Beogram 4002 (5513): Another Beogram From Houston - Electrical Work

Continuing the Houston Beogram 4002 restoration with the electrical work, here is the reworked main PCB.

Before

After

Some details to point out are the replacement of the speed select relay and the trimmers for the platter speeds (33.33 and 45).

The old trimmers for the platter speed adjustment have trimmer adjustment slots on the top and bottom.
The Beogram 4002 main board mounts where its trace side faces up so the speed adjustment is made from the underside of the board.  That is why the new trimmers are mounted upside down...so their adjustment screw can be accessed from the trace side of the board.

Here is the trace side of the main board.  You can see the speed trimmer adjustment screws.

Next are the replacements of the Beogram 4002 main board Darlington transistors IC1 and IC4.
Other Beolover Blog posts show the replacement of these two transistors with new transistors as a precaution against the aging Darlington transistors the Beogram 4002 uses for its power supply (IC1 21V) and the solenoid actuation (IC4 30V).

Another possible transistor replacement is 1TR3.  It is in the arm detector circuit the Beogram 4002 uses to determine if a record is on the platter or not. On the Beolover Blog restorations of Beogram 4002 (and 4004) turntables we check the 1TR3 transistor Hfe value to make sure it is good (typically, above 500).  If it is not, we change the transistor.  

Regardless whether 1TR3 is replaced and whether the lamp in the Fixed Arm (Detector Arm) is replaced, we do change resistor 1R26 from a fixed value (1Mohm) to a trimmer (usually a 2Mohm or 5Mohm trimmer).

This photo shows a 5Mohm trimmer for 1R26 installed on the trace side of PCB1.

The new trimmer will remain there for a while until I finish the rest of the electrical restoration work and am ready to test the Beogram.

The Beogram output board is next.
Here is the before photo of the output board.

Here is the after photo.

The only changed components on the output board are the Beolover replacement muting relay, a new electrolytic capacitor for C1 and the addition of a toggle switch to select whether the Beogram chassis ground is connected to the signal ground.  We have found that different grounding conditions can exist with various preamplifiers that result in ground loops.  Those cause hum in the phono signal and is noticeable during record play.  The grounding switch provides another option to handle that scenario.

Next, I replaced the incandescent lamps for the platter speed indicator panel.

Here is a little bit closer look at the original lamps...

...and the Beolover replacement lamps.  

The incandescent lamp in the Fixed Arm for the Detector Arm circuit was also replaced with a Beolover replacement LED assembly.

I changed out the Beogram 4002 control button panel with a refinished one.  The old panel was in bad shape.

The button panel contacts were oxidized pretty good so I cleaned them up to a nice shiny finish again.

Here is the button panel and contacts reassembled.

As I started installing the floating chassis components back into the Beogram 4002 cabinet, I replaced the original reservoir capacitor with a new Beolover capacitor assembly.  This new Beolover replacement component is very easy to install.

I also replaced the original DC Platter Motor with one that Beolover had refurbished for me.

Now I could install the circuit boards back into the Beogram 4002 cabinet and make some electrical adjustments.

With the electronic components re-installed in the Beogram cabinet it was time to tackle the adjustment of the Detector Arm Circuit.  The new 1R26, 5Mohm trimmer needed adjustment so the DC voltage measured at the 1TR3 collector is 4 VDC.

With the steady state voltage adjusted on 1TR3 the new 1R26 trimmer can be moved to its final position on the component side of PCB1.

Another PCB1 adjustment is with trimmer 1R88.  This trimmer adjusts the voltage at the collector of 4IC1 which is the sensor for tangential arm position. That is the sensor that receives the infrared light from 4D1 as the position slide moves between light and sensor during arm movement.  The light, sensor and position slide detect the arm set down points for various size records as well as the run-out groove detection.

The 4IC1 collection should be set to 5 VDC when the clear portion of the position slide is between the sensor and lamp.

So far all of the adjustments and measurements have worked out as called for in the service manual.

I continued on with the calibration of the tone arm groove tracking.
For this adjustment I like to make a crude adjustment with the bracket connected to the base of the tonearm which holds the aperture slot controlling light to the tracking sensor.

Note that I also make this crude adjustment with the eccentric tracking adjustment screw in its center position.  That is so I have room to make the fine adjustment in either direction.

I adjust that bracket so the arm can be lowered and it just stays put (it doesn't try to move forward on its own).

After that is achieved I loosen the set screw for the eccentric (fine) adjustment screw.

A test record can now be placed on the platter (without a platter belt).
The tonearm is advanced to a center track on the test record and lowered.

The arm should not try to advance.

Manually rotating the platter with the test record should result in the Beogram servo motor turning to advance the arm within one revolution of the platter.  It should advance the arm every platter revolution after that.

The eccentric adjustment screw is used to dial in that calibration.  
Extra fine adjustment can be made with the small trimmer on top of the Beolover tracking sensor lamp assembly.  Those adjustments increase or decrease the intensity of the LED used as the sensor light source.

This photo shows an adjustment with the eccentric screw.

Using an oscilloscope I measured the empty platter signal at the 1TR3 collector to check the health of the sensor output.

Two last measurements before actual record play are to re-install the platter belt and adjust the 33.33 RPM and 45 RPM platter speeds.

For these adjustments I use the Beolover RPM Tool as the measurement instrument and the two speed trimmers on the main, PCB1 board.

The owner of this Beogram 4002 also would like the Beolover Remote Commander installed as part of the restoration.

My next steps will be to first, finish reassembling the Beogram 4002, then some quick test play of some vinyl records.  

After that I will install the Beolover Remote Commander and resume testing record play on this Beogram (the real fun part).

Beogram 8002 Type 5633: Sensor Checks

Now that the Beogram 8002 is past the electrical restoration and the power supplies check out, I set up for some testing of the various sensor signals the Beogram uses to operate.

Those signals are the platter speed sensor, arm position sensors (there are two) and disc detector sensor. 
All four sensors are optical sensors whose signals the Beogram 8002 microcomputer depends on for operation.

With basic operational tests of the Beogram I can tell that the sensors appear to be doing their job but I like to measure the signals with an oscilloscope to really make sure.

Most of the measurement points can be made right at the microcomputer IC (2IC1) and that is a good place to start as it is the destination of the sensor signals. 

Here are the Beogram 8002 sensor signals I usually look at.

Note that the Disc Detector signal at 2IC1-33 is a two-state signal at this point...meaning that the signal is either at +5V or at 0V, depending on the Disc Detector sensor signal generated on PCB 1.

Here is a portion of the Beogram 8002 schematic that shows the Disc Detector circuit.

Notice the highlighted, yellow circle I placed on the diagram.
That is the node between 1R72 and 1R71 where I measure the raw sensor signal that controls 1TR16 to produce the Disc Detector signal at the microcomputer board.

Here is the probe for that measurement.

I measure the Disc Detector Sensor signal at the 1R72/1R71 node and at 2IC1-33.

I also like to look at the Beogram 8002 "SO" switch state on PCB 1 (P2-1) and at 2IC1 (pin 32).

Here are the oscilloscope measurements at Standby mode and at press Play mode for the SO switch, platter speed sensor and the two arm position sensors.

The sensor signals look healthy so I am pleased with that.
Something to observe about the signals after the Play button is pressed is the empty activity on the two position sensor signals until the SO switch is cleared.

When the Play button is pressed the Beogram 8002 servo motor starts moving the tangential arm assembly towards the platter.  At the same time, the platter drive motor starts rotating the platter.
You can see from the platter sensor signals that the platter speed is ramping up quickly by the change in the sensor signal pulse width.  During that time the tangential arm assembly is still physically in contact with the SO switch, keeping it in the off position. When the tangential arm assembly clears the SO switch, the switch engages and goes to +5V.  That is when the arm position sensors are enabled (have power) and start being counted by the microcomputer (uC) and is how the Beogram 8002 uC keeps track of where the arm position is.
The position counts are how the Beogram knows where the set down points are located, where the end of record position is and where to return to when the Pause function is used.

The SO switch location has a mounting screw that allows it to be moved, slightly.  That adjusts the point where the position counter begins and affects where the actual set down point is located.

If a Beogram 8002 loses power during record play, the arm is raised as power to the lowering solenoid is lost.  When power returns to the Beogram, the uC will not know where the arm position is and will return the arm to the Standby position.

Here are the oscilloscope measurements at Standby mode and at press Play mode for the Disc Detector sensor.

The (blue trace in the photo) Disc Detector signal at 2IC1-33 shows how it changes with raw Disc Detector signal at the 1R72/1R71 node (on PCB 1).

During the period before the SO switch is cleared the Disc Detector sensor initializes.  
It is then inactive as the arm moves towards the platter.  The Disc Detector sensor reaches the platter well after the SO switch is cleared and the pulses generated by the sensor lamp reflection off the Beogram 8002 platter cause the Disc Detector signal at 2IC1-33 to go to 0V.  That locks out the Beogram 8002 from lowering the tonearm.

I didn't find any faults with the sensor signals or with operation of the Beogram 8002.
I can move on to the final adjustments to the cartridge tracking force and the record tracking sensor.

Beogram 8000: Sensor Checks and Adjustments

The problem with the Beogram 8000 detector arm not detecting an empty platter turned out to be a simple adjustment to the sensor source lamp positioning.

The Beogram 8000 schematic shows the detector arm circuit and what the expected sensor signal should look like.

Measuring at C19 I could see the signal was not as strong as it needed to be.

I used a flashlight to apply more light and saw that the sensor output increased up to a working level.

The easiest thing to adjust and check at this point is the sensor source lamp position. I opened the lamp and sensor housing to move the lamp down a little inside the housing.

That did the trick. The detector arm now functions normally with the correct output voltage on the detector circuit.

While I had my Beogram test setup in place and my scope warmed up I checked the tangential arm position sensors. Here is the circuit diagram.

Both sensor outputs look good and the Beogram has been working as it should with regard to the tangential arm movement.

Another sensor output to check is the platter speed. This Beogram still has the original, plastic printed tacho disk. Let's see how it measures. Here is the circuit and measurement point.

Here is the sensor output for 33⅓ RPM and 45 RPM.

Last are the forward/reverse scanning LDR sensors. This is the adjustment I always add my custom connector for so I can check the LDR voltages at any time without having to completely open up the Beogram again.

Here is the circuit.

Here is a photo of the test connector I always add for the this LDR adjustment. This photo is from an earlier Beogram 8000 restoration project. It is the same on this current project. The wire colors used on the test connector were chosen to match the Beogram ribbon cable for those signals.

Here are the forward and reverse LDR measurements after adjusting them to be between 0.6 VDC and 0.7 VDC.

I believe this Beogram is working correctly electronically. Now I need to check the tonearm balance and the distance to platter surface. After that I will check the cartridge tracking and, if necessary, adjust the linear tracking sensor. I should be playing records on this Beogram real soon.