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Old 27th March 2006, 01:08 AM   #1
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Default Troubleshooting Electronic Turntables

Just some FYI. I have owned a Thorens TD-125 MkII turntable for over thirty years. I have always loved the sound of analog recordings. This particular Thorens model incorporates a built-in oscillator to drive the synchronous 24-pole motor via a drive belt to play the records at the various speeds. The motor's frequency is changed to drive the platter's RPMs. This built-in oscillator is a Wein-bridge variety and as such requires a Negative Temperature Co-efficient device in its feedback loop to stabilize the threshold of oscillation. The problem is that the miniature grain-of-wheat lamp is mounted in a socket using silver-plated terminals. Great for conductivity, bad to tarnish and corrosion. After a phone call to the importer of these turntables, the techincian suggested the removal of the socket and then directly solder the lamp's leads to the PCBoard. As stated earlier, this turntable has been in service for over thirty years. This condition may be something to look for if your turntable gives intermittent operation and it has a built-in oscillator/amplifier that directly drives a motor. Note the TECHNICS SL-1200 is a totally different animal. Its principle of operation is based upon a direct drive of the platter with Hall-effect devices sensing rotational position and speed.
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Old 27th March 2006, 01:09 AM   #2
anatech is offline anatech  Canada
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Hi Hector,
Most techs mess those up. Nice table, I have one too.

-Chris
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Old 27th March 2006, 01:55 AM   #3
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Default Thorens TD-125MkII Turntable

To Anatech,

Do you have any suggestions for maintaining a shine on the platter. I have tried SemiChrome Polish and various semi-abrasive cleaners. Looks good for a while then goes dull and shows fingerprints. Someone told me that since this is pewter (not really sure if it is) that it cannot be chrome electroplated. Would welcome any suggestions for a polished looking platter like the ones on the vintage AR turntables on my Thorens. And besides a chrome polished platter would match the finish of the SME 3009 tonearm.
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Old 27th March 2006, 02:03 AM   #4
anatech is offline anatech  Canada
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Hi Hector,
The best thing to do is leave it alone. Those platters take a greyish satin finish and stay that way. It's the way it is. I've had a number of Thorens tables myself and never had the urge to change this.

My tables always had the original arms, I didn't care for the SME and other's too much. They are a satin aluminum finish.

Sorry I wasn't much help to you.

-Chris
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Old 27th March 2006, 09:39 AM   #5
sreten is offline sreten  United Kingdom
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Hi,

polish the platter and then clean any grease off, then spray with clear laquer.

/sreten.
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Old 28th March 2006, 04:57 PM   #6
kevinkr is offline kevinkr  United States
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I recently polished the edge of the platter on my TD-125, it took about 30 minutes to do. I used a standard metal polish I purchased at the hardware store - the polish you use should be compatible with aluminum. I used MAS which may or may not be available elsewhere.

I think it looks a lot better than when it was dull looking, gives the table a new lease of life. (Well not really - I wasn't going to get rid of it just because it looked dowdy.. LOL) Note that if there is pitting due to corrosion this will not get rid of all of it, although it does work quite well. I much prefer the platter shiny, helps me over look the fact that my TT is not a whole lot younger than I am.

Now it matches my shiny SME3009 series II arm..

Some other tips, if your neon lamp takes a while to light it may be contact oxidization between the bulb and its socket. Just remove the strobe assembly lens closest to the sub-platter (2 small machine screws) and the bulb pops right out. Don't overtighten the screws when you put it back together. The bulbs appear to be a custom type made for Thorens and I believe they are basically unobtainable these days. Based on amount of use these typically get I can't imagine them actually failing, just the afore mentioned oxidization issue.

I would also recommend replacing the electrolytics in the power supply with new ones. 3300uF/35V ought to work just fine. I have two of these tables and the electrolytics have not failed yet, but large amounts of ripple on the rails may degrade speed stability.

The complete service information for the original TD-125 is available online, just google for it. All of the information required to recalibrate the oscillator after repair is provided - follow the procedure to the letter!
(I can forward the TD-125 manual to a limited number of people if you email me at directdotheatedatgmaildotcom)

The speed control on the thorens td-125 is very well designed and almost nothing seems to go wrong with them, other than the pots. I have many friends who use these 30yrs+ tables and they still run fine.

Re, the weinbridge oscillator bulb: Silver oxide is highly conductive, and should not be causing any problems, that is why silver is used in the first place. It may look nasty, but I would say leave it alone. You don't know whether or not the socket mass itself alters the thermal time constant of the bulb or not. I wouldn't even remove the bulb from the socket - I haven't tried in any I've worked on (I am not totally sure they can be removed safely) and see NO need to. Worst case if you absolutely have to, clean it with Caig Labs de-ox-it..
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Old 29th March 2006, 03:42 AM   #7
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Default Thorens Platter

Kevin,

Thanks form the information. I had hard soldered that oscillator lamp more than twenty years ago. With respect to the strobe lamp, you may want to clean the mating surfaces with a very light abrasive such a a whitening toothpaste or Go-Jo with pumice. Having performed service and repairs at major recording studios along the Gulf Coast, humidity takes it toll on silver-plated contacts (patch bays, switch contacts, tube sockets, etc.) And while silver is just one step below gold in terms of conductivity, the high resistance from the dark oxide can cause havoc in low-voltage, low current circuits. Anyway, thanks for the polishing suggestions. I have tried Happich SemiChrome polish which leaves a beautiful finish, but was looking for something more permanent (I'm basically lazy and I don't do windows). If anyone is interested I've done a simple modification on a Advent 300 receciver that converts the conventional voltage source power amplifier stage into a current-source type. The idea came from seeing the schematic for Genelec 1060A studio monitors I repaired for a Loyola's Recording Studio.
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