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Old 18th December 2006, 06:20 PM   #1
pjpoes is offline pjpoes  United States
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Default repairing adjust Thorens TD125 wein bridge oscillator

Hi I really could use some help with this. I was having some speed stability problems and rumble, so I decided to recheck my Thorens. When I removed the cuircit I found a bunch of problems. I need help with how to adjust it, I'm using the manual, but anyone with further info would be great. More important, I need help with the cuircit. It uses a neon bulb as a resistor with positive temperature characteristics in the feedback loop. Mine is burned out, and I'm wondering if thats the best way to handle that? Would an actual resistor be better, or an LED and resistor, or shoudl I just go find a replacement lamp for it.

Now for the tweeky parts. I have replaced the power supply caps with new rubycon ZL's I had around, same value but higher voltage. I haven't yet touched the film caps, but I am thinking of doing so with some modern parts. Will it make much of a difference to use real good parts such as say Wima film caps isntead of just the more comon readily available film caps. Or for instance, the new Dayton Film caps at 1% tolerance, which might work best since accuracy is so important, and the parts are radial lead anyway.

Final thing, is using a higher quality opamp of any benefit. It currently has an LM709CM opamp, and I have some opamps I pulled from something else I could put in it. The ones I have are NE5534AN opamps. Would those work as a drop in replacement by any chance? Also, are the same sorts of tweaks that work on a normal amplifier good for this, such as ceramic caps across the power leads, and such. Does doing things to reduce noise on the output of this amp have as big an effect as it does on a regular audio amp.

Last, should I replace the transistors with modern ones? They are located in red sockets, so I believe they are readily replacable. Maybe a modern one would be better at this job, be more stable?

Any help on any part of this problem is appreciated. Minimally I need to get it working right. I always wondered why I couldn't get rid of the DC component when adjusting it before, I think I know now.
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Old 18th December 2006, 06:32 PM   #2
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Hi pjpoes,

Quote:
It uses a neon bulb as a resistor with positive temperature characteristics in the feedback loop. Mine is burned out, and I'm wondering if thats the best way to handle that? Would an actual resistor be better, or an LED and resistor, or shoudl I just go find a replacement lamp for it.
You want to source the original bulb if at all possible..it's not very likely to be neon. The oscillator uses the temperature coeffecient to stabilize the output. Usually it works with the feedback resistor as a voltage divider, as the bulb heats up its resistance increases, cutting the gain until it finds it's "happy place"...different filament=different "happy place"

As far as the rest, what ever it takes to keep your audiophilia neurosis at bay

-Casey
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Old 18th December 2006, 06:34 PM   #3
pjpoes is offline pjpoes  United States
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Thanks, further help on that bulb would be appreciated. All it says is that its a 5 volt bulb at .02 amps. so I'm looking for a .1 watt 5 volt bulb? So I imagine they don't use that sort of feedback system much anymore, anyway to update it to something that wont require this maintenance?
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Old 18th December 2006, 07:04 PM   #4
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Ok I found a part from Eiko bulbs that looks like it will work. Its a part number 6022 and is rated at 5 volts and .02 amps, or .1 watts. Its a little under 2 dollars or so for each one. I'm ordering the minimum order of 10 so I guess I will have a few around. I hope this and some final adjustments fixes the problem.
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Old 18th December 2006, 07:06 PM   #5
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pjpoes-

Quote:
All it says is that its a 5 volt bulb at .02 amps. so I'm looking for a .1 watt 5 volt bulb?...anyway to update it to something that wont require this maintenance?
Bulbs of this size usually don't come with a wattage rating, you need to find a 5v 20ma bulb, or as close as youy can get. In theory you can replace it with a cds/led opto-isolator, but the circuitry involved will be way more complicated than the entire oscillator now...best to just buy a spare or 2 when you find it. Think about how long that original bulb lasted...they run far below the rating.

-Casey
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Old 18th December 2006, 09:12 PM   #6
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Thanks I found a suitable replacement I belive, and I'm just going around now and fixing up what I can. I'm checking the resistor values and capacitors against the repair manual and finding that the Resistors have drifted quite a bit in some cases, I believe as much as 20% in on particular case (I removed them to measure them). I have ordered some replacement resistors and capacitors. I decided to use Mica caps for the very small values, Phillips Polystyrenes for some inbetweens, and some .1mf metalized film caps I had around.

For now I will leave the opamps and transistors alone, though I am considering ordered some replacement transistors. I found some new versions of the same part number that have better on/off specs than the ones in it, and it would only cost about 5 dollars to change them.

There is a part of me that is, at this point, thinking I should fix this up, and then turn around and sell it. I will probably wait and see how it sounds after the repair, maybe I will be happy.
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Old 18th December 2006, 10:09 PM   #7
AuroraB is offline AuroraB  Norway
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In general terms, there should be no need to change resistors and film caps, as their aging characteristics should be well within a normal tolerance. As for the speed accuracy, each speed point has its own trim to compensate for minor changes in component tolerances. A resistor being 20% off due to aging, is quite unusual, unless it is heated above normal ambient temp.
I don't know about the tolerances used in these TTs, but 10% resistor tolerance was quite normal for production units in those days- 20 was actually also quite normal, unless the design demanded higher precision. The manual doesn't spec tolerance either. I have a Mk-II sittting on the sheelf myself, awaiting a minor service. It's otherwise working quite OK.

EDIT: Had to read your post again....comments above still, valid, but I don't think you will gain much by changing the opamps, provided you find one that fits. The original 709s used have frequency and offset compensations, that I don't think will fit for a 5534. Have you checked the datasheets to see that the pinning matches??
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Old 18th December 2006, 10:43 PM   #8
AuroraB is offline AuroraB  Norway
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Oh- I suddenly remembered..
along with the manuals, there is also a picture of the MK-II board here.
http://www.theanalogdept.com/manuals.htm

original resistors are 5%. Original film caps are of good quality.

Thinkin' about it, I don't think I have seen a 5534 in 14 pin DIL...?
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Old 19th December 2006, 08:13 PM   #9
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I have two 8 pin dip style opamps, not 14 pin. Unless I missed something.

Actually at this point, all tweeks aside, I just need to get it working. Not sure whats going on, but I can't seem to get it to a good adjustment. I have a fairly inexpensive Fluke meter, maybe 80 dollars, and the readings I am getting are all over the place. Maybe you would know better, the adjustable pots for adjusting speed are not changing things in any predictable fashion. If I make a slight change the voltage might change 1mv or it might change 10 volts. The voltage seems to go up and down all the time, some times drasticly. I can not get the dc offset correct either, at the moment the 33 rpm speed is reading over 1 volt of dc offset, and somehow that is giving minimal vibration, but still too much.

At the moment the only parts that were changed out were the power supply caps with Rubycon ZL 1000uf at 35 volt caps, which I had laying around. The old ones might be fine, and I can try putting them back in, I just figured why not, I have them here.

If someone can help me better understand how the pots work, that would be great. Each speed has three pots. Two give the adjustment of the AC voltage, and thus speed, the third adjusts DC offset. First, should I have all of them moved to one side or the other, or in the middle when I start making adjustments? Next, for the two that make AC voltage adjustments, what does each one do. Does each handle one phase of the output? If so, should I be checking one on the Black and Blue wire, and one on the Red and BLack wires. Oh yes, besides being unable to get the voltages right, or the DC offset correct, the 45 rpm speed wont go to the specificed 8 volts. I can get about 7ish.

I have no oscilliscope, so this is the best I can do for now. Any chance the power supply caps I put in are no good, and I didn't realize it? How can I test this if I dont have a meter that tests capacitance?
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Old 20th December 2006, 01:33 PM   #10
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pjpoes-

Not having your table in front of me, I could only guess at what the problem is.

I would suggest you download the service manual and go from there.

One thing I'm wondering, based on your description, is if you are measuring the ac voltage, rather than frequency. A wein bridge in the speed control circuit would indicate an ac motor. In which case, the speed is controlled by the frequency of the oscillator...the bulb will control the voltage if all is well.

-Casey
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