Beogram 4002 (5503): Unit Switches Immediately to 45 RPM After Pressing Start

The Beogram 4002 (5503) that I am currently restoring exhibited a strange phenomenon when I tested it the first time after rebuilding all its functional parts (more about this process in a subsequent post):

After pressing START the carriage began to move while it immediately switched to 45 RPM. Pressing the 33 key switched it back to 33 RPM, but only as long as the key was pressed.

This immediately suggested that either TR3 or TR2 had an issue. Let's have a look at the relevant section of the circuit diagram:

This is how the speed selection works:

The base of TR3 is connected to the collector of TR4, and the base of TR4 is connected to the collector of TR3. Resistors 5R1 and 5R2 act as pull-up resistors together with the light bulbs IL1/2. I guess the resistors were added to keep the deck working even if a bulb burned out.

This causes a flip-flop behavior: When the base of TR3 is shorted to ground via the 45 switch in the keypad or TR2 (that is activated by the carriage as it travels towards the set-down point for singles), TR3 is turned off for a moment, causing its collector to go high, which in turn pulls the base of TR4 up, and that drives the collector of TR4 to GND since TR4 turns on. This then keeps the base of TR3 permanently grounded and the deck is running at 45 RPM (via switching TR6 on which changes the oscillation frequency of the Wien Oscillator to drive the AC motor at a higher RPM).

The opposite happens when the 33 key is pressed. This causes TR4 to be off and TR3 on, and so TR5 is turned on running the oscillator at a lower frequency.

I removed TR3 and checked it with my transistor tester:

Apparently it had turned into a voltage divider!...;-).

I replaced TR3 with a generic BC547B and that fixed the issue.

Beomaster 8000: Input Selector Trouble - Tuner Audio Mixed with Phono or TP1 Inputs (Pt. 2 - Conclusion)

I thought this was fixed! But it wasn't! Last year I reported about a strange intermittent issue with a Beomaster 8000 that I was restoring: When switching the input to Phono it sometimes mixed in the tuner signal that both turntable signal and tuner signal were audible at the same time...a pretty useless situation. My investigation back then determined that there was a discontinuity around P82 on the uProcessor board, which occasionally caused the input switches on the preamp board to malfunction. P82 carries the control signals for the two electronic multiplexers that control which input is routed into the preamp-circuit of the Beomaster. I implemented a bunch of jumpers, shown here:

and the problem appeared to have gone away.

As it turned out recently, this was an illusion as the Beomaster again showed this issue when I tried to play a lovely 1950s vinyl by Cal Tjader of which I bought a significant number recently. 

My initial thought was that maybe the ribbon cable that connects the input selector pins of the uProcessor to the preamp board had an issue, and so I ordered a custom manufactured 5-lead ribbon with appropriate 0.1" headers and put it in. Here is an impression of the end plugged into the uProcessor board:

And a photo of the preamp end:

While this new cable made a great connection and looked very pretty, it did not cure the problem. But at least another potential root cause was ruled out.

Back to the bench it went. I opened it up (again) and wiggled P82, and there it was (again)...depending on the pressure on the brand new connector the input behaved correctly or malfunctioned. I took the board out and had a closer look. All the jumpers were in good shape, so the problem had to lie somewhere else. However, all the traces checked out o.k. (again) and so I was a bit at a loss.

I put the board back in and played the Beomaster for a while scratching my head. Then it occurred to me that such problems are often related to ground connection issues. I prepared a jumper cable from the preamp board to the uProcessor board. Then I induced the issue and subsequently made a separate ground connection between the boards with the jumper. And, eureka, the input out of a sudden behaved normally. With the jumper in place I was not able to produce the issue again. I took the board out again and inspected it more. And I finally understood how the grounding plane is setup on the uProcessor board. I realized that this board is actually a double sided PCB, even though it has no etched traces on the back side. However, they used the continuous Cu layer as a ground plane from which several connections are made through the board to the traces. This is hard to see since the surface just looks a uniform green and there is no obvious mention in the manual where they show the PCB layout. After this it became clear to me how the ground connection to the preamp board was made. This shows the pertinent part of the board:

C67 connects through to the patterned side of the board and so makes the ground connection that connects to pin 2 of P82. The picture shown on top shows the backside of this area. I had soldered a jumper in from the C67 leg to Pin 2 of the header to bridge potential cracks in the trace connecting the two. What I did not consider was that the problem was rather caused by a 'cold' solder joint on the other side causing an intermittent contact. I resoldered the area and then the problem went away. Another demonstration that most control issues in the 8000 are caused by problematic solder joints on the uProcessor board.

Back to testing it for a bit longer, but I think it may finally be ready for its journey to its new owner in the UK.

Beomaster 8000: Input Selector Trouble - Tuner Audio Mixed with Phono or TP1 Inputs

*************************Achtung: There is a follow up to this post. **********************

Oh well...I ran the Beomaster 8000 that I finished up recently for some time in my Beosystem 8000 setup and all was well, but then it developed a strange issue: When switching from FM tuner to Phono the phono signal was audible, but the tuner signal was still there in full strength. So instead of switching from one input source to another the Beomaster decided to mix them together instead! So back to the bench it went!

After I opened it up I discovered that I could provoke the issue by wiggling P82 on the uprocessor board. P82 transmits the microcontroller's wishes to the input switches on the preamp board. So this made immediately sense: The FM input did not get turned off when opening up the Tape 1 or Phono inputs due to an intermittent connection (the audio input switches on the preamp board are basically gates that open and close depending on high/low control signals from the processor).

First I thought it was a plug issue, and I cleaned the contacts a second time and bent the header bins a bit to give everything a bit more mechanical intimacy, but the problem persisted. So I came to the conclusion that the issue must be something more serious, like cracked traces or bad solder joints. This photo shows the original condition of the P82 header from the solder side:

Close optical inspection did not reveal anything abnormal, but I was able to demonstrate that at least Pin 1 had an issue since I was able to 'disconnect' it from the component it is connected with by flexing the board a bit. So I put in a magnet wire jumper and put the board back in. After firing the Beomaster up, well, no change...o.k. I took the board back out and this time I jumpered all five solder points to their next neighbors:

Magnet wire is really very helpful for this type of fix, since the polyurethane coating burns off when heated with the solder iron. This allows to make safe wire connections in tight spaces.

After this fix, the problem seemed to be gone. So I put the Beomaster back together...let's see if it is stable now! I usually test these babies for a few weeks until I send them back to their owners. There are just too many components and interconnections that can develop problems, and often they are intermittent. The only way to make reasonably sure that the experience on the customer end is a happy one is to give them a good testing under normal operation conditions. But I sure hope that this was the last post about this particular Beomaster 8000...;-)!

Beogram 4002: Replacing the RPM Indicator Bulbs with SMD LEDs

I continued working my way through replacing the lightbulbs in the Beogram 4002 (5513) that I am restoring right now. Today was RPM indicator day. I took this as an opportunity to further improve my LED replacement approach. In an earlier post I described how to replace the incandescent bulbs with the red and green LEDs of a RGB through hole 5 mm LED. This worked fairly well, but I was not 100% happy with the homogeneity of the back illumination of the RPM trimmer scales. The issue with standard through hole LEDs is their directed emission pattern out of the top of the LEDs. This made it necessary to sand their bulbs into a matte surface and also painting the tops black to prevent a too high emission on one side of the trimmer scales. 

My latest approach is based on my current approach to implanting LEDs for the position scale illumination of Beogram 4000s. I changed the PCB shape to fit behind the bulb covers of the 4002 bulb housings, and I adapted the current limiting resistors (10k for green and 2.5k for red) to the 21V operating voltage and the different illumination angle of the diffusing cavity. I made a short video about this updated approach:

Beomaster 8000: Inputs Not Working

A Beomaster 8000 that I rebuilt last year arrived a few days ago for trouble shooting. The owner told me that he suddenly lost all the inputs. No matter what input button was pressed nothing was audible, while the display would show the right input etc...This indicated to me that the uProcessor was working properly. So I opened it up with the plan to look into the chain of command between the uProcessor and the input selector chips. Since both channels of any input did not work, I hoped I could rule out a failure of the input selector chips. 

Once I had the unit open, I turned it on. Everything worked. That is of course frustrating if the problem is intermittent. I started methodically wiggling the connections between the uProcessor and the input chips. Nothing came up, everything was stable. 

Upon further reflection (while listening to some nice NPR jazz on the frustratingly working Beomaster) I determined that an interruption of the power supply of the preamp board would have a similar effect. So I checked the connections between the power supply board and the preamp. And indeed, when wiggling P46 that carries the power rails to the preamp board, I was able to get the inputs to turn off occasionally. So there was a bad contact somewhere. Here is a picture of the area:

The colorfully wired vertical plug is the one in question. This area is also prone to cracked traces on the circuit board due to the multitude of connectors that torque the board in that corner. A bit of a design weakness.

I took the board out and inspected for cracks and the like, but was not able to find anything obvious. So I resoldered all the headers and cleaned them and put the board back in hoping that a hairline crack in one of the header pin solder points was the root cause. After reconnecting all plugs, I fired the Beomaster up and wiggled the plugs in that area again, and nothing happened anymore. Seems everything is stable now. So it seems this issue may be fixed...but of course with intermittent stuff, one never really knows until one knows.

Beogram 400x: Replacing the Tracking Sensor Light Bulb with a Drop-In Ready LED Assembly

I used my current Beogram 4002 (5513) restoration project as a chance to redesign my earlier approach to replacing the incandescent light bulb in the linear tracking sensor with an LED. In my earlier approach (see here), I reused the bulb housing and just replaced the light bulb part with a 3D printed LED assembly that I had designed as an early (and later abandoned) iteration of my Beomaster 6000 4-Channel incandescent bulb replacement method. While this worked well for the Beogram, it had two significant issues: (1) it was not a drop-in ready solution and required a lot of detail-Dremeling of the original housing to get the glued-in bulb housing separated and replaced, and (2 - probably more important) that the LED position was not very well defined and also not impact/vibration stable. No(2) is clearly an issue when shipping the Beogram, i.e. not suitable for a professional restoration.

So I set out to design a part that is drop-in, 100% mechanically stable, and that is precise enough to only require a small adjustment of the tracking mechanism, which can be done by just turning the executer screw (larger adjustments require a cumbersome realignment of the sensor aperture). The adjustment process is shown in the video below.

Here is a picture of the original bulb housing:

It is interesting to note that the bulb is slightly misaligned, i.e. oriented to one side. I suspect that this reflects 1970's production tolerances, and that most tracking sensor bulb housings will have bulbs at various locations. The obvious consequence is that the installation of a bulb replacement will require the adjustment of the tracking mechanism after installation of the replacement part, independent of the part being an original-style incandescent bulb or a LED replacement.

I designed a part resembling the lower part of the original bulb housing, which is responsible for keeping stray light out of the tracking sensor. Then I combined this part with a custom designed printed circuit board that is responsible for operating the LED. 

Here is a picture of the two parts before assembly. The LED is a standard white 0603 SMD LED run with a 2.5 kOhm resistance that it can take the 21V rail of the Beogram. The LED, resistor and the solder points of the leads fit into appropriate cutouts of the housing part:

Here is a picture of the running LED after epoxying the circuit board to the housing. The LED is precisely situated in the middle, i.e. very close to the bulb filament position of the original part:

I updated my earlier video about the tracking sensor light bulb replacement with the new part. It goes through all the steps of replacing the original bulb housing and discusses the tracking sensor mechanism. It also shows how a misaligned sensor behaves, and how to adjust it properly:

Beogram 4002 (5513): Replacing the Speed Indictor Light Bulbs with RGB LEDs

While I am waiting for a 3D print of the redesigned LED light assembly for the tracking sensor of the current Beogram 4002 (5513) I rebuilt the speed indicator scale lights with RGB LEDs. I essentially followed the approach demonstrated in my recent video:

Here are a couple pictures of the actually rebuilt unit:

Original set-up with incandescent bulbs (covers taken off):

And after replacement. There are two resistors since I am using the red and green LEDs of a RGB LED to get a naturally appearing light reflection on the red scale markers. Amber LEDs make an incandescent-like glow, but they do not contain red light, i.e. red scale markers show up gray and even invisible in absence of natural room light - this is discussed in more detail in the above video):

As usual, I further refined my approach a bit: One of the issues with LED replacements of incandescent scale lights is their considerably more narrow radiance pattern. Light bulbs emit fairly uniformly in all directions, while LEDs usually have a narrow emission angle. This is usually combatted by sanding the LED bulb, but some of the directional pattern remains, even after this procedure. In the above video I used tape to shield one end of the scales from too much light, which allowed me to achieve a mostly identical scale appearance as with the original light bulbs. And this worked fine for my personal Beogram 4002, but for a professional 'commercial' restoration I felt this is not a permanent solution, since tape ages and the adhesive can loose its force etc...Since my main interest is to make these beautiful turntables 'future proof' (Imagine Charlton Heston digging it out next to the Statue of Liberty on the beach, cleaning it and plugging it in and the scale lights work...;-),

I needed a more permanent solution. So this time I decided to carefully apply some black acrylic paint to the end of the bulbs and some to the exposed side. Here is a picture of my efforts:

After a few trials slowly adding more painted areas I ended up with this degree of coverage. It is important to also cover a bit of the side wall of the bulb cylinder to absorb some of the photons in that direction to get an even scale illumination.

Beogram 4002/4004: Speed Selector Circuit and Replacement of Its Incandescent Light Bulbs with LEDs

The last incandescent light bulbs are now gone in my Beogram 4002. I decided to completely replace all light bulbs since I like the longterm stability of LEDs. Furthermore, with red-green LEDs one can achieve a very authentic incandescent feel (much better than with single-color yellow or amber LEDs). I made a video about the process. The video also explores the speed selector circuit and it shows the relevant circuit diagram sections. Here it is:

Here are a few pictures of the LEDs after installation, and the look of the scales with LED illumination (for comparison - the insert in the video title shows an amber LED, which did not produce a satisfactory outcome - more in the video):