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).
The first step was replacing the cracked original plastic brackets that hold the RPM panel in place. They often crack when people pry out the RPM panel in an off angle:
Luckily there are perfectly reproduced replacements available at the DKsoundparts store in Denmark. This shows the two new replacements together with the extracted original parts:
All one needs to do is transferring the metal springs to the new parts:
This pictures shows them installed:
And here bolted into the new keypad:
The new brackets need to be adjusted that the RPM panel is flush with the left keypad side:
Before I put in the new keypad I installed the Beolover Commander remote control receiver board:
It plugs directly into the keypad jack of the main PCB. This shows the keypad installed with the Commander auto-repeat indicator LED bolted in under the keypad screw:
My customer also wanted a new Beolover Carriage Motor for Beogram 4000, 4002, and 4004 installed. This upgrade makes the carriage run more quietly while drawing less current, which protects the H-bridge transistors. This shows the original motor still in place:
I unsoldered the leads and took it out. This shows the original motor still in the enclosure together with the new replacement:
The new replacement fits perfectly into the space with the two provided rubber rings for damping its vibrations:
This shows it bolted in with the leads soldered back on:
The final upgrade was a new hood. This shows the new hood together with a new aluminum trim strip:
The first step of the transfer is unbolting the old hood from the metal hinge. The screws are easily accessed after prying off the aluminum trim on the sides of the hood. This reveals the mounting screws:
The centered alignment of the new strip is a snap when using the Beolover alignment aides (orange):
I usually use a blue tape strip for making a pencil alignment mark:
Once the alignment has been marked, the hinge can be bolted into the new hood and the trim can be installed:
A crucial moment is the bending of the side parts. The Beolover bending block that is included with our trims helps getting a precise corner fold:
This shows one of the corners:
Beolovely! And here a picture of the completed hood assembly:
This Beogram was now not only perfectly restored on the inside, but also in perfect cosmetic condition on the outside! Here a few pictures showing off its lovely looks:
