Tips for success with re-building your GFA-565 circuit board… in no particular order…
Be prepared for a lot of work! It’s more than people expect.
I am happy to answer questions about your repair, but before you contact me, please do read up as much as you can on the repair of the GFA-565. There are many edifying discussion threads on the topic at DIYAUDIO.COM. This one and this one in particular. It’s a lot of reading, but you’ll find important tips that may save you time and hassle.
Go slow, it’s faster.
Here’s the latest board revision!
They are now two-layer boards, though the layout is still almost exactly as original. (No need to mess with a known working layout.) The two layers allow me to add a few new features: All the small staple-shaped jumpers are now built in as a trace on the top layer. The two longer black wire jumpers are still required. There are now pads to mount surface-mount CMXSTB400 stabistor diodes in place of the original KB262 and KB362.
Parts: Here’s a spreadsheet with part numbers from Mouser for all known equivalents. Quantities are exactly enough to populate one board. Order extras if you need to match transistors or resistors. You don’t need to replace everything on the board, but I recommend at least replacing all resistors and small signal diodes. (except for the stabistors, keep those if yours are OK. They are delicate, so be careful de-soldering.)
Resistors listed are all 0.1% tolerance in symmetrical applications, and 1% elsewhere. Dale RN55C is my preference, but they are not available in all resistance values, so TE Connectivity YR1 0.1% series resistors are used where Dales cannot be had. You may find that stocks have changed since this writing.
The TO-92 voltage references labeled “Adcom J2” are often ruined, being located right next to the leaky caps. I usually replace all four. The other voltage reference on the board is usually fine. (The 2.5V Adcom J6 or LM336)
Notes: (In no particular order)
- Error in the service manual: In the parts list, the first mention of R114 should read R144.
R144, R145 1/4w/499ohms
R114, R145 1/4w/499ohms
- Reusing original parts: Be cautious about re-using the original components. The bad capacitors can spray electrolyte all over the board, and a drop of spittle can create a conductive path between component leads, especially on transistors. All re-used parts should be run through an ultrasonic cleaner. For a solvent, I use a 50/50 mix of denatured alcohol or vodka, and Simple Green. Rinse the parts in water afterwards. Your nose can tell you if there is still electrolyte remaining on a component lead. Just heat with a soldering iron and sniff. The smell is very distinctive; a bit like rotten antifreeze. Once you smell it, you’ll never forget it.
- STABISTORS: D105 through D108 (KB262 and KB362) are listed incorrectly in the service manual as “Varistor Diodes”. They are actually stabistors; a type of diode that has a particularly steady forward voltage drop versus current—about half as much drop as a normal diode like a 1N4148. Each stabistor diode is actually a package of stacked diode junctions in series, with approximately 0.6V forward drop per diode. They are named for the number of junctions in the package. KB262 has two diodes (1.2Vf), KB362 has three (1.8Vf), etc. There are no modern replacements available in through-hole wire-leaded packages. However, (after much research) I found surface-mount versions available from Central Semiconductor called the CMXSTB200 (two diodes) and CMXSTB400 (Four diodes.) The three-diode version, the CMXSTB300 is not in stock anywhere, but you can just use the four-diode version and ignore one diode. My circuit board uses the CMXSTB400 for all four diodes. Ideally, just use the original stabistors. They are seldom a problem. They are physically fragile, so be careful. More on this topic here at DIYAUDIO.
* Testing stabistors can be done with the diode check on a multimeter. They should read around 1.2v for KB262 and 1.8v for KB362. There should be no reading in reverse. UPDATE and NOTE: The latest boards have spots to mount the surface-mount stabistors instead of the original through-hole KB262 and KB362. Use the CMXSTB400 to replace both KB262 and KB362. The extra one or two diodes in the chip go unused.
- The original heatsinks run really hot. I recommend swapping them with the larger ones provided in the parts list. Unfortunately, the mounting hole is higher than the original, so the leads won’t reach on the original transistors. Also, the screw is too wide for the TO-126 mounting hole. So I recommend tapping a new hole at the same height as the original heatsink. Countersink the hole after tapping. Original mounting screw is M3/0.5mm.
- Matching transistors: If you replace the MPSA13 and MPSA63 devices, they must be matched. I recommend it anyways, as the factory matches are only so-so. This is an involved topic, and is covered here at this thread on DIYAUDIO. User ‘Cogeniac’ posts a good schematic to build a matcher, and he may offer boards for sale at some point.
Ideally, the cascode transistors Q103, Q104, Q107, Q108 should also be matched. The Fairchild KSP42 and KSP92 transistors appear to be good replacements for the cascodes C3478 and A1376 but these are untested and unconfirmed at the time of this writing. (April 2017) Also, the leads on these are EBC and not BCE so the leads must be bent. This can be confusing and lead to incorrectly connected transistors. Quadruple-check your work on this to avoid major failure.
- I like to get rid of the wire-wrapping posts and solder the wires directly in. When I put the amp back together, I install the output modules last, so no need for the wire-wrapping posts.
- It’s a good idea to refurbish the soft-start board while you’re at it. The original 25W 4.7R in-rush protection wirewound resistor often burns out. I replace with an aluminum-cased 50W resistor. IMPORTANT: The original resistor is held in place mechanically as well as by solder. In the event of a melt-down, it should not collapse and short to the chassis. The replacement resistor should be mounted in such a way that it will not fall through or collapse if it melts. I use 12ga solid copper wire arranged like the photo below. You could also mount it to the chassis and run short wires to it.
(Ignore the fact that the photo above is an 8-ohm resistor, it’s for a special project) Also recommended is to replace R501 3.3K with a higher-wattage 1/2w resistor. The original runs hot enough to turn brown.
- The OEM board headers and plugs for the BIAS and BIAS COMP connections are kind of crappy, and the plastic gets brittle eventually. I replace them with the Molex Nano-Fit system. You need to buy three parts; Headers, Receptacles and Pins. Part numbers are in the spreadsheet. Assembly and crimping of the wires to the pins is supposed to be done by robot, but if you have keen eyes, a fine-tipped soldering iron, and a steady hand, you should be able to do it. The Nanofit headers have a tiny plastic locating pin on the bottom that should be clipped off so the header sits flush with the board.
- If you replace the 2SC3298/1516 TO-220 drivers with On Semi MJE15032/33, you may have an oscillation issue, which is solved by also replacing the 2SC3907 that forms the second stage of the darlington, with On Semi NJW1302/3281.
- The output zobel network’s resistor and capacitor can be mounted directly to the binding posts. I think this is much better than the ten inches of wire used to run it back to the control board as original. Part numbers for a nice polypropylene cap and non-inductive resistor are in the spreadsheet.