Beogram 4002 (5503): A New Arrival From Oklahoma - A First Look

I recently received a Beogram 4002 (Type 5503) from a customer in Oklahoma for a full functional restoration. It arrived professionally packaged in a Beolover-provided shipping container and so all went well. I extracted it from the box and put it on the bench:

The hood is still intact without cracks in the hinges area, but is badly scratched on the top. It could be polished or replaced with a perfect looking new reproduction hood:

The deck itself is in pretty decent cosmetic condition. The aluminum surfaces only have minor dings

and the keypad shows only light use traces:

It could last for many more years if finger contact is avoided by either using a soft cloth for operating it, or installation of a Beolover Commander remote control module, which enables controlling all functions of the Beogram without using the keypad.

The plinth is also in very good condition with nearly pristine corners:

I removed the aluminum panels and had a look:

As usual the transport lock bushings have completely deteriorated as is evident by their many fragments strewn around the enclosure:

This shows one of the locks. The bushing is completely gone:

The plinth guiding washers are also cracked:

A look at the carriage revealed that some work had been done previously as is evident by the presence of a  Beolover tracking sensor light source (red) and the absence of the plexiglass "ruler" which allows the control system to detect the carriage position:

Luckily the ruler and all mounting screws were included in the shipment. Beolovely!...;-)

The small bent metal strip next to the screws is from the carriage end switch. I hope it can be reinstalled.

After this inspection, I plugged the unit in and pressed start. The carriage started moving and the AC platter motor came to life. All good signs. Of course the carriage did not stop at the LP set-down point, but that is no surprise without the 'ruler'.

In summary, this should be a straight forward restoration. Stay tuned!

Beomaster 1900 Type 2904: Solving the Voltage Difference Between the Channels

I ended the previous post with a problem regarding the output of the Beomaster 1900 amplifier.
Specifically, the Left channel output signal was about 20 percent stronger than the Right channel.

At first I thought the problem might be with the output amplifier transistors...either the Left channel or the Right channel.  However, I also had to keep in mind that the amplifier sounded good and while there were measurable differences, the distortion and frequency response measurements were very similar to other Beomaster 1900 units I have seen.

Still, I felt that a 20 percent difference in output between the channels was a little much.
So I did some audio signal tracing and measurements of the test signal through the various stages of the Beomaster 1900.

At the point of the input signal selection, right before going into the volume control, both the Left and Right channels are identical.

After the volume control, the difference begins...but it isn't always 20 percent.

Studying the Beomaster 1900 volume control again, I realized that the light source controlled, LDR volume control assembly had to be the most likely culprit.

The Beomaster 1900 (and 2400) volume control is an interesting design and is worth looking at here.

Owners love the sleek design of the Beomaster 1900 and in the mid-seventies, the touch control buttons were quite a modern marvel.  Source selection and Standby mode selection were simple on/off state changes while the up/down volume control was a more complicated matter.

The volume control has two parts to the control.

There is the control from a human finger holding the volume button up (or down).  Then there is the actual attenuation control of the audio signal.

First, the detection of a human user wanting to change the volume level.

This circuit and description show how the Beomaster 1900 takes commands from the volume control up/down buttons and turns them into different voltage levels.

The selected voltage level is used by the actual volume control of the Left and Right audio signals.
The way that works is the selected volume level voltage controls the amount of light a small lamp emits on a set of four LDR (Light Dependent Resistor) devices.

The LDR volume control assembly containing the light source (lamp) and the four LDR devices looks like this.

Disassembled, the Volume Control LDR Assembly looks like this.

As you can see, the volume control assembly comes apart and the discrete components can be replaced.
Martin Olsen (Beoparts.com) supplies a rebuild kit for these that consists of the lamp and four LDR devices.

The Beomaster 1900 Service Manual describes how the light source and LDR volume control works.

Here is the Beomaster 1900 schematic section that shows the Left and Right audio signals passing through the volume control and to the output amplifier.
The 8002293 device shown in red is the LDR volume control assembly.

The schematic above shows four key measurement points I used in looking at the audio signal.
TP200 and TP300, before the volume control device.
TP201 and TP301, at the entry to the output amplifier.

An important thing that stood out from the Service Manual description of the volume control device was that full volume occurs with a minimum amount of voltage on the lamp (typically 1 VDC) while the lowest volume level is typically with about 5 VDC applied to the lamp.  

That is opposite of what I would have thought before diving in to the inner workings of the volume control.
What that means though, is that the high end power (maximum output) of the Beomaster 1900 that I want to measure the performance of is at the lowest light source level.

Combining that information with what I observed in the Beomaster 1900 performance...
The Left and Right channel outputs at the speaker load were much closer to each other at output levels below 5 Watts.  That is when the light source is emitting more light.  From 5 Watts and up (to the maximum 20 Watts), the Left channel output got further away from the Right channel. That is when the light source is emitting very little light.

That looked to me like a volume control calibration problem.

I ended up playing around with three different volume control assemblies (two shown here).

Eventually I settled on the one I could adjust the best.

I have to mention that rebuilding this volume control device is more involved than simply de-soldering the old components and soldering in the new ones from the kit.

The calibration technique I used was to connect the lamp terminals of the volume control device to a bench DC power supply.

From there, I applied voltages from 1 VDC to 5 VDC and compared the resistance of each LDR device.  I wanted the L&R sets (R200/R300) to be close to each other and (R201/R301) to be as close as possible to each other.

To adjust the LDR devices to accomplish that requires moving the LDR devices a bit towards (or away) from the light source.  Even tilting them a bit changes their value.

The adjustment procedure means applying a voltage to the lamp and measuring the LDR resistance with the metal cover on the volume control assembly.  To adjust an LDR means having to remove the metal cover, readjust the LDR, then replace the cover to measure the resistance again.

The procedure is quite tedious and I ended up decided on a source voltage of about 1.3 VDC as the target level I wanted to adjust the LDR devices to. 

That corresponds to a pretty high volume level and wasn't too difficult to measure with a DMM.

When I was finally happy with my result, I reinstalled the volume control device and retested the Beomaster 1900 amplifier.

The THD measurement immediately showed an improvement.
 

The Left and Right channel outputs at maximum rated audio power into 8 ohms are now within almost 6 percent instead of over 20 percent.

The THD measurement for both channels at that maximum output level is in the 0.03% range.
That is a very good value for this amplifier model.

The Frequency Response measurement for the two channels also shows the signal amplitudes being closer than before the rework on the volume control LDR device.
The Frequency Response measurement is within the limits I have previously seen on the Beomaster 1900 units although the Right channel does appear to have a better response trace below 1KHz than the Left channel.

I will move this Beomaster 1900 into a listening room for better real world listening tests (than out in the workshop).  Unless I don't like something I hear during those tests, I will consider the bench testing complete.

Cosmetically there is still the issue of peeling veneer from the plastic side panels.
I need to see if I can save those while the Beomaster 1900 is in the listening room.

Beogram 4002 (5513): Brand New Hood Fatally Damaged Due to Bent Hood Hinges! - Straightening Hinges Out and Installation of Another Hood

I recently restored a Beogram 4002 (Type 5513) for a customer in Florida. This unit also received a new hood, which was procured from the beoparts-shop in Denmark. These hoods are exact replicas of the original ones, i.e. do not have any seams from glueing laser cut plexiglass panels together. They are die-cast like the original ones. Very awesome!

Unfortunately, in this particular case the a strange thing happened: The newly installed hood slightly chafed on the hinge back when closing it. This happened about halfway between fully open and closed. When I installed it I did not really take this issue seriously since it was not very noticeable. I basically put it in the 'in the 70s the manufacturing tolerances were a bit looser than these days' bin and tried to forget about it...;-).

However, blissful ignorance was not an option in this case: After taking his Beogram home, my customer sent me this dramatic picture a few days later:

The hood had spontaneously cracked off one of the hinges. A fracture occurred right at one of the two screws. At this point I thought the chafing was probably the root cause since it put a bit of stress on the hood, but I did not understand what caused the chafing. The metal parts looked perfectly o.k. on first glance.

I sent an email to Martin Olsen who designed these replica hoods and asked him if he knew if something like this had happened before. He answered that "I have an interesting Beogram hood on the bench now from a customer who couldn't make it fit properly. The hood would fit reasonably well one side. At the other side it would sit too high and too far forward, enough to show gaps. Opening the hood, I can see that one hinge buts up against the hinge block as it should, but the other has a gap of 2mm.

I took out the hinges, and they appear to be different, in that they have different angles."

The plot thickened! I removed the hinges from the troubled hood and also the hinges from another hood that I was working at that point on that did not have this issue. This allowed me to directly compare the hinges of the cracked hood with those of a 'normal' hood that did not chafe. This is what I found: Indeed the troubled hood had hinges that had a smaller angle between the two shanks than the normal one. This shows two of them in direct comparison:

It became clear that I needed to bend the bad hinges back into the normal shape. To do this I needed a way to precisely determine when the correct angle was restored. So I designed a 3D printed template that I iterated a few times until it perfectly fit into the normal hinges. The perfect fit was established by being able to put a bolt through the alignment hole that I designed into the block to line up with the hinge pivot point. It is evident that the smaller angle of the chafing hinge prevented the holes from lining up.

I took the bent hinges to the workbench and used a vise and adjustable pliers to open the hinge up a bit.

I carefully bent it by small amounts until I was able to put the bolt through the hinge into the template block:

I did this for the other hinge, too, and then put the hinge assembly back together. One side was lacking the plastic slider block, so I installed a 3D printed replica:

Then it was time to install another one of these awesome hoods as well as a new replica 4002 aluminum strip:

I used my alignment tools to place the strip exactly centered:

And then I bolted the hood to the hinge and glued the strip into place:

And I am happy to report that this hood did not chafe anymore! Beolovely!

Of course I am wondering what may have cause the hinges to deform in the first place. The only idea that came to mind so far is that the troubled hood was opened a bit too far and with a bit of violence. Considering the way the hoods are constructed, this would indeed reduce the angle between the hinge shanks if bending occurred. Treat your Beograms more gently, people! Pretty please?!?!...;-)

Beogram 4002 (5523): Repair of an Intermittent Sensor Arm Light Source

Recently a Beogram 4002 (5523) returned to my bench, about 14 months after I restored it in May 2022. The reason was a 'not working sensor arm LED'. This made me concerned since this deck was the first to receive my redesigned sensor arm LED assembly.

Luckily, when I set the deck up on my bench and plugged it in the LED was working normally! This indicated some kind of intermittent issue that somehow 'fixed itself' during shipping. 

I wiggled the plugs and cables a bit but to no effect. The LED stayed on! Only when I started to flex the main PCB a bit around P4 where the components of the sensor arm light power supply are grouped nearby, I was able to get the LED to turn off sporadically. 

A closer inspection of the solder points relevant to the sensor arm circuitry revealed a bad solder point on the collector of the transistor that powers the light, TR5:

The collector of TR5 is connected to the 24V power rail via 1R36, which powers the LED.

I re-soldered the point, and that solved the problem. 

This Beogram restoration was done before I realized that the H-bridge power transistors of the carriage drive also need to be replaced preventatively. They seem to frequently go bad after using the decks again for a while, so I decided to add their replacement to the Beolover '400x restoration canon'. Therefore, I used this occasion to update the main PCB in this respect. This shows the original BC144/143 cans in place:

I replaced them with more modern 2N3019 and 2N4033 components. They have the same package, i.e. drop right in:

I will play this deck a bit before it goes back to my customer in Colorado to make sure that there are no other issues.

Beogram 4002 (5513): Full Functional Restoration and Installation of Beolover Commander Remote

This post describes the work I did on the Beogram 4002 (Type 5513) that I recently received from California. See here for my initial assessment of this unit.

This shows the unit with the aluminum panels off as I received it:

As usual I started with the platter motor since the oil infusion of the bearings can take up to 72 hrs. This shows the extracted motor:

I took it apart and removed the bearings:

The bearings are the two small donuts on the black pad. I immersed them in motor oil and pulled a vacuum. Immediately vigorous bubbling started:

The bubbles coming from the bearings are air that is drawn into the oil due to the vacuum. This makes room for oil to diffuse into the porous bearing material. While this process was going on I focused on the remaining restoration tasks. First I removed all 'motion critical' parts from the carriage assembly for cleaning:

This shows the removed parts:

And after cleaning in an ultrasonic bath:

Beolovely! Then I put everything back together. A critical update is the replacement of the damper gasket:

They tend to harden over time, which can make the arm lowering process inconsistent. Another task to do while working on the carriage is the replacement of the tracking sensor light source. The original incandescent bulb sits in the black box under the arms:

I replaced it with a Beolover LED based replacement (left):

The SMD LED is placed in the same approximate location as the filament of the original light bulb.

This show the LED replacement implanted:

And here a shot of the restored carriage assembly:

There is one more mechanical part that needs to be re-lubricated. It is the damper to arm linkage, whose pivot point is located on the sensor arm assembly. This shows the arms from the back. The linkage pokes out via the V-shaped cut on the small arm that is attached to the counterweight assembly:

The re-lubrication of the pivot point requires that the sensor arm assembly is removed:

While working on this I also re-glued the small copper plate that eases the lateral movement of the tonearm when it is up. This small plate usually is about to fall off since it is only attached with decaying double sided tape:

This completed my work on the carriage. 

The next step was restoring the PCBs. As usual I started with replacing the two power Darlingtons that are mounted on the solder side of the main PCB. It is best to replace them while the board is still installed, which makes their correct placement easy. This shows the original TIP125 (IC4) which is responsible for activating the solenoid:

I replaced it with a stronger TIP107 since this transistor is probably the most stressed electronic component of this circuit:

After also replacing IC1, I removed the PCB:

This is a detail shot of the 'RPM section' comprised of the original Siemens RPM relay and the two RPM adjustment trimmers in the corner:

I replaced the relay and the trimmers, all electrolytic capacitors and the remaining power transistors (especially replacing the original H-bridge transistor is advisable - they often go bad once the decks are used again for a while). This shows the rebuilt board:

And again a detail shot of the RPM section:

The new 25-turn precision RPM trimmers are installed in a way that the adjustment screws are accessible from the solder side of the board, allowing adjustment while the board is installed.

The next step was restoring the output board. This shows it in its original condition:

And with a new output relay, a new capacitor for the relay activation timing, and a switch (red) that allows connecting system and signal grounds in case there is an audible hum during record playing. This feature is often handy if a Beogram is used with an amplifier with RCA inputs:

After having removed the boards and the keypad I also removed the main reservoir capacitor. This shows the original unit:

It is a dual-capacitance design, which was used for Beograms that could be upgraded with a CD-4 output board. I also removed the plinth, which had a metal fixture that was about to fall off:

This shows the completely empty enclosure after vacuuming out all the fragments from the completely decayed transport lock bushings:

It is important to remove such fragments since they can get lodged under the floating chassis and impede its floating motion.

After the enclosure was prepared I started the re-assembly of the components. First I installed new transport lock bushings. My replacement bushings are designed in two parts. This makes it very easy to install them: 

Simply stick one half into the lock opening from the bottom, and the other from the top, and that is it:

This shows one of the locks re-assembled:

After the floating chassis was back in place I installed a new reservoir capacitor assembly. Since the original dual-capacitance units are no longer manufactured, I came up with a 3D printed replacement part that holds two standard capacitors:

This shows the assembly installed:

The next step was installing the re-guled plinth. Since the deck had lost four of its plinth guidance washers, I replaced them with 3D printed nylon replicas:

This shows one of them installed:

Next I focused on replacing the remaining incandescent light bulbs with LED based assemblies. This shows the sensor arm compartment pulled out from the arm tube. Next to it is the Beolover LED replacement with its alignment template:

I removed the bulb and installed the LED board:

This shows it in action:

The remaining bulbs were in the RPM adjustment panel. This shows it belly up. The bulbs are behind the two covers on the back:

This shows the bulbs after I removed the covers:

I unsoldered the bulbs and pulled them out. The two LED replacement boards are shown up front:

They solder directly to the bulb terminals:

They do not interfere with the original covers, i.e. they can be replaced:

Now it was time to focus on adjustments:

The first step is usually to adjust the platter that it is parallel to the arms travel and at the correct height. Once that is done the floating chassis needs to be adjusted that the platter is flush with the aluminum panels and centered inside the round opening in the main panel. This can be a bit of a challenge and several iterations are necessary until everything is straightened out properly. 

After this is done one can focus on calibrating the tracking weight as well as the arm lowering limit. My first step is usually to replace the flimsy locking washer on the counterweight screw with a nut, which allows locking the calibration in place so it survives shipping. 

On this deck someone had beaten me to this issue by glueing the screw into place:

Messy! I used the hot air blower of my soldering station to soften the glue, which finally allowed me to remove it and with that the screw so I could install the nut:

Then I was able to calibrate the tracking weight to yield the proper 1.2g when the small weight dial is set to 1.2. The scale on this dial is notoriously imprecise, i.e. it is best to check the weight occasionally with a digital scale:

After the tracking weight was properly set, I adjusted the arm lowering limit:

This is an important adjustment since it is the 'fail safe' in case the control system develops an issue and the arm gets lowered on an empty spinning platter. The limit prevents a crash of the needle onto the ribs of the platter.

On to electronic adjustments. First I adjusted the bias of the sensor transistor to get 4V at its collector:

After the trimmer was set, I removed it and soldered it into place 'below deck' on the component side of the main board:

Then I adjusted the tracking sensor feedback:

At this point the bubbling of the motor bearings had stopped, and I extracted them from the oil:

Then I reassembled the motor and implanted it back into the enclosure. Then it was time for a 24 hrs RPM stability test using the BeoloverRPM device:

The BeoloverRPM allows measuring the RPM in 10s intervals for extended periods of time. Ideal for detecting intermittent RPM variations. In this case the motor performed perfectly and I measured this curve after about 24 hrs:

This is probably one of the best RPM curves I ever measured on a DC motor Beogram! Beolovely!

Unfortunately, the measurement of the sensor response was decidedly un-Belovely:

The curve was flat, indicating that the sensor did not respond at all!

I removed the sensor compartment and had a closer look. What I found was that one of the electrodes of the photo cell that measures the reflected light from the platter had come off:

I removed the photo cell:

This will need some more attention down the road. I decided to 'borrow' a restored sensor compartment from one of my working Beograms and install it in this unit so I would be able to run and test it, while I work on a solution for the broken photo cell. With the replacement sensor compartment I was able to measure an excellent sensor response, indicating that the circuit works well:

The final step of the functional restoration was the replacement of the corroded original DIN5 plug

with a modern all-metal unit with gold plated terminals:

My customer wanted a Beolover Commander remote control module installed. This shows the module in place:

It is a simple plug-and play installation, which can easily be done 'in the field' without soldering or other special tools. Please, see here for more information.

Since this deck came with a hood that was beyond repair, a new hood needed to be installed. This shows the replacement hood (from the beoparts store in Denmark) with a properly labeled reproduction aluminum trim strip:

I installed the hood on the original hinge, and glued the aluminum trim into place.

And then it was finally time for a test spin of this restored Beogram 4002! I selected one of my favorite CTI label releases (CTI 7073), 'Crawl Space' by Art Farmer, which he recorded in 1977. Of course this album was cleaned ultrasonically on a CleanerVinyl ProXL setup before play! This shows the Beogram together with this lovely album:

A perfect combination! Stay tuned for an update on the photocell replacement. Once that has been accomplished this deck will be ready for its trip back to California.