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
Hello All,
Trying to diagnose non dropping tone arm, so I am following the flow chart in the manual.
It says "start with 25/30 cm record" > "does pickup arm lower at 25/30 position" > (no) > "is C of 1TR6 high at lower position?".
Problem here is that with platter on, I can't get to C of TR6. Not sure there is even room for a croc clip.
Any suggestions please?
Also, what does "high" mean? Voltage at this point measured with platter off is in the range 30 to 40 mV.
Thanks,
Simon
Hi,
Check out Beolover's video on his blog that shows the Beogram 4002 tonearm detection and lowering circuit. He shows the circuit diagram about half way through the video and towards the end of the video he shows the voltage at 1TR6. The "high" at the collector appears to be an expected voltage of about 15.5 VDC according the service manual. When 1TR6 is turned off (no base current), the voltage at the collector drops to about zero volts.
You can make measurements involving the tonearm without the platter on the Beogram. With the platter removed you can get your probes in place. The arm detector won't have any strobe activity so it will think there is a record present and try to drop the arm at the full size LP position.
-sonavor
Thanks.
Oscilloscope would be useful here!
Hello,
I've checked out Beolover's excellent video.
I'm finding it difficult to check the resistance of 0PH1 (on the detector arm) using a DMM as I am getting negative resistance measurements where I should be expecting around 8k ohm, though I am reading around 50k ohms on the ribs, which sounds about right if I could trust it. If I reverse the leads, I am getting inverse readings (high when it should be low and vice-versa). I am using Pin 11 on Plug 4 and the negative (Ground) side of the big 4000u cap.
Is there a centralised grounding point on this (main) circuit board?
I am getting 0,61v at the base of TR3 and 3,43v at the collector.
Any advice on how to check the photoresistor function would be appreciated.
Cheers,
Negative resistance ??Hmm.. I smell money here...
Martin