This is a follow up to my recent post about the redesigned Beogram Commander remote control board, which now works in both (DC-motor) Beogr...
Wednesday, August 26, 2020
My current Beogram 4000 project is coming to an end and it is time to summarize the work that was done. I already made two posts about this restoration discussing some special topics. The first one was about alleviating some 'human creative interference' (a missing collector insulator on an enclosure mounted transistor ) and a dead motor driver transistor which caused a significant stress on the power supply of this deck. The second one was about the design of a 3D printed replacement for often cracked or broken off 'old style' MMC cartridge mounts found in earlier issues of the 4000.
This post will summarize all the other work that went into this unit:
Usually I start out by rebuilding the carriage, which involves removing and cleaning all the moving parts. This shows the arm lowering mechanism:
I removed the linkages and also the carriage rods and spindle for cleaning:
I put everything through the ultrasonic cleaner:
A lot of old lubricant leaving these parts! And finally ended up with clean parts:
While having the carriage upside down it is a good moment to remove the solenoid activated switch terminals for gold coating. This shows the switch PCBs before rebuilding them:
And after plating them with nickel and gold:
Nice and straight, and shiny! Beolovely!
After I put everything back together it was time to replace the tracking sensor light bulb
with an LED based update:
The small blue box is a trimmer that allows tuning the intensity of the LED, which is very convenient for adjusting the tracking feedback. I also replaced the cracked original plastic carriage pulley with a nice machined aluminum replacement:
I get them from a B&O aficionado in Vienna. Let me know if you want one. I'll be happy to get you in touch.
The next step was rebuilding the electronic system and the AC platter motor. This shows the original condition of the reservoir and motor capacitors and the motor itself:
I took the capacitors and the motor out and opened up the motor for an oil infusion:
This shows the motor after the infusion. I use 3D printed parts to bolt the two can halves back together (originally the motor is held together by rivets, which need to be obliterated if one wants to open it up):
I re-installed the motor and then replaced the 24V Zener diode, located next to the motor. This shows the original 1W type (grey cylinder to the right of the red resistor):
These Zeners are a bit under dimensioned and they get a bit warm, causing the voltage to exceed the specified 24V. I usually replace them with a modern 5W device
Which keeps the voltage rock solid at 24V.
Then it was time to install the new big capacitors. I use a 3D printed fixture to hold them neatly in place:
After this chapter was finished, I focused on straightening out the remaining switch terminals. This shows the badly oxidized terminals the carriage position switches:
The third and last batch of terminals are in the keypad cluster:
Also very black. I removed all the terminals and plated them with nickel and gold:
It is always satisfying to see them in this pristine golden state! I soldered them back in. This shows the renewed carriage switches:
And the keypad switches:
Beautiful! I hope they will remain that shiny for a long time!
While inside the keypad I also replaced the four light bulbs with LEDs. This shows the original condition:
These are the boards for backlighting the position sensor and the RPM trimmers:
The final task of the keypad cluster restoration was to glue the strobe mirror back in. They pretty much always are loose. This shows the mirror like I found it:
And back in its position:
This concluded the keypad. I moved on to the main PCB and replaced all the electrolytic capacitors, the RPM relay and the RPM trimmers. I also replaced TR14 and its biasing resistor. TR14 serves to amplify the sensor signal. They often go out of spec and then the record detection circuit looses its 'eyes'. An issue with the circuit is that the transistor is biased with a single resistor between base and collector, which usually does not yield a collector voltage matching the specs. This shows the 25 turn trimmer I use to replace this resistor:
I usually install it on the solder side of the board first so I can adjust the collector voltage to 1.8V while the board is installed. Then I move it below deck. This shows the entire board after completing all these tasks:
At this point I replaced the sensor arm light bulb with an LED setup that folds into the bulb compartment:
This shows it 'in action':
The LED is a warm white LED which produces enough red photons to give the B&O logo a realistic red-orange backlighting.
It is always a good idea to check the sensor signal above an empty spinning platter to make sure that the sensor gets enough light etc...This shows the signal I measured, which is very nice matching the trace shown in the service manual:
At this point I decided to qualify the platter drive system and I measured the RPM for about 24 hrs with the BeoloverRPM device:
This shows the curve I measured:
This looks pretty good. The AC motors usually have a very stable RPM performance.
The next step was to update the signal path. I started out by adding a switch that allows connecting signal and system grounds. This can be helpful for suppressing hum issues, especially when using adapters for RCA inputs on a non-B&O amplifier. This shows the switch installed on the output terminal, which gives convenient access to both grounds.
This Beogram 4000 came converted to RCA:
Not very pretty, but at least they broke out the system ground contact. I reinstalled a nice all-metal DIN5 plug with gold plated pins:
In my opinion DIN is superior to RCA since a good DIN cable like the Beograms use them the system ground is carried along as a braided shield around the cable. In combination with a DIN5 input on the receiver you get a fully EMI shielded connection. RCA in contrast uses the signal ground as shield around the signal leads, while the system grounds on both ends are connected with a separate wire. Not an ideal solution.
On to cosmetic issues: This 4000 came with a disjointed plinth:
Proper re-glueing required removal of the metal fixture from the side part. This can be done by heating the part in a standard oven at the 'warm' setting, which usually yields a temperature around ~70-80C. I usually wrap the part in aluminum foil to distribute the heat more evenly:
After an hour or so in the oven the metal part can be easily removed without damaging the wood part. Before re-glueing the parts the old contact cement needs to be removed from the metal parts. I usually soak the part in isopropyl alcohol for a few hours. This is easily done by wrapping a alcohol drenched paper towel around the glue covered parts and then prevent evaporation with a secondary wrap of aluminum foil:
Once the parts are prepared they can be clued back together. I glue first the wood parts together with some white wood glue. This shows my custom designed assembly braces holding the parts together while the glue is fully curing:
Once the wood parts are joined the metal part can be glued in using contact cement. See here for a more detailed description of the entire process.
When I installed the restored plinth I used new cabinet guidance washers to replace the original cracked ones:
The final step was to re-paint the hood hinge which had the usual corrosion often found in 4000s:
This can be alleviated by removal of the hinge part, sanding the offending area and spray coating it with satin black spray paint. This shows the removed hinge:
After removal it became apparent that the plexiglass was cracked on one side in the attachment area:
This is a frequently encountered problem with these hoods. There is just a lot of stress on this area, while it is weakened due to the holes. Luckily this can be fixed if noticed early before complete fracture occurs. I installed thin stabilizing patches to strengthen the area on both sides of the cracked area
Then it was time to bolt in the repainted hinge and glue the trim sides back on:
I use contact cement for this process and custom designed clamps to join the trim firmly with the Plexiglas. I did the entire process also on the other end of the hinge, since if there are cracks on one side the other is not far behind.
And that concludes the restoration of this hood (my customer decided to polish it himself - a nice workout in the garage coming up, my friend!!...;-):
And then it was finally time to take this restored Beogram 4000 for a first spin!
I selected 'Mysterious Traveller' by Weather Report (Columbia 32494, 1974). Of course this vintage record was thoroughly cleaned on a CleanerVinyl Pro System together with an Elmasonic P60H dual-frequency ultrasonic cleaner, which restored its sound to a like-new condition.
I recently bought most Weather Report studio recordings, and the Traveller is probably my favorite. Pretty smooth, yet quite avant-garde. A perfect combination for testing a Beogram 4000! My favorite track is 'Nubian Sundance'. What a pretty sight!!:
I will play it a bit more and then it will be time to send it on to California to its new owner! Beolovely!!
Monday, August 24, 2020
Beogram 8000: An Exploration of the Platter Speed Sensor (Includes Oscilloscope Traces for the Most Important Signals)
I recently received a Beogram 8000 as a parts unit for repairing another Beogram 8000. I wanted to exchange the circuit boards in the hope to alleviate a strange issue that I sadly was not able to figure out. So I rebuilt the boards of the donor 8000 and then plugged them into the other unit.
And then my fun exploration of the platter speed sensor began...;-). Here is what happened: I plugged the Beogram in, and pressed Play. Immediately the platter ramped up madly to about 100 RPM and the "33.33" never appeared (it shows that a stable speed has been reached by the system). The display stayed at "33".
Head scratching etc...ensued. The first thing I did was plug the board into the other Beogram to see if its platter would also go out of control. It did not. So what gave?
Well, it turned out that some of the Beogram 8000 with old style boards (i.e. with the two piggybacked small boards on the upper end of the main board) did already come with the speed sensor used in the later Beogram 8002. This are the two circuit snippets in comparison:
First the older version:
Note R46 and R47. They are on the main board in this circuit. Now look at the corresponding Beogram 8002 circuit:
Here, the resistors are on a small board attached to the sensor located under the sub platter. We also see that the value of R46/R2 changed and that R47/R3 carries an "X" now, indicative for the somewhat "semi-exploratory beta release approach" applied for these boards (I do not know of any consumer product that has so many different versions and slight undocumented changes than these B&O circuits). I think the X is a sign that they used different IR LEDs and photosensors during the production run of these turntables.
Anyway, lets have a look at the two sensors as they are mounted in the Beogram:
Again, first the old style setup: Front
And now the newer type with the added circuit board:
You can see the two resistors on the front of the board. The upper one is for the IR LED and the lower one for the sensor:
This is how it looks from the back:
Due to this difference the main board needed to be modified for the new sensor version. This shows the relevant square inch of the board configured for the old style sensor:
Note that R46 (big resistor that is mounted horizontally) is present and also R47 (red red orange 22k on the left, vertical). Now let's have a look at the board configured for the new sensor style:
R47 is simply missing, and R46 is replaced with a jumper wire!
And there lies the danger when replacing boards without checking what sensor type is installed:
When an old style sensor setup is plugged into a board configured for the new sensor like it happened to me, the LED is directly connected to the +15V rail via the jumper wire, which will probably cause it to be super bright for about 1 microsecond and then dark forever. Well, some people say it is better to shine brightly for a brief moment than to be a dull boring light for a long time, but in this case I get to figure out what modern LED can replace the original LEDs...;-).
I suspect the OP240 may be a good starting point since it can be used to replace IR diodes in other spots in this vintage of B&O. We will see...probably another post coming up sometime in the near future...;-).
Anyway, I ended up transplanting the new style sensor along with the newer boards and that yielded a working setup (again). For the sake of documentation I measured the relevant signals in the speed sensor setup:
This is the signal right at the sensor (P4-pin7):
This signal is cleaned up to a square by the opamp IC1 which is configured as a 5V comparator:
The above trace was measured at pin 14 of the opamp. This signal is subsequently divided down (R51/R50) that the micro controller can digest it, yielding this signal at P6-pin 2:
And that is it about the speed sensor of the Beogram 8000!