Sunday, November 27, 2005

Living together with a Revox A77

Joe wrote: "hello, I am looking for a schematic for a revox A77. I plugged mine in today for the first time in 25 years, and a resistor fried in the switch relay circuit board. If you could be of any help, I would appreciate it. thanks, Joe"

The A77 is probably the best tape-recorder ever made. Using good tape like the Maxell it outperforms all recent DAD-recorders.

BUT after 30 years most electrolytic capacitors have dried out and many resistors changed their value. Record & reply heads will have to recalibrated.

There is no easy way to revise an A77, it will take months of work. And a lot of surfing the Internet to find certain original components...

But when it works as it should, there is no better recorder.


Saturday, November 19, 2005

The Quad 405 Loudspeaker Protection Circuit

This post contains pictures that might shock sensitive Quad-perfectionists. If you read ahead it's on your own risk!

I never felt very happy with Quad's LS protection circuit.

In fact all it does is short-circuiting the outputs if there is a DC-component in the speaker-signal.

Word goes that in the 405 MK II the protection circuit has been improved. I'm sorry to say that's not a fact. The only thing that changed is that it was integrated on the circuit-boards. In the 405 MK I it was soldered to the LS-plugs.

2 Weeks ago I got an e-mail from Olivier, who bought a 405 on Ebay I revised and sold on Ebay a few months ago:

"Bonjour, je viens de faire l'acquisition d'un ampli Quad 405 modifié par vos soins à Chagoumy (pseudo ebay). L'essai chez lui s'était avéré positif. Je ramène l'ampli chez moi et clac: bruit énorme sur la voie pire. Le tweeter des 104/2 (voie droite) grille !! Je ne comprends pas. Je rééssaye sur un autre système. Idem."

One of the BDY77 endstage-transistors was grilled and the 405 has the bad attitude to put -50 Volt on the speaker-outputs when this happens. Unfortunately in Olivier's 405 someone had replaced the protection-circuit with 4A fuses. This will protect the speakers against overload but not against DC-voltage.

Olivier's 405 is up and running again but it was time to think about a better protection-circuit. A circuit that monitors the output and switches on the speakers after a few seconds when everything is OK and that switches them off immediately if there is a DC-component or when the power is cut.

I heard about the Velleman K4700 protection circuit (see and surf to "projects"), so I built one and tried to put it in the 405.

But where?

There is not much space in a 405 so I had to screw it on the top-cover. Anybody who has a better idea will be rewarded with a box of Belgian Chocolates.

Anyway, it's ugly but it works fine ;-(


Wednesday, November 16, 2005

Quad 405 Tansformer rumble ....

Some older 405's have a problem with the transformer. It produces a rumble (not in the speakers but in the transformer itself).

There is no way to fix this problem except by replacing the transformer.

Of course you can order a new transformer from Quad but this is a quite expensive solution. Therefor this alternative: one can fit a toroidal transformer (300VA 2x 35V). This also offers the advantage of better stability and somewhat less noise, limiting the cost to about 90 Euro.

The only problem: to fit it in you will need a special mounting plate. I found one on Ebay in the UK, Stuart Adamson had them made by a steel manufacturer. I also had to move the fuse because it was in the way (see the picture).

But now my 405 works fine again ;-)

PS. The transformer you need is a Toroidal 2x 35 or 2x 36 Volt, 300 VA (not less!). Make sure it will fit in.


A 300VA toroïdal transformer and the mounting-plate are now available in the Dada Electronics Webshop.