roger2
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I picked up a non-working KA-7100 a few months ago on eBay for a good price. The previous owner had correctly diagnosed speaker relay issues, but did not know how to repair the amp. Upon searching old threads, I found several references by EchoWars about how this amp is prone to relay failure due to aging components. So I felt it likely that I would be able to repair the amp simply by replacing components in the relay circuit, which indeed was the case. I believe this amp has been a workhorse judging by the way she looks inside and out.
As I worked through this project, my approach was to study existing threads about the KA-7100 in order to have an idea of common areas of failure. The issue mentioned most frequently in old threads is the speaker protection relay circuitry as noted above. Another area of concern is the regulated power supply, both the components on the main PCB as well those serving the control board.
In addition to addressing common areas of concern, I made component upgrades and a couple of conservative modifications. Also, a couple of observations (in regards to the power switch and PCB supports) are included in this thread.
I hope that this thread can be helpful to others working on a KA-7100
Overview of work done:
-power supply received special attention in refreshing and beefing-up of components
-heatsinks added to regulator series pass transistors
-AC power switch's unused contacts employed, new safety cap installed
-electrolytic and tantalum caps all refreshed and/or upgraded with film
-previous repair (by prior owner) of Qe10 redone with a better sub
-new Bournes trim pots
-new speaker relay, relay driver transistor, and diode
-output transistors received new mica and grease
-several stressed-looking resistors replaced
-all zener diodes replaced (power supply, main amp, control and phono boards)
-missing foam PCB supports replaced
-DeoxIT D100L used to clean pots, switches, fuses/holders, RCA's, speaker terminals, face plate, knobs, switch covers
-bias and DC offset adjusted to spec
Acknowledgements and thanks:
-EchoWars: numerous previous posts/threads that serve as valuable and useful resources. One in particular regarding KA-7100 power supply will be linked in this thread where appropriate.
-leesonic for this thread: Another Kenwood KA-7100 recap which, for anyone recapping a KA-7100, can be used as a general guide with great pics and his complete parts list included.
-ManyMoonsAudio for assistance in a separate thread where I had questions about the AC power switch. See post 14 below.
As I worked through this project, my approach was to study existing threads about the KA-7100 in order to have an idea of common areas of failure. The issue mentioned most frequently in old threads is the speaker protection relay circuitry as noted above. Another area of concern is the regulated power supply, both the components on the main PCB as well those serving the control board.
In addition to addressing common areas of concern, I made component upgrades and a couple of conservative modifications. Also, a couple of observations (in regards to the power switch and PCB supports) are included in this thread.
I hope that this thread can be helpful to others working on a KA-7100
Overview of work done:
-power supply received special attention in refreshing and beefing-up of components
-heatsinks added to regulator series pass transistors
-AC power switch's unused contacts employed, new safety cap installed
-electrolytic and tantalum caps all refreshed and/or upgraded with film
-previous repair (by prior owner) of Qe10 redone with a better sub
-new Bournes trim pots
-new speaker relay, relay driver transistor, and diode
-output transistors received new mica and grease
-several stressed-looking resistors replaced
-all zener diodes replaced (power supply, main amp, control and phono boards)
-missing foam PCB supports replaced
-DeoxIT D100L used to clean pots, switches, fuses/holders, RCA's, speaker terminals, face plate, knobs, switch covers
-bias and DC offset adjusted to spec
Acknowledgements and thanks:
-EchoWars: numerous previous posts/threads that serve as valuable and useful resources. One in particular regarding KA-7100 power supply will be linked in this thread where appropriate.
-leesonic for this thread: Another Kenwood KA-7100 recap which, for anyone recapping a KA-7100, can be used as a general guide with great pics and his complete parts list included.
-ManyMoonsAudio for assistance in a separate thread where I had questions about the AC power switch. See post 14 below.
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