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Beogram 8002 Repair

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sonavor
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sonavor posted on Sun, Dec 30 2012 12:21 AM

This Beogram 8002 was a recent find. I picked it up as a learning unit because one of my primary, everyday turntables is a Beogram 8002 I bought way back in 1983.  That original 8002 is still working well.  I had a B&O technician look at it about eight years ago and he made some adjustments.  That was before I took on this vintage audio hobby.  When this recent aquisition became available I decided the price was too good for me to pass up.

In the picture you can see the lid assembly is off.  The main spring and lid attachement came apart.  I am going to fix that first.

Electrically, this new 8002 powers on and the turntable starts spinning as soon as it is plugged in.  None of the pushbutton controls work.  So this could be a real troublesome repair.

 


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tournedos
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sonavor:
The electrolytic capacitor I hate replacing the most on these BG8002 units is the one inside the processor housing (C28, 47uF, 10V).  It is in a difficult place to remove and replace.  You have to be very careful.

Seems similar to the cap inside the processor board shieldwork in pizza box Beomasters. They are very dificult to remove because the circuit board with a heavy ground plane conducts all heat away from the soldering spots. I tend to just cut the old cap off, leaving enough stubs on the board to solder the new cap on them. Much easier and helps to avoid circut board damage.

--mika

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sonavor
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I am going to monitor some other signals next.  In looking at the result so far, here is some more detail of the problem.

In starting from a standby/stopped position, pressing Play causes the most stress on the problem.  That results in the brightest flicker on the dim bulb tester and often results in the turntable immediately returning to stop.  Occasionally the Play is able to get passed that startup surge and continue on.  However, when it reaches the point where it needs to accelerate to 45, I see a dimmer flicker on the tester and the 8002 returns to stop.

When I don't press Play first and begin by using the Turn button, I only see an initial, dim flicker on the tester and the turntable does turn.  Pressing Play after that doesn't cause any stress on the circuit that results in a dim bulb flicker.  Changing speeds to 45 is the next time I can see a surge.

When I begin with the Turn button then 45, then Play, the tonearm will travel its full distance as it should and there is no dim bulb flicker after the speed change to 45.

-sonavor

sonavor
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sonavor:

I am going to monitor some other signals next.

I moved the scope probes to the positions shown in the diagram.  These measurements are at the transformer secondary that are used for the 8002 drive.  Probe 1 is on transformer connection P7-5 (b) and is near zero when the 8002 is in standby (stopped with power on).  Probe 2 is on transformer connection P7-11 (c) and measures around 30 VAC, 60 Hz in standby mode.  When I press Turn or Play, the signal increases at (b) and decreases at (c) then they settle out.


sonavor
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Here are those same signals across a wider time range at start up and stop.  I need to rerun the Stop test because I forgot to note whether that includes a speed change up to 45.


sonavor
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Continuing with the oscilloscope monitoring of the transform secondary that connects to the 8002 drive circuit.  With the scope probe 1 (Red) connected to (b) in the schematic and scope probe 2 (Yellow) connected to (c), I measure the following for the entire startup sequence for this BG 8002. Since this captures a good startup, it means that the dim bulb tester only barely lit up on the start.  The two pictures below make up the full start from standby to a steady turning of the platter at 33 1/3 RPM.  I believe this is the correct behaviour for the 8002 here but I still have yet to find the source of the current surge I am seeing. It is also a mystery why sometimes the surge at startup is greater than other times.  It does that randomly as far as I can tell.


sonavor
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As another quick test to try I swapped out the tonearm motor and belt from my spare BG8000 unit. I had to use both the belt and motor assembly because the pulley capstan on the 8002 has a larger diameter than the 8000.  It turned out to be a deadend though.  The problem remains the same.

-sonavor

Søren Mexico
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John, I think you are a little of track here, start and speed change are powering the main motor, the tonearm motor does start and stop but there is no speed change, I think the power surge must come from the main drive circuit or motor.

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

sonavor
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Søren Mexico:

John, I think you are a little of track here, start and speed change are powering the main motor, the tonearm motor does start and stop but there is no speed change, I think the power surge must come from the main drive circuit or motor.

Yes, you are quite right. I was just looking for something to try and since that motor was turning during part of the operation I decided to just check it. 

Anyway, I think I have gone over this Beogram 8002 pretty thoroughly now.  The power rails are good.  The solder joints all look fine...I have resoldered all of PCB1 since I started this. The voltages on the AC power to the motor drive looks like it is working okay.  There is just the power surge when the drive accelerates - at the start and when increasing speed.  So 0 to 33 and 33 to 45.  That makes sense that those would be the times that demand the most load.

I was thinking about flipping the switch on my dim bulb bypass and seeing how the turntable performs.  That will be at the risk of blowing another transformer fuse.  However, I have spares.  My bulb that I currently have in the dim bulb tester is a 60W bulb.  The rated power consumption of the Beogram 8002 is 15W.  Instead of bypassing the dim bulb part , I decided to change the bulb to a 150W bulb (because that was the next size I had around to try).  In that test, the bulb never illuminates and the turntable goes through its motion like it should.  Of course that was what I observed back at the start of this project, right after the recap and before my first transformer fuse blew.  Maybe the 150W is too big for this situation? Does that mean the 60W bulb is interfering with normal operation of the BG8002 or is it protecting it?  I left the 150W bulb in the tester and ran the turntable for about an hour.  I didn't experience any problems...other than the nuisance problem of extra segments in the display illuminating.  I'll continue and run the turntable for a longer period of time tomorrow and see if the transformer fuse holds up. 

-sonavor

chartz
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chartz replied on Sat, Feb 23 2013 10:29 AM

Undersized transformer perhaps. They always hum slightly I find, on Beocords or Beograms — not earlier 4000 series, they are completely silent.

Bean counters I hear?

John, just put the whole thing back together and send it to me for my birthday! Big Smile

Jacques

Søren Mexico
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Is it possible to change the main motor with the one from the 8000, or did you take the motor apart for cleaning and lub. And did you try running with an Ampmeter on the motor mains, variations during run will show if the bearings are OK or not. As the failure is "uneven" dry or bad bearings could cause it.

And of course a  small transformer will make it even more important that the motor is running without problems, When I start up machines after installations or big repairs, I always check the Amps and voltage on all 3 phases during the first start up and test run, this saved my but a couple of times.

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

sonavor
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The Beogram 8002 doesn't use a conventional motor with bearings and a shaft.  It has a tangential drive system. The base platter as shown in the picture below is actually the rotor in this system.  The platter that the record rests on goes on top of the rotor.  The transformer looking thing that the rotor passes through is the stator.  So the motor in this system doesn't have any moving parts.  There are just bearings that the turntable hub rotate on (I've not taken that apart and don't intend to). 

In these two pictures I show the rotor in place with the stator (1st picture) and the rotor removed (2nd picture). 


sonavor
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chartz:

Undersized transformer perhaps. They always hum slightly I find, on Beocords or Beograms — not earlier 4000 series, they are completely silent.

I don't think it is a transformer problem because I tested already using a transformer from another unit.  My birthday is coming up and I was hoping to make this turntable my present :).

-sonavor

sonavor
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Søren Mexico:

Is it possible to change the main motor with the one from the 8000, or did you take the motor apart for cleaning and lub. And did you try running with an Ampmeter on the motor mains.

I do have the option to change out the stator assembly with the 8000 but I think that will be a last resort.  My dim bulb/variac test device has a built in ammeter and voltmeter.  The ammeter scale goes from 0 - 10 amps so it isn't for making low current measurements. However, I can observe that current to the 8002 is far below 1 amp.  When the turntable starts up or the speed changes I can see a very faint wiggle on the needle. 

So I am still trying to figure out what affect the dim bulb tester might have had on my testing.  Before using the tester I blew out the transformer fuse twice.  That was back when I had just done the initial recap of the 8002 and was testing the operation of the turntable.  Leaving the 8002 on standby, it began rotating the platter by itself and then lost power (fuse).  Since then I have been testing with safety of the dim bulb tester.  The bulb I used was a 60W bulb.  The 8002 is rated at 15W of power usage.  I observed that on startup and speed change the 60W bulb would briefly (1 second) illuminate at different brightness levels.  When I switched to a 150W bulb, I no longer see any illumination of the bulb and the turntable operations appear to work.  Today I am operating the turntable with the dim bulb bypassed.  Operation so far is normal and I am going to leave the 8002 in standby most of the day to observe it.

-sonavor

Søren Mexico
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Sorry didn't do my homeworkCool, the rotor has bearings and there you may have the problem, or was it possible to get some lube into it. The power surge is at start up and speed change and not always the same. It will be interesting to see changes in the surge after running the deck some hours.

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

sonavor
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I am thinking now that the surge I am seeing is normal. My oscilloscope measurements didn't show any shorts to ground.  I didn't post any pictures but I also monitored the 15 VDC rails during the speed changes and they remained steady. Today I found a 100W bulb so I removed the 150W from the dim bulb tester and put in the 100W.  It also doesn't show anything during the surges and the ammeter barely wiggles.  Next, I swapped the 100W bulb with a brand new 60W.  The 60W draws current and the turntable problem returns.  So even though 60W is four times greater than the BG8002 rated 15W, it appears to be affecting the testing of the turntable.  One thing I can try is to take my working BG8002 and use the dim bulb tester (with a 60W bulb) and see if I get the same result.

-sonavor

Søren Mexico
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Will be interesting, I have problems getting bulbs higher than 60 W down here.

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

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