Beocord 5000 (4715/4716): Exchanging the Belts

Once the tape mechanism was extracted from the Beocord 5000 (4715) that I am currently rebuilding it was time to replace the limp decaying original belts. I pretty much followed the procedure that I am showing in my YouTube video:

I improved the procedure in one point, however: I realized after doing it the first time that the alignment of the eject lever relative to the tape mechanism is crucial to get the proper action point when pressing the eject key once the unit is completely reassembled. Last time it took me quite a while to get this properly readjusted, since I only marked the position of part #256 ("lever") with a pencil, which was not precise enough. This time, I put a couple dabs of white paint where the part connects to the drive mechanism. Here is a picture:

This resulted in a much more accurate reassembly once the belts were in.

Here are a couple pictures of the newly installed belts:

I did the bottom belts first:

Here is a shot of the belt that drives the spindles and the free-wheeling mechanism. I also put a new light bulb in there - the old one looked pretty black, indicating little remaining lifetime:

After I put the drive back together and plugged it in, it turned out that the take up spindle would only occasionally feel like doing its job, usually after a making a clicking noise. It turned out that the retaining ring that holds the spindle assembly together (part #173 on the schematic below) was not seated properly. This caused the axle to have to much play at the top (top part of sequence #168), which resulted in random disconnects from the second part from the top in #168. Since this part engages with the cassette tape, this caused the tape not being wound anymore during play. A crumpled tape was the result. Luckily, I expected such issues, and used an old unwanted tape that has already a bunch of crumply regions...;-).

Here are a couple pictures of the part taken when I extracted it for inspection:

The small wheel at the bottom drives the counter, and is below the center plate of the mechanism. The above picture shows how I extracted the part after removing the retaining ring that holds the counter wheel in place, and the entire mechanism together. 

After I put everything back together a second time, the mechanism started to work normally. On to the recap of this unit.

Beocord 5000 (4715/4716): Extraction of Drive Mechanism

Experience makes life easier! This is my second Beocord 5000 restoration, and so I further streamlined my 'tape mechanism extraction' procedure that I outlined in my earlier post about replacing the belts in these units where I also posted a YouTube video about this process. The first step is to open the unit up. This time I took the plexiglass cover off since it needs to be polished anyway. This makes it much easier to extract the tape mechanism. I also lifted the preamplifier and radio amplifier boards up, since this unit will receive a full recap. Here is a picture of the 'unfolded' unit:

Inspired by earlier comments by 'chartz' of Beoworld.org that one can leave the solenoid in the enclosure when extracting the mechanism I realized that one does not even need to loosen the solenoid fixture if one takes out the screw that holds the bottom cover of the drive in place before taking the mechanism out (I wondered last time why there are three access holes in the front of the enclosure, but did not realize the purpose at that point - live and learn...;-).

That way one does not loose any of the alignment, except for the height of the tape mechanism. This issue was tackled by putting some small dabs of white paint on the two screws that hold the mechanism up relative to the solenoid bracket. Here is a picture of the right screw, and the screw that holds the bottom cover (that is the one that sits in the square cutout of the solenoid bracket):

When I took this picture, I was still of the opinion that I needed to move the solenoid a bit forward to get the tape mechanism out, so I marked the position of the bracket relative to the enclosure at the bottom, too. But as said above, this is not necessary. All that is needed is to mark the position of the screws that hold the mechanism to the solenoid bracket (the slightly rusty screw near the center of the photo). 

So, in summary, all one needs to do to get the drive out is to take these three screws out at the front, unplug the plug that connects the heads. After driving the counter screw at the back all the way out the drive can be lifted up and unplugged from the connector at the back and taken out (this pic shows the solenoid bracket loosened - I was still in the learning process...;-):

The next step is replacing the belts.

Beogram 4002: Apple Remote Controls All Functions of Turntable

I recently developed a remote control receiver that plugs into the keypad connector socket on the main circuit board of Beogram 4002 (5513 model) turntables. I have been testing it now for a few weeks, and it works very reliably. So it is time to post about it.

The implemented receiver board takes over the keypad without interfering with it, i.e. the Beogram can still be operated as usual via the keypad. I decided on using the latest Apple IR remote control that still comes with the Apple TV. I was attracted by its sleek aluminum surfaces, which match the 4002 style well. The sparseness of its buttons also almost perfectly matched the application at hand.

My main motivation for the development of the remote system was to preserve my perfectly restored keypad from wear and tear. In addition, I always thought the cueing function of these Beogram models could use a bit of improvement. For my taste the "<" and ">" buttons result in too slow cueing if one wants to skip a couple tracks, while using the "<<" and ">>" buttons requires quick action to stop the movement with either the "<" and ">" buttons or the tone arm up lift button. This makes it difficult to effectively place the arm at the start of a certain track.

The implementation of the remote control allowed me alleviate this issue by programming a dynamic cueing function that automatically switches to the higher cueing speed after ~2 seconds of pressing the cueing button, and that also automatically terminates when the button is released. This makes it much more straight forward to precisely hit the transition groove between two tracks.

I made a video demonstrating the remote control. It also shows some details about the implementation of the receiver board:

The remote board is based on the Atmega 328p microcontroller (aka "Arduino"), which receives its commands via an integrated 38 KHz IR receiver (IRM 3638). The board essentially 'imitates' the pressing of keys via transistor networks that make 100ms connections in lieu of the switches. Since this is fully parallel to the keyboard switches, they keep working in this scheme. Here are a few pictures of the setup:

Bottom side of the receiver board. Arduino controller and male header that plugs into the keypad socket of the Beogram main PCB can be seen. The small SMD parts are the transistors etc...:

 Top side of the board: The female header connects the original keypad connector:

This shows the board installed:

This shows the remote receiver as it peeks out from underneath the cabinet:

The next step of this project will be the design of a more professional board layout and get it manufactured instead of winging it with Fe-chloride, a laser printer and my wife's iron...;-). The issues with this prototype are for example that the in service programming port conflicts with the fixture that receives the standoffs of the top aluminum panel (hence the strip of plastic over it in the above pic - I was lucky that I could still install the panel...;-). Also the remote receiver connects a bit too far to the right, also interfering with this fixture...obviously, I did not consider its existence when I designed the prototype..;-). I also would like to have some more space for the keyboard cable, i.e. the female connector could be moved a bit further away.

In the meantime, back to my current restoration projects!

Beomaster 8000: Signals at IR Remote Receiver Test Points

After the remote receiver came alive again in the Beomaster 8000 that I am currently restoring, I decided to take some measurements at the test points for future reference. I thought this memo might come in handy the next time there are some problems in this area.

Below is the relevant section of the circuit diagram. The IR sensitive diode 6D27 changes its impedance when IR light impinges. This drives the damped 41kHz resonator formed by L1/C2/R1. IC1 drives the signal into the input of IC2 (TP5), where it is amplified. The signal emerges from the amplifier at pin 3 where it has a 12V amplitude (TP7). The 41kHz modulation is removed in the filter formed by R16/C27 and a cleaned up digital signal containing only the bits of the remote codes is forwarded by TR25. TR26 finally changes the signal to a 5V compatible signal via a pull-up on the processor board (PCB#9)

It is interesting to note that this entire circuit is nowadays integrated into the receiver package (example IRM3638), which has pretty much the same form factor as the simple IR diode D27 that is used in the Beomaster 8000. Amazing progress in a couple decades!


This is the table of remote codes from the service manual:

It is evident that all codes start on a 1, which represents the start signal for the transmission and its evaluation in the microcontroller. It is interesting to note here that the signal of the remote is fed into pin 24 of the 6500/1 microcontroller, which is on its port B. Only Port A is interrupt capable on this processor (S. A. Money: Microprocessor Data Book) and it is completely used for the keyboard readout. This suggests that the remote function is fully software implemented. Anyway, after this trip down the memory lane (I am a proud Commodore 64 veteran...;-), here are the oscilloscope signals that I measured while pressing continuously 'Volume Up' (10100111) on the Terminal:

TP5 (input to amplifier) - pretty strong 700 mV signal, which is no surprise, since the Terminal was about 20 cm away from the IR diode (note the 20ms timebase...the other two pics below were measured at 10ms):

TP7: Output of Amplifier (signal is still modulated, but amplitude is now ~12V):

TP8 (after removing the modulation and translation to 5V a clean digital signal emerges):

Beomaster 8000: Rebuilding the Power Supply Board (PCB#6) and a Happy Remote Control Receiver

I always recommend to recap the power supply (PCB #6) board whenever I open up a Beomaster 8000. The golden reservoir cans are often prone to fail at this vintage. Another frequent candidate to interrupt the Beomaster bliss is 6C35 that is mounted directly next to 6IC4 at the voltage regulator heat sink, where it gets nicely warm (but not necessarily cosy for electrolytic capacitor standards...;-). The procedure is largely uncomplicated. PCB #6 can be left connected to the heatsink, and simply pulling it up vertically after removing the plugs from the board gives enough access to the capacitor solder points. The golden cans are usually glued to the board with some kind of silicone-type goo. It can be softened up with a rework heat gun (or a blow dryer) until the cans come loose. This shows the board before I started working on it (note the imposting yellow 'Sprague' capacitor in the top left corner of the board...this helped kill the remote received. See at the bottom of my last blog entry for a better picture of the 'modification' of this board - not sure what the nut who worked on this was up to...it all does not make much apparent sense):

 This shows the board after rebuild with quality 105C caps and removing the offending capacitor:

After this I woke the Beomaster 8000 with the Beolab Terminal by pressing '0' and it came on! All the other functions also appear to work remotely. It appears the remote receiver is back in the game! Beobliss!!

Out of interest, I did some measurements at the remote receiver test points TP5 (input of IR remote amplifier TDA 4050), TP7 (output of TDA 4050) and TP8 (cleaned up 5V digital signal) for further reference. I will post the oscilloscope shots in the next blog entry.

Beomaster 8000: Rebuilding and Testing of the Output Stages and First Power-Up

When I get a Beomaster 8000 of unknown provenance on my bench, the first thing is to check the main 10A fuse, and the color of the emitter resistors in the outputs (crispy brown indicates a burnout of the output transistors, the same can be said for an open circuit main fuse). The 8000 I am currently working on was negative on both counts, and so I proceeded to rebuild the output boards with new quality 105C electrolytic capacitors from name brand Japanese manufacturers (Nichicon, Rubycon et al.). I also put in 25-turn quality encapsulated Bourns trimmers to control the quiescent current of the outputs. This is probably the most important operation in any Beomaster 8000 restoration. Hot heat sinks during modest volume play, or, worst case, smoke coming from the back of the receiver are usually related to the failure of the often corroded original single turn 100 Ohm trimmers that were standard back then. These old trimmers also make it difficult to adjust the quiet current to its prescribed 18 mV equivalent voltage drop across the output resistors since they drift thermally and over time. 25-turn trimmers are much superior, and allow a precise adjustment that will stay put for a long time.

Here are the PCBs after updating them (the original look is shown in this post):

Left channel:

 

Right channel

Below are the 25-turn trimmers before implantation in comparison with one of the original single turn trimmers. The 25ers need to be extended on the swiper lead to fit the old footprint. Also, it is important to orient them properly, so one gets clockwise increase of the quiescent current when turning the adjustment screw. If this is done reverse, then down the road another tech may get confused and accidentally fry the output transistors while adjusting the trimmers. Semiconductors behave exponentially, i.e. once the quiescent current threshold is crossed, the current will increase swiftly.

Once I had the new parts installed, I disconnected the power rails and the three plugs that are on the boards. Then I ran the PCBs with three external power supplies. While doing so, I adjusted the quiescent current to the correct 18 mV voltages at the test points:

Left channel:

Right Channel:

Both stages took in the prescribed ~100 mV via each of the main ±54V rails, so all is good now in Output Ville!

Now that the outputs were certifiably stable, the first power-up of the Beomaster could be attempted without worries about producing 'magic smoke'. Before turning it on, I reconnected the clipping light control line to its proper place (see heat shrink on red wire below the indicator light that is on - someone just cut this wire for whatever amateurish reason...gotta love it when dilettantes find a screwdriver (or wire cutter for that matter) and wonder how best put it to use...;-)

I connected my two bench-speakers and then the Beomaster came on with a health double relay click, indicating that the start-up sequence worked properly and that the microcontrollers were doing their job as they should. I tuned to a station, and I got some decent FM reception of a local station, which played nicely on the speakers. Unfortunately the displays of this Beomaster are in need of fixing. They show the frequently observed missing segments issue:

I tested all the functions of the Beomaster, and it seems that everything is working except the remote control. None of the buttons provoked any response by the Beomaster.

I poked around a bit and finally realized that the dilettante did not only cut off the clipping wire, but also did some strange experiments with the remote receiver. Here is a picture of a part of the power supply board which shows some 'additions', notably a Sprague capacitor soldered to D25 in the output of the TDA 4050 IR preamp, which has no obvious function. Furthermore, pin 1 on P48, which carries the remote signal is bent upwards, i.e. the signal is effectively disconnected from the microcontroller remote input. No surprise that the remote does not work...I will check this out when I rebuild the power supply. Let's hope that the remote just works when disconnecting the cap and reconnecting the output to the connector.

Beomaster 8000: First Peek Under the Hood - Some Remarkable Findings

Today I opened up the Beomaster 8000 that just arrived to do an initial assessment of the interior. Here are a few images of what I found:

Maybe most remarkable is PCB #9 (u-computer board), which looks uncharacteristically new. In difference to all the other 8000s that I opened up so far, this one is also a higher quality, thicker polymer material. Pristine green gloss!! It seems that this Beomaster received a replacement at some point, potentially in the 1990s.

Strange are the two white/red/blue striped cutoff jumpers that are soldered on to the board to wire connector for the volume encoder. Maybe they were used to check the encoder signals. A frequently found problem on these boards is that some of the vias malfunction. Some of them affect the volume encoder  functionality. Maybe the techs tried to figure it out, and then gave up and simply replaced the board.

The hypothesis that the Beomaster saw some care in the 1990s is supported by the electrolytic capacitors on the right output PCB, which have a typical 90's style light blue/silvery look:

Compare with the left channel, where the usual original 1980s models are apparent:

A good sign on both of these output boards is that the emitter resistors of the output transistors are not brown. This makes me hope that the transistors are still o.k.

Another thing I found is this cutoff wire (red) to P70 of the display PCB. It controls the clipping indicator. This probably explains why the seller mentioned an issue with the clipping light.

Aside from a missing screw for the control panel, this is the only major mechanical damage I found so far:

This is the left receptacle for the control panel cover hinge. Maybe I will find the missing piece of plastic under one of the PCBs and I can reattach it to give this hinge the support it needs. When I opened the Beomaster, the cover would not come up by itself, and it was stuck to the glass panel...this has probably to do with this loose hinge.

And finally a glance at the power supply board, which exhibits the usual golden Made in Germany reservoir cans that I have found on their last legs in most Beomasters that I opened up so far. Let's see if we have all the voltages when I turn this unit on for the first time (after I rebuild and test the output stages):