Beomaster 8000: Step One - Rebuilding the Output Amplifiers

While we wait for Nick's awesome specially made pulley for the Beogram 4002 (5501) that I just finished up, it is time to get started on the Beomaster 8000 that will also go on to the UK once restored. It recently arrived and I gave it an external inspection, which suggested this 8000 is an excellent starting point for a full restoration.

My first step in any Beomaster 8000 project is to rebuild the output amplifiers. They are the most crucial part of the restoration of the 'power' part of the unit. A failure of their often corroded quiescent current trimmers usually neatly kills all output transistors with a bit of smoke emission on that channel before the main fuse protects the transformer (and the breaker of the house grid on which the 8000 resides while this burnout happens). These old trimmers often go open circuit during transport, so my approach is to not even turn the unit on when I receive it, but I go straight to the amplifiers and rebuild and test them to make sure that the above does not happen.

Here are a few impressions from this effort on this unit:

This shows the right channel as it came:

We see from the two clunky (white) emitter resistors that the above mentioned disaster already must have happened at some point. When the transistors burn out, the emitter resistors usually brown like a chicken in the oven due to the immediate heat emission. But usually this does not affect their performance down the road since they are wire wound types. An ugly replacement like seen here is definitely not an improvement in the Beolover's eyes! Also, whoever did this did not learn 'The Lesson'...he did not replace the trimmer! I always put in 25 turn precision encapsulated units to prevent this from ever happening (again). Multi-turn trimmers drift only very little and so the quiescent current adjustment is very stable over time. This is not the case for standard single turn trimmers, and that is one of the reasons that the 8000 often presents with one or two hot heat sinks, if it runs at all.

This shows the rebuilt board with new electrolytic 105C type capacitors and prettier resistors and the 25-turn trimmers:

Beautiful! I did the same to the left channel, which did not have new emitter resistors, but (slightly browned) original resistors. And then it was time to run these babies from external power supplies to test for any silicon failures, cracked traces and the like. This shows my hookup:

And here with the multimeter connected to the test points at the emitter resistors after powering the board up:

When installing a new trimmer make sure that the resistance is close to zero. This may be counterintuitive, but in this setup it turns off the output transistors preventing significant current flow between the +/- power rails. This shows my bench supplies at this point:

The left two are the - and + rails, which draw 60-70 mA while the quiescent current trimmer is close to zero Ohms. The right supply provides the 15V control voltage that is used by the Beomaster to enable the output by controlling the constant current source used for biasing the output transistors.

Adjusting the quiescent current means to ramp up the resistance in the potentiometer to set the working point of the output transistors that there is a 18mV voltage across the two emitter resistors:

At that point the power rails draw 0.1 (+) and 0.11(-) A:

I did the same for the right channel:

All good now in the output amplifier department! On to rebuilding the power supply board before giving this unit a first spin!

Beogram 4000/4002/4004 Relays: Professionally Manufactured PCBs!

I recently redesigned the Beogram 4000/4002/4004 relay adapter boards to be manufacturable by a professional PCB supplier. I started filling orders with the new boards. Today I made a bunch and I thought they look quite Beolovely!

Here is an impression:

They are available for both the old-style Siemens and the newer vintage National relays that are found as RPM and/or signal path grounding relays in the various 400x models. The boards simply drop into the footprints of the original relays. The relay featured on these broads is a modern encapsulated relay made by Omron, a major Japanese manufacturer. They are rated for a minimum performance of mechanical 50,000,000 and 100,000 electrical operations under load (1 Amp). Since the current switched in the Beogram applications is minuscule, they will last probably close to the mechanical performance limit, meaning that they will probably outlast the rest of the turntable by far. 

These blog posts give an impression about their installation in various Beograms:

Beogram 4002 (5521): Replacing the Electrolytic Capacitors, New Relays and New RPM Trimmers

Beogram 4000: Replacement of a Stuck RPM Relay with a Modern Encapsulated SMD Relay

Beogram 4002 (5513): Further Improvement of the RPM Stability Through Replacement of the RPM Trimmers and the RPM Relay

There is also a video about replacing the RPM relay in a Beogram 4000:

Beogram 4002 (5521): Replacing the Electrolytic Capacitors, New Relays and New RPM Trimmers

It was time to replace the electrolytic capacitors of the Beogram 4002 that is currently on my bench. These units have many tantalum capacitors, and it is a good idea to replace them at this age. They can die a fiery death due to the highly oxidizable nature of the Ta electrodes in them. Here is a picture of the original board. All the blue and red 'dots' are Ta capacitors:

Another standard item of my restorations is to replace the trimmers and the relay in the DC motor RPM control network. Oxidized trimmers can cause drift of the platter speed depending on the temperature of the deck (and the moon phase...;-), while the relay is not encapsulated and hence prone to oxidized contact issues after almost 40 years of service.

I recently developed a replacement relay assembly for the Beogram 4000 and early 4002 models based on a modern encapsulated SMD signal relay. However, this Beogram is a later model, which was already fitted with a more modern relay made by National. This relay is still available as new original stock (NOS) via vendors on AliExpress and similar Asian outlets. However, modern relays are much superior in their design due to modern encapsulation technology and the promise to last for many decades. It is good to keep in mind here that NOS parts are still 30-40 years old and old technology, they just were not used. But oxygen hat its go on them, and so we can expect that the contact tabs are almost as oxidized as in the relay installed in this Beogram 4002!

Hence I modified my design to fit into the later models, too. Here is a picture of the revision of the drop-in replacement relay assembly:

This picture shows the 25 turn 5k trimmers prepared for installation:

This is a picture of the main PCB after replacing everything:

A detail of the relay and RPM trimmers together with the original relay:

This shows the trimmers from the top side of the PCB:

It is important that they are accessible from the top, so one can trim the RPM while the deck is running without having to take out the main PCB.

When replacing the capacitors, of course the main reservoir capacitor of the power supply also needs to be replaced. This shows the original can:

And here the replacement:

I designed a 3D printed adapter enabling me to clamp a modern 4700uF capacitor in the space of the original unit.

After the main PCB I did PCB #8 that sits underneath the keyboard, and which contains the output relay. Here is a shot of the original unit:

I usually install a new relay, replace the time constant capacitor for the relay, and also install a grounding switch that allows to tie system and signal grounds together if necessary to squash humming issues. Here is a picture of everything installed:

After these replacements I put everything back together and then I tested the unit...all went well. This concludes the electronic rebuild. On to the adjustments and alignment of the platter, chassis and the arms!

Beogram 4000: Redesigned Drop-In Tracking Sensor Light Bulb Replacement

It was time to replace the tracking sensor light bulb in the Beogram 4000 that I am restoring right now with the SMD LED based replacement light source that recently I developed. I used this occasion for redesigning the part with the aim to incorporate the intensity adjustment trimmer into the light shield of the source to hold it firmly in place. I did not like that the trimmer was only held in place by the leads in my previous design, which resulted in a slightly wobbly appearance when adjusting it. In other words, this part was not yet fully up to Beolover standards!

So I designed a new circuit board which allows to directly solder the trimmer to it. This, however, required the concurrent redesign of the 3D printed housing part to enable the trimmer to protrude from the PCB. Here are some pictures. This shows the original light source.

This Beogram had the optional 100R trimmer installed that is mentioned in the supplement of the service manual. This allows the adjustment of the intensity of the light bulb which is a great feature that much simplifies the adjustment of the tracking feedback circuit sensitivity. 

This shows my redesigned part from the light source side. The LED is powered by 6V as it would in the Beogram 4000:

This shows the part from the other side prepared for insertion into the tracking sensor

The potentiometer protrudes through a opening in the plastic part and looks now fully integrated. Lovely! The negative lead is curled around one of the bolt holes. This makes the ground contact (the Beogram chassis is tied to the system ground). This setup essentially copies the original setup with the 100R potentiometer. The other lead is bent up to be able to go into the 6V rail solder point on the PCB from below:

After the installation I adjusted the tracking sensitivity to the prescribed 4 rounds of the record before the tracking mechanism kicks in after the needle is put into the groove. A few months ago I made a video about the tracking mechanism and the adjustment process:

Beogram 4000: Replacing the Keypad Light Bulbs with LEDs

The next step of my restoration of the Beogram 4000 that is on my bench right now was to replace the light bulbs in the control panel assembly with LEDs. I like to do this to increase the longevity of the unit and to guard against shipping issues. Incandescent light bulbs do not like vibration...Here is a picture of the opened up keypad:

The two bulbs at the top end are the bulbs that illuminate the unique position indicator feature of the Beogram. The vertically implemented bulbs below illuminate the RPM trimmers. I recently developed SMD LED based PCBs that replace the two position indicator bulbs (these parts are available - just send me an email). This shows the boards before installation:

The RPM trimmer bulbs can be replaced with standard red LEDs and current limiting resistors. Here is a picture of the implanted LEDs:

And after starting up the Beogram:

The position indicator LEDs are red-green LEDs tuned to appear with an incandescent sheen. This ensures that there are red photons to properly illuminate the red scale indicator.

This shows the LEDs at work after closing the panel back up. The red indicator is missing, since I did not reinstall the panel since I still need to restore proper functionality of the keypad switches. The unit has some issues with the slow scanning functions, which is an indication that the switches are corroded.

Very beautiful! What a nice design.

While I was in there I also glued the strobe light mirror back on (it often does off at this age):

On to the keypad switches!

 

Beomaster 8000: Recapping the Tuner Boards

The final boards to be recapped in the Beomaster 8000 that I am currently restoring were the FM tuner boards. Here are some impressions of this effort. This shows the boards with the original capacitors:

And here after replacing all electrolytic capacitors with 105C grade Japanese units:

With these boards it is especially important to clean the contacts between the boards and coat them with DeoxIT. I had several cases in the past where one channel did not work on the tuner, which could directly be traced to a contact issue between these two boards. After I put everything back together I tested the Beomaster and everything works fine now!

This concludes the recapping of this Beomaster. On to the displays and the uProcessor boards. On into the heart of the dragon...;-)

Beomaster 8000: New Electrolytic Capacitors for the Control Panel and the Preamplifier

I made some progress with the recap of the Beomaster 8000 that I am working on right now. Today I did the control panel and the preamplifier boards. The control panel pcb is bolted to the control panel fixture, and one needs to unsolder the leads to the signal strength meter. After that is done the PCB can be extracted:

Here is a shot of the board after replacing all electrolytic capacitors with 105C grade Japanese units:

As usual, I also reflowed all solder points on the wire-to-board connectors and coated the pins with DeoxIT D100 for longterm stability.

After I put the control panel back together I moved on to the preamplifier board. It seems one 10uF capacitor had already been exchanged at some point. These red electrolytic cans are often problematic at this age.

I replaced all of them with new ones and also did all the connectors:

Here a detail shot:

On to the tuner boards, which will conclude the recap of this Beomaster 8000!

Beomaster 8000: Rebuilding the Power Supply Board

After replacing the original main reservoir capacitors of the Beomaster 8000 that I am restoring right now it was time to do the remaining power supply. The golden reservoir capacitors often fail at this age. Another often found dead capacitor is the 5V rail cap, since it is directly mounted on the 5V regulator (on the heat sink on the right side of the Beomaster enclosure), and therefore sees elevated temperatures. Here is a photo of the original power supply board:

and after replacing all the electrolytic capacitors with quality 105C grade Japanese units:

Before putting the assembly back into the Beomaster I also cleaned and reflowed all headers, and coated the pins with DeoxIT D100 for lasting oxidation protection. On to rebuilding the remaining boards...