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.
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.
Hi pjpoes,
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
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
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.
pjpoes-
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
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
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.
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.
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??
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??
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...?
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...?
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?
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?
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
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
Thank you. I have the manual actually, and that is what I am using. Its where i got my voltages from.
According to the manual, I need to adjust the pots to get the desired voltage, then adjust a third pot to get the dc offset as low as possible. However the manual is vague, and I have been unable to do what they say.
If you look at the manual, it says to measure the Ac voltage, get it to 8, 5, and 2.5 volts, then measure the dc voltage for each one, and get the DC offset as low as possible. Problem I am having is that, even when I get the correct voltage, the DC offset is too high, and often doesn't stay still at all. I was able to get the 33 rpm speed measuring correctly, low DC offset and correct voltage, however, it is running too slow at the moment, so apparently I need to increase the voltage slightly. That was actually my fault, I forgot to move the thumbwheel adjuster to the middle as the directions instruct, I was hurrying after I started to make some headway. The 16rpm and 45 rpm's are still way off, and I have been unable to get those right.
According to the manual, I need to adjust the pots to get the desired voltage, then adjust a third pot to get the dc offset as low as possible. However the manual is vague, and I have been unable to do what they say.
If you look at the manual, it says to measure the Ac voltage, get it to 8, 5, and 2.5 volts, then measure the dc voltage for each one, and get the DC offset as low as possible. Problem I am having is that, even when I get the correct voltage, the DC offset is too high, and often doesn't stay still at all. I was able to get the 33 rpm speed measuring correctly, low DC offset and correct voltage, however, it is running too slow at the moment, so apparently I need to increase the voltage slightly. That was actually my fault, I forgot to move the thumbwheel adjuster to the middle as the directions instruct, I was hurrying after I started to make some headway. The 16rpm and 45 rpm's are still way off, and I have been unable to get those right.
Are the supply voltages across C2 and C4 balanced, stable and at what voltage?? Both 12-15VDC and stable?? Did you change the lamp, and with what characterstics of the new lamp??
The motor volages from each output vs. ground should be balanaced and stable. If one half is stable, the other not, this of course indicates a fault in that half. If both are unstable, a fault in the supply seems more logical. Changing the 709 with a 5534 might be possible, if you compensate for the pin skew, but no guarantees given , at your own risk. I would get a pair of new LM/uA 709s.
The motor volages from each output vs. ground should be balanaced and stable. If one half is stable, the other not, this of course indicates a fault in that half. If both are unstable, a fault in the supply seems more logical. Changing the 709 with a 5534 might be possible, if you compensate for the pin skew, but no guarantees given , at your own risk. I would get a pair of new LM/uA 709s.
I haven't checked C2 and C4, I will have to do that, thank you. Was that in the manual, and did I just miss that? The bulb is the original, I was incorrect when I said it was blown. When the power supply cap blew it blew paper and oil all over the board, and covered the lamp. As a result I thought the bulb was blown as I couldn't see the filament. However, when I began testing it, the bulb lit up, and once things became more stable, it lit up correctly. However, I did order new bulbs, ten of them, with identical specs, that I could try just in case.
As for the halves of the motor, unless I missed something, the manual doesn't tell you to do that. However, I figured it made sense, but wasn't sure if they should be the same. However, they were the same balanced as you put it, so that seems to be working correctly.
I think this turntable is good enough to warrant putting a little money into. I have contacted a local repair shop that has plenty of testing equipment, and so I will be having them take a look at it if they get back to me. I think I need to see the waveforms on an oscilliscope and get a better perspective of whats going on. I think it will make adjustments easier, and help identify possible parts problems.
At this point, as I said before, the 16rpm and 45 rpm speeds are adjusted correctly for voltage, but DC offset is very high. I also find that if you adjust DC offset until the motor runs smoothly, and smooth is relative here, the DC offset is sometimes in the multiple volt range. I think I got lucky with the 33 rpm, and I'm guessing that the key to getting the right speed and low DC offset has to do with the right ratio of the first two pots, and then using the third to fine tune the DC offset. Somehow I stumbled upon that right ratio, and was able to further fine tune it.
Mind you, DC offset isn't stable. I don't know if it should be, but it goes rom .001mv up to 15mv, but seems to average in the 5mv range. Thats only true of the 33 rpm speed too.
Do you have a better understanding of what each one of the pots does in adjusting things. As I said, there seems to be two pots that adjust the voltage, and onef or DC offset. Is that correct. I couldn't understand fully what the manual meant when it said to adjust them, it doesn't give mention of three, it only talks about two.
As for the halves of the motor, unless I missed something, the manual doesn't tell you to do that. However, I figured it made sense, but wasn't sure if they should be the same. However, they were the same balanced as you put it, so that seems to be working correctly.
I think this turntable is good enough to warrant putting a little money into. I have contacted a local repair shop that has plenty of testing equipment, and so I will be having them take a look at it if they get back to me. I think I need to see the waveforms on an oscilliscope and get a better perspective of whats going on. I think it will make adjustments easier, and help identify possible parts problems.
At this point, as I said before, the 16rpm and 45 rpm speeds are adjusted correctly for voltage, but DC offset is very high. I also find that if you adjust DC offset until the motor runs smoothly, and smooth is relative here, the DC offset is sometimes in the multiple volt range. I think I got lucky with the 33 rpm, and I'm guessing that the key to getting the right speed and low DC offset has to do with the right ratio of the first two pots, and then using the third to fine tune the DC offset. Somehow I stumbled upon that right ratio, and was able to further fine tune it.
Mind you, DC offset isn't stable. I don't know if it should be, but it goes rom .001mv up to 15mv, but seems to average in the 5mv range. Thats only true of the 33 rpm speed too.
Do you have a better understanding of what each one of the pots does in adjusting things. As I said, there seems to be two pots that adjust the voltage, and onef or DC offset. Is that correct. I couldn't understand fully what the manual meant when it said to adjust them, it doesn't give mention of three, it only talks about two.
Have you measured PSU ripple?? Tens of mV is actually quite acceptable for this circuit.
As for changing the film caps, I wouldn't bother. You don't need 1% accurrcy, as the trimpots will fix that for the frequency. You need freq. stability, and that is plenty OK with the original caps. ( I used alot of those back in the 70-80s).
If you get the voltages right, and the strobe looks stable, I think you are OK. I am sorry that I cannot be more helpful at this time, but I have no time to dig into my own till after X-mas..otherwise I could have given you my measurements to compare-
BTW- do you have the original arm..?
Mine came without, but I have an SME tucked a wawy..
As for changing the film caps, I wouldn't bother. You don't need 1% accurrcy, as the trimpots will fix that for the frequency. You need freq. stability, and that is plenty OK with the original caps. ( I used alot of those back in the 70-80s).
If you get the voltages right, and the strobe looks stable, I think you are OK. I am sorry that I cannot be more helpful at this time, but I have no time to dig into my own till after X-mas..otherwise I could have given you my measurements to compare-
BTW- do you have the original arm..?
Mine came without, but I have an SME tucked a wawy..
pjpoes-
My first thought here, is that your manual and contol card are different revisions. You might try downloading the manual from different sources..they might not be all the same. This has worked for me before.
As for your dc offset, It's starting to sound like your oscillator opamp is leaking..before you lay out the lettuce for a service bill (your table is certainly worth paying to get it fixed if you have to), I would throw in a pin-for-pin replacement and see if your problems disappear..I would socket it as well while your at it.
-Casey
My first thought here, is that your manual and contol card are different revisions. You might try downloading the manual from different sources..they might not be all the same. This has worked for me before.
As for your dc offset, It's starting to sound like your oscillator opamp is leaking..before you lay out the lettuce for a service bill (your table is certainly worth paying to get it fixed if you have to), I would throw in a pin-for-pin replacement and see if your problems disappear..I would socket it as well while your at it.
-Casey
First, thanks all for the help. No I don't have the original arm, I have no idea if my grandfather bought it with anything other than the ADC arm on it. I think he did change it, but I don't recall for sure anymore. Given that the arm came out in the 80's and the table in the 70's, I think he must have.
As for different revisions. I have one source for the service manual, its The Analogue Depot. There are multiple boards, but all of the boards appear to have three pots per speed.
If one of you have adjusted one of these before, please tell me what each pot does. Like I said, mine has three sets of pots, for the three speeds, and each set has three ports. That means I have a total of 9 pots. Is that the same as you? I would imagine as much. Again, as I said before, based on my reading, and trial and error, it appears that the first two change the AC voltage, and the final one changes the DC offset. My assumption is that the first two each change the voltage of each "leg" of the motor, and the final one tries to align the phase of the two voltage swings, so that they are precisely out of phase at the right amount, I assume 90 degrees. However, the instructions aren't clear on how to adjust the first two pots in order to get the correct voltage, while getting a suitable low dc offset. I can adjust it so that the voltage and DC offset are close to correct, however the motor then doesn't run smoothly. If I adjust it so the motor runs smoothly, then the DC offset is way off. I also am yet to get the speed correct for playback yet, even when the voltages are correct.
As for different revisions. I have one source for the service manual, its The Analogue Depot. There are multiple boards, but all of the boards appear to have three pots per speed.
If one of you have adjusted one of these before, please tell me what each pot does. Like I said, mine has three sets of pots, for the three speeds, and each set has three ports. That means I have a total of 9 pots. Is that the same as you? I would imagine as much. Again, as I said before, based on my reading, and trial and error, it appears that the first two change the AC voltage, and the final one changes the DC offset. My assumption is that the first two each change the voltage of each "leg" of the motor, and the final one tries to align the phase of the two voltage swings, so that they are precisely out of phase at the right amount, I assume 90 degrees. However, the instructions aren't clear on how to adjust the first two pots in order to get the correct voltage, while getting a suitable low dc offset. I can adjust it so that the voltage and DC offset are close to correct, however the motor then doesn't run smoothly. If I adjust it so the motor runs smoothly, then the DC offset is way off. I also am yet to get the speed correct for playback yet, even when the voltages are correct.
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