Krell KAV-150a Power Amplifier Repair & Restoration

Krell KAV-150a Power Amplifier Repair & Restoration

This beautiful Krell KAV-150a power amplifier is the partner of the KAV-300i I restored last year. Come along as I repair and restore this one.

The Krell KAV-150a is another product from Krell’s more affordable KAV line, from the mid 90’s. Whilst certainly more affordable, this gear is most definitely not cheaply built, sharing the same stunning machined aluminium casework and stainless steel fasteners as other Krell gear.

Krell KAV-150a

The KAV-150a could be partnered with the KAV-300i, like this KAV-300i I restored recently, or a dedicated Krell or other preamp. It has plenty of power too, pumping out 150 watts per channel, continuously into 8 ohms, more into lower impedances.

Krell KAV-150a

If you’d like a slightly different look at this job, check out the KAV-150a restoration video over on my YouTube channel.

Specifications, courtesy the HiFi Engine

Power output: 150 watts per channel into 8Ω, 600W into 8Ω (mono)
Frequency response: 20Hz to 20kHz
Total harmonic distortion: 0.25%
Input sensitivity: 1.8V
Signal to noise ratio: 112dB
Speaker load impedance: 4Ω (minimum)
Dimensions: 475 x 92.5 x 387.5mm
Weight: 11.4kg
Year: 1998

Problems

This unit, like the KAV-300i I worked on earlier, didn’t appear to have been overhauled or even serviced. It was apparently suffering from bass distortion and I certainly found some major issues with leaking capacitors and the resultant corrosion this caused to nearby components.

Krell KAV-150a

Krell KAV-150a
KAV-150a, prior to commencement of repairs and restoration.
Krell KAV-150a
Yep, this is bad and needs some careful attention. More on this later.

Repair & Restoration

Like most repairs and restorations, I begin with disassembly and cleaning. That involves cleaning of boards, chassis connectors etc. Then I refurbish the boards that need work and repair any damage. Finally, I reassemble everything and test the unit. Let’s work through each stage.

Disassembly & Cleaning
Krell KAV-150a
First steps are always disassembly and comprehensive cleaning. This usually means washing, as it allows me to see what’s going on underneath all the crud. It also cleans away the corrosive electrolyte from leaking capacitors.

Krell KAV-150a

Krell KAV-150a

Krell KAV-150a
This KAV-150a is pretty dusty, but most of this came away with high pressure air and a soft brush I use for this purpose.
Krell KAV-150a
This is the soft-start and line voltage selection board. This part of the amplifier runs relatively cool and only needed a light clean.
Krell KAV-150a
First board out is the input board.
Krell KAV-150a
That leaves this large mainboard, and this is where most of the work lay in this unit. Note the all original output devices on original thermal pads. These are fine to stay, but I always re-torque the output device fasteners.
Krell KAV-150a
Washing the input board…

Krell KAV-150a

Krell KAV-150a
Next, the mainboard comes out…
Krell KAV-150a
This allows me to clean away the rest of this dust.
Krell KAV-150a
Mainboard removed from chassis. All capacitors are original, with the possible exception of the row of 10 black caps in the middle.
Krell KAV-150a
I had discussions with Krell about these kludges that appear to be factory.
Krell KAV-150a
I don’t remember working on any other Krell gear that used Jamicon capacitors, so I still don’t know if these are factory.
Krell KAV-150a
These Nichicons certainly are factory…
Krell KAV-150a
As are all these. Sadly, many of these smaller capacitors are leaking badly.
Krell KAV-150a
Like these two. Both are dead, have zero capacitance and high resistance, causing them to leak.
Krell KAV-150a
The leaking resulted in this mess. The electrolyte you see here is corrosive and damages both circuit board traces, and components it comes into contact with. Luckily the damage is somewhat confined in this case, affecting the precision operational amplifier bottom right, and a few other components.
Krell KAV-150a
Smaller capacitors are always most affected by heat and age, these two are also not well.
Krell KAV-150a
Overall, this is a ticking time bomb in electronics terms and must be resolved.
Krell KAV-150a
My cleaning regimen helps minimise current and future damage. I use a process that neutralises and removes the electrolyte. You can see that happening, bottom centre. Some electrolyte seeps into the board though, working its way along inner layers. This damage cannot be undone, but we can minimise further damage with careful attention now.

Krell KAV-150a

Krell KAV-150a
Cleanser doing its work on capacitor electrolyte.
Board Refurbishment

Now I can see what I’m dealing with, faulty or damaged components make way for new parts.

Krell KAV-150a
This op amp is badly affected by corrosion from leaking electrolyte.
Krell KAV-150a
Here, I’ve removed the opamp and several other components nearby. That allowed me to further clean the affected area and, using a glass fibre pen, scour the remaining corrosion from the board and legs of mildly affected parts.
Krell KAV-150a
Here I’ve just removed the Zener diodes D39 and D40. I replaced these with new parts, along with the op amp, C43 and C45.
Krell KAV-150a
Main board ready for new parts.
Krell KAV-150a
This is most of the new capacitors.
Krell KAV-150a
All the small parts installed. Note the new op amp, Siemens film caps and premium Panasonic FC and FM electrolytics.
Krell KAV-150a
Lovely Nippon Chemi Con LXY capacitors…
Krell KAV-150a
And more Nippon Chemi Con KYB series used here. These are all amazing parts, super long lifespan, super low ESR, high temperature.
Krell KAV-150a
Nichicon main filter capacitors round out the refurb of this board.

Krell KAV-150a

Krell KAV-150a
As always, neatness is very important to me. I even straighten all the transistors and other parts, something slightly OCD going on there…

Krell KAV-150a

Reassembly, Testing & Adjustment

All the remains now is to put everything back together and do the final testing and adjustment.

Krell KAV-150a
Main board reinstalled in chassis, speaker wires connected.
Krell KAV-150a
Input board in place…
Krell KAV-150a
Wiring and all fasteners now secure.

Krell KAV-150a

Krell KAV-150a
These are all the parts I removed and replaced with new ones.
Krell KAV-150a
Soak testing after bringing the amplifier up on a variac. Wires are mine, speaker cables mixed with bias test leads.
Krell KAV-150a
Monitoring bias in left and right channels, over time.
The Restored Amplifier

Overall, I am very happy with this job. The amplifier is running very nicely now, very stable and she sounds great. The images below show the KAV-150a after final testing, adjustment and cleaning.

If you would like me to take a look at your Krell amplifier, don’t hesitate to get in touch.

Krell KAV-150a

Krell KAV-150a Krell KAV-150a

Liquid Mike

At 10 I was pulling apart electronics and by 13 I'd have Dad's hi-fi in pieces when my parents went out! Later, I started Liquid Audio, a specialist electronics repairer known for detail-focused service, repair and restoration of hi-fi electronics & turntables. Keeping classic hi-fi gear alive and well is what we do. Our mission: to deliver TLC for classic Japanese, American and European hi-fi stereo equipment. In my spare time, I ride motorcycles, travel, listen to music and research interesting topics.

This Post Has 2 Comments

  1. Keepee

    Thanks.

    1. Mike

      No problem!

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