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...;-)!

Beomaster 8000: Repair of a Damaged In DIN7 Phono Jack That Had Lost All Four Panel Mount Tabs

One of the weak spots in Beomaster 8000s are the phono jacks in the input box under the control panel. The plastic of these jacks becomes brittle over the years, and all it takes to break out the plastic tabs that hold the jack above the input box panel is a firm push to get the DIN7 plug from a Beogram into the jack. And then the jack is inside the connector box, and the cable probably not even connected. And this is how this sorry outcome looks like:

Usually, only the top (outside) tabs break off, and there is a fix for this with two 3D printed inserts that replace these tabs. A detailed report on this repair and a video is posted here. This shows the tabs that I installed in this Beomaster:

These tab replacements are available to other enthusiasts via the Beolover Shapeways store. By the way, they also repair the jack on Beocord 8000 and 9000 units. They are also often broken out.

Unfortunately, this Beomaster's DIN7 jack posed a new challenge: It's inside tabs that hold it below the panel (so one can pull the plug without dislodging the jack) were also gone.

Since the original jacks are impossible to find these days, and even replacements from donor units are very hard to come by, I had no other recourse than trying fixing this with yet more 3D printed parts!

But first I needed to repair several disconnected leads, which probably were a casualty of a previous repair attempt. Several jumpers were broken off:

Luckily, while repairing the contact box of another Beomaster 8000 I made an extensive sketch of all the wire connections in this box, and so I was able to restore the connections without trouble:

After this I designed the replacement tab for the right side of the jack. Since it is pretty much impossible to print flexible precision parts on this size scale, I resorted to a two-pronged approach, where I first designed a static tab without spring force that would be glued onto one of the sides of the jack. This would allow to insert the jack diagonally into the cutout together with the outside tab replacements. Then, once seated, the plan was to glue another static tab into the other side of the jack from the inside of the box.

Alright, that was the plan...and here is the execution:

This shows the right side tab:

It fits into the side 'cavity' of the jack that is shown here:

This shows the part seated (with a dab of epoxy underneath):

I clamped it for an hour until I could be sure that the epoxy was fully hardened:

Then I inserted the jack together with the top tabs:

The next step was to design a tab for the other (left) side of the jack that I could get into the 'cavity' from the inside of the connector box:

I made it considerably longer than the other one. This gave me enough 'meat' to handle it on the inside and guide it into the space between jack and panel. I put some double sided tape on the contact surface and then inserted it into the space:

The part fits very snugly, and my hope is that the double sided tape will hold it in place reliably once pressed in. This will allow extracting the jack again if necessary in the future, in case there are further repairs necessary etc...I always thrive to make my repairs and updates 'reversible' in case a better solution comes along in the future.

The tight fit required me to use tongue and groove pliers adjusted to fit the task:

And this is how it looked after the repair was completed:

Almost like new! Beolovely! I gave this assembly a bunch of plug-in and -outs with a tight fitting DIN7 cable, and it seems to hold up nicely. But I will recommend to my customer to hold the jack down while pulling out the plug. Looks like we are getting close to buttoning this baby up and giving her a first spin!

Beomaster 8000: Step Seven - Replacement of the Speaker Switches with Modern Encapsulated Units and Laser Cut Adapter Plates

The final 'standard restoration task' for a Beomaster 8000 is to replace the speaker switches. My own personal statistics are that in about 50% of all Beomaster 8000s these switches will go bad in the near future if they are used regularly, like for headphone listening etc...They are usually corroded on the inside and frequently when a Beomaster is used again after a long period of storage, they break after a few uses and then the outputs are no longer connected to the speaker jacks. This shows the speaker switches flipped out of their 'compartments' for accessing the solder points:

Note the brown PCB-like base plates that hold the contacts...this switch style has long gone out of fashion in favor of fully encapsulated modern designs that are capable of millions of switchings before they need to be replaced. Unfortunately, these modern switches do not have this base board anymore, and hence they do not fit into the compartments under the heatsink cover. I designed laser-cut adapter plates that mimic the original base plates while snugly fitting the footprint of the modern switches:

This shows them after soldering them in:

And situated in their compartments:

All good now in switch land!

Unfortunately, when I gave this Beomaster a subsequent testing of all functions, I realized that the phono jack in the contact box was broken out and also disconnected. Very beo-unlovely! The next post will report about my repair process of these DIN7 jacks.

Beomaster 8000: Step Six - Rebuilding the uProcessor Board

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ACHTUNG: Please, note that the crystal exchange procedure described here can damage the microprocessor chips on the uProcessor board. It is recommended to remove the chips before replacing the crystals and the capacitors. Due to the inherent capacitance of these devices, high voltages can be present between their terminals, which upon release, can burn out the gate that forms the oscillator together with the crystal. Make sure you short circuit the leads of the parts before installing them.

*********************************************************************************

After completing the display board rebuild with new SMD LEDs in the displays themselves, as well as replacing the incandescent light bulbs in the indicator cabinets with SMD LED based assemblies, it was time to do the uProcessor board.

The uProcessor PCB is the main source of operational trouble due to a few shortcomings in its design. In the 80s double sided PCBs were only beginning being standard in consumer electronics, and the earlier 8000s have boards without through plated vias. This made it necessary to insert metal plugs into the vias and solder them. Many 8000s have startup trouble due to these solder joints cracking. Luckily this Beomaster 8000 already had an updated board where the via plugs were replaced by soldered in colorful wire jumpers. This shows the processors, the jumpers and the crystals:

Another source of trouble are crystals that stop working reliably. This can cause severe malfunctions in the start-up circuit potentially damaging the main transformer. I started replacing them as a standard restoration item since I had a malfunction in one of my own Beomaster 8000s that caused the uninterruptible power supply (UPS) I run this Beomaster on to go in protection mode. This indicated that very high currents were flowing in the main transformer while the malfunction occurred. More here about this phenomenon. When the exact same condition happened in another Beomaster that I restored a few years ago, I took it as a sign that this is an issue that may be prevalent in many 8000s.

So here we go. This shows the old big crystals replaced with smaller modern 2MHz units. 

You can pretty much use any 2MHz crystal, as long as you also install the correct capacitors. The original crystals use 12pF capacitors to resonate, while modern units usually have 18-22pF. If you look closely, you will see that I also exchanged the two small capacitors directly underneath the crystals. They are now 18pF. 

I also replaced the two electrolytic capacitors on the board with high quality 105C grade Japanese units.

Here are some more photos: This is a detail shot of crystal X1

and a picture showing the solder side:

This shows the solder side of X2 for reference:

After this I plugged everything back together, and it was time for a test run! This shows the display board in all its glory:

Beautiful! Very Beolovely! By the way: One can 'provoke' the clipping light (left most) to come on, too, by setting the tuner to a station and then cranking up the volume. Above about 5.0 the clipping light usually comes on (make sure you do not have speakers connected during this test...;-).

On to replacing the speaker switches!

Beomaster 8000: Step Five (2) - Rebuilding the Displays and Indicator Lights with SMD LEDs

This is a continuation of this post about rebuilding the display board of a Beomaster 8000:

After 24 hours the LEDs were all still in business, and so I decided to put the displays back together. My current method is to press the display covers onto the PCBs with carpenter clamps and then use black hot glue to seal the sides of the display units. This keeps them together reliably, while it is still reversible - with some effort the glue can be peeled off to open them again. I like solutions like this since no one (and no electronic component) is 100% perfect, and so it is nice if one can repair things (again) at a later date if necessary.

After putting the displays back together I usually run them for another 24 hours to make sure that the assembly process did not inflict damages on the SMD LEDs:

While this test was running, I did the rest of the display PCB. I replaced the two electrolytic capacitors with 105C grade Japanese units and then I went on to replace the four incandescent light bulbs that illuminate the indicators (clipping, filters, mono, and manual tuning). This shows the right two bulbs in their reflector boxes:

The original bulbs usually have very weak leads since they corrode over time. The slightest motion of the bulbs can break them off. Therefore, I developed a SMD LED based solution to replace them. This shows them in place of the bulbs with the reflector box removed:

There need to be two versions of these SMD LED boards since the reflector cases are mirrored for the left side, i.e. the polarity is reversed (which plays a role for LEDs while bulbs are impervious to the current direction): 

And here with the reflectors put in place:

After the 24 hrs test I soldered the displays back into the PCB:

Before I test this board in situ I will restore the microcontroller board...more about this in the next post of this project.

Beomaster 8000: Step Five - Rebuilding the Displays and Indicator Lights with SMD LEDs

No Beomaster 8000 restoration is complete without rebuilding the displays. They will all fail eventually, and most in the near future. And since those beautiful large LED displays are a main design feature of this lovely receiver, nothing is more frustrating than having a few missing segments. No matter how good the condition of the enclosure, when the displays are not complete it just looks sad! Not beolovely at all!

Luckily the displays are large enough that one can update them with 0603 packaged SMD LEDs, which can still soldered fairly well by hand. For this to happen one needs to extract the displays from the display board:

Once liberated, the next step is to open them up and scrape off the original LEDs, which are directly bonded to the PCB...truly a 1980s design!:

Once the pads are prepared it is time to line up the SMD LEDs and solder them into the places of the original LEDs:

The critical element here is to place the SMD LEDs precisely into the areas that are underneath the light guides in the LED assemblies (white parts with long slots in the picture above) when assembled. If they are not centered, the LEDs will get damaged during re-assembling the displays since the light guides will exert lateral pressure on the LED packages, which can result in them breaking off their pads. I do this by soldering the LEDs on only one side, then checking where the LEDs come close to the light guide slot walls, and then I correct the position of those that are a bit off. Once all are properly in place, I solder them on the other side.

After soldering all the LEDs in, it was time to give the boards their first 24 hrs test. For this I have a breadboard setup that mimics the wiring on the display PCB:

Let's see if they are still all on tomorrow!

Beomaster 8000: Step Four - Rebuilding the FM Tuner, Preamplifier and Control Panel PCBs

After completing the power supply restoration of the Beomaster 8000 that I am rebuilding right now, it was time to do the remaining boards. While it is relatively rare to find a dead capacitor on the FM tuner, preamplifier and control panel PCBs (they run relatively cool), it is a great idea to replace them anyway while one is in there. That brings everything on the same 'capacitor clock' (for another 30 years countdown...;-). It is also a great idea to re-solder all the wire to board headers since they tend to develop bad solder joints over time. Here are a few impressions of my progress:

This shows the board under the control panel switches and sliders in its original condition:

And after replacing the capacitors:

On to the preamplifier board:

In original condition:

and after rebuilding it:

The FM tuner has two boards, the front end and the detector part:

This shows them after exchanging the capacitors:

On to the display and uComputer boards!

Beomaster 8000: Step Three - Replacing the Main Reservoir Caps

No power supply restoration of a Beomaster 8000 is complete without also replacing the four big cans in the back. They are the reservoir capacitors for the output amplifier ±54V power rails. One 10000 uF capacitor per polarity and channel - massive! That gives the Beomaster 8000 its excellent output stability at 150W per channel. Awesome!

Here are a few impressions of my process:

This shows the original capacitors on the right side:

I unsoldered and pulled them out and replaced them with modern 105C grade Japanese units. As usual, modern capacitors are a bit more compact than 1980s types. That is why I use 3D printed adapters to fit them into the capacitor bays of the Beomaster 8000. This picture shows the original capacitors and the modern replacements with their 3D printed adapters:

The adapters and capacitors are available as a kit, just send me a message using the contact form on the right

After the installation it turned out that I broke off one of the resistors at the thermal switch for the right side:

This happens frequently when pulling out the capacitors, since the resistors are in the way, and they cannot take much bending. I replaced both resistors with new ones:

Then it was time to turn this 8000 around and do the left side:

After replacement:

After this I measured the capacitance of the original caps and of course they were out of spec (they usually are at this point):

4000uF is less than half of the specced 10000uF, but this would still have worked at moderate volume settings. But of course, the caps would have failed in the near future. So it was the right thing to replace them now for piece of mind. 

An then it was finally time to turn this Beomaster 8000 on for the first time:

And of course, one of the segments in the input display was a no-show. No problem, since I usually rebuild these displays anyway with SMD LEDs to ensure stability in the years to come. These displays all will fail in the near future, i.e. it is almost better to buy a Beomaster with failed display for less money and then fix it. That way one has a Beomaster with displays that will likely last another 30 years.

On to recapping the rest of the unit!