ex BTW kost!

De eindversterker en voedings printen zijn opnieuw bestukt en hier en daar aangepast.

Alle electrolytische condensatoren zijn vervangen en alle transistoren zijn vervangen. R709/710 van 10k naar 6k8 voor verminderde input gevoeligheid.

This Sansui amplifier is a sturdy built one, however, the schematic available is "less accurate" at best.

Some modifications were made just the way I wanted it.

The odd Crowbar type loudspeaker protection was replaced by a relay circuit, and the power and amplifier boards were repopulated.

All electrolytic caps were replaced, partly having bigger capacitance and/or voltage rating.

All transistors were replaced, the small ones using KSA992 and KSA1845. Additionally, for less sensitivity, R709 and R710 go from 10k to 6k8

Generally, all replaced resistors are metal film types, tested for being same value for left and right channel.

Eerst uit elkaar halen en printen leegmaken/ First, take it apart.

sansui au666 kabelsalat

Lege versterkerprint en een opnieuw opgebouwde, nog niet gemodificeerde.

Bare main amplifier circuit card, and an unmodified newly populated one, before deciding on modification.

A few resistors on the boards were of the wrong value and adapted for the AU999, some resistors were obsolete (literally going to nowhere) and taken out.

Probably, Sansui simply used circuit cards intended to be put in AU999 amplifiers during production.

The nulling circuit is modified to get nulling within a few millivolts instead of the large 100mV max deviation in the specifications of this amp. The use of well matched modern input transistors makes this possible.

Bestukte printen / populated boards. The tone amp was not totally taken apart, only transistors, electrolytics, and a few resistors were exchanged on this one, the resistors on specific positions where noise contribution of the old ones was expected.

sansui au666 circuit cards

Het schema van de gemodificeerde hoofdversterkers, het vreemde filter is ook verwijderd. Het blijkt een "warp filter"te zijn.

The schematic of the modified main amp. The combined filter, which was a low frequency rolloff higher than 4 hertz is removed and the high-pass which influences frequencies below some 50 hertz already, also.

A nice gentleman put the SPICE simulation of the filter for me on the audiokarma website. It can be seen for people registrated only. See answer in post 4 here on Audiokarma

It seemed, nobody knew why this EXTRA filter was in, and restorers tend to take it out on the AU-666 and also on the AU-999 where it found in some of them but not all..

In the end, it was concluded it was intended as some "warp filter" to compensate for warped records, something you see hardly ever used.

sansui au666 schematic

Veel bedrading aan de onderkant, en de situatie na revisie.

There is a lot of wiring at the bottom, I wanted to route it as neat as possible.

Vieze voorversterker, en het vergelijken van het beveiligingsprintje met het schema, het printje bevat minder onderdelen en 2 zelfs verschillende triacs, in het schema staat echter een thyristor schakeling....

The dirty preamp, and the protection board being compared to the schematic. The schematic specifies a SCR setup, the circuit card in this amp contained 2 different triacs and less other components.

sansui au666 crowbar

Het nieuwe prototype beveiligingscircuit en het inbouwen van het definitieve.

A picture of the protection relay board prototype, and one of the final one being mounted in the amplifier.

The relay is a totally sealed type which is used in aircraft/military equipment also. New price of such relays is well over $100

sansui au666 loudspeaker protection sansui au666 luidsprekerbeveiliging

Schema van het beveiligingscircuit / Schematic of the protection circuit.

The lamp was defective and its rubber was burnt, so a LED and a nice very professional Bourns potentiometer to tweak LED brightness were used, instead. Picture on the right: The testing of the main amplifier boards.

I use a regulated, current protected power supply and an extra filter board including rectifiers for this, to eliminate possible mistakes and resulting damage or unnoticed deterioration.

Het eindresultaat / The result.

Click here for the TU666

Check out this Sansui.us site

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