Sunday, January 10, 2010

Revox Manuals can be downloaded now

A collection of the "Most Wanted" Revox User Manuals and Service Instructions are available in pdf-format in the Download section of the Dada Electronics Website now.

You will find documentation for the Revox A720 - A740 - A77 - B285 - B77 - B780.

These are the original manuals from Studer. They are large, so you will need a High-speed Internet connection, but the resolution is excellent.


Friday, January 08, 2010

Extra repair/mod information concerning the Quad 44

Correction for the 7815 pin layout!!!

In the first series of the 44 (before serial 12.000) the output mute relay was fitted on the tone control board. There is an official Quad mod to decrease the switch off time of the relay: a capacitor of 4u7 (35V minimum) between the +HT rail and the base of Tr501. To prevent switch off noise, always apply this mod. On the tone control board M 12512 iss 8 (somewhere between serial 2401 and 12000) the cap was fitted in the factory.

In some cases the relay will rattle, Quad refers simple to: HT problems. Modern OPA’s draw substantial more current than the very efficient TL071’s. Also they pull more current from the negative supply in respect to the positive supply.

If you look at the relay circuit diagram, it is a critical design. With a total supply voltage of 30V the relay “on” voltage is exact 24V minus the Vce of Tr501. But the voltage is not 30V, there is only 28.8V max at hand.

To cure this, I placed one diode in series with each of the 15V zeners (D400 and D401). The same way Quad builds the 8.2V supply for the Switch board. The next step is reduce R400 and R405 to 5 ohm’s and increase the zener and transistor circuit current by reducing R402 and R403 to 1k5 Ohm’s. In most cases the supply will now deliver enough current to feed the hungry OPA’s and the relay. It is good practice to replace all the zeners (also the 7.5V and the diode D403) by 1,3 Watt versions.

If the rattling still occurs: increase the relay voltage by decreasing R550 to 22k ohm.

The last (final) step to get the relay silent is the following: replace the negative supply by installing a 7915 Voltage regulator. Until now I had to do this once (a time consuming warrantee case for Stefaan).
Remove R402, R404, R405, Tr402, Tr403 and D401. Replace R405 by a wire link; replace Tr402 with a tripod wire link. Place the 7915 in the position of Tr403 with “legs” 2 and 3 (input and output) crossed. Don’t use the metal nut and bolt to fasten the regulator.

Topview 7915

The same kind of operation can be performed to rebuild the positive supply with a 7815 voltage regulator.

Topview 7815

Remove Tr 400, Tr401, R400, R401, R403 and D400. Replace R400 by a wire link; replace Tr401 by a wire tripod. Place the 7815 in the same position as Tr400 on the copper side of the Pcb. With ‘legs’ 2 and 3 (output and ground) crossed. Place the 7815 in an upright position.

Modern 24V relays are functionally compatible, not pin compatible, see the drawing for correct placing. The relay is in an upright position on the component side of the pcb. Use always a relay with high impedance, 1K Ohm or more.

Other useful mods.

In Quad 44’s below serial 23.000 the volume can not be reduced to zero, like in a 34 or 44’s above serial 23.000. Remove R502 and R503, replace R500 and R501 by a wire link, and replace R552 and R553 by 10 k Ohm. In this way the series resistor is reduced, this brings the gain of this stage to the original value.

In the Radio module and the first series of Aux modules two extra resistors are placed in the OPA circuit. R202, 204 and R203, 205. Why they did this? In this way the circuit is unity gain, but the OPA runs at a higher gain. In those days OPA’s weren’t always stable at unity gain. Remove R204 and R205 and replace R202 and R203 by a wire link.
In the Aux module: remove R606, 607 and replace R604, 605 by a wire link.

Joost Plugge