For tests with a scope etc, if I want to connect the DUT to a 100V-100V isolation transformer, is this getting close to what I would need?
This is Swallow SBE series, and the manufacturer has another series, SA, without the earth terminal, but it looks like all of the models in that line only do step-up or step-down.
I only have access to ungrounded electrical outlets for 2-prong plugs, so the earth terminal on this would be unused anyway.
This is Swallow SBE series, and the manufacturer has another series, SA, without the earth terminal, but it looks like all of the models in that line only do step-up or step-down.
I only have access to ungrounded electrical outlets for 2-prong plugs, so the earth terminal on this would be unused anyway.
Sorry, I posted right after Osvaldo's comment, but actually I was looping back, in my own mind anyway, to a comment over a week back about the need for an isolation transformer. Something I have left too long, and have to take care of, for the various projects I've been doing and new ones that I will be doing.
Just checking back in with where I'm up to with the turntable. No progress as such, but the old damaged rotor has been replaced. The speed issue is, as expected, unchanged.
After reading up on safe use of oscilloscopes I have lost my nerve on using one to test equipment for the moment. The best move might be to get some differential probes when I have scraped together a little money. I have no direct way to ground the scope. I only have access to a ground wire connected to a buried rod that I put in a few years ago in case I needed it. I guess I can check it by connecting the wire to the pigtail on a powerstrip. If it tests out, I can ground from the back of the scope to that. The other part of the test setup would be the turntable connected to an isolation transformer. The transformer would be connected to an ungrounded outlet. If it's worth anything in terms of safety, I can use a two prong GFCI plug either on the mains and plug the transformer into that, or on the secondary side of the transformer, but I asume it adds nothing useful.
After toying with the turntable for a while today, I noticed that the speed shown on a phone app does go back and forth an rpm or so. It shows quite a neat sine-like pattern on the graph, with a tendency to creep slowly upward over the course of the measurement. Adjustment of the VRs on the top or under the turntable doesn't seem to have much visible influence on this. I suppose this pattern is the low frequency oscillation in the servo circuit mentioned by Osvaldo.
Just as general info, here's a picture of the motor I was able to find. I placed some arrows to show where you can see the cutaway of the rotor, which is an inverted cup on the outside of those coils.
After reading up on safe use of oscilloscopes I have lost my nerve on using one to test equipment for the moment. The best move might be to get some differential probes when I have scraped together a little money. I have no direct way to ground the scope. I only have access to a ground wire connected to a buried rod that I put in a few years ago in case I needed it. I guess I can check it by connecting the wire to the pigtail on a powerstrip. If it tests out, I can ground from the back of the scope to that. The other part of the test setup would be the turntable connected to an isolation transformer. The transformer would be connected to an ungrounded outlet. If it's worth anything in terms of safety, I can use a two prong GFCI plug either on the mains and plug the transformer into that, or on the secondary side of the transformer, but I asume it adds nothing useful.
After toying with the turntable for a while today, I noticed that the speed shown on a phone app does go back and forth an rpm or so. It shows quite a neat sine-like pattern on the graph, with a tendency to creep slowly upward over the course of the measurement. Adjustment of the VRs on the top or under the turntable doesn't seem to have much visible influence on this. I suppose this pattern is the low frequency oscillation in the servo circuit mentioned by Osvaldo.
Just as general info, here's a picture of the motor I was able to find. I placed some arrows to show where you can see the cutaway of the rotor, which is an inverted cup on the outside of those coils.
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I had a bit of a look.... The flutter could be caused by a slight drift in the values of the components that I circled.
The differential input pair compares the 5V (the +15V DC rail must be stable + the R842 & R841 must not drift) with the mains supply voltage, whose representation is present at point No 8 (check: all other values I circled in red; that R837 in particular). I think it is normal to see the variations in voltage drop across R839 - the problem is that these variations are either too large, or not large enough.
I'd use a can of freeze spray as well and do very short bursts of focused freeze air... there might be some cold solder joints (hot spots developing), as well. You could check for the cold joints if you look very closely at the soldering points with a magnifying glass while bending the PCB ever so slightly.
Good luck.
The differential input pair compares the 5V (the +15V DC rail must be stable + the R842 & R841 must not drift) with the mains supply voltage, whose representation is present at point No 8 (check: all other values I circled in red; that R837 in particular). I think it is normal to see the variations in voltage drop across R839 - the problem is that these variations are either too large, or not large enough.
I'd use a can of freeze spray as well and do very short bursts of focused freeze air... there might be some cold solder joints (hot spots developing), as well. You could check for the cold joints if you look very closely at the soldering points with a magnifying glass while bending the PCB ever so slightly.
Good luck.
Also, because it is an analog circuit, the root cause could be oscillations that vary in amplitude (like parasitic oscillations)... You'll need to look for these with an oscilloscope. Due to the nature of this circuit, you must use either an isolation transformer to power it, or a differential probe on your oscilloscope.
Setting the oscilloscope across R847 you surely can see the sine wave that the speed is modulated. So mooving the tip back in the signal path you will found it with variations in the shape and amplitude. Its like as kind of motorboating, in place of offending a speaker, in this case, a motor. As already mentioned, I would start replacing caps but with values close to the originals: don't put 10uF where was a 1uF. This will cause future complexity of the issue.
Thanks for the suggestions. For the resistors and caps, are you saying I should check their resistance and capacitance, respectively, against the specs? Or do you mean taking measurements during operation?
Resistors are easy to measure. Any resistor out of tolerance or suspicious (cracked, overheated, etc), should be replaced. IMO all lytics would be changed with no doubt prior to start any failure tracing. After replacing all, keep the motor running say, half an hour to let lytics "accomodate" into your circuit.
C814 and 813 were replaced by me. All red caps (I don't recall the manufacturer) in the images I posted on one of the previous pages are my replacements. I did all the electrolytics a few weeks ago, and way back, in the earliest troubleshooting, the 4uF cap in the power supply, which probably didn't need changing after all. I still have the original somewhere, and a new axial I ordered that is a bit nicer than that generic PRC thing I've put in there for the moment.
I'll check those resistors. Might be a few days before I report back on those.
I'll check those resistors. Might be a few days before I report back on those.
I'm making reference to my post #18. IMO those small caps are the most probable culplits, athough I can't say 100% sure. I'm not so pedantic.
Regarding motor run capacitor, if the original part is good, I would return it to its job and save the new for an eventual future build.
Regarding motor run capacitor, if the original part is good, I would return it to its job and save the new for an eventual future build.
You think so? Those are ones that I've replaced. All the electrolytics. The maker of the new ones is Wurth and I put in that order with Mouser less than a year ago. I installed them some time last month.
The old motor capacitor will eventually go back in then, if it still checks out. No point in wasting it.
The old motor capacitor will eventually go back in then, if it still checks out. No point in wasting it.
Yes. I didn't really expect changing the caps to have much effect, but there weren't many to do, so I thought it was worth a try.
I have set this turntable aside for a while as I go ahead with a kit build and a recap project that have also been waiting.
I tested (with multimeter) the earth wire I installed to an electrode buried outdoors; that's the only earthing I'm going to get in this house, but it will do for the purpose of testing. I also got the isolation transformer I was asking about upthread. I haven't wired up the secondary side yet but will do so before I start again with this turntable, and then do some testing with the oscilloscope. It might be a month or so before I get back to this. I have plenty of information and things to try based on the advice offered in this thread so far, but of course, may be back with further questions...
I tested (with multimeter) the earth wire I installed to an electrode buried outdoors; that's the only earthing I'm going to get in this house, but it will do for the purpose of testing. I also got the isolation transformer I was asking about upthread. I haven't wired up the secondary side yet but will do so before I start again with this turntable, and then do some testing with the oscilloscope. It might be a month or so before I get back to this. I have plenty of information and things to try based on the advice offered in this thread so far, but of course, may be back with further questions...