Thorens TD165-bad TP60 galvanic corrosion issue - diyAudio
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Old 8th February 2007, 05:32 PM   #1
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Question Thorens TD165-bad TP60 galvanic corrosion issue

This week, I have completely stripped down a Thorens TD 165, serial number 0550xx. It is in fairly good shape and I plan to tweak/restore it to its former state of grandeur! (albeit the 165 was Thorens’ bottom of the line table in the mid 70’s!). Now, on to some background leading up to my questions.

The massive TP60 headshell on end of the TP11 tonearm looked great until I removed it and saw all the corrosion underneath. I was a little surprised since I assumed the headshell was anodized aluminum with stainless steel screws and threaded receiver block for seating the cartridge. There is also considerable corrosion where the little lifting handle connects to the headshell. The corrosion was so bad the threaded block couldn’t move or slide for cartridge adjustment. I scratched my head and eventually decided to carefully lift the Thorens nameplate on the top of the headshell and was able to dig and scrape it out from there. A quick cleaning and inspection with a magnet showed that only the screws and the lifting handle were stainless steel. The threaded block appears to have been unfinished die cast aluminum(?), and I’m now assuming the headshell itself is painted rather than anodized aluminum.

My questions are as follows:
1. Has anybody experienced galvanic corrosion in the headshell before?
2. What successful measures have done to mitigate the corrosion?
3. Are my assumptions on the types of metals used to construct the headshell correct?
4. Finally, the engineer in me needs to know the cause! I remember the galvanic series from chemistry and that aluminum, zinc, and magnesium make up one end of the reactive continuum. If this is an isolated case, what would act as the catalyst? Are the screws and the lifting handle not really stainless steel? Was the table exposed to a humid or saline environment? Could galvanic corrosion be indicative of a problem in the tonearm wiring?

Thanks in advance... I didn't find this one in the archives!
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Old 8th February 2007, 06:21 PM   #2
kevinkr is offline kevinkr  United States
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If it got wet or was stored in a damp basement this could happen.
I have somewhat similar problems living near the ocean.

I can't remember whether or not this headshell is detachable or not, but if it is I'd replace it if possible.
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Old 8th February 2007, 09:49 PM   #3
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Default TD165 headshell corrosion

Hmmm... so it sounds like this wasn't a typical flaw seen in all Thorens aluminum headshells?

I believe the table came from the state of Washington, so it could have been exposed to damp, humid, or even saline conditions.

I am keeping my eye on eBay for a replacement TP60 headshell, but until that happens, I'm going to have to clean it up and proceed with what I've got.

Thanks
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Old 8th February 2007, 10:04 PM   #4
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Default thorens headshell

A good way to clean the shell is with phosphoric acid (naval jelly) IF it truly is aluminum and stainless only. If it's ZAMAC or some other crappy zinc based pot mettle alloy, gentle cleaning with vinegar and an acid brush may loosen and dissolve the corrosion, but go slow and neutralize with baking soda when done.

The advantage of the phosphoric acid is it will passivate the aluminum and stainless, dissolve the corrosion products, and not touch the metal. Nitric will also work on aluminum/stainless, but it's highly toxic and corrosive to handle, so I wouldn't reccomend it for safety reasons.

It does sound like it got wet... aluminum and zinc based alloys will corrode with white crusty products if even a little electrolyte is present over time (like the salt in your sweat) so if it was near the coast and stored in a basement or garage, that's probably what happened.

good luck.
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Old 8th February 2007, 11:13 PM   #5
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Default Thanks gold plater!

I'll carefully give the vinegar a try.

For the threaded block to react as it did with the aluminum, I suspect it's a die cast zinc alloy.

If it weren't so darn small, I'd probably try a machine a new one out of aluminum or steel for kicks.

Don
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Old 9th February 2007, 08:17 PM   #6
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Steel in contact with aluminum, even stainless steel, always creates the possibility for electrolytic corrosion. In the Navy, steel in contact with aluminum would rapidly result in the aluminum corroding to a white powder. You had to have a barrier between the steel and aluminum. If we had to have steel in contact with aluminum we used cadimum plated steel. When we were told not to use cadmimum plated hardware due to environmental concerns and to use stainless steel instead, we ignored the directive and kept on useing the cadmium plated hardware. Stainless steel in contact with aluminum causes as much corrosion as regular steel.
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Ray
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Old 11th February 2007, 03:34 PM   #7
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Default Headshell made of Magnesium!?

I read on one of the Thorens pages at theanalogdept.com that the TP60 was magnesium!

Ouch! Magnesium is even more reactive than aluminum.

Thanks to all who who posted a response.

Don
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Old 13th February 2007, 08:38 PM   #8
bulgin is offline bulgin  South Africa
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Default Thorens TD165-Bad galvanic corrosion issue

Hi sjogren

The advice you have been give here is basically sound and correct. I machine stuff on lathes and milling machines in my workshop and also have built an anodising setup and keep nitric, sulphuric and phosphoric acids around my shop.

Nitric acid is used to pre-clean aluminium before anodising and will corrode ferrous metal including some stainless steels.

To 'chemically polish' aluminium, phosphoric acid shiould be heated to 100 degrees C and the aluminium part immersed for seconds at a time until you are happy with the result.

This is dangerous stuff and gives off noxious fumes!!!

You may also try a brass wire wheel in your drill or lathe to remove corrosion, together with naval jelly or vinegar. Protect eyes and wear protective clothing.

bulgin
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