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Beogram 4004 tone arm will not stop when record present: All help and input appreciated!

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aavatech1
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aavatech1 posted on Sat, Mar 20 2021 3:28 AM

Greetings, I've pulled my 4004 off of the shelf now that I've got time to work on it. Thanks to the very instructive videos from the beolovers site and the wealth of knowledge here I'm making steady progress. Currently I'm having an issue with the tone arm not lowering onto a record. It just goes from be beginning toward the spindle and then back again. The speed does indeed change from 33.3 to 45 RPM at the prescribed point and all push button controls work fine. The tone arm itself has been serviced, moves freely and can be balanced. With the aid of the beolovers videos I have replaced the tracking arm bulb with another 3mm / 6V - 40mA bulb that I had on hand. Originally the 1M Ohm resistor provided +3.6V at the collector of 1TR3 without the platter inserted. I replaced the 1M Ohm resistor with a 2M Ohm, variable resistor and now I have +4.02V at the collector of 1TR3. The bulb is positioned well and below you can see the output from the collector of 1TR3 with the platter inserted and spinning. I'm getting approx. 4.02VDC and the associated waveform seems OK. It's not the ~+6V mentioned on the beoworld videos and I"m not sure why. 1TR3 (BC548-C) has an hfe of 550 and is new so I'd think it should be working fine. I've also replaced 1TR4, 1TR6, 1TR7, 1TR17 & 1TR18 with new correct transistors.The "Photo Darlington" 4IC1 has been adjusted for the required +5.0VDC as well. I put a scope on the base of 1TR17 to see if it was dropping the +20V to ground when the black bars on the lucite transport slide passed the photo sensor and a "trend plot" is also shown below. 1TR17 does drop the +20V to ground when the black 30cm, 25cm, 17cm  & run off stripes pass the sensor. Yet and still the arm drop circuit will not drop the arm. I've also tried adjusting the slide in and out from the sensor but to no avail. My rail voltages are a hair under +20VDC (+21VDC spec) even though I've installed a new 1IC1 (TIP120). One caviat; I'm having a hard time getting my hands on the 1TR5 lamp transistor (BC533-25) and that has not been replaced. I'm getting +5.4V on the collector (+9V spec) /+6.1V on the base/ +5.3V on the emitter.  I'm kinda at a loss here and any input would be appreciated. The arm not lowering is the only thing between me and a working unit and it's making me crazy! Thanks for reading through all is. I'll offer any additional info I can.

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sonavor
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Use the "<<" or "<" button to manually position the arm over a record then try the "v" button.  Do you hear the solenoid click as if it tries to lower the arm?  Or is it silent?  If you hear the click of the solenoid then your issue is likely a mechanical one.  In that case I recommend manually checking out the arm lower mechanical operation and see if you can spot the problem.  If you do not hear the solenoid engage then the problem is electrical. Either the solenoid is not being commanded to engage or the solenoid is broken.

-sonavor

aavatech1
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Hi, Sonavor, it's not that the arm won't lower, it will, the problem is that when start is pressed the arm won't stop at the edge of a record. The arm advance (>) and lowering (V) mechanisms work fine. When start is pressed the tone and sensor arms just travel from the starting point to the spindle and then back again. The arm will not stop when a record is present.

Thanks much for your reply and I'm not realizing that I should have titled my post differently. Apologies.

M

aavatech1
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Hi, Sonavor, it's not that the arm will not lower, it will, the problem is that when Start is pressed the sensor and tone arms just travel from the start point to the spindle and back again. I'm realizing now that I should have worded my post title better. I've just changed it. The arm travel button (>) and the lowering button (V) work just fine so I can move and lower the arm manually. I just that the arm won't stop when a record is present on the platter.

Thanks for your reply, I appreciate it.

M

sonavor
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I see.  Since the arm will drop and a record will play then the fixed arm sensor is working.  The problem is with the signal from the position sensor not working.  

What is the voltage doing at the node of 1D41, 1R82, 1D42 and 1C33 when the set down events occur?

Has 1C33 been replaced?

-sonavor

aavatech1
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Hey sonovor, I just saw your message. Been wrapped up in a fix for a friends daughter (Pioneer cassette deck) I'm done now but will have to get those voltages tomorrow. I'll post them a.s.a.p. Thanks for the guidance. M

aavatech1
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OK, I finally got back to Beogram. 1C33 had been replaced but more on that in a second. I've included images of the trend plot of the voltages at the node of 1D41, 1D42, 1C33 & 1R82. Whether the meter is set to "Min, Max or Average" the "scope" scale itself is the same. Measurements were taken with and without a record on the platter. I didn't really get what they meant by "Run-Off Stop" over the graph on the schematic so I just hit start and let the arm travel to the spindle and back again. Compared to the graph on the schematic which shows four "peaks" reaching up to approx +3.5V, the peaks that occur when the black stripes pass the sensor on my unit seem only to be reaching approx. 500mV. In regard to the 1C33 cap, I did replace it but I only had a 1uF / 50V to replace it with. I still have the original tantalum which measures 1023nF and I ran this test with each installed. The results were identical for each. I'm still waiting for actual 1uF / 35V caps to arrive as well as others.I also tested 1D41, 1D42 & 1R82 and all tested good.

The first group of peaks in the trace in the image is the arms travel without a record on the platter, the second and third are with a record on the platter. They're pretty much the same. Thanks again, sonovor, for the assistance.

sonavor
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The run-off stop signal is for detecting the run-out groove on a record so the tonearm will lift and return before travelling all the way to the ES (end stop) button.

My understanding was that your Beogram isn't setting down on the 30cm and 17cm points of the scale.  Those are sold black so the voltage from the sensor will be a specific voltage for those positions. Not pulses like the run-out stop.

Is that still the case?  That the arm won't stop and lower automatically at the 30cm position?

-sonavor

aavatech1
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Yes, the problem is that the arm will not set down on the 30mm & 17mm points of the scale.

I think the info I last posted is my misunderstanding. When you asked "what the voltage doing at the node of 1D41, 1R82, 1D42 and 1C33" I thought you were referring to what happened to the voltage, at the node, when the black stripes <b>crossed</b> the sensor. Which is what the trend plots represent. Are the voltages you requested the voltage present when the sensor is blocked, that is stationary, in front of the sensor? If so I can provide that (those) measurements as well. My apologies for the misunderstanding. And thanks again for the help.

PS. does the cartridge have to be in place for the set down function to operated properly? In either case, whether the cartridge is on or off, the arm does not stop or engage the set down function.

Mark

Dillen
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Dillen replied on Wed, Mar 24 2021 5:56 PM

Is 1TR6 collector frozen near ground on a scope?
Check 1C19.
It's a tantalum capacitor.
If you replaced it with an aluminium electrolytic, I suggest you fit a tantalum instead.

Martin

aavatech1
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Hi Martin, did you mean "frozen near ground on a scope" during the slides travel, with the sensor blocked by a black band or just at power up?

The 1C19 was replaced with an electrolytic but I'll check and replace it with the original tantalum. I'm working on another parts order currently for new ones.

aavatech1
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OK, with the original 4.7uF tantalum back in 1C19 (it tested good on an Extech 380193 LCR meter) I get +12.3V - +12.9V during the travel of the slide from start point to spindle and back again. This is without the platter installed.                        

Dillen
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Dillen replied on Wed, Mar 24 2021 7:22 PM

So 1C19 is not the problem.
With the platter installed, can you see the signal from the sensor arm opto corresponding to the radial streaks on the platter?  

Martin

aavatech1
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Yes. I believe that the second scope image, in my first post above, shows around +4.2 VDC. No matter how I position the new bulb the resulting votlage is basically the same.

sonavor
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sonavor replied on Wed, Mar 24 2021 11:13 PM

aavatech1:

Hi, Sonavor, it's not that the arm will not lower, it will, the problem is that when Start is pressed the sensor and tone arms just travel from the start point to the spindle and back again. I'm realizing now that I should have worded my post title better. I've just changed it. The arm travel button (>) and the lowering button (V) work just fine so I can move and lower the arm manually. I just that the arm won't stop when a record is present on the platter.

Hi, the above quoted post and the posts you are communicating with Martin seem contradictory or describe a very strange problem.

The Beogram fixed arm record detector circuit makes it so the analog logic in the Beogram will not lower and play a record when the platter is empty.
The black ribs of the platter cause the light sensor to generate a pulse signal that is interpreted as an empty platter.
It would be unusual for that circuit to fail in a mode where the Beogram thinks the platter is empty as no signal means there is a record present.
I suppose that would be possible but that contradicts the fact that you can manually scan the tonearm to a point on a record and lower the tonearm to play it. That tells me the record detector circuit isn't failing in some unusual state.

It still appears to me that the problem is in the logic that resolves the carriage position and isn't registering that the tonearm is at the 30cm or 17cm points.

In answer to one of your earlier questions, yes a phono cartridge needs to be on the tonearm with the tracking force set to something high enough for it to lower. Even without a cartridge installed though, the Beogram would normally stop and try to play a record at the 30cm point if a record was on the platter (or if the platter is completely removed).

At the end of playing a record manually does the tonearm lift and return automatically during the run-out groove or does it go to the end of the run-out groove and return because of the end-stop (ES) switch?

-sonavor

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