KLH Model Eight FM Radio (1962)



The KLH Model Eight (8) FM radio is a prized collector's item, scarcer than the Model Twenty One that succeeded it. Both were designed by Henry Kloss with one goal in mind: to create the world's best-sounding FM radio in a compact tabletop cabinet.


Designed by Henry Kloss and built in Cambridge, Massachusetts, the Model Eight was produced from 1960-1965. Kloss had earlier co-founded the Acoustic Research company, creator of the acoustic suspension speaker (a speaker sealed inside an air-tight cabinet).

The Model Eight was Kloss's first radio. Using a transformer-type power supply, it has a hand-wired chassis. The Model Eight employs seven tubes (types 6U8A, 6BS8, two 6BM8, and three 6AU6A).

This set produces amazing sound for a radio of its size and it stands alone among tube radios of its time. Mine sits in my office and I listen to it nearly every day. That's quite a testimonial from someone who owns a houseful of radios.

Below you will find the Model Eight owner manual and schematic diagram. To save either PDF file on your computer, right-click on the icon and choose Save Target As.


KLH factory literature is rather scanty. If anyone has found a more detailed service manual, kindly send me an email.

You can read more about the Model Eight's development and features in this article from Antique Radio Classified magazine.

First Look

Here is my Model Eight as found, before I had done any cleanup or restoration. The radio and speaker are in identically sized cabinets, which I stacked for these photos. The front controls are simple: power, treble, volume, and tuning.


From behind, you can see that the speaker has a back cover but the radio does not. The two are linked by a 30-foot long cable. Big cleats on the speaker's back cover make it easy to stow excess cable:


Notice the corner joints in the rear cabinet views. These are early Model Eight cabinets, made of solid walnut; later cabinets were made of veneered wood.

The radio's chassis is compact, with a few rear connectors. Two upper terminals let you connect an external FM antenna. At lower left are two jacks for the speaker cable. The tuner output jack can be used with an external amplifier. The multiplex jack is for use with an external stereo multiplex decoder (and stereo amplifier).

Trying Out the Model Eight

When I powered up the radio using my variac, I found that it worked nicely, with good sensitivity, excellent tone, and not a trace of hum (which would signal bad filter capacitors).

The only defects were some scratchiness when turning the volume control and, less often, the tuner. These can easily be remedied with a little electronic cleaner when I take the chassis out of the cabinet. When I get to that stage, I can also check the filter caps with my capacitor tester to make sure they're up to snuff).

The cabinets are in nice shape, overall, with only a few little boo-boos that will be easy to eliminate. The speaker grille cloth has some visible stains. If those can't be cleaned away, I'll replace the cloth with a comparable linen fabric.

Cleaning Up

My Model Eight's chassis was pretty grubby, covered with greasy dust.

The view underneath was more inviting. No cleanup needed here.

The build quality of this radio is impressive. The solder joints are extremely neat and every one was tagged with a green marker, possibly a quality control measure to show that it passed inspection. The box at upper right in the photo is the tuner cage, soldered shut to exclude dirt as well as RF interference.

After dusting off the air variable capacitor, I lubricated it to eliminate intermittency and scratching sounds when tuning. The yellow arrow points to the ball bearing structure at the front of the tuner frame:

With the chassis tilted forward, I put a tiny drop of light oil into the ball bearing mechanism using a toothpick, and then turned the tuner all the way back and forth a few times to spread the oil. I left the chassis tilted forward for a couple of hours to let a little oil migrate down into the vernier tuning mechanism.

The arrows in the next photo show two more areas that got a wee bit of CAIG electronic lubricant:

As with every restoration, I also cleaned every pin on each of the seven tubes and applied electronic cleaner to the Treble and Volume potentiometers.

I let the radio sit overnight, to make sure all of the electronic cleaner had evaporated from the potentiometers. When I tried it the next day, it sounded fabulous. The cleaning eliminated all noisiness from the tuner and controls.

Cleaning the chassis took much longer. Its plating was oxidized as well as dirty. I began by mopping off dirt and grime with isopropyl alcohol, using rags, paper towels, and Q-tips. After that, I brought back the shine using metal polish and pieces of non-scratch plastic pads. I cleaned off the polish with damp paper towels and then polished with a cloth. This photo shows the chassis after I had done a strip near one end.

Now, the chassis looks much more civilized.


Of course, the electrons whizzing inside don't care whether the chassis metal is dull or shiny. I don't bother with this in many projects, but the Model Eight is so nicely built that I like bringing it closer to its original appearance.

Cabinet Restoration

Let's start by cleaning the Model Eight's front panel. This painted Masonite board cleans up easily with Windex and a soft cloth. Don't scrub it with anything abrasive, lest you ruin the paint.


The speaker grille cloth is trickier to handle. In this earlier photo, you can see a stain meandering across its lower part:

Grille fabrics are not intended to withstand wetting, and there's no way to know whether this stain came from spilled coffee, alcohol, or something else. We also don't know what sort of dye (if any) was used in the original fabric. If the dye is water soluble, wetting may have unpredictable results.

Notice the lighter appearance of the cloth over the circular holes for the speaker drivers. Perhaps air moving through those openings kept dust from settling on those parts, whereas the rest of the cloth was glued stationary to the board, where it could collect more dust, cooking fumes, etc., over the decades, eventually darkening with grime.

In many old radios, the grille cloth is lightly glued around the edges of a big opening, and with care, you can remove it for gentle washing. Not so with this cloth, which is glued to the mostly-solid Masonite speaker board. Trying to peel it off would likely destroy it.

Nothing ventured, nothing gained. I began with clear, warm water, dabbing it on with a clean cloth and then pressing off the excess with another cloth. Following that, I left it under gentle heat from a lamp, to see how it looked after drying.

The first attempt wasn't encouraging. Dabbing with water lightened the stain somewhat, but it also lightened the surrounding area (duh), so the stain was still evident.

I suppose the lightest areas on the cloth represent the original color, more or less. One approach would be to keep dabbing the entire cloth, until it had a pretty uniform light color and the stain was much reduced.

I didn't do that because the cloth had begun to shrink. Look at the lower left side, and you'll see a space starting to open up between the cloth and cabinet, whereas the cloth is tightly pressed against the cabinet in other areas. The cloth was also loosening a bit in that corner.

After the water treatment, I went back over the cloth with naphtha lighter fluid, a solvent that evaporates without residue, paying special attention to the darker areas. This lightened the stain so that the cloth looked presentable.

Since the radio and speaker cabinets had only minor scuffs, I freshened them up by applying Howard's Restore-a-Finish. This treatment isn't suitable for a badly damaged finish, but it works for concealing the little stuff.

Electronic Restoration

For an older tube radio, I would typically replace the electrolytic and paper capacitors as described in my recapping article. For this 1960s set, I took a wait-and-see approach. After playing it for a while, the radio performed beautifully. When I monitored key voltages, everything looked hunky-dory, so I decided against "shotgunning" its capacitors for no reason.

At the first writing (early 2013), the Model Eight had been playing happily in my office for about two years. The audio quality was outstanding and the tuning was as stable as a rock, without a trace of drift. Perhaps it would need service someday, but so far, its performance showed no sign of flagging.

2015 Update

In 2015, my Eight's power switch finally failed. This is a weak point in this model and mine had been flaky for a while. Sometimes, it would click into place normally. Other times, it refused to switch no matter how hard you turned the knob. It eventually quit working altogether, failing in the Off position so that I couldn't turn the radio on and off by plugging and unplugging the cord (or installing a line switch on the cord).

You'd think that a simple rotary power switch would be easy to find, but this one was not. Most consumer radios combine the power switch with the volume control, and other equipment typically uses a toggle or rocker type switch. None of the major suppliers stocked a plain rotary switch of the right size.

Here's the switch in question, made by Clarostat, with the part number 12 on its back plate. Around the perimeter is another number, 140-6046; the 60 means it was manufactured in 1960, and the 46 stands for the 46th week. (Yet another code, CM 26254, was of no use in finding a replacement.)

The photo shows that a previous "repairman" tried to open the switch by prying apart its case with an awl or screwdriver. That bent the case but didn't open it, since it's held in place by two thin tabs bent over slots. You can see a dent in the lower half of the case, perhaps from gripping it in a vise or heavy plier. The flat portion of the shaft has also been rounded off, when someone tried again and again to force the switch to operate.

On eBay I found a replacement for the lower half of the switch (part 12), which contains the actuated portion that makes and breaks the electrical contact. Here's a photo of the old and new parts, along with the upper half that holds the knob shaft.

When I assembled the switch with the new half, it seemed to work outside the chassis, but every time I installed it in the chassis, it quit working. The actuated mechanism refused to click into place, no matter how hard you turned the shaft. After disassembling, checking, and reinstalling the switch a couple of times, I finally gave up on it. The flaw—whatever it was— seemed to be in the bent-up shaft portion of the switch, and for that I had no replacement.

Another option would be to use a volume control that combines a power switch with a potentiometer. I'll use the switch portion and leave the potentiometer terminals connected to nothing. The trick will be to find one that fits in the available space. Most power/volume controls have two sections stacked one behind the other, and as you can see here, the original switch is about the same depth as a potentiometer alone:

After rummaging in the pot drawer of my parts chest, I found a volume/power control that would fit. In the next photo, I have cut the control shaft to length and I'm ready to file a flat spot on the shaft for the knob:

After some careful filing, the knob fits just right. In this photo you can see the control's two sections. The pot section is cased in metal and the switch is cased in black plastic at the rear.

In the next photo, I'm slipping the new switch into place. It barely fits, if you first remove a screw from the fuse holder and slide it aside.

Unseen on the opposite side of the control are the three potentiometer terminals, which will remain harmlessly disconnected.

Replacing Electrolytic Capacitors

While testing the switch under power, I noticed that a 2-watt resistor connected to one of the power supply filter capacitors overheated and showed signs of burning. In the next photo, that resistor is marked with an arrow; a second arrow points to the can that contains four 60-mfd electrolytic capacitors that filter AC ripple out of the power supply:

I disconnected those resistors and checked the electrolytics with my EICO 950B capacitor tester. They leaked like crazy! One of them had basically short-circuited. No wonder that resistor had overheated. I knew I had been pushing my luck by playing this radio with its original 53-year old electrolytics in place. It's a lucky coincidence that I detected the problem before one of them failed catastrophically and ruined the radio's power transformer.

Other articles in this website document capacitor replacement exhaustively, so I won't bore you with details. I removed the electrolytic can in preparation for wiring four 60-mfd electrolytics onto its base.

The next photo shows the dried-out innards that I pulled out of the old can. Notice the crumbly powdered junk on the workbench. When this capacitor was manufactured 53 years ago, that powder was a moist paste that formed the capacitor's electrolyte. Now it is garbage, a certain sign that the capacitor has failed.

The next photo shows the four new electrolytics stacked and wired with their leads going through holes in the can's base. After I confirm that the radio works normally, I'll glue the emptied can back over this assembly.

While I had the chassis on the bench, I also replaced its one other electrolytic, a 5-mfd/25-volt unit. Low-voltage electrolytics don't typically cause serious problems in this type of radio, but better safe than sorry.

This photo shows the radio with the new power switch, resistors, and electrolytics wired in place. One arrow points to a new resistor and another points to the capacitor assembly, ready for the can to be reinstalled.

I tested the radio's voltages and gave it a nice long bench test to confirm that everything worked properly. It played like a champ!

Now I have returned the Model Eight to its place of honor in my office, right next to my computer where I play it every day. With luck, it shouldn't need further service for a good many years.

Twenty One, Anyone?

I also own a KLH Model Twenty One, which I keep in my workshop:

The Twenty One is a solid-state set with speaker and radio combined in one cabinet. It is also easier to find than a Model Eight. Otherwise, there isn't much to choose between the two. Both testify to Henry Kloss's dedication to quality. Someday, when I have too much time on my hands, I might play them side-by-side and do a "Pepsi challenge," but I suspect that I wouldn't hear much difference!

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