Kenwoord KA9100 Amp Repair Journey

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Yuri December 21, 2021
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This is a log of me repairing my KA91001 with a speaker popping / no sound issue.


Research

The first step is to research the issue's origin, which I found the answer here
TLDR:

Popping in the amp section of a KA-9100 is usually due to Q5, Q7(L-ch) or Q6,Q8(R-ch) (2SA912) going intermittent. Replace them with KSA1220AY. It won't hurt to replace Q1-4 (2SC1775) with KSC1845 or 2SC2240. Don't buy transistors on ebay.

So the two things I need to replace are

  • 2SA912 at Q5 Q7 (L) and Q6 Q8 (R)
  • 2SC1775 at Q1 - Q4 according to manual2, you can also just look at the board itself which have prints on them.

While I'm at it I might as well replace the caps on the Amp board too.


Sourcing material

Oh boy if I say looking for the material is hard.... for a Amp that is older than my mother XD
The KSA1220AY mentioned in the origional post is way obelete in 2020 and is no longer produced(officially anyway)
KSC1845 is the only one I can found in stock, and limited aval at best.
and as always... Don't buy transistors on ebay!

So after looking around, and waiting for almost forever, I decided on a paring that works, not the best but it should do.

I also replaced the caps on amp boards with

  • EEU-FC1C470B
  • ECA-1VEN101
    No photos provided so please note where and what value of cap you took off! you can also refer to manual for the info, see foot note.

Getting the job done

to do this you'll need a steady hand and a GOOD solder iron, here I recommand a T12 iron that has at least 45-60w output.

For me personally I recommend TS80/TS100 or the Pine Pencil, both are definally worth your money while being way less bulky compare to other stations

The other supplys can also be bought online. the things you need include

  • a solder sucker / desolder braid / desolder station (Which I have but is hella expensive)
  • a bowl of flux, thank me later
  • a solder removal tool for your iron, sponges are not ideal, I like the scrubber more
  • a spool of solder, make sure you get enough, any grade will technically be fine
  • pair of leg/wire snippers
  • (optional) a fume hood

and with that being done, lets get started.
First remove the machine from the case if you have one.

the wooden case will be fragile so be careful when sliding it out, there's four screws with washers on the bottom to hold it firmly in place so remove those first

Second remove the top screws holding down the Amp pcb

if you don't know which one then take a look at it... should be two on each side holding down the pcb, this is VERY IMPORTANT as if you don't you risk damaging the pcb when you try to take it off from the back!

Third flip the thing so it's back side up. and remove the back plate.

be gentle with it, also be careful don't cut yourself on the edges.

now you need to remove the screws holding the two amp and it's heatsink off.
no need to mess with the screws holding the TA100WA because the TA100WA is stuck on there pretty firmly

Spoiler: there's two screws on the back panel on the very left and right, and four holding the heatsinks in the middle section, as well as four holding the front panel to it on the two sides where they meet.

While you're at it, remove the metal cable channel just to make life easier

And now take a few photos of the two pcb! it will tell you where the colored cables goes when you put it back, as well as the solder points for reference

Remove all the "pins" that's soldered to both side by first heating it up with your soldering iron until the solder fully melted, BE GENTIL!!!! you can pull the contact pad off if you're not careful enough!

if you do pulled the pads oof, congrats idiot, now you need to fly a wire from the closest pad that whe traces are supposed to go. it's sample tho, just look at the pcb.

Remove the two amp boards from the main body after the pins have been pulled
now use your preferred solder remove method to remove the solder on the pin pads, also the solder for the components you want to replace (Caps, Transistors)
Take the desoldered components off and put them in a small bag or tray for safe keeping.

Slot in the new components, LOOK AT THE POLARITY ON PCB!!, and bent the leg on the solder side so it stays in.

Now solder all the new components in, careful don't heat them up too long or you'll kill them. Snip all the excess legs off with snippers.

Solder the pins back into their origional place.

do a pre-assemble test, make sure the relay clicks

assemble everything back together, be careful not to bent the PCB.

Done! Enjoy~

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