$80.00 – $380.00
- Adcom GFA-565 replacement circuit board.
Replacement circuit board for Adcom GFA-565. (Original model)
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.
My solution is a new board!
UPDATE: (Sept 24, 2018) This is a completely new board version with a number of improvements. The old version was basically an exact reproduction of the original, with components in the same places, and the same copper traces.
The new version is a whole new design. The layout is no longer identical to the original, though it remains physically compatible. Some components have been moved to take advantage of the two-layer layout.
- Traces are routed on the top and bottom layers. This allows for shorter, wider traces, better spacing between conductors, and better component placement.
- Improved labeling. Lots of thought went into making component placement as confusion-free as possible. All components are labeled with their value and type of component. Inputs and outputs are labeled, as are jumpers for LEDs and thermal breakers. Populating the board goes faster, with fewer errors.
- Better 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.
- ECB, EBC, CBE confusion no more! The new boards will accommodate original or new transistors with any pinout. The original 2SC and 2SA type transistors have leads arranged in ECB format. These are no longer available, and the modern equivalent replacement transistors are either EBC or CBE. (KSP42/92 and BC550/560, respectively.) Previously, this meant bending the transistor legs all goofy in order to fit, and it was really easy to make a mistake. The new board has pads arranged in an ECBE format that will accommodate either! Much less confusing.
Read more here at my original blog post.
This is a job for an experienced amplifier tech. If you have not fixed amplifiers before, the GFA-565 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 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, that service is available for my usual hourly rate of $60/hr.
All parts available from Mouser, Digikey and other suppliers. (No affiliation), and if you buy everything on this parts list, it’s around $110. That’s absolutely everything you’ll need, not just for the input board, 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 for $40. (When you buy a board.) Matching 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 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.
|Dimensions||6 × 6 × 1 in|