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Thursday, December 17, 2015

BeoloverRPM: Beogram RPM Calibration Device

Sadly, the Beogram 4002 that I restored a little while ago did not pass muster. It had initially come  to me with an indication of sudden and gradual RPM fluctuations and I thought I had fixed the issue by replacing the RPM relay and the trimmers as well as the DC motor, which showed a strange feedback signal whenever the fluctuations occurred. But not so. While the phenomena were somewhat alleviated, some spontaneous variation still occurs occasionally. Per my customers description it happens maybe once per LP, i.e. it needs to be treated as an intermittent issue. Which are notoriously difficult to fix. 
I decided I needed to be able to measure and quantify the issue before I would be able to attempt another stab at fixing it. The Beolover never gives up, but this one is a tricky one. The last few days I worked on designing the "BeoloverRPM" device, which now allows me to quantify and monitor the RPM of a Beogram with fairly high precision (I think it maybe about 0.03%) over time. I also implemented an automatic fluctuation detection mechanism based on monitoring the standard deviation of the measurement. 

I designed a sensor head, which uses the black ribs found in the Beogram 400x and 800x models to detect the RPM of the platter. Here are a few impressions. The front is curved to fit the platter circumference, and the reflective optosensor unit extends via a fork over the platter

I implanted two M8 bolts into the back of the unit to give it some more weight so it would sit securely next to the platter:

Here is is shown in action measuring the platter of the Beogram 4000 that I just finished up:

It appears to be floating due to rubber feet inserted into the bottom of the body (I was inspired here by the Beogram 8000's rubber bumpers that dampen the impact of its hood when it closes)
The most crucial design parameter was the distance between sensor and measured features. Its optics are fixed, and I determined the best measurement resolution results at a distance of 9 mm. I printed several versions of the housing until I got that distance about right.

Once I had the sensor and verified that I could use it for RPM measurements I sat down and programmed an Atmega328p (aka Arduino) to measure the RPM and do some statistical evaluations. At this point the firmware is able to log the RPM and the standard deviation in timed intervals (10s in the example below). It also detects sudden deviations through monitoring the difference between the running mean of the standard deviation and the current standard deviation. This difference is quite sensitive and a proper threshold allows to hone in on a certain level of fluctuation.

Here is an impression of the printout that is generated:

The ***-marked % deviations were the result of slightly touching the platter with my finger a couple of times. It is interesting to note that the RPM shows as ~33.71 even though I calibrated the RPM of the Beogram 4000 using the AC motor frequency as specified in the Service Manual. It is obvious that this calibration is about 1% off. Naturally I wondered if a measurement error would be responsible for the discrepancy, but a test of the BeoloverRPM device on my Beogram 8000 revealed a proper measurement of 33.32 RPM, which is very close and possibly even at least partly a result of the Beogram 8000 being slightly off (the Service Manual specifies a 0.02% RPM accuracy).
As a next step I will implement an external trigger function for my oscilloscope that I can measure the vitals of a turntable at the moment a fluctuation occurs. Hopefully, this will allow me to figure out what the issue is with the fluctuating 4002! This is Beolove!

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