Beogram 4004 (5525): Replacement of Sensor Arm Bulb with LED Assembly and Calibration of Sensor Amplification Circuit

A Beogram 4004 that I restored a few years back returned to my bench for some TLC/adjustments. Along with the adjustments it was decided to also update the restoration to the latest Beolover standard, i.e. the sensor arm bulb needed replacement with an LED assembly, which was not yet available during this Beograms's first visit.

This shows the sensor arm bulb cabinet pulled out together with the flexPCB based LED replacement circuit:

After extracting the bulb I installed the LED assembly and fired it up:

The LED that is used on the board is a warm white LED, i.e. it has enough red emission to make the B&O logo light up nicely.

The next step of this procedure is to adjust the biasing of TR3 properly. TR3 is responsible for amplifying the weak signal of the photo diode that is in the sensor arm. By design the base of this transistor is biased with a 1MOhm resistor (R26) pulled up by the collector. Unfortunately, this biasing scheme depends on the transistor gain Hf, which can vary quite drastically across several transistors of the same production run.

As a consequence one rarely finds a Beogram 4002 or 4004 that has the proper manual-specified 4V at the collector. This issue can be corrected by adjusting R26 until 4V are present. This can be done by replacing R26 with a 2MOhm multi-turn trimmer, and adjusting for 4V. This shows the trimmer installed from the solder side of the board that it can be adjusted while the Beogram is running:

After adjusting the collector voltage to be 4V I unsoldered the trimmer, and installed it from the component side:

After this it was time to measure the sensor signal at the collector of TR3:

The amplitude of the signal is 6.3V, exceeding the manual specified 6V, which is good. It should not be smaller, but a bit larger is o.k. Important here is also that the signal drops (that correspond to the passing of a platter rib) go all the way down to 0V. 

This result indicated that the sensor circuit was working properly, i.e it was time to test with and without a record present. All went well. I will play it a bit more and then it will be time to send it back to its owner!

Beogram 4004 (5525): Leveling Platter, Final Adjustments and a Spin with Freddie Hubbard's "First Light"!

The functional restoration of the Beogram 4004 (5525) that I am restoring right now is coming to a close. After I rebuilt the DC motor, it was time to do the platter and tonearm adjustments. The platter needs to be leveled with the aluminum plates first before the arm lowering limit can be set. There are two videos on my YouTube channel that show how to adjust the platter bearing and the subchassis (in a Beogram 4000, but the process is very similar in the 4002).  This picture shows the arm lowering limit adjusted:

When lowered the needle should miss the lower parts of the strobe bars on the platter. This is an important adjustment in case the record detection mechanism fails to disable arm lowering. This is the main reason of the existence of the lowered sections. They are wisely located at the set down points to prevent damage to the needle. There is a video on my youTube channel that shows how to adjust the lowering limits.

 The next step was to adjust the tracking weight. As usual I installed a M3 nut to replace the flimsy circlip that is supposed to hold the counter weight screw in place. This upgrade makes the tracking force adjustment 'shipping survivable':

And then I calibrated the tracking force for the 1.2g point:

There is also a video about the tracking force calibration process.

The final adjustment was the tracking feedback sensitivity. I used my new 'favorite most unliked record': The soundtrack 'Gaily, Gaily' by Henry Mancini and adjusted the tracking sensor alignment:

As usual the (blue) light intensity trimmer on my SMD LED based light bulb replacement part proved useful for fine-tuning the feedback. I posted a video that shows the tracking sensor adjustment procedure in detail.

And then it was finally time for a first test drive of this Beogram! My love affair with CTI Records continues and chose one of my recent acquisitions, Freddie Hubbard's "First Light" album from 1971, which is rapidly becoming one of my favorite records. Lush arrangements by Don Sebesky complement Hubbard's precise yet sensuous trumpet play. Just awesome! What a great (and in my opinion under appreciated) direction in the evolution of Jazz! Check it out! The above discogs link has a few YouTube tracks posted.

It was interesting to listen to this record with the Axel Schürholz MMC20 cartridge (with aluminum cantilever). It sounded clean and crisp! Very awesome! This is Beolove!

Beogram 4004 (5525): Restoration of DC Motor with Re-Infused Oilite Bearings

The final step of the restoration of the platter drive system in a DC motor Beogram (551x/552x models) is an overhaul of the motor itself. These motors came with Oilite brass sleeve bearings, which contain oil inside their porous brass material. After 30-40 years of operation this oil supply runs dry and needs to be resupplied. Dry brass bearings are the foremost cause of strong RPM drops in the DC motor Beograms. RPM increases are usually associated with oxidized RPM relay contacts or defective RPM trimmers. See here for a summary.

In the following I will show a few impressions of the process as I performed it on the Beogram 4004 (5525) that I am currently restoring. If you are interested to do this on your own, I recently made a video that demonstrates this process in detail. 

This shows the motor in place:

Three screws later the motor was on the bench:

To get at the bearings one needs to completely take it apart:

The bearings are the two small 'donuts' at the bottom of the photo. I immersed them in SAE 30 oil in a mason jar and evacuated the jar with a FoodSaver pump. Immediately the characteristic bubbles formed that indicate that the oil immersion process had begun:

After 24 hrs the process had stopped and it was time to reassemble the motor. It can be a bit of a challenge to get the bearings back in without damaging the motor. The concern with the bottom bearing are the very thin wires that connect the windings of the feedback coils. They easily break off and then some micro-solder-surgery becomes necessary to get things right again. The top bearing's challenge is to reinsert it in a way that it sits tight in its place. I designed two 3D printed fixtures to simplify this process. They allow me to apply a high but very controlled force on the tabs that hold the ring in place that presses the bearing into its spring:

After the bearing is in place I usually test it with a toothpick

It needs to move but not wiggle or be loose in any way. 'Tight fit' is the keyword here.

After the motor is reassembled it is time to test it before going further:

My benchmark is a current smaller than 30mA at an applied voltage of 5V and that the motor rotates in clockwise direction. If this is not the case, then the brush carrier plate needs to be realigned until the correct values are achieved.

After passing this test the motor can be reinserted into its noise reduction housing and implanted back into the Beogram. I usually give a restored platter drive system a 24hrs test with my BeoloverRPM device to make sure everything is o.k.. This shows the result before and after for this Beogram 4004:

The black curve is the initial test. Strong RPM spikes can be seen after a few hours of operation. They point towards issues with the RPM relay or RPM trimmers. Since there were no significant RPM drops I am speculating that the motor still would have had a few hours left on its bearings. Not surprising since the keypad looks fairly pristine on this deck and the usual finger patterns are absent, suggesting that the deck was not used much in its earlier life. But I think it is a good idea to do a full restoration of the drive system once the unit is on the bench. This will ensure trouble-free operation down the road.

The red curve was measured after the motor rebuild and exchanging relay and trimmers. The curve is very good and well within the ±0.0025% wow and flutter spec stated in the service manual.

Beogram 4004 (5525): Replacing the RPM Indicator Light Bulbs with LEDs

No DC motor Beogram 4002 or 4004 restoration is complete without replacing the RPM indicator incandescent lightbulbs with LEDs. The purpose is not only longevity, but also DC motor stability. Since the user accessible RPM trimmers in the RPM indicator panels are actively participating in the motor control, their operating temperature is of concern. Light bulbs emit most of their energy as heat and therefore significant temperature changes occur. Replacing the light bulbs with LEDs lowers the heat load dramatically and therefore the trimmers do not change their temperature as much. This can significantly decrease RPM changes of the DC platter motor. See here for a summary of DC motor RPM issues and their cure.

So, after rebuilding the PCBs of this Beogram 4004 (5525) it was time for this procedure. Here is an impression of the original light bulbs in the RPM control panel:

The left one was exchanged at some point with a different type. The correct bulbs are difficult to find these days, one more reason to switch to LEDs. This shows my custom designed replacement boards:

These assemblies are available to other enthusiasts. This video shows how to install the LED boards. They fit precisely into the gaps in the plastic assembly that holds everything together:

The boards feature red/green LEDs tuned to emit light looking like incandescent light. I elected this design strategy to make sure that red photons are present for the illumination of the RPM indicators, which are made from red plastic. Amber LEDs such as used in some restorations of classic audio components when replacing bulbs are not suited for this task since they do not emit red light. LEDs are usually very monodisperse in their emissions, which makes them different from incandescent light that contains a wide spectrum of wavelengths.

The design of the boards permits to reinstall the bulb covers, retaining the original look even 'under the hood':

Beogram 4004 (5525): Replacing Electrolytic Capacitors and Rebuilding the DC Motor Control System

After rebuilding the arm lowering and tracking mechanisms of the Beogram 4004 (5525) that I am restoring right now, it was time to rebuild the two main PCBs. I always replace the electrolytic capacitors with modern 105C grade major Japanese manufacturer units, and I also replace the original relays and RPM trimmers with modern encapsulated units. This ensures stable functioning of the DC motor control system since it is very sensitive to contact impedance in the relay, as well as resistance fluctuations of the usually corroded RPM trimmers. Replacing these with 25 turn modern units ensures that the RPM can be adjusted precisely, and that the adjusted RPM is stable over time.

These two pictures shows the main PCB in its original condition:

The original National RPM relay and trimmers in detail:

And after restoration:

My custom designed replacement relay boards are based on modern SMD relays and a breakout that has the exact pinout of the original relay:

These drop-in ready replacement relays are available to other B&O enthusiasts, just send me an email or use the contact form on my blog. This shows the relay and the trimmers installed:

I always orient the trimmers that their adjustment screws are flush with the through-holes in the PCB, and that RPM increase corresponds to clockwise turning:

After completing the main PCB I turned my attention to the output (#8) PCB. This shows it in original condition:

The main difference to 4002 output boards is the remote control section to the right of the black mounting standoff. The 4004 can be controlled via the Beomaster 2000 using an early form of beolink. Essentially start, stop and arm lift can be controlled.

This shows the output relay in detail:

And after replacement of capacitors and relay:

As usual, I also installed a grounding switch next to the output cable jack. This switch allows the connection of system and signal grounds in case grounding issues cause humming. An example is the use of a DIN5-to-RCA adapter without system ground breakout.

The last step was to replace the original power supply reservoir capacitor. I developed a 3D printed adapter for a modern, smaller unit that fits exactly into the capacitor bay in the enclosure:

Pretty, isn't it?? This is Beolove! On to the DC motor restoration with oil infused oilite bearings...

Beogram 4004 (5525): Restoration of Arm Lowering and Tracking Mechanisms

As usual I started the restoration of this Beogram 4004 (5525) with the arm lowering mechanism and the tracking system. Here is a picture of the solenoid and the damper that control the arm lowering:

I removed the linkages and the damper for cleaning and lubrication:

This video shows how to rebuild the tone arm mechanism in detail. After putting everything back together I turned my attention to the linkage that connects the damper to the arm. For getting to this one has to take out the sensor arm> This shows the sensor arm from the back:

and after removal:

Then I removed the circlip and the spring that hold the linkage down and removed it:

This video shows how to get to the damper to arm linkage and re-lubricate it. After putting everything back together it was time to adjust the lateral parallelism of the arms:

Then I replaced the original tracking sensor light source with one of my SMD LED based replacements to ensure trouble free operation down the road. This shows the original fixture:

After taking off the light source assembly the tracking control aperture becomes visible:

Then I installed the replacement:

It has a potentiometer which allows easy fine tuning of the tracking feedback via changing the light intensity of the LED in the assembly. This part is available. Just send me an email or use the contact form on the side bar of the blog. There is a video that shows the tracking sensor adjustment procedure in detail. 

The next step was to exchange the wobbly original plastic pulley with a new aluminum pulley provided by Nick (I'll be happy to get you in touch with him if you need one!). This shows the original pulley:

And the new one:

Allright! All good again in the arm lowering and tracking departments!

Beogram 4004 (5525): First Impressions

A Beogram 4004 (5525) just arrived for a restoration. It came double boxed:

I never have issues with shipping Beograms when double boxing is used. Another key ingredient to a successful shipping experience is to stay away from USPS. FedEx works much better.

The Beogram came with a MMC20CL cartridge that had received a restoration by Axel Schürholz in Germany. I never saw an example of his work. I really like the little box with the precise foam cutout that houses the cartridge:

The cartridge itself was rebuilt with an aluminum cantilever

i.e. technically this is not a MMC20CL anymore since they originally came with a single crystal sapphire cantilever that has a significantly lower weight than an aluminum cantilever. This results in a higher frequency bandwidth due to the higher resonance frequency. 

The deck itself is in quite nice condition, although there are a few marks on the plinth:

Unfortunately, the keypad also took a hit at some point and has a fairly deep mark on it:

Other than that the unit is original under the hood (I apologize for the blurry image - forgot to shorten the exposure time):

The hood is an unmarked one:

Not sure when exactly they used these, some 4004s do have a "Beogram 4004" label on the left side of the aluminum ribbon. The hood has the usual age-appropriate scratches. 

No problem to sand these out and then do a thorough polishing job to get it shiny again.

After the visual inspection I performed a basic functional test, and it seems everything works more or less. The motor is a bit loud, which concurs with the reported incidence of RPM variations, i.e. this motor will need to get re-infused Oilite bearings. Other than that the arm lowering mechanism is a bit sluggish, which can be fixed with a cleaning and re-lubing job of its mechanical parts. The transport lock bushings are also on their last leg and should be replaced.

In summary, this deck is a very good candidate for a restoration. It will perform like new again once I am done with it.