Beomaster 2400 Type 2902 - Canada Project Bench Testing To Collect Some Reference Data

Listening to this restored Beomaster 2400 has been very enjoyable. The amplifier sounds as good as any Beomaster 1900 and 2400 receiver I have tested in the past.  But that is a little subjective isn't it?

In my ongoing effort to collect some quantitative data so I can compare my restoration results I returned to using my QuantAsylum QA401 Audio Analyzer and some fixed resistive, dummy speaker loads.
Since I last used it QuantAsylum has released a couple of software upgrades that will help me with my amplifier testing.

If you are not familiar with the QA401 Audio Analyzer, it can output stimulus signals into an audio amplifier and measure the outputs either from preamplifier outputs (which a Beomaster 1900/2400 does not provide) or output amplifier speaker outputs.

A nice feature of the QA401 analyzer is that it can do single-ended measurements or differential measurements. I have experimented with both in the past. For this test setup I went with differential measurements. 

Choosing the measurement points based on the expected output is important.  For that reason I constructed my fixed resistor, dummy speaker loads like this -

...and here are the connections to the QA401 Audio Analyzer.

I have a dummy speaker load assembly for two amplifier output channels.
I decided on 8Ω fixed loads.  If I had to it would be quite easy to desolder and resolder my configuration for 4Ω fixed loads. 

An important reason for having multiple series load resistors in the fixed load assembly is because the QA401 inputs have limits to how much voltage they can handle. I have tested amplifiers on my bench that have 20 Watt outputs all the way to amplifiers that have 250 Watt outputs. With the Beomaster 2400 I am only looking at 20W into 8Ω.

My dummy speaker load assembly is constructed like this (from the positive lead to the negative lead):
4Ω to 0.08Ω to 2Ω to 2Ω

That gives me a total 8.08Ω for the fixed resistance load across the amplifier speaker outputs.

The 0.08Ω resistor is in there for the large 100 plus watt amplifiers. I need a small sensing resistor to connect my QA401 differential measurement probes without exceeding their input level limits.

Determining what the sensing load I am going to use is important in the configuration of the test measurement system of the QA401. It has a popup context menu for setting the Input Gain level of the measurement. If I was going to measure across the full 8.08Ω loads then I would use an Input Gain value of 0.00 dB. However, as I explained above, the QA401 has input voltage limits so it is common to be measuring across one of my individual (smaller) resistor segments. When doing that you must calculate the Input Gain value to put in.

For example if I am concerned about the amount of voltage going into my QA401 measurement then I could begin by measuring the amplifier output using the small 0.08Ω sensing resistor.
In using that I calculate my Input Gain value using the formula 20*log10(0.08/8.08)) which gives me -40dB.

Also note there is the option to display the output as Vrms or Watts.  The actual load impedance value field here is for the QA401 to calculate the values it displays. This is the impedance the amplifier sees, not the individual sensing resistor value.

Why not just use the 0.08Ω sensing resistor for all amplifier measurements?
The reason to not do that and the reason for having the other measurement point options are for sensitivity of the measurement. Basically the resolution for accuracy.  According to the QA401 documentation and forum discussions you get the most accurate measurements when your test signal is not too close to the maximum input the QA401 can handle and not too small either.

For the Beomaster 2400 and its 20 Watt power output rating across an 8Ω load I am going to make my measurements across one of the 2Ω resistors in the assembly.  That means a QA401 Input Gain setting of -12dB.

Continuing on with some actual measurements...
I wanted to first check the QA401 measurement setting configuration and my earlier service manual adjustment of the Beomaster 2400 volume level (for the Medium Volume Preset switch position).
Remember that test setup asked for a 200mV, 1KHz test sine wave into the Tape inputs.
Turning the Beomaster 2400 on using the Tape source selection should result in 100mVrms at the speaker outputs at the medium volume level setting on the Beomaster.

Here is that check.

Note that I set the QA401 measurement units to volts so I could see what it measures for the speaker outputs. The voltage output levels are in the 100mVrms range. I was hoping they would be closer but this is the first time I have recorded that setting so perhaps that is typical. I will start noting that on all of my Beomaster 1900 and 2400 restorations from now on. 

The bench tests I really want to collect data on is the total harmonic distortion (THD) at 1KHz and the frequency response. For all of these measurements I will need to use the Beomaster Tape source as the point of entry for the test signal. I decided for these tests to keep my Tape test signal at 200mV.
On the signal generator of the QA401 that calculated out to be -13.8dB for the generator configuration.

That value was determined using a generator test option on the QA401.
I set the QA401 generator amplitude value while measuring the signal the QA401 was putting out using a DMM.

The bench testing generator is now configured to go along with the measurement configuration.

I began the bench testing with the THD test option on the QA401. The generator is sending a 200mV, 1KHz test signal into the Beomaster 2400 Tape inputs. I increased the volume level on the Beomaster 2400 while watching the QA401 left and right measurements coming off the speaker load assembly.
I set the measurement units to watts for these tests and here are my results with the Beomaster producing 20 Watts of power.

Less than 0.02% THD.  That is a very good number and that is at the maximum output level for this amplifier.

I left the Beomaster 2400 at the volume level that produced the 20W output (at 1KHz) and removed the test signal.

Next, I setup the QA401 for a frequency response test using that same volume level.
I let the QA401 do a single shot sweep frequency response test from 20Hz to 20KHz and saved the result. 

Another nice measurement. The swing of the signal from 20Hz to 20KHz for each channel stays within 1.5dB from the 1KHz point. 

That is all I will collect for now. I believe I have a repeatable test configuration I can use on future Beomaster 1900 and 2400 receivers.

Beomaster 4400 Type 2419: Final Reassembly and some final restoration tasks

This has been a marathon of a Beomaster 4400 restoration. Every time I thought I was ready for the home stretch some new problem would pop up.

With the testing out of the way this Beomaster has been playing constantly without any issues what-so-ever. So it is finally time to wrap this project up. Of course there were a couple of final repairs I had to make for this project to be complete.

The first was a known issue regarding the lenses for the slide controls. The old lenses were badly yellowed and foggy. I had already ordered a replacement set from Martin Olsen at Beoparts.

You can see how dark the background is in the picture above through the original lenses. The replacement lenses show what they should look like.

Reassembly of these components is a little tricky. The metal parts are very thin and probably had some special tool to mount them on the plastic slider at the factory. As such I found it very difficult to reattach them without any metal tabs breaking off. I ended up enlisting the aid of some very thin, double-sided tape (the kind used in modern cell phones) to put them back together.

The next issue was deterioration of the Beomaster 4400 cabinet feet. Almost all of the Beomaster 4400 units I have seen have feet that look like these.

You can see that there is also an accompanying rust problem.

This is the only part of the Beomaster cabinet that has any rust so I think the problem starts with the mounting screws of the feet.

I removed the old feet, sanded the rusty spots away, then spray painted the base plate (both inside and out).  I didn't want to lose the original wording and graphics on the cabinet base so I taped them off.

The rust is gone so I moved on to attaching the new feet.

The new feet have metal sleeves for the attachment screws so the screws don't compress the rubber as they are tightened. I threw the old feet and old screws away and used new mounting screws for the new feet.

The third and last issue I had to deal with was a broken FM tuning dial catch for the FM tuning slider.
All of these Beomaster 4400 units have a hard, brittle piece of glue that attaches the FM tuning slider to the tuning cord.

If you are careful with it the hardened piece of glue will remain on the cord and can be reused.
In this case it was too brittle and broke off.

There is not enough there to fit into the FM tuning dial catch.

The broken catch is always a blob of glue so I think the factory determined the position of the catch, set the tuning dial fork there and glued them together.  Whatever the type of glue they used was it does not bond to the slide control fork so it can be removed for service.

Over time however, those glue pieces break.

Looking for a solution that does not involve any permanent gluing to the slider fork I came up with using a small nylon washer and some epoxy to build me a new catch.

I marked either side of the tuning cord with a red marker before I removed that last remnants of the old glue piece.  That gives me the location for my new catch.

I used JB Weld fast drying, dark gray epoxy so I could easily see it.
I ended up having to cut some of the bottom part off because the catch was a little too big.
Note that this task is really something that should be done prior to having reassembled the Beomaster.
At this point though, I did not want to go through a whole disassemble and reassemble operation.

In the end my makeshift catch does a good job.

It fits into the tuning slider fork and the FM tuning slider operates the main FM tuning great.

Finally I could finish the reassembly of the Beomaster 4400 and get on with the best part...some listening tests.

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 :-).