Thanks for purchasing a board from me. I hand-assemble each one to exacting standards, and it has been fully tested.

Prepare for an involved but satisfying DIY project. You’ll be tearing the amp down to the bare chassis, so if you are planning other upgrades, like re-capping the amp, now is the time to do that as well.

The board is provided WITHOUT the big capacitors and one of the AC snubber caps installed. Do not install them until later. This is to allow access to solder connections for the bridge rectifiers, which cannot be done until the board is mounted and aligned in its final position.

Tools:

Besides the usual basic electronics tools, you should have:

• A powerful, regulated temperature soldering station. The board uses large, wide copper traces that suck heat away quickly. (For maximum current capacity.) Use a large tip for soldering the big caps.
• 5.5mm nut-driver or socket for the bridge rectifier mounting bolts
• 1/8″ drill bit for mounting board stand-offs
• MK1 only: Stepper bit to drill a 7/16″ hole, for re-locating transformer mounting bolt.
• MK2 only: 3/16″ drill bit for re-locating four transformer mounting bolts.
• Small blade screwdriver for pushing levers on the WAGO cage-clamp terminals.

SAFETY WARNING: The GFA-555, as supplied by the factory, is a 2-wire appliance with no safety earth connected to the chassis. Since you are modifying the amplifier, it is easy to make a mistake that could be dangerous to yourself or to anyone who comes in physical contact with your system.

It is STRONGLY recommend that you convert the amplifier to a properly earth-grounded appliance. Install a 3-wire power cord, or a standard power inlet jack such as an IEC or, my favorite, a Neutrik Powercon.

The earth ground lead from the power cord or input jack should be wired directly to a crimped ring terminal. (It can be soldered as well, but it must first be crimped.) The ring terminal should be bolted to an un-painted spot on the chassis, and secured with toothed lock washers. There may be specific regulations for this connection where you live. This earth ground bolt should not be used for any other connection or purpose. It should not be shared with the mounting flange of another component like a transformer or capacitor clamp.

Above all other considerations, it is vital that there be a direct, low-resistance, and high-current-handling connection from the amp chassis to safety earth ground. If this single connection is done correctly, then you or anyone who comes into contact with your equipment are much better protected from potentially fatal mistakes. This connection exists to ensure the chassis is always “earthed”, and never becomes energized with a dangerous voltage potential, such as when a live wire touches the chassis inside.

Also IMPORTANT. Do not remove the original AC input fuse holder. There is no fuse on the power supply board. Wire the OEM fuse holder in series with the incoming HOT from your power cord or input jack. From there, wire it to the LINE terminal on the board.

This board features an earth-loop ground breaker as described by Rod Elliot. This prevents the hum often experienced with three-prong audio equipment. With this configuration, noise and RF rejection is superior to a 2-wire appliance.

First:

Begin by stripping the chassis of all components. We’ll be doing everything over from scratch. Take the feet off too. We should be down to a bare piece of U-Shaped sheet metal.

Installing the board stand-offs:

Placement of the board and transformer is different for the MK1 and MK2. Additionally, some early-model 555’s have a canned and potted toroidal transformer, and these also have a different placement.

• MK1 with un-canned toroidal transformer: (Most common type) We’ll use the mounting hole in the bottom left of the board as our “registration point”. (The hole next to the small power transformer) You’ll notice the chassis has a row of ventilation slots cut into it, that run front-to-back. Set the board down on the bare chassis, and align this mounting hole with the far left side of the first ventilation slot. The board standoff in this position doesn’t need a hole drilled, as it actually mounts in the slot.
  

  

  
  Align the board as squarely and accurately as you can, and using an automatic center-punch or marker, mark the chassis through the other four mounting holes. Also mark or center-punch the large holes near the bridge rectifiers. Set the board aside and drill these holes with a 1/8″ bit.
• MK1 with canned toroidal transformer: (Early-models, usually with vertically-mounted input boards.)
  Make a mark with a sharpie or center-punch at 34mm from the left side of the chassis, and 61mm from the front. Set the board down on the chassis and align the “CHASSIS GND” mounting hole with this mark. Align the board as squarely and accurately as you can, and using an automatic center-punch or marker, mark the chassis through the other four mounting holes. Also mark or center-punch the large holes near the bridge rectifiers. Set the board aside and drill these holes with a 1/8″ bit.
• MK2: We’ll use the mounting hole on the left side of the board, near the ground loop breaker, as our “registration point”. There should be an existing hole in the chassis at 29mm from the left, and 56.6mm from the front of the chassis. Set the board down on the bare chassis, and align this mounting hole with this factory hole. Align the board as squarely and accurately as you can, and using an automatic center-punch or marker, mark the chassis through the other four mounting holes. Also mark or center-punch the large holes near the bridge rectifiers. Set the board aside and drill these holes with a 1/8″ bit.
  

  

Locating the transformer:

Both MK1 and MK2 need to have their transformers re-located. Drill the mounting holes, but don’t actually mount the transformer until the board is in place.

MK1 with un-canned toroidal transformer: (Most common type)

Drill a 7/16″ hole to re-locate the large transformer carriage-bolt. I recommend using a stepper-bit, as it will cut cleanly and not leave a huge snag in the metal.

Placing the hole: Viewing the chassis from the front; drill the hole centered at 86mm from the front edge of the sheet metal, and 86mm from the right edge. The 7/16″ hole is meant to be larger than the bolt shaft, but smaller than the square protrusions under the head of the carriage bolt, that are meant to prevent the bolt from turning. These square edges need to be pulled through the sheet metal. Find a socket or some kind of strong metal tube that is larger than the maximum diagonal width of the square underneath the bolt head. Put the bolt though the chassis, the socket, and add a washer and the nut. Tighten the nut until the bolt just pulls through the hole and the round head of the carriage bolt is made flush with the chassis underneath. Kind of like using a punch.

Set the transformer aside and now you can start mounting the board.

MK1 with potted toroidal transformer: (Early models, usually with vertical input board)

The transformer is potted in a metal can with four M4 mounting bolts around the base. Position the transformer so that, viewing the chassis from the front, the front edge of the transformer is 16mm from the front edge of the chassis sheet metal, and the right edge of the transformer is at 76mm from the right-hand wall. Mark the holes with a sharpie, and drill them out with a 3/16th bit.

Set the transformer aside and now you can start mounting the board.

MK2:

The transformer is potted in a metal can with four M4 mounting bolts around the base. Position the transformer so that, viewing the chassis from the front, the front edge of the transformer is 10mm from the front edge of the chassis sheet metal, and the right edge of the transformer is at 84mm from the right-hand wall. Mark the holes with a sharpie, and drill them out with a 3/16th bit.

Set the transformer aside and now you can start mounting the board.

Mounting the bridge rectifiers:

The bridge rectifiers attach to the chassis for heat-sinking. Use the supplied thermal pads, and mount them to the chassis as shown. Their leads should be bent skyward.

Use a razor blade to check for flatness where the bridge is to be mounted. (For maximum thermal performance.) Sometimes this spot is bulged because of the transformer bolt being over-tightened. Scrape the razor blade perpendicular to the sheet metal and look at the scratches in the paint to check for high spots.
If it’s not flat, set the chassis bottom flat against a wooden surface, and peen it with a hammer. Hone the surface flat if you want to get really picky.

 

Mounting the board:

In order to get the bridge rectifiers soldered in place, in the proper alignment, we’ll be mounting and un-mounting the board before finally installing it.

Attach the five M3x12mm stand-offs with the M3 lock-screws. Leave them a little loose for now until everything is in place.

Lower the board down onto the standoffs, being careful to align the rectifier pins with their holes.

Tighten the standoffs from the bottom, and then screw the board down.

The board and bridge rectifiers should now be aligned properly, and can be soldered in place. For now, the big caps and the right-side AC bridge snubber cap are left off, so you can solder the bridges in place…

Next, remove the board with the bridges in place. They are now aligned exactly as they need to be. Check that the solder joints have flowed through to the bottom and touch them up if necessary.

Next, solder in the AC snubber cap. It’s a little tricky; you have to poke the soldering iron under the bridge rectifiers.

And finally solder the big caps in place. Use a large soldering tip.

Mount the transformer:

MK1 with un-canned transformer: Install the transformer with the primary wires towards the front. Tighten the center bolt, but don’t go crazy with the torque. We don’t want to squish the transformer’s wires too much.

MK1 with canned transformer: Install the transformer with the wires to the left.

MK2: Install the transformer with the primary wires towards the front.

Mount the board.

Screw the board down on to the five standoffs, and install the bolts through the bridge rectifiers. It’s best to install the screws loosely, then tighten them once they are all in place.

Electrical connections:

The WAGO cage-clamp terminals are activated with the push of a small blade screwdriver. Strip 5-6mm from the end of each wire, push straight down on the lever, insert the wire and release. These are extremely awesome connectors. They won’t come loose in a hundred years.

Primary connections:

All line-voltage wiring should use at minimum, 16ga, 300v, 105C rated wire. The WAGO cage-clamps will accept up to 12ga.

• There are two terminals labeled “SWITCH”. Wire them to the switch. 🙂 You might want to just jumper these terminals for now, until the amp is up and running, and ready to have its face-plate installed.
• “LINE” should be connected to the output of the fuse holder on the back panel. A black wire is the convention. There is no fuse on the board, so utilize the original fuse holder.
• “NEUT” should be connected directly to the Neutral wire from the AC cord or power jack. A white wire is the convention.
• Your earth ground wire to the chassis should be at least the same gauge as the primary and neutral wires. Green or Green/Yellow Stripe is the convention.

Transformer primary connections:
Caution: The wire colors are different on the MK1 and MK2, as well as the early MK1 with the canned and potted transformer

MK1 with un-canned transformer: (Most common)
1: Orange
2: Red
3: Brown (This wire goes through a thermal breaker embedded in transformer)
4: Yellow
5: Black
6: White
7: Blue

MK1 with canned and potted transformer. (Early models, usually with vertical input board)

*Careful, there are two orange wires coming from the transformer, one is a primary and the other is a secondary. We’re dealing with the primary wires here.

1: Blue
2: Brown
3: Orange
4: Jumper to pin 3, using a thin wire, 22ga or so. Twist this wire together with the orange wire from the transformer and solder so that both wires will fit into the terminal. This wire simply powers the soft-start circuit.
5: Empty
6: Empty
7: White

MK2:
1: Orange
2: Red
3: Brown (This wire goes through a thermal breaker embedded in transformer)
4: Yellow
5: Blue
6: Black
7: White

Don’t have color-coded primary wires on your transformer? Some very early rack-mount 555’s have a different transformer, made by Pierdant, and it has colored bands, the meaning of which I am unsure of. Examine the original terminal block to determine which wires are neutral and hot, and wire them to appropriate terminals 4, 5, 6, and 7, which are marked “0V and 120V” for neutral and hot, respectively. Also, terminals 3 and 4 need to be jumpered together, as these early transformers lack a thermal fuse connection.

Secondary connections:

Phase of the secondaries should be matched for lowest noise. There are two pairs of orange and red secondary wires. Make sure that the orange wires go to “SEC1”, and red to “SEC2”. The transformer center tap terminal is labeled “XFMR CNTR TAP”.

Testing the board:

Before connecting the rest of the amp, you should test the power supply board. Bringing it up slowly on a variac is a good precaution, to make sure capacitor voltages are as expected at a low voltage level. Absent a variac, use a dim bulb tester.

Fortunately, there is no way to install the caps backwards. Yikes.

The relay should click in about 1 second after power is applied. The input voltage needs to be at least 75VAC for the relay to click in.

At full AC voltage, there should be about 80-85V on all four power rails.

DC connections:

There are B+ and B- terminals for both channels. You’ll need to install longer wires on certain output modules, and others will be clipped shorter.

“BYPASS GND” is for the black ground wires that come from the output modules of MK2 only. (Or if you have installed local supply bypasses on the MK1, which I highly recommend. I use Panasonic ED or EE 100uF/250V)

“SPK GND L&R” go to the negative speaker terminals. It doesn’t matter which.

“Input Board Ground” goes to the ground wire on the input board. You’ll need to install a longer wire than original. I suggest upgrading this wire to 16ga or even 14ga.

Bringing the amp back to life:

At this point, you may have spent a lot of time re-capping and upgrading the amp’s input board and output modules. It’s a good idea to bring the amp up slowly on a variac the first time you try it. The board is variac-friendly; There is no need to short out the thermistor as you bring it up on the variac. It may heat up as you do, but its resistance will drop as it does so, and so it should not overheat or overload.

The end:

That should be about it. Please let me know if you have suggestions to improve this documentation.

Thanks!
Chris