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4002 tonearm lifts

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Jim Carr
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Jim Carr posted on Wed, Feb 10 2016 3:40 AM

Hi! I have a 4002 type 5503. The tonearm will suddenly lift then drop back during play.

It will do it usually once while playing a 33 and can do it any time. It is just a momentary lift then right back down without any input from me.

I have one other issue that is it wants to set down way before it reaches edge of the record.

Other than those two things it performs great.

Thank you for any help.

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sonavor
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One more thing. Read about my solenoid measurement carefully. Always unplug the Beogram with adjusting and setting up the scope probes. It is easy to want and setup and get to the measurements quickly. You can short something out that way...don't ask how I know that.

-sonavor

Jim Carr
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I am familiar with the solenoid circuit as I had to replace the original levers on the 4002 as they were plastic and would crack then the solenoid would go up in smoke! We had customers come in horror when they would see the smoke coming out of their brand new decks! I replaced the one in mine for the first owner back in 75 and never had to work on it again until now.

I have checked that part of the circuit and everything seems good. I think the problem starts around TR 16 as that is where the voltages go off.

sonavor
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If you are confident that the solenoid is good then that leaves the control circuit which does involve 1TR16. 1TR15 and 1TR16 control turning 0TR4. They are controlled by 1TR14. I would monitor the base of 1TR15 next. Set the scope to read the "tonearm down" level which the schematic shows to be around 0.5 VDC. So you will need to verify what the up and down levels are. If you can get the down level set you should be able to put your scope in single mode and the trigger to go on a voltage rise. Also set it so the scope records some time prior to the trigger so you can catch the whole event. That should give you a reading of an unwanted "tonearm up" signal there if that is what is occurring. Test your set up with a real command from the Beogram control panel.

There are several Beogram things that feed into 1TR14 which can signal the tonearm circuit to lift. So the actual problem could still be elsewhere than 1TR15 and 1TR16. Monitoring and catching the event at the base of 1TR15 will tell you for sure that it is an unwanted lift signal coming in (rather than some instability of the lift circuit). Another place to monitor for an event is node between 1D19 and 1R51.

Another test you can do to check the tonearm lift is to remove the belt and platter completely. Let the tonearm lower (to an invisible platter) and gently tap the tonearm to watch the tonearm motor move it forward. See if you can get the unwanted lift to occur that way so you can find a convenient method of measuring with your scope.

-sonavor

Jim Carr
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I have checked all of those points which took weeks to do and here is what I get.

Starting with the base of TR 14 no change at all. The base of TR 15 drops from .5 to .3 (it's already in cutoff so it does nothing) The collector of TR 15 & base of TR 16 goes from 4.7 to 17 Volts! As well as the Emitter and collector of TR 16 (as if it is shorted) As the emitter of 16 is tied to the base of 0TR4 it gets the 17 volts too.

I have checked all the connecting wires for 0TR4, grounds etc. The worst part is it only lasts for 1 to 3 seconds then may not do it again for hours.

I have the platter off so the tonearm is just hanging down. I also have tried hitting every switch, touched the tonearm, lifted the board up and moved it around, stared at it, threatened it, offered it new vinyl, nothing seems to make it happen it just does when it wants too.

With the voltages I see around TR 16 and that nothing happens at the base of TR 14 I do not think it is from any switches or sensors. I also disconnected the ground wire for the tonearm optics and that does not make the arm lift but will keep it from lowering.

I have a few points to monitor today and hope it shows something. Ill keep you posted.

Jim.

Jim Carr
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Well the problem has not shown itself for weeks now. I finally put the covers back on and I am just playing it. I do have 1 question, where can I get a new belt? I bought one from Vintage Electronics and it seems too tight. It measures 16.75 in when laid flat and I think they should be 17 in.

 

Søren Mexico
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Get one from Dillen on this forum he has the right ones

Collecting Vintage B&O is not a hobby, its a lifestyle.

Dillen
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Well, not quite there yet with this one, but working on it.

Martin

Jim Carr
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Please let me know when you have them!!

Jim Carr
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Well I'm still at it. My deck worked just fine for about 2 months then the problem came back. I have replaced transistors 1TR14, 1TR15 & 1TR16 with no help. Also when I monitor the base of 1TR14 there is no change at all when the problem happens. The base of 1TR15 drops from .5 to .3 mv. so I do not think that is the cause.The base and emitter of 1TR16 goes to 17 volts as does the base of 0TR4 but the tone arm solenoid activates and lifts the arm which I do not understand.

The other weird thing is at the base of 1TR15 I see a 10mgz signal that I do not think should be there although it's there while the deck is working fine and goes away when the problem hits. I am attaching a picture so maybe someone out there that has a 4002 can look at theirs and see if they have it too.


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