Tips for success in re-building your GFA-565/585 circuit board.

Be prepared for a lot of work! It’s more than people expect. But it is very satisfying, like a big puzzle, and the result is a truly spectacular amplifier.

This is an advanced electronics project, so if you haven’t had much experience fixing amplifiers, the GFA-565 is absolutely not a good place to start.

You’ll need a fairly well-equipped electronics laboratory, with a good soldering station, oscilloscope, signal generator, and preferably a distortion meter. A variac is pretty much essential for powering the amp up the first time.

I want as many people to succeed with their repairs, and I am happy to answer questions about the board itself, to clear up any confusion about what parts go where and such, but if you need help actually troubleshooting the amplifier, I need to charge my usual hourly rate. ($75/hr) Please read up as much as you can on the repair of the GFA-565. There are many edifying discussion threads about repairing the GFA-565 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. the DIYAUDIO community is super helpful, and much of what I’ve learned about the GFA-565 comes from there.

Go slow, it’s faster!
It’s very easy to place a component wrong. To prevent errors as I go, I populate the components in roughly the same order as they are listed in the service manual. (But do the surface-mount stabistors first!) For each component, I count out exactly how many of each component I’ll need, and then place them. That way, if I have previously mis-placed a component, I should be left holding an extra component.

Service manuals:
GFA-565 Service Manual
GFA-585 Service Manual

Hoppe's Brain GFA-565 Input Board


Click here for the GFA-565 and GFA-585 parts list. 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 with new parts, but many components are so cheap, you might as well.

Resistors listed are all 0.1% tolerance in symmetrical applications, and 1% elsewhere. Dale RN or CMF is my preference, but stocks fluctuate, so TE Connectivity YR1 0.1% series resistors are used where Dales cannot be had.

Notes: (In no particular order)

  • Error in the GFA-565 service manual: In the parts list, the first mention of R114 should read R144.
    R144, R145 1/4w/499ohms
    R114, R145 1/4w/499ohms
  • Error in the GFA-585 service manual: Right-Channel output board: Under the heading “Resistors, Oxide Metal-Film. Delete line referring to R204 and R205. These resistors do not exist on the right board, but they do exist on the left board.
  • Reusing original parts: Be cautious about re-using the original components, especially around the high-impedance parts of the input stage. The bad capacitors spray electrolyte all over the board, and a drop of spittle can create a conductive path between component leads, even on the undersides of transistors. Clean between the leads with a sharp cotton swab. 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 and dry thoroughly. Your nose can tell you if there is still electrolyte remaining on a component lead. Just heat the component lead 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. To familiarize yourself with the smell, try heating up a component pad around one of the bad caps on the original board. Gross, huh?
  • Adcom J2: 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)
  • 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 with an especially steep current-versus-voltage curve, which is good for regulation. 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 packages. However, (after much research) I found surface-mount stabistors are still available from Central Semiconductor. The CMXSTB400 has four stabistors in one package, and the circuit requires two diodes with two junctions, and two diodes with three junctions, so my board uses the CMXSTB400 to replace all four stabistors. (The extra diodes simply go unused.)

Test the success of your surface-mount soldering, by checking the stabistors with your meter’s diode check function. You should read 1.2V on the KB262 and 1.8V on the KB362.

But you can just re-use the original stabistors if they are good. They are seldom a problem. They are physically fragile, so be careful desoldering. Also, be careful about the markings on the package; they are opposite what you might expect. The KB262 and KB362 diodes that come in the epoxy-blob package—unusually—have a black stripe to mark the ANODE not CATHODE. Don’t take my word on this. Confirm polarity with your meter’s diode test setting. More on this topic here at DIYAUDIO.

  • The original heatsinks run really hot, especially on the GFA-585. I recommend swapping them with the larger ones specified in the parts list. It’s the closest fit I could find, but unfortunately, the transistor mounting hole is higher than on the original heatsink, so the leads on the original transistors wouldn’t reach. Also, the supplied 6-32 screw hole is too large for the TO-126 transistor. You’ll need to drill and tap a new M3/0.5mm hole just below the 6-32 hole. Countersink the hole after tapping.
  • 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. The DIYAUDIO transistor matching jig is a community effort; The forum moderator ‘Anatech’ designed the circuit, and user ‘Cogeniac’ designed the original circuit board, and I’ve developed my own forked version of that board. At some point there will probably be a group buy for boards. The project will also likely go open-source.
    Ideally, the cascode transistors Q103, Q104, Q107, Q108 should also be matched. Fairchild KSP42 and KSP92 are confirmed to work beautifully.
  • I sell matched sets of input transistors for $50, or $40 with a board. This is a complete set of 8 matched transistors, enough for one amp. 2x MPSA13, 2x MPSA63, 2x KSP42, 2x KSP92.
  • EBC, CBE, BCE confusion!
    Original transistors 2SC3478/2SA1376 and 2SC1815/2SA1015 have ECB pin-outs. (Left to right, looking at face of transistor.) Replacement transistors KSP42/KSP92 are EBC, and BC550/BC560 are CBE instead of ECB. My boards are designed to accept either new or old types. The transistor pads have an extra emitter pad, accommodating either transistor pinout. Align the face of the transistor with the flat printed outline.
  • My GFA-565 boards contain two additional diodes that are not present in the original… D120 and D121, 1N4148. These are reverse-protection diodes around Q113 and Q114. These diodes were introduced in the GFA-585, which came out after the 565, so I reckon they were added for a good reason.
  • I like to get rid of the wire-wrapping posts and solder the wires directly to the pads. When I put the amp back together, I install the output modules last, so no need for the wire-wrapping posts.
  • Board headers: The original JST board headers and plugs are often corroded from age and capacitor electrolyte spittle. They can cause mysterious problems. Replace both headers and plugs! If you bought an assembled and tested board from me, it will come with new cable assemblies for the bias comp transistors, as well as the bias enable connection and signal input.  For the signal input, I prefer to just solder the wires directly to the board.
  • It’s a good idea to refurbish the soft-start board while you’re at it. The original 25W 4.7R in-rush protection wire-wound resistor often burns out. I replace it 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 as in photo below. You could also mount it to the chassis and run short wires to it.
    (Ignore the fact that the photo above shows an 8-ohm resistor, not 4.7, 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.
  • Don’t forget to short out the 4.7R soft-start resistor if you are bringing the amp up on a variac. If you don’t have a variac, at least use a DBT. (Dim bulb tester) But I really, really recommend using a variac.
  • Speaking of variac testing, the amp’s bias circuit will not engage and un-mute the amp until you hit about 40VAC input.
  • If you replace the 2SC3298/1306 TO-220 drivers with On Semi MJE15032/33, you may have an oscillation issue, which is solved by also replacing the 2SC3907/1516 that forms the second stage of the darlington, with On Semi NJW1302/3281. I do recommend replacing these as a matter of course, as the breakdown voltage headroom is improved.
  • The output zobel network’s resistor and capacitor can be mounted directly to the binding posts instead of on the input board. I think this is much better than the ten inches of wire used to run it back to the input board as original. There’s no reason for it to be on the board; it connects to nothing there. Part numbers for a nice polypropylene cap and non-inductive resistor are in the spreadsheet.
  • It’s also a good idea to run the feedback wire straight from the binding posts, instead of from the screw at the center of the output bus as OEM. The feedback point should come from a point close to the Zobel, for best stability, and for the feedback network to best include the Zobel in its corrections.

Testing the board:

I recommend testing the board before you hook up the output section, and risk blowing out twenty expensive output transistors.

The best way to test the board is outside of the amp, hooked up to a lab power supply.

Conditions required to operate the board without the rest of the amp connected:

  1. Override the bias delay: The bias delay consists of an opto-coupler and switching transistor located on the power supply relay board, whose job it is to turn on the amp’s bias circuit after a short delay, in order to mute the amp while it powers up and stabilizes. This is the purpose of the long wire that goes from the power supply board to J106 on the input board. We’ll simply bypass all that. All that’s required is a jumper from the center pin of J106 to point 17, and that will energize the current source circuit.
    GFA-585 Left Board: Jumper from center pin of J107 to point 6. (Solder to the bottom of the board, not the pin of the connector.)
    GFA-585 Right Board: Unfortunately, the voltage divider that energizes the bias circuitry, only exists on the left board. No problem, just connect two 39K metal-oxide resistors in series, tie one end to B+, the other to ground, and tie the middle point of this divider to the center pin of J157.
  2. The thermal protect LED does not need to be connected. It doesn’t hurt though.
  3. Connect an LED to test the clip light functionality.
  4. Output and feedback hack: Solder a pair of standard 1/4W 1K resistors into points 3 and 4 and tie them together on the other end. (Y-Connection) This is your output. From there, run a short wire to the feedback input at point 7.
    GFA-585: Drive signals are on J105 and J155. Feedback goes to points 13 and 8, left and right boards respectively.
    GFA-565 Output and Feedback hack
  5. Thermal-compensation bias transistors: These are the small 2SC3478 and 2SA1376 transistors that are mounted to the output modules, and connected with long wires to J104 and J105. They’ll need to be connected while you test, so just set the output module heatsinks on your bench near the board. (With nothing else connected to the output modules.)
  6. Power supply requirements: The board has two negative and two positive voltage inputs. (Fused lines feed the current source circuitry, and un-fused lines feed the amp circuit.) These can simply be wired in parallel for the test. Positive and negative voltage inputs are conveniently labeled on the board as Fz B+, FZ B-, UnFz B+ and UnFZ B-. (Fused and un-fused)
    Power supply ground connects to point 18. (GFA-585 use either point 1 or 5 for either left or right.)
    Configure your lab power supply as a +/-30V bi-polar supply with a common ground. +/-30V is plenty to operate the circuit. If your power supply allows you to slowly ramp up voltage, that’s a good idea.
  7. Connect your test signal generator to the input on J103. (GFA-585 J101 and J151)
  8. Connect your scope to the Y-point of the two 1K resistors you’ve hung off the drive outputs.
  9. Powering up: Set your signal generator at 1KHz, 100mVrms. Power up, and hope that none of the magic smoke escapes. If you’ve got it right, you’ll be looking at a nice clean sine wave on the scope. Distortion may be higher than the final amp, but it should be less than 0.1%. You shouldn’t see any wave deformation on the scope. DC offset should be less than 20mV. If the board seems to be working well, try a higher supply voltage, up to the full operating voltage of +/-85V, if your supply goes that high. Don’t get shocked! Check the voltage on pin 6, the output of the DC servo op-amp. If the amp were theoretically perfectly balanced, you would read 0V here. But because NPN transistors tend to have more gain than PNP, a typical amp will read up to +/-4V. If you read something close to 14V, positive or negative, that means the servo is trying as hard as it can to correct some DC imbalance, and something is wrong.
  10. Check DC Operating points: Download this spreadsheet, which I use to check each board I assemble. It contains DC operating points as well as a other parameters you can check.
  11. Bask in the glow of the LEDs and your accomplishment.

Testing the output modules:

Please do not skip this step! You must be certain your output modules are good before applying power to the amp. This GFA-565 is direct-coupled from one stage to the next. If you try to power up the amp with blown transistors on the output modules, the “Domino Effect” may ripple backwards through your amp, blowing up everything all the way back to the input stage. It’s a big job to repair the damage.

Often there are blown resistors on the output modules. Usually, the output transistors are fine. Here’s how to test everything.

  1. Lift one leg of R201, R202, R301 and R302. Test these resistors and replace with suitable metal-oxide types if they are bad.
  2. Test all the other resistors on the board, including the 10R base stoppers and emitter resistors. (Hint, most meters do not read the emitter resistors accurately, due to their low impedance. If they read a little high, they are probably actually fine. An LCR meter works better.
  3. Lift one leg of the reverse-protection diodes D201 and D301.
  4. Using your meter’s diode check, you should see the usual, approximate 0.5-0.6V drops from B-E and B-C on all transistors. Q202, Q203, Q302 and Q303 will show 0.6V from C-E in one polarity only. All the output transistors should read open-circuit from C-E in either polarity. If you read 0V anywhere, you have a shorted transistor.
  5. Check C202 for a short circuit. (Rare but it happens.)
  6. Test your bias comp transistors in the usual manner.

If you find a bad transistor or transistors.: When power output transistors fail, they tend to do so very obviously, and will usually either blow out completely shorted, or completely open. If yours test OK on the diode check function, and show the usual 0.5-0.6V voltage drops, and no shorts, they are probably good. That is no guarantee. The most important thing is that none are shorted, because that’s how input boards get smoked.

If you find that you just have one or two blown transistors, it is recommended that you replace the whole set of ten, as they need to match. I recommend On Semi MJ21193/94, but there are many good substitutes. If you order from a reputable source like Mouser or Digikey, you will probably receive a whole set with the same batch code. Manufacturing tolerances are so good these days that a set from the same batch code will likely be decent matches.

Putting the amp back together:

If all seems well, you can assemble the rest of the amp with some confidence.

Good luck! Please let me know if you have any suggestions or corrections to this documentation.