Description

NEW VERSION! (October 2022) Based on customer feedback, I present this completely new, improved input board for the GFA-565. This version is substantially easier to build, install and troubleshoot, and its performance is a little better too. If you really want the old version, which looks more like the original board, here you go.

This is the cure for leaky capacitors and DC offset problems in the original Adcom GFA-565! Nearly every GFA-565 ever made was affected by a bad batch of leaky capacitors that cause speaker-blowing DC offset to appear on the output. You can try to clean up the original board, but the DC offset issue often persists. The electrolyte actually soaks into the fiberglass, making it ever-so-slightly conductive. This affects the balance of the high-impedance circuitry around the input stage and DC servo. Amps that seem fixed can actually develop DC offset later on, as the electrolyte continues to spread through the fiberglass. It’s a frustrating problem to work on. A new board is the best solution.

Features:

• Optimized, symmetrical PCB layout: The GFA-565’s circuit—designed by the legendary Walt Jung—is a picture of perfect symmetry; A complementary mirror-image from positive to negative halves. The new layout reflects this symmetry. It is made larger than the OEM board, and arranged in a more symmetrical fashion. Impedances seen by the circuit, that are caused by the board, are therefore more evenly balanced. (NOTE: The OEM board is a perfectly good design, but it’s single-sided and they would have been under budgetary pressure to make it small, or to meet a certain size so they can get a certain number of boards on a panel. Thus, the OEM board is a bit cramped. Again—not complaining—Adcom’s circuit board quality was better than similarly priced competition at the time.)
• Solder-less installation: Something I kept hearing from customers, is that if something goes wrong and the magic smoke escapes, and the board needs to come out for repair… All the wires need to be de-soldered, and it’s hard to reach them without burning components, wires, or chewing up the PCB pads. It’s a little awkward to work with the older board with so many solder connections, and a major bummer to have to re-do them every time the board needs to come out. On the EBFA-565, all connections that used to be solder-pads, are now right up front on WAGO cage-clamp terminal blocks. These spring-loaded clamps grip the wire very, very firmly, and make an oxygen-tight connection that will never come loose. (Screw-terminal blocks can come loose over time as the copper squishes.) The board can be removed for troubleshooting or repair in a few minutes, and reinstalled with no damage to PCB pads. …so much better. And it makes the wiring neater too.
  
  
  
      Inside a WAGO Cage-Clamp
• Top-shelf components throughout: Kits and pre-assembled boards come with Dale RN and CMF resistors, 0.1% tolerance resistors in key locations, polypropylene capacitors, high-endurance electrolytic capacitors, silver-mica capacitors, etc… The good stuff.
• Compatible with old parts from the original board, so if you are building up from a blank board, you may recycle many of the un-damaged original parts. (With a few exceptions, see below)
• Two-layer, plated through-hole FR4 fiberglass circuit board.
• Traces are routed on the top and bottom layers.
• Ground planes cover the top and bottom layers.
• High-impedance traces are given wider isolation from ground planes, other traces and pads.
• Symmetrical layout makes it easier to build, and less error-prone, because it’s easier to locate the components.
• More room to work: Things are less crowded. It’s easy to probe any point on the board, and it’s easier to assemble and re-work.
• Clear labeling: Every component is labeled with its part number and value.
• An annotated schematic, loaded with troubleshooting tips, DC operating points, etc, is included with the board, printed on tabloid size 11×17 paper. Download it here.
• Convenient test points for measuring the most important circuit parameters, referred to expected values on the schematic.

• Improved heatsinking. The TO-126 VAS transistors in the original design run very hot. The heatsinking of this new design is much more effective and keeps temperatures below 60C.
• Bypass caps are added to the +/-13.8V supplies
• A 0.1uF polypropylene bypass cap has been added in parallel with the 10 ohm isolation resistance between amp ground and the input section. This improves high-frequency coupling between these ground potentials. This feature was added in response to a customer who experienced oscillation because of substituting a wire-wound resistor in this position. The added inductance was causing the oscillation, and a metal-film resistor fixed the issue. If a little bit of inductance can cause this issue, I figured that adding a bypass cap in parallel would prevent that potential issue, and it may improve performance a smidge.
• Includes new cable assemblies for signal input, bias compensation transistors mounted on the heatsinks, and bias enable from the soft-start board. These cables are often corroded from capacitor leakage, or life in a humid environment. New cables are pretty essential for a good restore job, but they are difficult to make by hand because the connector pins are meant to be crimped by a special machine. I had these cables specially manufactured to include with EBFA-565 boards.
  

• STABISTORS: These components are rare and getting rarer, so the board supports three types of stabistors.
  • OEM through-hole KB262 and KB362. (Little epoxy blobs) You may re-use them if their leads are not corroded. Caution: Anode is marked with black stripe instead of cathode as you would expect. Double-check with multimeter diode-check function.
  • CMXSTB400 SOT-6 SMD Recently gone end-of-life.
    
  • Nexperia BAS17,215 These are the only stabistors in production AFAIK. Single-diode SOT23 package, and you need 10 diodes for a board. Fortunately SOT23 is pretty easy to solder for SMD.
    

Deleted features:

• • 2-pin JST Board Header Connectors J101 and J102: These are meant to provide power to the exceedingly rare balanced input option board. If someone wants to install an EBFA-565 board into a balanced input GFA-565, these power connections can be bodged in, using test points as pads for connecting power wires.
  • 2-pole pin header J109: Connected across the 4.7uF input capacitor. I think this was used for production testing and is not needed.

Performance:

Performance of the EBFA-565 is slightly improved over the BFA-565, which itself, is almost certainly an improvement over the OEM board. I can’t definitively prove this because I have never tested a working OEM board, as they are all so ruined. However, the measurements I have taken can be compared to those in the service manual, and to measurements taken by others. This article goes into details of my performance validation testing, and the results are excellent as expected.

EBFA-565 versus BFA-565 (Validating a new design)

Skills Required:

~AND~

https://hoppesbrain.com/gfa-565-circuit-board-replacement-notes/

Support:

30 minutes tech support is included with every purchase of a board kit. Time beyond 30 minutes is billed at $75/hour. This includes time spent researching your issue, writing emails or talking on the phone. I am happy to provide technical support, but please anticipate paying for for my time as part of the cost of your project. Tech support time is always billed, whether the issue is yours, mine or one of my suppliers. Alternately, consider asking your question on a forum such as DIYAUDIO.COM. Send me a link and I may comment on the thread.

Parts:

All parts available from Mouser, Digikey and other suppliers. (No affiliation)

EBFA-565 Parts List

That’s absolutely everything you’ll need, not just for the input board, but for a thorough refurbishing of the whole amplifier, including power supply capacitors, higher-voltage driver transistors, parts for the soft-start board, power switch, etc. Likely, you can re-use many original parts to save money. I also sell matched sets of input transistors, along with their cascodes, 8 matched transistors in total for $40. (When you buy a board.) Getting oneself set up to match these devices is a significant project in itself, so I sell matched sets. I use a test jig to sort and match transistors, based on this discussion at diyaudio.com. You can reuse the original transistors if they are good, but they often have electrolyte goo on their leads, so if you do re-use the originals, make sure you clean them really, really well, and that the leads are not corroded. Corrosion from electrolyte contamination can work its way up the leads of components, and into the body of the device.

Parts that cannot be recycled from an old GFA-565 board:

These things are either new or different on the EBFA-565. If you are buying a bare board and sourcing your own parts, or recycling old parts from a GFA-565 board, you will need at minimum:

• Heatsinks: 2x Aavid Thermalloy 513002B02500G
  A common heatsink profile, there are many substitutes.
• C123, C124 WIMA MKS2C031001A00MC00 0.1uF 63V 5mm L/S
  Non-existent on OEM board. Substitute any good poly cap. These are power supply bypass caps for the +/-13.8V supplies.
• C130 KEMET R71MF31004030K 0.1uF 420V 10mm L/S
  Non-existent on OEM board. Substitute any good polypropylene cap 250V rating or higher. This is a bypass cap for the 10 ohm buffering resistor between input ground and amp ground.
• C112, C113, C116, C117 KEMET F461BK105K160C 1uF 160V 15mm L/S
  OEM 10mm L/S Polyester caps will not fit 15mm L/S footprint on the EBFA-565. Substitute any good polypropylene cap 100V rating or higher.
• WAGO Cage-Clamp Terminal Blocks 233-505 or Bussmann/Eaton EM280205
  Non-existent on OEM board.
• All five electrolytic capacitors. See parts list.

Any parts that are recycled from an old GFA-565 board should be inspected—under magnification—for corrosion on the leads.