REVOX A740 POWER AMPLIFIER, a good story

charles

AVForums Super Veteran
Joined
Feb 13, 2011
Messages
1,476
Reaction score
130
Location
Pretoria
REVOX-A740 STEREO POWER AMPLIFIER, a good story

This is not meant for the Vintage section as this amplifier may possibly be one of the very well made audio amplifiers, even when comparing it with today?s ones.

A couple of months I obtained a very in-pieces a Revox A740 audio amplifier. I had it at the back of a bakkie on a thick sponge when I took it home. It is a very heavy and big amplifier. I is very well designed and manufactured. Balanced and unbalanced inputs and a a so called ?double VU meter display?. 

I have done lots of restorations, mainly on valve amplifiers, but this one looked a good candidate for a spare/scrap components application. The top and one side cover were missing. All the wires of the big mains transformer were cut off the voltage selector and there were lots of other faults.   

I had a good look at it and though maybe, maybe something could be done with this highly collectable Revox audio amplifier. 

After a discussion with Dewald regarding the possible restoration of the amplifier, he indicated that there may a possibility to do it.
Well, after some time Dewald did the impossible, he restored it to its former glory, now it is working perfectly. For this I am very grateful to Dewald, a real professional restorer.

Dewald will now continue the story with some photographs and a discussion regarding the whole restoration process.   

 

Family_Dog

AVForums Grandmaster
Staff member
Administrator
Moderator
Joined
May 17, 2006
Messages
11,934
Reaction score
967
Location
North West
Charles, looking forward to this, glad you were able to save this beauty!!

-F_D
 

ghostinthemachine

AVForums Grandmaster
Joined
Jan 9, 2009
Messages
5,346
Reaction score
255
Thank you for the challenge Charles!

I will load photos tomorrow when I get a chance but I have made a video that's loaded on my YouTube channel and can be viewed here:
https://youtu.be/HlQPLkgfW3k

Apart from the amplifier being in a terrible physical state as Charles mentioned it had numerous electronic faults, which I believe prompted the breakdown of the unit. The previous technician failed to pinpoint the real issue.

The amplifier uses a pair of positive voltage regulators in the predriver power supply. The way they are connected enables the designer to obtain a positive 65 and negative 64 volts DC rails. If this is not known then the tech will be lost. Next is the grounding of these two regulator systems are done on the amplifier boards and if that is also not known then the tech will never be able to get a proper reading of the supplies. That out of the way... Repair and restoration could begin.

The missing lid was recreated from images off the web in a CAD program and was then made by a metal fabricator. The new replica lid fits well. A pair of output transistor cover replicas were also made.

LEDs where fitted to the VU meters in place of the festoon bulbs.

This is a fine example of a pedigree power amplifier. It tests extremely well on the Audio Precision analyser. 0.006% THD, 1 watt into 8 ohms. Frequency response is very linear.

The topology is a complete complementary system from input to output with global feedback. There is only a blocking capacitor on the input.

Protection is a very simple crowbar system on the secondary of the power supply that knocks the mains fuse out in a flash.

There are no relays anywhere. The attenuators are true stepped type with a resistor ladder.

The VU meters are just wonderful. They work on a negative movement system where they sit at 0 when powered down and when the power is turned on they get pulled down to the far left hand side. Their ballistics are insanely quick.

It is a wonderful sounding amplifier worthy of both the recording studio and highend home system. Not to bad for an amplifier from the mid 70s.
 

HC1

AVForums Member
Joined
Oct 12, 2021
Messages
88
Reaction score
37
Location
pretoria
Congratulations Charles and Dewald.
What a gem. A piece well worth rescuing.
I could just stare at it for hours on end.  :clap:
 

HC1

AVForums Member
Joined
Oct 12, 2021
Messages
88
Reaction score
37
Location
pretoria
Agreed HB.
Dewald has superpowers.
He did the impossible and saved my Marantz 4140:notworthy:.
 
Top