Beomaster 1900 (Type 2903): Testing The New Bass, Treble and Balance Indicator Masks

The original display indicator masks for the bass, treble and balance settings on this Beomaster 1900 have deteriorated from long term exposure to the incandescent lamps. I measured up to 118°F on the tone control lamp lenses.

Since the bass, treble and balance lamps display their settings mechanically, by sliding the indicator mask across a fixed lamp source, these are a good case for switching from the hot, incandescent lamps to cooler operating LEDs.

Here is picture of the original incandescent lamps in their bare, illuminated state.

A design goal for a replacement lamp is to produce a reversible substitute part. The LED version of the lamp should just solder on to the same two board pads that the original lamps use. No modifications to the board should be necessary.
Each of the original lamps has a 68Ω current limiting resistor in series with it. For my replacement I decided to go with two LED devices and a load resistor to limit the current further and to control the illumination of the LEDs.

On my test circuit I used a 68Ω resistor (for the existing resistor in the Beomaster), two white LEDs and a 5KΩ trimmer resistor to come up with a good load resistor value. I ended up with a 1KΩ load resistor a and the results are a nice substitute for the original bulb. The illumination of the red indicator lens is nice and uniform.

My final replacement version of the LED assembly looks like this. I put clear heat shrink over the load resistor and color coded heat shrink to mark polarity of the LED (red for positive, black for negative).

Now for a quick test.

The test passes and showed me that I needed to bend the LED leads a little different in order to fit the lens cover assembly for the lamps.  Here is the board with all three replacement lamps in place.

The lens cover assembly fits back on and the new bass, treble and balance indicators are restored.
Before showing the new LED replacement lamps in action here is what the new indicator masks look like.

Here are the new bass, treble and balance indicators with the Beomaster 1900 turned on and playing.

The temperature on the indicator mask is much better now. These newly printed masks should last a long time.

Of course I will have to pull those three slider potentiometers again once Martin's new contact housings arrive. In the meantime I will put this Beomaster through a burn-in test. I will let the local classical radio station play for a few hours.

Beogram 4004 (5526): Final Adjustments and Test Drive with Freddie Hubbard's High Energy!

After restoring the work keypad of the Beogram 4004 (5526) that I am restoring right now, it was time to do the final adjustments and then give it a spin. As usual I started out by adjusting the subclasses and the platter to be accurately positioned vertically and horizontally within the enclosure The platter needs to be flush with the surrounding aluminum panels, the arms horizontal in the right distance from the platter, while the sub-chassis needs to be centered between the transport lock brackets. Not an easy feat considering the many variables that go into solving this 'equation'. After that was done, it was time to adjust the tracking sensor:

The picture shows the adjustment of the light intensity of the Beolover tracking sensor light source that I implemented earlier. A convenient 'upgrade' that makes it much easier to adjust the tracking feedback precisely. After establishing proper tracking, I went on and adjusted the arm lowering limit:

When the arm is lowered without a record present, the needle should miss the lower parts of the black ribs on the platter by about a mm. This is a safety feature to protect the needle in case the record detection mechanism is malfunctioning.

After that was done, I did the final adjustment, which is the tracking weight. The first step is to upgrade the standard B&O design, which uses a locking washer to hold the arm counter weight in place:

This design may be sufficient if the deck is adjusted after setting it up in your living room, but any weight calibration done with it will not survive the rigors of shipping. Therefore, I usually replace the washer with a M3 nut and a standard washer:

This allows to secure the counterweight in place once everything has been calibrated.

The final step was to adjust the tracking weight with a digital scale:

And then it was finally time to give this lovely Beogram 4004 a spin. I selected a recent acquisition to my rapidly expanding vinyl collection, Freddie Hubbard's High Energy. I bought it via Discogs in near mint condition. After ultrasonically cleaning it with my CleanerVinyl Pro, it sounded like new. This album is usually underrated by the jazz reviewer set, who think that smooth sounding jazz is something that is inferior. All I know is that after I bought it it became rapidly one of my favorite Freddie Hubbard albums...I guess music critics are a bit like oneologists: It is helpful for their resumees if they have strong public opinions...;-). I generally follow my own advice when it comes to music and so here we go:

What a lovely sight and sound! Beolovely!

Beogram 4004 (5526): Restoration of the Keypad

The Beogram 4004 (5526) that I am restoring ring now had the usual wear pattern of the keypad. The coating gets worn down over time and one starts to see which keys were used most (you guessed it: START is among them...;-). This shows the keypad as I received it:

After recoating it looked like this:

Beolovely! On to final adjustments and then some testing!

Beomaster 1900 (Type 2903): Replacing The Bass, Treble and Balance Slider Potentiometers

Now that the Beomaster can power back up I performed the 15 volt supply check and adjusted the No-Signal Current setting for the left and right channels.

Since the left and right channel No-Load Current adjustment went smoothly I expected the Beomaster to be able to finally play music again. I used an ipod Nano as the source to the Beomaster Tape input and a pair of Beovox S55 speakers for the outputs. To my surprise I had no sound in the left channel. After some quick debugging I found that the Nano source signals were present in the Beomaster up to the balance slider. It is common for the bass, treble and balance slider potentiometers to develop problems in these receivers.

Checking the balance slider I found that it was the culprit.

Note: The tabs shown by the red arrows below have to be bent straight in order to disassembly the potentiometer.

I thought about doing a creative repair with some glue but figured there must be a better solution.
As it turns out, Martin at Beoworld makes a replacement part for the black slider housing that moves to change the output. That black slider housing is the piece with the broken tabs the potentiometer contacts are secured with. I ordered a set of three replacement parts as all three slider potentiometers (bass, treble and balance) have a broken contact mounting tab.

Since it will take at least a week for the parts to come from Denmark I wanted to keep going with the Beomaster restoration. I pulled the bass, treble and balance slider potentiometers from a Beomaster 2400. The Beomaster 2400 turned out to have nice, intact slider contact assemblies so they will do nicely and I have new assemblies on order. In the exchange of parts I will only take the contact assembly as the Beomaster 2400 Bass slider potentiometer is 20KΩ and the Beomaster 1900 Bass slider potentiometer is 50KΩ.

I cleaned the potentiometer contact strips and contacts with a fiberglass brush and some Deoxit Fader Lube.

With the Beomaster 1900 slider pots cleaned and rebuilt it was time to test the Beomaster 1900 for sound again.

Sly's voice came out beautifully on both Beovox S55 speakers. All of the slider potentiometers really needed that restoration.

Beogram 4002 DC Motor Restoration - Oil Infusion of Bearings and New Spark Snubbers

I recently received a DC motor extracted from a Beogram 4002 located in Arlington, VA for restoration:

As usual, the first step was disassembly to extract the bearings for oil infusion:

The bearings are the two small donuts on the black pad up front. I immersed them in motor oil and pulled a vacuum. Immediately the typical bubbling of air from the porous bearing material started:

After about 24 hrs the bubbling came to an end indicating the bearings were full with oil again, and it was time to reassemble the motor. I extracted the bearings and let the excess oil be absorbed into a paper towel:

I used my 3D printed tool for inserting the top bearing:

and put in the bottom one, too:

After installing the rotor it was time to test the motor with my BeoloverRPM device, which allows the logging of the RPM over extended periods of time. 

The blue graph shows the result after the oil infusion:

Far from being stable, this curve indicated that the spark snubbers had a problem and that at least one of them was intermittently short circuiting. I extracted the rotor again

and removed the three spark snubbers:

This picture shows the original snubber 'ring' together with the prepared modern SMD units. I soldered the SMDs in between the coil terminals, paying attention that they did not protrude into the commutator region (otherwise they can interfere with the brushes):

Then I put the motor back together and ran another 24 hrs test. The result of this measurement is the red curve in the above graph. Much better! This motor seems to be working again! Time to send it back to Virginia!

Beomaster 1900 (Type 2903): Trying Out The Recapped Beomaster

I partially reassembled the Beomaster 1900 so I can check out my recap work and begin the amplifier adjustments. It is always an anxious moment when you first re-apply power having completely disassembled a unit.

This Beomaster powered right up to Standby mode and touching the FM5 terminal post put the receiver right into that mode (no FM antenna is connected yet). A great feeling for sure.

To get from the disassembled recapped boards to this first power on test I prepared the PCB 2 board for the No-Signal Current adjustment.

That adjustment will require measuring the voltage across the left and right channel emitter resistors while adjusting trimmers R259 and R359. I decided to install new trimmer resistors for those two devices. I didn't want to discover those old trimmers are a problem and have to remove PCB 2 again.

Here are the new adjustment trimmers installed.

The next step was to re-attach the four heat sinks for 2IC200/300 (TIP 141 Darlington pairs) and 2IC201/301 (TIP 146 Darlington pairs). The original mounting used thermal paste/adhesive to mount and insulate the transistors to the heat sink. For the re-assembly I decided to use a Thermalloy thermal interface product. It doesn't dry out like the paste and is less messy.

Tomorrow I will test the power supply voltages and do the No-Signal Current adjustments. Then I will see if I get output from the amplifier section.