This amp belongs to a friend of mine who has owned it for almost 20 years now. He was not the original owner, but it's been in use since he first bought it at a garage sale for only $10 in the mid 90s. I offered to take it home and give it a good cleaning, but after opening up the top, I found out that it would be relatively easy to do a full restoration including a whole recap of the unit.
I started by doing a full inventory of the parts needed—it ended up requiring a total of 39 electrolytic capacitors including the two giant filter caps beside the transformer, and I decided to also replace the relay as it was causing issues with audio cutting in and out. Conveniently, this amp used the exact same relay as the KR-6400 I recently fixed up (part number: MY4-02-DC24).
I went with Panasonic caps rated for 105°C for the power supply and filter caps. For everything else, I stuck with the original Elna brand (Silmic II type) which are considered "audio grade". Numerous people have told me that audio grade caps are nothing more than a marketing ploy, but they weren't much more expensive than the generic kind, so whatever.
For anyone curious, here's a cheat sheet of the capacitors required. All caps are radial unless otherwise noted:
- 4x 330uF 35V
- 1x 100uF 25V
- 1x 100uF 16V (non polar)
- 1x 10uF 50V (axial)
- 2x 15000uF 63V (filter caps)
- 6x 1uF 50V
- 4x 10uF 25V
- 2x 33uF 25V
- 6x 47uF 10V
- 2x 47uF 16V
- 2x 47uF 50V
- 1x 100uF 10V
- 3x 100uF 16V
- 2x 100uF 63V
- 2x 220uF 6.3V
I went with the actual readings printed on each individual capacitor rather than what's listed in the service manual, just in case there were any changes to the design after the manual was printed. I also bumped up the 6x 47uF 10V caps to 16V just to make the order process a little simpler (for a total of 8x 47uF 16V caps).
The power supply board was recapped first, then I did the filter caps. The original filter caps were absolutely massive, so I had a hard time finding ones that would fit nicely inside the bracket mount. My wife had the good idea of padding them with some foam for a snug fit. I ended up buying a sheet of black foam from Michael's for only 99 cents.
It's a smart idea to take a photo of any wiring you intend to snip, so you can use it as a reference when putting everything back together. Don't screw up that polarity, or you'll be sorry.
The replacement caps had two extra pins used for mounting to a standard PCB, but I snipped them off because I would be using the original mounting bracket.
Like I said, the originals were enormous compared to the modern equivalents.
And a close up of the foam job. A nice snug fit.
It's important make sure your replacement caps fit on the circuit board nicely. You should be mindful of the diameter of each capacitor, especially if you are upgrading the voltage rating on any of them. Luckily, modern capacitors are almost always smaller than vintage ones, so it's not too big of a deal. I will admit that I made a mistake, and forgot to take the height into account when replacing a capacitor on each amplifier board.
I ended up having to bend it upwards a bit to fit it inside the chassis. I wasn't happy with this result, so I decided to order replacements that fit properly. The ones that needed replacement were the two 100uF 63V caps (one on each amp channel). I made this mistake because Digikey didn't have any Elna Silmic II 63V caps in stock, so the closest kind I could get were 100V—much taller. I ended up ordering two Nichicon replacements instead. They were also "audio grade", so I figured it wouldn't make a difference.
With all the capacitors for each board successfully replaced, I was done.
I was extremely happy that the recap process was a complete success, but I noticed there was another small issue that I was worried about. With the amp powered on, even sitting idle, I could feel a wave of heat rising from inside. I was worried that I had installed something incorrectly, but it turns out that the power supply board has several resistors that run fairly hot. Taking a closer look, you could see they were kinda toasty, so I decided to replace all of the large resistors on the power supply board while I was at it. Here's a closer view from a photo I took earlier in the process:
I upgraded each resistor to 3 watt resistors instead of the 1 watt originals. Hopefully this would stave off any future toastiness. Here's the part list of resistors I replaced (all are Panasonic 3W):
- 3x 680 ohm
- 1x 820 ohm
- 1x 470 ohm
- 1x 390 ohm
- 1x 1k ohm
With the resistors replaced, they were still a little warm but would hopefully last considerably longer with the bump up in wattage rating.
All that was left to do now was confirm the bias current matched the specs in the service manual and this thing would be good as new. The manual gives you some pretty wonky instructions for measuring the bias current, but I found out through the help of folks at Audiokarma that you can alternatively measure the voltage across pins 8 and 5 on each amp board. Then you just turn the adjustment pot until you get 30mV. Easy.
With everything finished, I gotta say that this amp sounds amazing. Incredibly clear, and powerful enough to power just about any speakers you throw at it without any distortion (a hefty 75 watts/channel). I wish I could keep it.