Replacing toggle/rotary switches on the Revox B77/PR99 & cleaning the Motherboard

Procedure level – Advanced

Tools Required

Small and large Philips screwdrivers
Q tips
Can of DeOxIt (Caig D5)
camera to take pictures of your work as you go.

Scope of this Article

While the Revox B77 and PR99 tape decks were designed to be run 24/7, and most are still very reliable after a thorough servicing, one of the more common problems is that toggle switches can snap off, and a poor contact between the switches, both toggle and rotary, can cause intermittent channel operation. This is very common on decks that have been stored for decades. While the switch contacts on the motherboard as well as the switches themselves are gold plated, oxidation and corrosion can start on the surface of each, causing this intermittent operation.

This article assumes you’ve opened up your Revox machine before, and know your way around it.

Take the back of the deck off.

This is done by removing the 4 screws that gold the back cover off, and pull straight back. The rear cover will slide right off the main deck chassis.

Remove the bottom PC board shield/cover

Two more Philips screws allow the bottom cover to slide off, exposing all of the PC boards. Turn the deck upside down so that the audio PC boards are facing you.

Pull out all of the audio PC boards.

Take a picture of the orientation and order of the PC boards before pulling them out, and put them to the side, in order. Depending on the options installed into your Revox, there may be a PC board slot that is empty. Make a note of that with your pictures, as it’s easy to get the board locations confused. The boards pull straight out. Make sure not to bend the pinc when pulling the boards out, or when you’ve put them aside.

**This Revox B77 has the sync board missing, and the bias/oscillator board will plug into that spare location if you’re not careful.**

Remove the motherboard

The motherboard is held to the chassis with 12 Philips screws, scattered throughout the large board. To be more exact, the motherboard actually bolts to the rotary and toggle switches, and not the chassis itself. Remove all 12 screws

**Picture of the motherboard as it sits in the B77. The PR99 is very similar.**

Several notes about removing the motherboard:

The big point is that if this motherboard hasn’t been removed in decades (and most haven’t), it will be stiff, and there’s a good chance the long speed switch will stick to the white plastic lever that moves it from side to side. Wiggle the front portion of the switch off the white lever with your fingers as you slowly remove the motherboard, otherwise you could bend the brown portion of the switch too far, and snap it (don’t ask how we know this!)

**Closeup of the brown slide switch that slides over top of the white lever that moves side to side when the speed is changed on the front panel.**

The white and black, and green and black wires that connect to two pins each near the top of the motherboard will restrict the motherboard from tilting down fully to access the back of it. Unplug those 2 pairs of wires, but again, take pictures so you remember where to plug them back into.

Once the board is wiggled off its mounting posts and the slide switch assembly, the motherboard can then be tilter down about 90 degrees to gain access to the back of the board, as well as the backs of the switches, that have the gold plated contacts that make contact with the motherboard.

**Motherboard tilted down about 90 degrees for easy access to the gold contacts on the board, as well as the backs of the switches.**

Taking a Q tip and a can of DeOxIt, load the Q tip fully with a few shots of the contact cleaner, almost to the point where the Qtip is dripping with the fluid.

Use the Q tip to gently wipe down all gold contacts on the motherboard. A small amount of residual DeOxIt on the motherboard is a good thing, it will continue to lubricate and clean the switch contacts.

**Residual dirt that came just off the motherboard contacts. Dirt isn’t conducive to making a good connection!**

Use the other side of the DeOxIt loaded Q tip to clean the backs of the toggle and rotary switch contacts. You will probably see a similar amount of dirt come off those set of contacts as well.

**Getting ready to clean the back of the rotary and toggle switches.**

Reassemble the Motherboard into the deck.

There are two round cutouts on the motherboard where the two pairs of wires slide through from the top to the bottom of the motherboard. Care must be taken to properly route those wires through those cutouts, and to not squish the wires between the motherboard and the chassis.

**Proper routing of the wiring from the top to the bottom of the motherboard, using the rounded cutouts on the motherboard.**

Each toggle and rotary switch has a small plastic flange that will fit directly into each screw hole of the motherboard. Care needs to be taken that those flanges do indeed fit properly into the motherboard before the motherboard screws are tightened.

**Proper orientation of the switch plastic flanges to come through the motherboard. Often they will ‘click’ into place, but sometimes not.**

It’s also very easy for one of the wires under the motherboard to become pinched under one of the switches. If that happens, move the wiring away from the back of the switch, and then reinstall the motherboard. Install all 12 screws, reconnect any wiring that you may have disconnected in the process, and reinstall the PC boards.

Put the metal shield back over the PC boards, ensuring that each PC board fits into the little cutout slot on the back of the metal shield, for proper securing of each board.

Test all functions of the deck prior to putting the back cover on, to ensure that all switches are working properly.