July 12, 2022 at 9:50 pm #6734
I am working on a beocord 6500 (type 4938) that is having issues performing any mechanical functions.
The capstan belt has been replaced, the pinch roller has been cleaned and functions fine.
What I am observing is that when pressing play, the tape head/pinch roller will raise but there is no motion from the reels. At the same time, I can hear the mechanism motor still trying to move. After a few seconds, the reel motor will very briefly spin and the unit will go into standby (I’m guessing due to excess current from the struggling mechanism motor).
It seems that mechanical failure as the cause is unlikely because if you attempt these functions when the unit has been off for a few minutes, it will go into play, turn the head, etc. However, after a few commands are entered, it will fail in the same way (reel motor doesnt turn, mechanism motor struggles, unit shuts off). I can turn the unit off for 5 minutes, come back to it and get another 10-30 seconds of functionality from it before it fails again.
On the control board, all caps tested good. There was a resistor that appeared to have gotten hot attached to IC1, an LM324N. I replaced both the resistor and the IC, no change in behavior. I also read an old beoworld post where a user described adding a 220nf cap and 1ohm resistor in series to ground from the outputs of the L272M’s on the board (per the recommendation of the L272M datasheet), this had no effect for me.
Has anyone encountered anything like this with these units?
Any help is greatly appreciated. Please let me know if I can provide any additional detail or clarify anything.
July 14, 2022 at 10:03 am #6753Die_BogenerBRONZE Member
- This topic was modified 1 month ago by jahmadeus.
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The LM324 is just an opamp … no power at all…
The L272 is controlling the motor, that is the power source.
Check the axles of the 2 spindles. Clean it… the grease is very sticky after this time. Like glue… check it by hand, you can turn it or not.
Disassemble the C-Clip on the top of the axles, then you can pull the wheels. Be carefull not to break it or loose it…July 14, 2022 at 11:04 am #6754
I was not implying that the LM324 was controlling the motor. Simply giving background. I replaced the IC and resistor due to visible heat marks on the components and board.
The two spindles seem to rotate easily by hand. The hang up and eventual shut down usually seems to happen before there is any movement from the ff/rw/play motor. However, I will disassemble them and give them a good cleaning and see if that can help the situation.
Thanks for the responseJuly 14, 2022 at 10:47 pm #6777
The spindle shafts were indeed a bit sticky. Cleaned them but as expected it does not help what I am experiencing. The unit does not struggle to turn the spindles, the mechanism motor (the one responsible for lifting/rotating the head) seems to want to keep rotating even when it has completed its rotation to a different mode.
The unit will then briefly spin the right spindle and shut off after it struggles. The switches and contacts in the tape mechanism seem ok. It is almost as if there is a logic problem or the motor is being driven incorrectly.
As I said, the mechanical functions seem to work perfectly when the unit has just been turned on. It seems that the problem could be electronic in nature rather than a problem with the transport itself. That said, I am not ruling out that it could be a mechanical issue, it just seems less likely.
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