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.  Its high viscosity is really good on Beogram 800x turntable lids.  The Rocol Kilopoise is supposed to be what B&O originally used on these dampers.  Its viscosity feels about half the viscosity as the PG-44A.

However, I ended up going with the Kilopoise filled damper on this Beomaster.  I felt the higher viscosity 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.  

Beogram 4004 (5526): Restoration of the DC Platter Motor and RPM Stability Test

After replacing the RPM trimmers, the RPM relay and the light bulbs in the RPM trimmer panel with LEDs, it was finally time to restore the DC platter motor of this Beogram 4004 (5526). This sequence is important, since one can only test the RPM stability performance of the motor properly after the entire RPM control circuit has been rebuilt. This shows the extracted motor:

The motor needs to be completely disassembled to get to the dried out Oilite bearings:

The bearings are the two small donuts on the black pad. I infused them with motor oil under vacuum:

The raising bubbles indicate that air is drawn from the porous brass bearing material. This creates space for oil to enter the bearing. After about 12-24 hours the process usually stops, at which point the bearings are again full of oil.

This shows the bearings after the infusion process:

After reassembling the motor I put it back into the deck and adjusted the RPM with the BeoloverRPM device:

Then I used the device to measure a RPM stability curve over 18 hours:

This motor showed an exceptionally stable RPM performance and can be considered ready for duty.

Beogram 4002 (5514): RPM Performance Measurement after Restoration of DC Motor and Control System

After rebuilding the DC motor and the control system of the Beogram 4002 (5514) that I am restoring right now, it was time to do a RPM performance test. I set up my BeoloverRPM device that allows the long-term logging of the platter RPM, and then did a 24 hrs test:

After the data was logged on my computer, I graphed it relative to a measurement that I took when I initially received the unit for restoration. This shows the two curves in comparison:

The red curve shows the performance before the restoration. The typical telltale sudden negative RPM spikes are visible that indicate failing brass sleeve bearings in the DC motor. This is very common for DC motor Beograms. In this case the spikes are still fairly moderate, but would have been audible since they are in the 1% range. The black curve shows the performance after rebuilding the motor and the installation of new RPM trimmers and RPM relay. The large spikes are gone and the variations are much smaller. There are still some variations, which are related to the intrinsically slightly erratic performance of the analog motor control system, which is affected by temperature changes and probably the moon phase (that was a joke...;-). Some Beograms show them others do not. At this point I do not have an answer why they sometimes occur. The good news is that they are fairly slow over time and also small enough that one cannot hear them. The short term 'wow and flutter' RPM changes (the high frequency 'noise' on these curves) are within the spec given in the service manual (0.05%).  So this Beogram is back in business.

Beogram 4000: RPM Performance Test

I finally did a RPM performance measurement on the Beogram 4000 that I recently restored. I always wondered how the AC motor Beograms perform in comparison to the DC motors, and also the 8000/8002 models with their computer controlled linear motors. So I hooked up my BeoloverRPM device and let the 4000 run for about 12 hours:

This is the curve that I measured:

The AC motor is performing very nicely and the RPM is very stable, but there is some initial drift as the oscillator and the motor warm up. I measured 36C on the motor and 46C on the transformer after the 12 hour run. 

It is interesting to compare the three Beogram platter drives. This graph shows the performance of a Beogram 4002 with DC motor (blue), the 4000 (green) and a 8002 (red) that I recently fixed up:

It is clear that the DC motor 4002 has the largest fluctuations. Interesting is that the linear drive of the 8002 is not significantly better than the AC motor 4000. Of course the computer controlled linear drive does not have the initial temperature drift seen in the 4000. But these measurements show how well-behaved a synchronous AC motor drive can be. The great advantage of this concept is that there is no feedback loop that keeps the RPM constant over time. The AC motor rather follows the driving oscillator, i.e. as long as the oscillator is stable, the AC motor will perform very stable, too. While this is all very interesting, one should not forget that even the DC motor variations are very small, and  that they cannot be noticed while listening to records.

Beogram 4002 (5513): More RPM Measurements

This is a follow up on yesterday's post about this Beogram 4002 (5513). I did an overnight RPM performance measurement with my BeoloverRPM device. This is the curve that I measured:

This result is pretty much as good as it gets for the DC motor versions. Minimal <0.05% variations over time satisfying the specifications stated in the service manual. The initial slope during the first couple hours is usually observed in the 551x and 552x models. I currently think it is a result of D13 (the Zener diode that stabilizes the DC motor supply) and R12 with which it forms a pretty stiff (i.e. high current) voltage divider to set the base voltage of TR2 heating up. After a while the temperature stabilizes at a certain value in equilibrium with the environment. The band gap change in the semiconductor of the Zener changes its Zener voltage and with that the motor slowly gets a bit more power until a sable condition is achieved. This 'ancient' voltage regulator design is a bit of a drawback of the DC motor versions of the 4002. But one should not get to worked up about it since the change is pretty slow, and only represents a total ~0.3% change, hardly noticeable by most listeners. I am thinking this phenomenon could possibly be alleviated via the adaption of the circuit to use a modern voltage regulator. 

Beogram 4002 (5513): Initial RPM Performance Characterization

This is a follow up to my initial post about this Beogram 4002 (5513). In the meantime I did a RPM performance evaluation of the unit using my recently developed BeoloverRPM device.

This measurement clearly demonstrated the suspected sudden speed variations. This graph shows the RPM vs. time over an approximate 24 hrs period:

The sudden negative spikes suggest that the DC motor control system and the motor itself needs an overhaul.