Description

Replacement circuit boards for Adcom GFA-585 (original model), set of two boards, left and right channels.

This is the cure for leaky capacitors and DC offset problems in the original Adcom GFA-585! Nearly every GFA-585 and GFA-565 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.

My solution is a new board!

Features:

• Two-layer, plated through-hole circuit board. Traces are routed on the top and bottom layers. This allows for shorter, wider traces, better spacing between conductors, and better component placement. The layout is slightly different from OEM, while maintaining physically compatibility.
• Improved labeling. Lots of thought and customer feedback has gone into making component placement as confusion-free as possible. To prevent mistakes, all components are labeled with their value and type.
• Thermal bonding of the input and cascode transistor pairs: Matched transistors pairs that make up the input stage should be thermally bonded with thermal paste and heat-shrink. These transistors have been located adjacent to make this easy.
  
• Use your original parts, or new, modern equivalents: It’s possible to build a completely new board if you buy all the parts on this list. Or, use some old components.
  • Transistor pads accept any pin-out, ECB, EBC or CBE. The original 2SC and 2SA type transistors have leads arranged in ECB format. These parts are no longer made, and the modern equivalent replacement transistors are either EBC or CBE. (KSP42/92 and BC550/560, respectively.)
  • 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.
      

More here at my original blog post about the GFA-565. (The 585 is uses the same circuit as the 565, but with a shared power supply and fewer output devices.)

How to Fix Leaking Capacitors in the Adcom GFA-565 for good!

Skills Required:

This is a job for an experienced amplifier tech. If you have not fixed amplifiers before, the GFA-585 is not really a good place to start. Given the complexity of the design, and relatively high component count, it is very easy to make a small mistake in assembly. Mistakes are to be expected, but you should have enough experience with amplifiers to troubleshoot any issues that arise.

Buying a pre-assembled and tested board is no guarantee of success. You must still possess the skills to troubleshoot a complex amplifier. Every component on the output modules should be checked before installing the input board, or the board could be damaged. Damage due to installation error is not warrantied.

I can provide limited tech support—answering questions about what parts go where, clarification on documentation, and such. But if would like help in actually troubleshooting your amplifier, by email, text, or phone, I’m happy to provide that service for my usual hourly rate of $75/hr.

All parts available from Mouser, Digikey and other suppliers. (No affiliation), and if you buy everything on this parts list, it’s around $200. That’s absolutely everything you’ll need, not just for the input boards, but for a thorough refurbishing of the whole amplifier, including 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. Matching these devices is a time-consuming 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 use 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 too much.

Also available: Complete cable assembly kit for the GFA-585. The original cables are often corroded, due to capacitor electrolyte leakage and the passage of time. These cables are not included with the BFA-585 boards, due to the expense of having them manufactured.