Harman Kardon ST 8 Turntable

snap, crackle and pop
FrankRH
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Harman Kardon ST 8 Turntable

Post by FrankRH » 15 May 2015 19:21

I was the Mechanical Design Engineer on the ST 5,7 and 8 turntable line.
I have a virgin ST8, in the carton, that I'm looking to sell. I'm located
on Long Island, New York.

These turntables were completely designed and assembled on Long Island.
All of the parts were American made in the New England and New York area.
Essentially, the units were constructed of Anodized Aluminum and Stainless
Steel, with very little plastic, except for the dust cover.

The purpose of the vertical drive belt was to break the static friction
of the carriage delrin bushings and the upper slide shaft. If this belt slips,
it could be over stretched and aged; or the pulley set screw may be loose.
Replacing this belt is easy, it's a standard neoprene sealing O Ring
that is available in serious plumbing hardware stores. Wiping the slide shaft with a
very light coating of fine oil can also help. The tracking shaft drive belt
is also a standard sealing O Ring.

Guest

Re: Harman Kardon ST 8 Turntable

Post by Guest » 15 May 2015 19:34

wellcome on board. nice to have a new member with a good background!

LesE
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Re: Harman Kardon ST 8 Turntable

Post by LesE » 16 May 2015 00:39

Welcome Frank. As an owner of an ST-7, it's great to hear from one of it's original designers. Personally, I feel that there is a great deal of beauty in the mechanical design of Rabco tonearm. While there are a lot of adjustments required, these turntables perform very reliably once everything is right. I have made some modifications to mine that replaced the AC line driven neon strobe with tri-color LED's that are driven from a quartz time based micro-controller. The strobe controller is tied into the soft touch controls so that the strobe color matches the color of the control lamps. In order to improve reliability, I also replaced the GE1302 incandescent lamps with custom made LED lamps. Since the strobe runs off of a quartz time base, I was able to replace the original strobe patterns with a single strobe ring that is independent of line frequency. Here are a few photos that I previously posted on Audiokarma:

Stop
https://i197.photobucket.com/albums/aa1 ... 37212c.jpg

33-1/3 RPM
https://i197.photobucket.com/albums/aa1 ... a23256.jpg

45 RPM
https://i197.photobucket.com/albums/aa1 ... f765b6.jpg

One of it's weak points, I feel, is the Papst DC motor which tends to drift somewhat since it lacks any type of servo control. I've also been working on a micro-controller based servo control system for my ST-7 but that project is on the back burner for the time being.

I've always hoped to one day own an ST-8 but unfortunately, importing a turntable has become too costly due to shipping charges, the unfavorable exchange rate and taxes. But in the meantime, I will continue to enjoy my ST-7. :)

Les.

FrankRH
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Re: Harman Kardon ST 8 Turntable

Post by FrankRH » 16 May 2015 15:42

Les:

Very cool, what you've done with the strobe. I personally like the looks
of the ST7 over the ST8.The basic external differences are the ST8 housing
is higher, giving it a fatter, bulkier look, and the black anodize.
Mechanically, the addition of the verticle belt (and two small pulleys),
to rotate the slide shaft was the only difference.There were some miscellaneous
electrical improvements also.

I also have an ST5, which looks just like an ST7.The ST5 has an AC motor with no
strobe, a belt shifter for 33 1/3/45, a mechanical end of groove lift device, and
an automotive dashpot for soft cueing. It's a little noisy but still functions well.
Even though it is a low end unit, it has a very crisp, museum quality look about it.

Frank

LesE
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Re: Harman Kardon ST 8 Turntable

Post by LesE » 16 May 2015 17:08

Of course, the other difference between the ST-7 and ST-8 is the tracking roller which is rubber on the ST-7 and metal on the ST-8. I've wondered if the change from a rubber to a metal tracking roller necessitated the addition of the rotating slide rod in the ST-8 or if it was simply for improved performance of the carriage assembly?

Another thing I discovered is that during assembly, the lamp sockets were installed in the PCB without concern for polarity since it wasn't important when using the incandescent lamps. But when switching to LED's, it necessitates the inclusion of a full wave rectifier to compensate for this. Getting the components to fit into a BA9 lamp base was challenging but I managed to get it done. In addition to this, it was necessary to emulate the 40 mA @ 6.3VDC characteristics of the GE1302 since it affects the motor speed. It was a bit of a pain to make them but they should last for the remaining life of the turntable. Of course, having the lamps working is more than cosmetic since the turntable will not operate at the proper speed if either the 45 or 33-1/3 lamps burn out.

https://i197.photobucket.com/albums/aa1 ... c995a1.jpg

https://i197.photobucket.com/albums/aa1 ... b43402.jpg

https://i197.photobucket.com/albums/aa1 ... f99edd.jpg


In addition to the mods, I also replicated the cartridge alignment tool using 3D printing. The cartridge alignment can be done without the tool but it makes the process much simpler.

https://i197.photobucket.com/albums/aa1 ... 967260.jpg

Now, if we could only replicate the arm wands. :-D

Les.

gvasale
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Re: Harman Kardon ST 8 Turntable

Post by gvasale » 16 May 2015 18:32

A modernized sl8e is also in order. :mrgreen:





Somewhere near 200 bucks too!

FrankRH
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Re: Harman Kardon ST 8 Turntable

Post by FrankRH » 17 May 2015 15:35

The stainless steel tracking roller replaced the ground urethane roller because
it was cracking and chipping. We were worried there might be more acoustic feedback
with the steel roller, but there wasn't. Some other small improvements in the ST8 were
the addition of leveling feet and a built in level. I'm in my Florida home now so I don't
have access to my notes, drawings etc. I'm working on memory only.

The 3D printed cartridge gauge is neat. Did you use AutoCad Inventor S/W to model it,
or was an actual part 3D optically scanned ?

Frank

LesE
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Re: Harman Kardon ST 8 Turntable

Post by LesE » 17 May 2015 16:57

FrankRH wrote:The stainless steel tracking roller replaced the ground urethane roller because
it was cracking and chipping. We were worried there might be more acoustic feedback
with the steel roller, but there wasn't. Some other small improvements in the ST8 were
the addition of leveling feet and a built in level. I'm in my Florida home now so I don't
have access to my notes, drawings etc. I'm working on memory only.
Thanks Frank. It's a rare opportunity to be receiving such insight from a member of an original product design team such as yourself.
FrankRH wrote: The 3D printed cartridge gauge is neat. Did you use AutoCad Inventor S/W to model it,
or was an actual part 3D optically scanned ?

Frank
For the cartridge alignment tool, I used SketchUp 8 which is free software to create the 3D model. I uploaded the model to Shapeways online printing service where it is available to the community. Since I didn't have an original tool, I relied on measurements provided by someone who had one. I'm told that it's not an exact duplicate of the original but the critical dimensions are correct ensuring an accurate cartridge alignment.

https://i197.photobucket.com/albums/aa1 ... tjrw0m.jpg


I've also created a 3D model for the RABCO carriage cover which can pop off and become lost.

https://i197.photobucket.com/albums/aa1 ... ris0t0.jpg


Les.

FrankRH
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Re: Harman Kardon ST 8 Turntable

Post by FrankRH » 18 May 2015 22:03

I see you care more than a little about your ST7.....Alright !

I have a bunch of miscellaneous spare parts; bushings, rods,
tone arms etc. I'll take pictures of these when I get back to
NY in about a month.

Another change in the ST8; the center aluminum disc on the platter mat
was eliminated and the mat was molded fully to spindle. Purely cosmetic.

LesE
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Re: Harman Kardon ST 8 Turntable

Post by LesE » 19 May 2015 03:55

FrankRH wrote:I see you care more than a little about your ST7.....Alright !
Yes, I certainly do. I have three other turntables but the ST-7 remains my main table. Personally, I have a preference for a high compliance cartridge with a low effective mass tonearm and the Rabco arm certainly does qualify as low mass. I also like the fact that there are no proprietary or unobtainable semiconductor devices in these turntables.

I feel that the ST-7/8 is great for someone who likes to tweak their equipment. For instance, the servo control that I mentioned previously started as an optically coupled turntable tachometer with a serial port to allow for the collection of speed data. With the speed data available, the next logical step was to have the processor adjust the motor speed to compensate for drift. On my initial prototype, I'm using a motorized ALPS pot to adjust the platter speed under the control of the processor. Having a removable top plate giving full access to the works also makes tweaking and testing much more convenient.

Les.

kmp14
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Re: Harman Kardon ST 8 Turntable

Post by kmp14 » 07 Jul 2015 20:22

Frank, I sent you a private message via this board. I am wondering if you still have the ST-8 and are still wanting to sell it. I would be very interested!

kmp14
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Re: Harman Kardon ST 8 Turntable

Post by kmp14 » 19 Aug 2015 16:27

LesE wrote:Of course, the other difference between the ST-7 and ST-8 is the tracking roller which is rubber on the ST-7 and metal on the ST-8. I've wondered if the change from a rubber to a metal tracking roller necessitated the addition of the rotating slide rod in the ST-8 or if it was simply for improved performance of the carriage assembly?

Another thing I discovered is that during assembly, the lamp sockets were installed in the PCB without concern for polarity since it wasn't important when using the incandescent lamps. But when switching to LED's, it necessitates the inclusion of a full wave rectifier to compensate for this. Getting the components to fit into a BA9 lamp base was challenging but I managed to get it done. In addition to this, it was necessary to emulate the 40 mA @ 6.3VDC characteristics of the GE1302 since it affects the motor speed. It was a bit of a pain to make them but they should last for the remaining life of the turntable. Of course, having the lamps working is more than cosmetic since the turntable will not operate at the proper speed if either the 45 or 33-1/3 lamps burn out...

Les.
Les, hoping you are monitoring this stale thread. I am interested in replicating your design for replacement lights for my ST-7 and ST-8. Where did you find/what are the specs of the LEDs you used? Where did you get the plastic housing you used as the bulb dome? If I get more ambitious I am ask for details on the colored strobe design, but for now I want to take stab at the bulbs. I am not an engineer, just a tinkerer so any tips and advice would really be appreciated!

kmp14
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Re: Harman Kardon ST 8 Turntable

Post by kmp14 » 19 Aug 2015 18:12

Les, found your thread on AK with all the details for the bulb mods. Thanks!

LesE
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Re: Harman Kardon ST 8 Turntable

Post by LesE » 20 Aug 2015 00:34

For the benefit of others, here are the assembly instructions for the custom LED for the ST-7/8 as posted elsewhere:
All of the following parts were purchased on eBay.

10mm Red LED (Diffused or Clear) 2.1 VDC – Qty 1
10mm Blue LED (Diffused or Clear) 3.3 VDC – Qty 1
10mm Green LED (Diffused or Clear) 3.3 VDC – Qty 1
10mm White LED (Diffused or Clear) 3.3 VDC – Qty 1
MB8S 0.5A 500mA 800V Bridge Rectifier SOP-4 SMD – Qty 4
BA9s Bayonet Base – Qty 4
200 ohm 1/4 W resistor – Qty 4
200 ohm 1/8 W resistor – Qty 3
330 ohm 1/8 W resistor - Qty 1
Heat shrink tubing
28 Ga solid wire
2 part epoxy glue


Assembly:

1/ Cut the LED leads short and solder the bridge rectifier DC leads to the LED as per the schematic.

2/ Connect the 1/8 W resistor to one of the rectifier AC leads.

3/ Connect a short length of wire to the other end of the 1/8 W resistor.

4/ Connect a short length of wire to the other rectifier AC lead.

5/ Slip a short length of heat shrink tubing over the rectifier and resistor to ensure that nothing shorts against the lamp base. Apply heat.

6/ Place the 200 ohm 1/4 W resistor in the base with one lead going through the bottom of the lamp base.

7/ Pass the wire connected to the 1/8 W resistor through the bottom of the lamp base.

8/ Solder the wire and resistor to the contact at the bottom of the lamp base. Cut off the excess.

9/ Solder the other end of the 200 ohm 1/4 W resistor to the inside of the lamp base. Cut off the excess.

10/ Solder the remaining wire to the inside of the lamp base.

11/ Mix a small amount of epoxy and partially fill the lamp base. There should be sufficient epoxy added so that a small amount squeezes out when assembled.

12/ Insert the LED with the attached rectifier and resistor into the lamp base. Clamp the LED and lamp base and allow the epoxy to cure.

13/ Test by sending sufficient current through the lamp to develop a 6.3 V drop. At this point, the current should be approximately 40 ma.
The simpler option is to use a preassembled LED lamp (6.3 V / 20 ma / BA9S bayonet base) and add a 330 ohm shunt resistor across the lamp base to provide the 40 ma required to obtain the correct turntable speed. For this application, the preassembled LED lamp must be non-polarized (ie. for AC usage).

Les.

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Re: Harman Kardon ST 8 Turntable

Post by Ol Dude » 18 Nov 2015 00:11

FrankRH wrote:.....I have a bunch of miscellaneous spare parts; bushings, rods, tone arms etc. I'll take pictures of these when I get back to
NY in about a month......
Bought my ST-7 new at Schaak in Southdale (Minneapolis) many moons ago. Maybe $325?

Great to find you, Frank! I made one sort of un-authorized modification. By drilling a hole I gained access to the speed control pot. Then I could record to tape, for later playback at 2x, and found quite an improvement in the recovered frequency response and low freq rumbles. Can't say how much was due to linear tracking VS pivot-arm turntables I had with 78rpm speed. Of course, I replaced the cartridge with a Shure with a 3mil (I think) 78rpm stylus. Anyway, it worked.

Did I see you have some of the arms left? Sure would have saved time going back and forth. Having since acquired a taste for some 20's and 30's performers, I've picked up a bunch of records I've yet to play!

I'd also like to do the LED light replacements (improvement ?) ....

Bill

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