ARCHIVED FORUM -- March 2012 to February 2022READ ONLY FORUM
This is the second Archived Forum which was active between 1st March 2012 and 23rd February 2022
I have a Beogram 4004 turntable that appears to do everything properly except drop the tone arm. The solenoid and arm mechanism is clean and operates very easily manually and the speed changes just exactly when it should as the arm moves across the platter.
I put an oscilloscope on the collector of TR6 expecting to see a pulse train as the arm moved over the platter ribs but got nothing. Maybe a pulse train if you consider 2-3 mv on the envelope a signal. I put the scope on the base of TR6 and it would register .6 vdc whenever it was over the rotating platter ribs. If a record is on the platter it stays at 0 vdc.
If I measure above R39 I get the full 22.5 vdc supply voltage.
I measure 125 ohms from the collector to ground but I don't know if I'm reading through a bad transistor or through R41 and a bad C19 without removing some components.
Before I do that I would like to validate that the photo sensor is working correctly if possible. My assumption is that the photo sensor should be sending a pulse train of some voltage to the base of TR6 which is used to turn TR6 on and off producing a much larger pulse train at the collector.
I haven't found any diagram that shows the tone arm photo sensor and how its output eventually makes it to TR6.
Does anyone know what the signal on the base of TR6 should look like -- shape and levels?
Of course if someone has a suggestion for how I can better zero in on the problem that's always welcome as well. :)
Howard
Circuitry referenced above: 145531.aspx
Never got any replies on this but did find my answer -- along with an unbelievable wealth of info on the Beogram 4004 -- at beolover.blogspot.com.