Harmon Kardon ST-8 Turntable On The Bench

SCompRacer

So I'm navigating retirement, no more stressful job distracting me. Wife says what's in those boxes over there? My basement is not like mhardy's stash, but lots of space is taken up with things. One was something I fixed for Carl, boxed and failed to ship. The other was an HK ST-8 turntable for repair for another friend. Not the best way to treat my besties. My apologies. Bad richee.

The Harmon Kardon ST-8 sports a Rabco Linear tracking arm. This one, needs all the belts, the motor doesn't know the words, it just sits and hums. Another common problem, the cue damper base is broke. Instead of using a large, clumsy carburetor dashpot like a previous model ST, it uses a vacuum air pot manufactured by the AirPot Corp. A graphite piston rides in a plastic tube, sealed on one end. The connecting rod is a ball link in center of piston that allows air to escape and pushes down easy. Upon release, the ball seals and a spring coaxes the piston to release slowly lowering the tone arm gently.

The base of the AirPot disintegrates with age. You can find posts in various forums about that. AirPot is still in business, and while waiting for a reply from them to see if part 61028-1 might still be available, I cobbled together an inexpensive fix that worked perfectly. A brass flat washer countersunk in center with a 3/8" long 8/32 brass flat head machine screw. A 5/8" rubber cap that fits snugly around tube. A hole is drilled in center of rubber cap. The flat washer and machine screw are secured via washer and nut to base. I had measured the clearance between piston and bottom of cylinder and had more than enough room for an .060” thick washer. Countersinking the washer for machine screw means screw is flush with top of washer. I turned the diameter of washer down to .610" so it does not contact sides of plastic tube.

Then, AirPot replied and guess what? While you may not be able to get a battery for your several year-old electric car, AirPot will still make one of these 61028-1 AirPot's for your 46 year old turntable. They said around ten per year are sourced. Price is high at $97, but it is a more professional fix than my dollar, half-fast repair and the owner prefers the new part fix.

Also have a broken adjustable foot to acquire or perform another half-fast fix.

pitdogg2

7.62x39 what a coincidence..... Now where have I seen that before?

Nice job Richee! Bout time you got your round too its in order.😁

Ricardo

Nice job Rich. And congrats on retirement!! I will join you in that stress free state at the end of this year...actually 59 working days excluding all that vacation I must take....but who's counting.

SCompRacer

Progress. Based on testing, the motor had proper voltages so I suspected a bad hall effect sensor that is unavailable. It’s actually inside the motor, you have to remove pulley and drill rivets to get to it. A post at AK with info from DIYaudio said find a motor from a Dual CS 701 or 721 and get the sensor from it.

Fortunately, the owner had an ST-7 in poor condition but with a good motor. A couple of screws, some soldering and we now have a motor that knows the words, it just doesn’t sit and hum. Cleaned speed adj pots, clean and lube bearing. New belts.

The auto stop did not work. The wires to LED are stiff with tubing around it. Wires were pressing against the top plate and perhaps with handling, it broke the retainer for LED. I recurved the wires and a dab of glue holds LED in place. I’ll have to build up some material around it or leave well enough alone.

The headshell is a piece of work. I said fitbits a few times attaching cartridge. No place else to go, this is the arm/headshell you are stuck with. You slide the headshell to adjust for proper stylus position.

I attached a respectable Acutex M310 IIE MM cartridge for now. It has great bass, articulate in the mids and nice detail in the treble. Originally a tool was supplied to aid in cartridge setup. If it is lost, you can purchase a 3D printed one from Shapeways. You can also figure out stylus position using math from a drawing with dimensions posted at AK.

https://www.shapeways.com/product/6FV7R7ATT/hk-alignment-tool-v3-1

$18.70 for the black one. $14.30 for the White one. You can get by without the tool but you’ll have some trial and error setup until you get the right shim for VTA. Four shims and five pairs of screws were originally supplied with table. The owner is searching to see if he has the missing shims. If not, guess it’s 3D printer time. It all depends on cartridge height.

I used a protractor from my linear air bearing arm to set stylus position. Basically, a linear arm stylus should be centered with spindle stub throughout arm travel. IF you have the tool, you can do VTA and stylus position on the bench.

A threaded foot was fabricated from a donor foot, the standard foot supplied with HiFi2000 chassis. I had the threaded piece. The rubber that secures the male threads to chassis had shrunk and they spun when adjusting footers. A little dab of glue to chassis and male threaded inner foot fixed that.

I replaced the cheap RCA's.

I still await the new AirPot cue damper, but everything functions. I'll upload short video.

SCompRacer

Oh, I am wearing cargo shorts so don't worry about bare legs in the beginning of video. Vinyl is Broken Bells self-titled debut album.

This is proper operation. At end of record, tonearm just lifts, does not return to start, and table goes stop.

https://youtu.be/ce902zOcJqk

pitdogg2

Incredible patience and skill.
Bravo 👏👏

SCompRacer

Replacement Airpot finally arrived! They really package well! Double boxed, wow.

skipshot12

Always thought the B&O Beogram was one of the coolest looking tt….

SCompRacer

All fini! Works great!

maxward

Looks good! Is that from the ‘70s?

SCompRacer

Thx! IIRC the ST8 was ~1979-82. There was a ST5, 6 and 7 before it starting in the early '70's. The ST8 did away with the arm rollers that melted and replaced the carburetor dashpot cue damper with a vacuum pot. The 8 also had a cartridge signal relay that engaged (muted cartridge signal circuit) when arm cued.

audioluvr

I never knew HK made a linear TT. Interesting 🤔

Goldenman

Thank You for your report. I found especially usefull that the original alignment tool can be rebuild and can be ordered. (The price is no longer 14,30$ but 17,19$) And there is a new Version 4 available.

In my turmtable the slide shaft belt is loose and I need to replace it. I took some time to clean all the pulleys and shaft.

I was surprised because bare wires were sticking out inside. My predecessor must have desoldered the muting board. They probably aimed to achieve a clean sound without any interference.

Do You have some photos with the tracking roller? I wonder how it suppose to look.

What also surprises me are two parts in the exploded diagram that are installed between the platter and the mat. They are numbered 162 (retaining ring) and 173 (curved washer). Does anyone know what functions they serve?

Goldenman

What also intrigues me is Chapter 14 in the service manual titled 'cable capacitance matching.' I haven't come across a situation where cables are replaced to match the cartridge capacitance. The manual states that the original cables have approximately 110 pF. Does anyone know which cartridges are compatible with this? Is it the majority or rather the exception? I'm particularly interested in MC-type cartridges.

SCompRacer

The snap ring is to retain the platter while the wave washer absorbs axial end play and makes for a tighter fit. It could be used due to manufacturing tolerance differences. All platters will fit on all shafts while the wave washer takes up the slop.

Regarding the muting relay, I did energize it and measured for any resistance through contacts. It tested good so I didn't have to attempt removing cover and cleaning contacts. I've cleaned relays before using paper towels soaked with contact cleaner and drying with clean towel. Sanding contact faces is not advised as it leaves a rough surface.

I don't have any pics of the tracking roller. The ST8 used a metal roller that did not disintegrate. I read that replacement rollers are available for earlier models. A pic above of an ST7 shows a roller that started to disintegrate on the shaft.

Regarding capacitance, cables have a capacitance per foot rating, so rating per foot times length in feet of cable. Also, you have to consider input capacitance listed for your phono preamp. Most are fixed, some you can change values. Are you using high or low output MC cartridge? High output MC are typically the same loading as MM cartridge. Low output MC are more sensitive to loading.