Beomaster 8000 Type 1903 - Arizona Restoration Project - Finalizing The Repairs

This Beomaster 8000 project is almost ready to wrap up. 

I left off with this Beomaster functionally working.  To continue that, I did some music listening with the Beomaster 8000 connected to an FM antenna, an iPod (in Tape 1 and Tape 2 source inputs) and with the speaker 1 and 2 outputs connected to my workshop Beovox S-55 and MC-120.2 speakers.

At first I thought the sound was okay.  Thinking back, that itself was a little odd.  Usually I am more impressed during the first listen.  Something seemed off with this Beomaster 8000 sound so I connected up my audio testing equipment.

Here is my test setup.
I mainly wanted to view the frequency response of the Beomaster at various output levels.
For that, I used a QuantAsylum QA-403 Analyzer and my two, bench dummy speaker loads (8 ohms).

The frequency response testing showed that the output amplifier had quite a drop off in output between 20 Hz and 100 Hz (the low end).

Here is a QA-403 frequency response test output with the Beomaster 8000 at its maximum rated output, 100 Watts into 8 ohms. 

A Beomaster 8000 should output its audio signals between 20 Hz to 20,000 Hz with only a ± 0.5 dB difference.

As you can see, at 20 Hz, the output levels are around -4 dB down from where the output is at 1000 Hz!

Looking at the left and right channel Output Amplifier modules (Module 5) I spotted a problem.
These Output Amplifier modules had already been re-capped previously by an electronics shop somewhere.  They did pretty good work but on the 100uF, 16V electrolytic capacitors (there are two per Output Amplifier PCB), the technician transposed the numbers for the capacitor.  Instead of grabbing 100uF, 16V capacitors, he (or she) selected 10uF, 160V capacitors.

One of those capacitors is for the Beomaster 8000 Output Amplifier feedback.  
The gain on the feedback is especially sensitive to the value of that capacitor and the 10uF capacitor results in the impedance of the capacitor at 20 Hz to be around 795 ohms instead of 79 ohms of a 100uF capacitor.
That really knocked down the gain of the amplifier output at the lower frequencies.

Here is a photo of the Output Amplifier boards as they were with the incorrect capacitor values.

The accidental selecting of the wrong capacitor values there didn't cause any catastrophic problem to the Beomaster but it did rob the Beomaster of its low frequency performance.  That is the problem I was hearing during my first listening tests.

Here are the boards now with the correct capacitor values installed.

Remeasuring the Beomaster 8000 frequency response at 100 Watts output across the 8 ohm dummy loads show a much improved low end.

The low frequency output is now very close to the -0.5 dB limit specified in the service manual.

The high frequency output is a little off from 0.5 dB at 20,000 Hz but for now, that isn't something I am going to address.

Listening to music on this Beomaster 8000 is now noticeably improved over what I was hearing before.

That problem and fix was an unexpected detour task on this Beomaster.  I had planned to fix the FM display segment problem and the broken lid lever problem discovered earlier in the initial assessment of the project.

Here is the broken lid lever problem.
 

The plastic lever had broken in half but the lid damper also had a couple of problems.
Highlighted in red, above...A plastic hook on the lid damper frame was also broken.
Plus, the damper was leaking damping grease.

One thing to note though, the damper and lever are for slowing down the opening of the lid, not the closing.

The Beomaster 8000 lid is sprung so that it pops open and remains open until someone manually closes it.

It turned out, this Beomaster 8000 lid had another problem besides the damper and damper lever.
The lid assembly itself, had a broken cover for the spring that raises the lid.

Here is a photo of the broken lid and the replacement (spare) lid that I have to replace it with.

I think that I can (later) design a fix for the broken lid assembly but for this project, this Beomaster gets a replacement lid.

For the lid damper fix I also provided a replacement damper from my collection of spare parts and a new, replacement lever that I got from DKSoundParts.

For the lid damper damping grease, I tried two different types.  I have used these two types before.
The Nye Lubricants PG-44A is the thickest damping grease I have come across.  It is really good on Beogram 800x turntable lids.  The Rocol Kilopoise is supposed to be what B&O originally used on these dampers.  It is about half as thick as the PG-44A.

I ended up preferring the Kilopoise filled damper on this Beomaster.  I felt the thicker PG-44A put too much pressure on the plastic lid lever and I didn't want it to break again.

Next was the FM display segment problem.

In my earlier testing of the Beomaster 8000 I noted that one LED segment and the LED period were not illuminating on the FM display.  I have highlighted those in the picture below.

On this project, the plan was to only fix the broken display segments, not go to the expense of replacing the whole set.

That being the case, I took a fully functioning FM display module from one of my spare Display boards.

I tested the faulty FM display and the replacement FM display on a test fixture I designed for these B&O LED display modules.

With my bench power supply I was able to increase the current and voltage to get the missing LED period to illuminate on the faulty display.  That took more voltage and current than it should and the display got a little warm.  Also...There is still the same missing LED segment.

The picture below shows both the old and new displays on the test fixture.

Here is the Beomaster 8000 with the replaced FM display module installed on the Display Board.
I tested it out as I also tested listening to the Beomaster FM tuner.  Both worked perfectly.

Now to carefully reassembly the Beomaster 8000 and do some more functional testing before returning it to its owner.  

I still need to connect up actual Beosystem 8000 components to verify that the remote control functions all work.