Beomaster 8000 Type 1903 - Arizona Restoration Project - A Couple More PCB Restoration Tasks

After the previous post I couldn't stop thinking about the Beomaster 8000 frequency response results.  The output was a little beyond the expected -0.5 dB at the two ends of the 20Hz to 20,000Hz frequency range.  Since the preamplifier board and volume control/filters board of this Beomaster had never been restored, I decided to go ahead and update those two PCBs.

Here is the before and after restoration of the Beomaster 8000 Preamplifier PCB.
The op amp devices and electrolytic capacitors on this board are all in the audio signal path.

I replaced the electrolytic and tantalum capacitors with nonpolarized audio capacitors.  
The op amps were replaced with TI OPA2134 op amps. They are more expensive than the original TL072 and LF353 op amps but the OPA2134 op amp has a very good reputation for usage in audio applications.

The Beomaster 8000 audio signal path leaves the Preamp PCB and goes to the Volume Control/Filters PCB.  The Volume Control circuit is first and the Filters circuit is second.  However, in the Beomaster 8000, the audio filtering (including Bass and Treble) is bypassed by default.  The Filters button inside the metal control panel lid engages and disengages the filtering.

The performance measurements I have been making have been with the filters bypassed.

Here are the before and after photos of the Beomaster 8000 Volume Control and Filters board.

Here is the Beomaster 8000 Volume Control and Filters board after being restored.
Like the op amps in the signal path of the Preamplifier PCB, I used the TI OPA2134 op amps here as well.

Now for the frequency response measurements again.

Quite surprising to me, there doesn't appear to be much improvement.  I was expecting a more noticeable result.

At 20Hz the signal is down just over 0.5 dB.  At 20,000Hz the signal is down about  0.8 dB on the Left Channel and just over 0.5 dB on the Right Channel.

As an FYI, I did adjust the Tape 1 input pots to make sure the Left and Right channel test signals are as identical as possible.

I decided that I needed to look at the frequency response measured at different points in the signal path.

First, just the preamplifier section.
I still kept the input signal source on the Beomaster 8000 Tape 1 source input.
For the output measurement though, I pulled the jumper bars between the Beomaster 8000 External Filter input and output RCA jacks.  This would show me the audio test signal only at the preamplifier stage.

My test cables picked up some noise from the line 60Hz but the frequency response is still very flat.
The output at 20Hz is only down about 0.02 dB and the output at 20,000Hz is only down about 0.04dB.

Next is the Beomaster 8000 Output Amplifier.
I used the Output Amplifier P39 connector for the audio input signal and I measured the output across the 8 ohm, dummy speaker loads.

My test setup encountered much less noise with this measurement.

The Output Amplifier frequency response measurement was also nice and flat.
At 20Hz the output is only down about 0.2dB while at 20,000Hz the output is only down about 0.05dB.

The Preamplifier and Output Amplifier boards checked out very flat as I would expect.

That means the Volume Control and Filters board must be the affecting the frequency response measurement I get with the end to end test.

Here is the Left Channel frequency response with the audio input at P27 of the Volume Control and Filters PCB and the output measured at P24.

Again, my test setup picks up the line 60Hz but you can see how much different the shape of the frequency response is.

I hadn't looked at just measuring the Volume Control and Filters board by itself before so I spent a little extra time.  Because the capacitors and op amps are all new and known good components, I wondered about the AD7110 (logdac) IC device itself which performs the Beomaster 8000 volume control functions.

Instead of jumping in and replacing the volume control devices (there is an AD7110 IC for each channel), I decided to take advantage of having a couple of spare Volume Control and Filter boards available.

Both of my other (restored) Volume Control and Filter boards yielded very similar frequency response waveforms.  It would appear that is a characteristic of this volume control design...but I will need to measure more Beomaster 8000 units to confirm or refute that.

For now, this as far as I will take this experiment.  The frequency response results I am getting are very close to the -0.5dB range specified in the Beomaster 8000 technical data sheet.  More importantly, the music output of this Beomaster 8000 sounds very good as things currently stand.  So I will start putting components back into place inside the Beomaster 8000 cabinet and do some final listening tests before shipping this receiver back.  

I have several more Beomaster 8000 units to restore so I will definitely be repeating these measurements for comparison.

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.  

A Pair of Beomaster 1900 Receivers for Restoration: Unit #1 Reassembly for Testing and a Power Problem

 The next step before testing the Beomaster 1900 was to reinstall the four output transistors that mount on the rear heatsinks. I had removed the heatsinks and the original thermal insulating paste. On the reassembly I like to use modern SIL-Pad thermal insulators. They do a great job and don't have the mess that thermal paste does.

Here are the parts laid out for reassembly.

I like the way Bang & Olufsen chose to mount the transistors to the heatsinks. They use curved, metal spring clips that press the plastic face of the transistor against the heatsink...sandwiching the thermal insulating material in between.

Here are the output transistors reinstalled.

Everything is installed that is necessary to start testing the Beomaster 1900.

It only took the amount of time for me to connect up the variac and dim bulb tester to try powering up the Beomaster to discover a problem.

The Beomaster 1900's +15 VDC power supply was not turning on. No Standby lamp illuminated.
I exercised the main power switch again to make sure it wasn't dirty but nothing.

I unplugged the power cord and started to check around the +15 VDC power supply.  I could not measure anything across its fuse.  I had noticed the fuse holders were badly tarnished but this unit had powered up prior to starting the restoration. I pulled the fuse and it was okay. I just couldn't get anything through it from the fuse holders.

Here are the fuse holder clips and where I removed them from on the board.

Age has not been kind to these fuse holders. I remember now that the original bridge rectifier had been replaced on this Beomaster and there was a large, charred mark under where the bridge rectifier had been mounted. It is likely some big power event had occurred sometime in the past.

In any case I need to order some of these fuse holders on the +15 VDC power supply line.  If I order them today Mouser will deliver them on Tuesday.

The power supply incident got me thinking about all of the input power to the Beomaster 1900. I had seen a bad power switch on a Beomaster 2400 in the past so I should look at this one more closely.

I am glad I did. The switch actually measures good and I did some cleaning on it with Deoxit.  However, I found a cut in the insulation on one of the main power wires delivering line power to the Beomaster.  It probably wouldn't hurt anything but I don't like bare wires where they are not supposed to be.

I will rework the wiring on the power switch and fix that problem while I am waiting for the fuse holder clips to arrive.

Here is the cleaned up and prepared power switch.

Beomaster 4400 Type 2419: Checking the amplifier right and left channels

Before I finish reassembling this Beomaster 4400 receiver I made a few amplifier measurements to make sure the left and right channels are close to each other in their audio performance.

My testing methods cannot be the same as what Bang & Olufsen used back in the seventies so I am not looking to hit those same performance measurement values. The goal of my performance tests are to make sure the left and right channels perform the same...or at least very close to each other and that they are similar to other Beomaster 4400 units that have come across my workbench.. If one channel is way off the mark from the other on the same test then I have to figure there is something wrong.

The tests I like to use in comparing the channels are a Total Harmonic Distortion (THD) test on the output amplifier sections and the preamplifier sections. For that I use a QuantAsylum A401 Audio Analyzer to generate a 1Vrms sine wave test signal at 1KHz. That signal goes into the Beomaster 4400 Tape 1 source input. I measure the output at the preamplifier outputs then again at the output amplifier outputs (across an 8Ω dummy speaker load). It is actually an 8.08Ω load as I use a 0.08Ω series resistor as a sense load to make the measurements with the QA401 analyzer.

Here is the Beomaster 4400 connected up for testing.

The distortion measurements through the preamplifier were very good as expected.

So were the power amplifier outputs.

The frequency response measurements were also good. These tests are made using a frequency response plugin that is part of the QA401 Audio Analyzer. It executes a series of pulses at different frequencies and measures the output. The stimulus is still Tape 1 and the output is measured across the 0.08Ω dummy speaker load. When the test is complete the QA401 plots the output.

The end points (20Hz and 20KHz) are in the 1.0dB to 1.8dB range of the flat line which is pretty typical with this test setup. Both channels measure the same.

Satisfied with the electrical work so far, I will continue on with reassembling the Beomaster 4400 now and then some functional testing with all of the source inputs and listening tests of course.  Those are the most enjoyable tests to perform :-).

Canada Beomaster 8000: Checking out the reworked boards

In the last post on the Canada Beomaster 8000 I left off having completed the restored display board.  With that last board completed I can finally begin testing what this Beomaster can do.

Since it has been a week since I last checked this Beomaster restoration I decided to recheck all of the power supply voltages, the no-load current checks and the DC offset checks.

Naturally I tested the display board now that it is reinstalled.

Everything looks like it is working correctly so I test played an iPod Nano into the Beomaster Tape 2 source input. As usual I connected my Beovox S-55 speakers I keep in the lab.

The Beomaster sounds like a Beomaster 8000.

I wrapped up the day by preparing the Beomaster for some upcoming performance tests.
Here is my test setup.
I am applying a 1KHz sine wave of 1.0VRMS to the Tape 2 source input of the Beomaster. I will measure and monitor the voltage across the dummy 8Ω speaker loads as well as measure the output with a QuantAsylum QA401 Audio Analyzer.

The test setup is ready and I ran a couple of preliminary tests.

Later this evening I will start running the performance tests.

Texas Beomaster 8000: Readjusting the output amplifier assemblies - Part 1

I was expecting to perform a quick task of verifying the output amplifier board no-load and DC offset adjustments (that I made earlier) were still good. Once that check was made I could start some functional testing of the Beomaster.

The Beomaster units don't always cooperate. In the case of the Texas Beomaster 8000 the left channel no-load current drifted upwards (from 18mV across the output emitter resistors) to the point of the clipping light illuminating and the protection circuit shutting the amplifier down. Disappointing to say the least.

I was able to adjust the left channel no-load current down where it stabilized at 18mV after a warm up period of a couple of minutes. The problem now is that the left channel was starting at around 6mV across the emitter resistors on power up where the right channel starts around 12mV.

I decided to check what music sounded like through the Beomaster. This is the first time playing a music source since the restoration work began.

Success. I can play music through the Beomaster TP2 source input.

A better description would be partially successful.

The volume level had to remain low and I could hear some distortion in the left channel speaker.
To determine what was going on I replaced the speaker connections from my Beovox S55 speakers to my 8Ω fixed dummy loads. I applied a test sine wave of 1.5Vrms, 1KHz to the Beomaster TP1 (Tape 1) source input and setup my oscilloscope to monitor the source signal and the amplifier output signal that is across the dummy speaker load (one channel at a time).

Beginning with the right channel here is what I got with the Beomaster in standby mode.

My measurement through my differential probes is a little noisy but will show me what I am wanting to see here.

This photo is of the Beomaster turned to the TP1 source and the volume level at 2.8.

I can see the 1KHz sine wave on the right channel speaker output as expected.

Now for the left channel.  The 1KHz sine wave looked okay up to around 2.2 on the Beomaster volume display.

Going above that results in clipping on the top section of the sine wave.

You can see the top of the left channel output signal is chopped off (even before the clipping light comes on).  That matches the distorted sound I was hearing when I had the Beomaster connected to my Beovox S55 speakers.

This means I will have to pull the left channel output amplifier assembly again and locate the fault. Hopefully it will just be some connection problem.