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.

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.

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.

KAV-150a, prior to commencement of repairs and restoration.
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
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.

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.
This is the soft-start and line voltage selection board. This part of the amplifier runs relatively cool and only needed a light clean.
First board out is the input board.
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.
Washing the input board…

Next, the mainboard comes out…
This allows me to clean away the rest of this dust.
Mainboard removed from chassis. All capacitors are original, with the possible exception of the row of 10 black caps in the middle.
I had discussions with Krell about these kludges that appear to be factory.
I don’t remember working on any other Krell gear that used Jamicon capacitors, so I still don’t know if these are factory.
These Nichicons certainly are factory…
As are all these. Sadly, many of these smaller capacitors are leaking badly.
Like these two. Both are dead, have zero capacitance and high resistance, causing them to leak.
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.
Smaller capacitors are always most affected by heat and age, these two are also not well.
Overall, this is a ticking time bomb in electronics terms and must be resolved.
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.

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.

This op amp is badly affected by corrosion from leaking electrolyte.
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.
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.
Main board ready for new parts.
This is most of the new capacitors.
All the small parts installed. Note the new op amp, Siemens film caps and premium Panasonic FC and FM electrolytics.
Lovely Nippon Chemi Con LXY capacitors…
And more Nippon Chemi Con KYB series used here. These are all amazing parts, super long lifespan, super low ESR, high temperature.
Nichicon main filter capacitors round out the refurb of this board.

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

Reassembly, Testing & Adjustment

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

Main board reinstalled in chassis, speaker wires connected.
Input board in place…
Wiring and all fasteners now secure.

These are all the parts I removed and replaced with new ones.
Soak testing after bringing the amplifier up on a variac. Wires are mine, speaker cables mixed with bias test leads.
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.

 

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