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4004 Tonearm Intermittently Returns Early

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Mark
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Mark posted on Wed, Jul 7 2021 3:03 AM

New member here, but Beogram 400x owner since the 1970’s. I have run into a issue I’ve not seen before. Intermittently, the tonearm will return prior to the end of the record. It does not do this at the same point nor is it only happening if I start the record at the beginning. Since it is returning, I’m suspicious of the RUN-OFF circuit. That circuit behaves properly at the lead-out groove but am wondering if a leaky 1C33 or an issue with TR16 may be the cause. Anyone seen this and its fix?

TIA,

Mark

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Mark
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Answered (Verified) Mark replied on Sat, Jul 10 2021 5:01 AM

John, thanks for the additional tips. Unfortunately, I do not have a storage scope which would have been ideal.

I have been able to eliminate the control panel and the remote board from the possibilities. I covered over the photo-darlington with a piece of black electrical tape. This eliminated the problem which point me to the LED circuit. In adjusting 1TR18 it appeared to be jumpy and cleaning it did not improve things. I swelled it with one of the </> ones as they were the same value and were more linear. Unfortunately, this did not solve the problem.

I then tested my servo motor theory and was able to get it to pick up by slowly moving the pully by hand without activating the motor. This left me with the 21v to the LED or the LED itself. I replaced the 4000uf cap even though the voltage read fine. I also added the ground bypass cap .01uf to the heatsink screw that was added on the 5526.

I have now played 10 sides without an early pickup! I'm not declaring victory yet, but it is definitely looking promising Smile

 

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Mark
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Mark replied on Fri, Jul 9 2021 6:48 AM

Here is some more info based upon further testing and troubleshooting.

Problem still occurs

1. with replacement C33

2. with replacement TR16

3. with swapped known working remote/muting board. 

Problem does not occur

1. when arm is simply lowered and not playing a record. 

2. when P5-8 pin is removed which disables activation of the arm position sensor.

At this point I can only come up with spurious noise due to the servo motor to a photo-darlington issue that is triggering the return. Any other ideas are welcome as this is a head-scratcher.

Dillen
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I've seen cases where the datalink remote control system in Beogram 4004 triggers from ambient noise if connected to an amplifier not supporting datalink.

Check if you have a refrigerator, switched flourescent light or similar running nearby or off the same mains phase.

 

Martin

solderon29
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There was a problem with the photo darlington device,and several mod's too,but I'm intrigued by the fact that the problem does'nt occur with another remote control pcb installed?

That suggests that the problem is on the original pcb?

Nick

Mark
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Mark replied on Fri, Jul 9 2021 4:19 PM

Dillen:

I've seen cases where the datalink remote control system in Beogram 4004 triggers from ambient noise if connected to an amplifier not supporting datalink.

Check if you have a refrigerator, switched flourescent light or similar running nearby or off the same mains phase.

 

Martin

Martin, thanks for the tip as that case was what I was trying to evaluate by trying a different board. I am not using the DIN cable at all having replaced it with RCA cables. So far that does not appear to be the case; however, if the disconnected photo-darlington test fails today, I’ll re-examine that possibility. I have the option to convert the control panel to the single connector version which would solve it. 

Mark
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Mark replied on Fri, Jul 9 2021 4:19 PM

solderon29:

There was a problem with the photo darlington device,and several mod's too,but I'm intrigued by the fact that the problem does'nt occur with another remote control pcb installed?

That suggests that the problem is on the original pcb?

Nick

Nick, thanks for the feedback. The problem does occur with another remote control board. This is why I've focused on the photo-darlington. What mods are you referring to? I have both a 5523 (the bad one) and a 5526 that works fine but don't see any difference in the circuits.
Mark
sonavor
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Desolder the button control panel switch contacts where they can only be operated by manually touching a wire to the contacts instead of pressing the buttons. In other words, you are manually making the switch contact to make sure the switches aren't accidentally making contact.

To go along with that...Disconnect the control lines from the remote control circuit that can affect the arm raising.

Other, more involved trouble-shooting things to try...

Try testing it with a multi-channel, digital oscilloscope if you have access to one (i.e. a four channel scope).
Set the trigger to a channel connected to one of the arm raising circuit nodes (whichever gives you the best signal to trigger on when the cue up command fires). Connect the other three channels to the control signals that can cause the arm to raise. Perhaps at the solenoid circuit (1IC4 ?) or maybe the nodes around 1TR15 and 1TR14. Then with the scope in single run mode you can arm it after the Beogram is playing a track. Nothing should show up on the scope until the arm is raised.

I you only have two channels then you can still run the test but it is a little more work.


John

Mark
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Answered (Verified) Mark replied on Sat, Jul 10 2021 5:01 AM

John, thanks for the additional tips. Unfortunately, I do not have a storage scope which would have been ideal.

I have been able to eliminate the control panel and the remote board from the possibilities. I covered over the photo-darlington with a piece of black electrical tape. This eliminated the problem which point me to the LED circuit. In adjusting 1TR18 it appeared to be jumpy and cleaning it did not improve things. I swelled it with one of the </> ones as they were the same value and were more linear. Unfortunately, this did not solve the problem.

I then tested my servo motor theory and was able to get it to pick up by slowly moving the pully by hand without activating the motor. This left me with the 21v to the LED or the LED itself. I replaced the 4000uf cap even though the voltage read fine. I also added the ground bypass cap .01uf to the heatsink screw that was added on the 5526.

I have now played 10 sides without an early pickup! I'm not declaring victory yet, but it is definitely looking promising Smile

 

solderon29
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The original problem was due to intermittant noise in the photo-darlington device.

I think the solution was to replace  the device with a different type.

You seem to have identified another cause though.

One for the gremlin book,thanks.

Nick

Mark
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Mark replied on Sat, Jul 10 2021 5:46 PM

solderon29:

The original problem was due to intermittant noise in the photo-darlington device.

I think the solution was to replace  the device with a different type.

You seem to have identified another cause though.

One for the gremlin book,thanks.

Nick

Which part is a suitable replacement for the photo darlington? I can now confirm that it is fixed Big Smile!

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