Krell PAM-7 Class-A Preamplifier Repair & Overhaul

Krell has produced some stunning gear over the years and this Krell PAM-7 is a beautiful example. Come with me as I repair and overhaul this lovely old girl.

The Krell PAM-7 preamplifier dates back to earliest days of Krell, but all the trademark features are present: thick boards, premium parts, class-A operation, separate power supply and the unique styling the company used back in the day.

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I particularly like the industrial grey brushed aluminium finish. Nothing else looks like older Krell gear and certainly, the mechanical and electronic build quality is extraordinary, even now.

Sonically, this unit gives nothing away next to the most expensive modern preamps. There are no new circuits and parts in 2019 that these guys didn’t understand and have access to in the ’80s.

This unit has all the best features: premium switches and pots, zero wiring, FETs, lots of local capacitance, discrete & configurable phono preamp, multiple regulation, class-A, all discrete circuitry, and on it goes.

Krell still have the owner’s manual available and the excellent staff in the Krell service centre are always kind enough to assist me with what data they have available – thanks Patrick!

Problems

This unit exhibited several issues and she had never been in for service until she came to me. Signals from the PAM-7 showed distortion and gain was low in one channel. Strange noises emanated from the phono preamp and it turned out that one of the 15V rails was sitting down at around 5V.

A couple of the on-board (as opposed to external) power supply capacitors were leaking and didn’t measure well. Obviously, the unit needed a repair and at least a partial overhaul at the same time. Despite the impeccable service life and fantastic performance of this wonderful preamp, it took some convincing her owner that this was a worthwhile repair.

Thankfully I was finally able to, so let’s go.

Fault Finding

First steps were to diagnose what was wrong. I traced the weird noises, distortion and low gain to a bad Zener diode voltage reference. This bad diode left one power supply rail low, and these asymmetrical rails caused the problems the owner and I heard.

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Very cool looking Krell gear, time to get inside…
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Covers and panels stored for later detailing.
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Here’s the unit, ready for some diagnostics. Once that’s done, the board can come out.
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I noted what looked to be leaking electrolyte from this big filter capacitor. A couple of these caps didn’t measure well, so I decided out with all four.
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Detail shot of one set of internal configuration switches. There are phono and line level options in the PAM-7.
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And here’s the external power supply. Very Krell, two transformers, generous filters caps of a different type to those used inside the pre and regulation. Everything here tested well, and the owner’s budget meant I left this alone apart from cleaning up a messy mains cable installation. I wanted to rebuild it, maybe next time!
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View of the main board after repairing the Zener diode, just to the right of the blue filter caps. Nothing much to see, so I didn’t get a close-up.
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After testing these original parts I ordered some Nichicon KG Gold Tune replacements. These are extraordinary caps, every time I use these I hear an improvement.
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Out with the old…(see the diodes..?)

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And in with the new…
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These caps just look so good, too. Awesome audio-grade capacitors from a legendary Japanese manufacturer – I highly recommend them.
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Power supply restored, note the additional new Panasonic FM capacitors in this shot.

Cleaning

Attending to all the little details that make something really look and feel like it’s had proper service is part of the value add that I provide for my customers. In this case, that meant a full clean of all panels, fascia, knobs, cleaning and treatment of connectors and lubrication of all switches and controls.

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This is such a lovely piece but has years of patina added to it. Time for a clean.
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Is there another logo with this attention to detail..? Those are actual screws holding the brass, engraved Krell emblem in place.
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The serviced Spectrol volume pot. Let’s talk for a moment about volume potentiometers. There’s lots of chatter in forums about ALPS Blue Velvet and similar, good consumer-grade potentiometers, but toys compared to this one. Few have heard of Spectrol and you wouldn’t unless you own and work on serious test and measurement gear as I do. This is a REAL volume potentiometer,  used in military, aerospace and laboratory gear. Extraordinarily expensive, here’s a PDF datasheet for those potentiometer aficionados…
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Overhauled board back in the chassis now and with all switches and controls cleaned and serviced. Astute readers will note that I left the original Roederstein capacitors on the left-hand side of the board in place. Don’t be concerned, all measured like new. They are exceptional capacitors, contribute to the great sound of these preamps, plus I was working to a tight budget here.
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You’ve gotta have clean knobs…
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A good sign they needed cleaning…
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Much better, all these details help when combined. Note Krell’s attention to detail – all the brushing is perfect and runs in-line with the pointer.

Results

This repair and overhaul were critically important for this PAM-7. Without them, I suspect she might have been tossed out, or taken to a butcher

Folks occasionally struggle with the idea that you need to periodically spend a little to have an expert look at beautiful equipment, like this. But this isn’t like taking a digital watch to a mall vendor to get a new battery. It’s much more akin to taking a Rolex to a good jeweller for its regular service. They’ll get it running perfectly again. Surely worth doing properly, right?! Remember, even Krell doesn’t make anything like this any more.

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Perhaps about to be sealed for the next 30 years..?!

I think the results speak for themselves. The unit now sounds better than ever. Her sonic behaviour is perfect and she sounds sublime, especially via that stunning, adjustable phono preamp. If only all hi-fi equipment was this beautifully constructed.

When I explained to her owner that to replace the PAM-7 with a modern ‘equivalent’ with similar sound quality and likely cheaper build one would need to spend well over $5000 AUD, I think it began to sink in.

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For example, you could buy a lovely Bryston BP6 for $5500, but that’s not really in the PAM-7’s league. How about a Musical Fidelity M8 for $6999 – do you imagine that this might sound better? Looking at the M8’s internals, I can guarantee it won’t.

So what does all this show us? Hopefully, it’s obvious that humans could build this stuff perfectly, decades ago. It perhaps teaches us that newer doesn’t mean better and in fact, often means the opposite. Most of all, I hope it shows that spending just a tiny fraction of the cost of something new, to have your beautiful old equipment professionally serviced, is some of the most sensible money you can spend.

And of course, if you would like me to look at your lovely vintage Krell equipment, you need only get in touch!

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Feel free to leave a comment and share your thoughts!