img 0370

Krell KRC-HR Ultimate Preamplifier Restoration & Repair

Come with me this time, as I dive deeply into the repair and restoration of one of the best solid-state preamplifiers of all time – the magnificent Krell KRC-HR.

NOTE – for a bunch of different shots and angles, check out my detailed Krell KRC-HR video overview!

If you’re reading this, you’ve probably heard of Krell and know something about the incredible range of premium, industrially constructed gear they made throughout the 1980s and 1990s. The Krell KRC-HR preamplifier is certainly one of the best pieces to come out of the Krell factory. Come along for a look inside to find out why.

img 9990

Top of the Heap

The KRC-HR was Krell’s finest preamplifier. This unit dates back to around 1994, but Krell sold them until the late 90s as far as I can tell. Perhaps my friend Patrick at Krell can elaborate on this…?

Like all Krell products from this period, the KRC-HR features a superbly made chassis, built out of cast, pressed and milled aluminium. She also features beautiful fibreglass PCBs, thick copper traces and premium parts throughout. Everything screams premium quality and is a world apart from standard consumer gear, I can assure you.

img 9988

The KRC-HR has a couple of features worth noting. First is the external power supply, housed in the smaller box. This provides regulated DC rails to the main chassis. The rails are further regulated down to the various voltages used in each block, in the main chassis. The external PSU gets warm, really warm, and it has no power switch, so you have to turn it off at the wall, or unplug it.

img 9989

The second feature I like is the add-on KSL phono board. Like the rest of the KRC-HR, the KSL board is superbly made, using the typical Krell gold pins to connect it to the main chassis. It uses +/-21V rails and a JFET input buffer. It also features lots of local power supply reservoir capacitors, MM/MC gain switching and a wide range of MC loading resistors, accessible via a pair of board-mounted switches.

Stereophile and others at the time liked this preamp very much. I suggest this KRC-HR review from Stereophile as a good starting point. If you’d like to see an overview of the Krell KRC-HR  in video form, check out my longest video so far, on YouTube.

Krell KRC-HR Specifications

Frequency response: 20Hz to 20kHz
Total harmonic distortion: 0.01%
Gain: 12dB (line)
Signal to noise ratio: 96dB (line)
Output: 14V (Pre out Max)
Dimensions: 19 x 14.5 x 2.75 inches
Accessories: remote control
Year: 1997
Sound: awesome 🙂

Problems

The Krell KRC-HR runs essentially in class-A as all good amplifiers do. This means it runs hot. The power supply dissipates a lot of heat, and, with no power switch, potentially stays on, 24/7. The capacitors, no matter how good they are, will suffer under this use-case scenario.

This unit came to me with a number of issues: one channel was operating intermittently, there were problems with the phono board, it had some shipping damage and many capacitors needed replacing. So let’s dive in then, starting with the best place to start any electronics repair & restoration – the power supply.

img 9991

img 9993

Power Supply

The best way to approach repairing and restoring any piece of electronics is to start with the power supply. Without a power supply, the unit can never work, and power supply problems will manifest themselves everywhere else in the unit.

This KRC-HR had experienced problems before and had been repaired, albeit not very well. This was nothing like the appalling work done on the Krell KRC in my Hall of Shame, but the repairs in this unit were crude nonetheless.

img 9882
From the outside, all looks good…
img 9883 e1530422624137
But at first blush, we see the layers of dust and grime. You may also spot the signs of earlier repair. Clue – look at the resistors in the middle of the board.
img 9884
Here’s a closer look. The blue pair of metal-film resistors middle-bottom are not factory. Anyway, the only way to deal properly with boards this dirty is to wash everything.
img 9885
Two toroidal transformers, three diode bridges and four nice big filter capacitors. I’m about to wash this whole board.
img 9887 1
You might be able to tell that someone has been in with a big, fat Weller soldering iron, low skills, and has lifted pads and a whole trace where that bodge wire is. This must have happened when that transistor was replaced. I repaired all of this.
img 9888 1
After cleaning, the board looks much better. I’ve removed the TO-3 devices and put them loosely back for the photo. I washed the heatsinks, and as you’ll see, remounted the transistors on new silicone thermal pads. Note the different date codes on the TO-3 devices, one was replaced at some point.
img 9889
Detail of the transistor and its heatsinking. This is an expensive way to do things, but it’s robust, dissipates heat well and should last a long time as long as they got all the calculations right!
img 9890
This is the hardware, removed from the board. We’ve got lots of little spacers to prevent shorting. These were brittle/broken, so I made replacements. I also replaced the fibreglass thermal pads with modern silpads.
img 9891
Here’s the board completely washed and dried.
img 9893
Detail of the new spacers I made. These keep the heatsinks and devices properly aligned. I make them out of a special high-temperature, high-stability rubber.
img 9892
A better look at a new spacer, and the fibreglass board. You’ll note that the area under both heatsinks is quite badly heat affected, but still mechanically and electrically sound.
img 9894
In this shot, you can see I’ve remounted the transistors on silpads and new spacers. You might spot one new cap already installed at the bottom, and I’m waiting on some parts for the two vacant spots at the top.
Krell KRC-HR
Here they are, installed. These are premium Nippon Chemi-Con parts. Note how clean everything is now, too.
img 9986
Sparkly, and now back in the chassis.
img 9883 e1530422624137
Before…
img 9987 e1530425539427
And after. Main filter capacitors measured perfectly, no need to replace them, but a good idea to replace smaller caps as I’ve done here, and attend to all the details.

KSL Phono Board

Let’s keep moving with this Krell KRC-HR restoration, onto the Krell KSL phono board. I replaced literally every electrolytic capacitor on the KSL board, they were all bad.

In addition, the gold pins that connect the board to the main chassis were slightly too long. This meant that, when the board was screwed down, it placed great stress on the pins and actually curved the board over time. I resolved this and a few other issues.

img 9998
This is the KSL phono board, a lovely design with lots of very nice caps. Sadly, heat has killed them all. Red switches in the middle control gain, white switches to the right control MC loading. These switches all needed cleaning and treatment.
img 9999
Four of the worst caps in terms of how they measured were these, in the middle of the board.
img 0206
The phono board washed and now featuring premium Vishay 8000+ hour axial capacitors. These are expensive, but up there with the very best parts one can use here. I also trimmed the interconnect pins, which were slightly too long. This allowed the restored board to sit flat on its standoffs and removed stress on the pins. I used wet & dry to take off any sharp edges from the trimming.
img 0205
Here’s the phono board finished, next to the main chassis which I’ve not yet worked on in this image.

Main Preamplifier Chassis

The last and biggest part of the refurbishment process was the preamplifier mainboard and chassis. The top board handles attenuation and switching. The bottom or mainboard houses the gain stages, buffers, protection, DC control and power supply.

Let’s go.

img 9992
Stunning design and layout. Note the almost total absence of wiring.
img 0207
In this shot, you can see the area underneath the phono board and the axial Sprague capacitors like those on the KSL phono board. I tested these and they were all dead. This convinced me to completely refurb this KRC-HR.
img 0208
These had very high ESR – around 3 – 5 Ohms. You need a way to effectively test capacitors if working on gear like this. The shotgun approach is ineffective, wastes time and often results in valuable parts being thrown away for no good reason.
img 0210
Look at how many caps are hiding underneath the control board! Luckily, not all of them needed replacement.
img 0214
Here is the mainboard, removed from the chassis, before I replaced any capacitors or reworked the board. Note the just two pairs of wires, superb quality RCA and balanced connectors, multiple paralleled transistors, Dale resistors and the large number of local reservoir capacitors.
img 0223
One of the very few Krell factory kludges.
img 0221
More evidence of someone being here before me. Again, note the damage to the pads. Luckily these holes are plated through and the boards are multilayered. Good electrical contact can be restored after these parts are carefully removed, by wicking solder down along the leads of components where the pads have lifted. But still, this is careless work.

img 0234

img 0242
More careless work. There really isn’t any need for this. It mostly comes from using too hot a soldering iron. In this case, I’ve replaced the capacitor.
img 0232
It might be hard to tell, but this pair of pads in the middle is after I’ve replaced the capacitor and re-worked the board, allowing for the damaged pad. This capacitor is securely soldered into place, even on the pin where you see the pad is missing. The through-hole plating and pad on the other side allows for effective re-work in situations like this, using the right techniques and equipment.
img 0241 1
By contrast, this is a pair of joints that only I’ve worked on, in the mainboard power supply. Note the straight pins, minimal flux and no pad damage.
img 0230
New, high-spec parts under where the phono board sits. This part of the circuit controls DC offset and protection. Again, note the new Vishay 8000 hour axial capacitors. Note also that a trimmer and an OPA27 op-amp have been replaced, likely by the person who worked on this unit previously.
img 0217
In this shot, you can see 8 slightly discoloured 1000uF caps in a block in the middle. I replaced these, along with 8 smaller caps you might see nestled in around them and further back.
img 0215
One cap in this group was leaking, the rest were fine but I replaced them anyway as they were obviously exposed to greater thermal effects than the others.
img 0227
Two of the 8 smaller caps I mentioned, replaced with Panasonic FM, superb parts.
img 0226
The block of caps I mentioned, here also replaced with Panasonic FM.
img 0228
Another view of the Pana FM replacement caps, smaller ones to the left, and 8 larger ones in the centre.
img 0225
The mainboard after the work done so far.
img 0209
This is the secondary power supply, where the +/-21V and +/-32V rails are regulated. The blue multiturn trimpot sets the +/-21V rails for the phono board. I decided to replace these 6 caps, they all measured slightly low for C and are critical as part of the power supply.
img 0237
Old Nichicon VR caps next to the United Chemi-Con KY parts. These are super low impedance, 4,000 – 10,000 hour, 105 degree rated parts, superb caps by any measure.
img 0239
New mainboard power supply caps installed, note parts dress and alignment.
img 0240 1
Same caps, from the inside. The trimmer sets the +/-21V rails.
img 0231
The underside of the restored board. I also repaired several RCA connectors exhibiting cracked signal pins. This particular unit suffered some mild shipping damage at some point, affecting the rear panel and connectors.
img 0244
Here are the parts I replaced in this Krell KRC-HR restoration. Lots of parts…
img 0246
For those who like order, here they are again!
img 0248
The reassembly itself took a couple of hours. I carefully cleaned all the pins like those you see here, re-torqued everything, resoldered the signal wiring and fixed small, residual faults. This, by the way, is the ultra-premium optical encoder that drives the precision resistive attenuator top-board the infamous ‘repairer’ in my Hall of Shame ditched in favour of a Silicon Chip kit…
Krell KRC-HR
Here she is, all back together, ready for testing and adjustment. You might also be able to see the wiring I replaced around the left-most phono input and ground connector.

Krell KRC-HR

img 0256
The main things I sorted out at testing were DC offsets from the phono stage and +/-21V rails to the phono board. I was able to trim out both of these parameters.

Final Testing & Listening

The Krell KRC-HR preamplifier restoration was certainly a big one, taking me many hours to complete, over several days. Having said that, it was a very enjoyable restoration and the results were well worth it.

The final testing involved sorting out a few more gremlins with the phono pins and loading switches. I cleaned and treated the switches and re-worked the now slightly shorter gold pins to restore good electrical connection across all of them.

The KRC-HR sounds better now than it probably ever has. I can say this because I’ve had the opportunity to listen to it over several years and subsequent to this restoration. Sonically, she’s transformed.

img 0262
Pretty with her lid back on.
img 0267
Thank you Krell, for building such a wonderful preamp in the KRC-HR. It continues to deliver great listening pleasure, all these years later.

img 0268

As always, thank you for reading. If you own a Krell KRC-HR or any other Krell preamplifier for that matter, don’t hesitate to get in touch if you’d like me to look at it for you!

Hi! Feel free to share your thoughts and if you need advice, visit our contact page.

%d bloggers like this: