EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: MTron on June 26, 2010, 03:50:34 am
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So, following my tube amp i decided to get this old turntable my gf's mom has, pretty old, say 70's-80's
The turntable works fairly well, but there are some issues. The most serious of which is the speed control. When running the RPM varies, not wildly, but enough to notice a change in pitch and tone in the music.
Also, there is a fine RPM adjust nob, which is just a pot under. Upon adjustment it takes some time for the rpm to settle on to its new setting.
Now i didn't have any of my tools with me but i figure that the supply voltage to the motor (a direct drive motor) is not quite stable. I opened it up and found quite a number of electrolytics on a single sided board. Would it be worth it to replace these electrolytics with new ones...could this solve any voltage instability?
Obviously im not going to de anything until i get it back home and really get into the diagnosis as well as taking some pics
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electrolytics are the first parts to go so it might be worth a go, unfortunately in the early days they seemed to not rate the voltages correctly, I've seen some hair raising combinations of voltage supplies and electrolytic cap ratings
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TT use a simple feedback loop to control speed.
There are 2 types of TT: direct drive and belt drive. Belt drives wear out and cause erratic speed, replacing it is all you need.
http://www.turntablebasics.com/
Because belt drives were notorious for wow and flutter, direct drives were state of the art in TT before CDs, today CDs and all are all DD.
If its direct drive, before you do any dissassembly, you need to determine the stable rotational speed of motor, or the platen mechanics, and check the support gearing for the TT if there are any. It might be that its rusty or dirty, and that's causing the erratic compensation.
If you like the look of the TT, a worse case repair is simply gut out the elctronics and replace it with a modern TT. The drives are better, smaller, more reliable and frugal, often they work off just ports.
http://www.newegg.com/Product/Product.aspx?Item=N82E16882021131&nm_mc=OTC-Froogle&cm_mmc=OTC-Froogle-_-Home+Audio-_-Rosewill-_-82021131
The only thing electronically good about old turn tables is possibly the stylus and its pre-amp, and the casing if it matters. Everything else is just too primitive for musicality.
The platter was often made of aluminum, and the weight made for inertia in starting and controlling the speed. Some are steel. Like a flywheel, once it got going, it also helped maintain the speed. Today's TT are plastic, and the speed is maintained purely by feedback. The advantage of heavy TT platter is immunity from room noise, and maintaining flatness to remove distortion from uneven record surfaces especially as the platen or discs age.
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If its direct drive, before you do any dissassembly, you need to determine the stable rotational speed of motor, or the platen mechanics, and check the support gearing for the TT if there are any. It might be that its rusty or dirty, and that's causing the erratic compensation.
My thoughts exactly. Before diving into electronics end of things, keep in mind it's a mechanical thing and any mechanical thing of that vintage is likely to have some issues. I'd be checking out all things mechanical before looking into electronics end of it.