Beomaster 8000: Ready for Some Bench Testing

Ever since I restored the power supplies, output amplifiers and preamplifiers on this Beomaster I have been exercising it with an iPod through TP1 & TP2 as well as playing the FM radio. The receiver has continued to play and sound very good as I added the restored FM boards, Filter & Tone Control board and the Microcomputer board. Now it is time to start putting the Beomaster back together and do some bench testing.

As I started the reassembly I tried the remote control for the first time. No functionality. So I will have to dive back in to the power supply board where the remote control receiver lives to see what is wrong. But first I decided to continue with some initial bench testing. I have been anxious to take some measurements and I don't see any reason to let the remote control problems delay that right now.

Before using any test equipment I decided to run a functional check pairing the Beocord 9000 that I restored earlier. These two will be returning together to their owner soon and I wanted to see how they play together for the first time since their restoration.

I used an original seven pin DIN cable for TP1 to connect it to the Beomaster. Pressing the play button on the Beocord turns puts the Beomaster into TP1 play mode as expected. Pressing the standby (off) button on the Beomaster turns both units off (and into standby mode). So they are communicating well. The Beocord also sounds great. In a later bench test I will add one of my Beogram 8002 turntables to the mix so I can both check out the phono input and do some recording on the Beocord.

Next up for this testing session though is to use my QA400 spectrum analyzer to check the Beomaster 8000 from its source input (TP1) to its output amplifiers (Speaker 1).

Beolover has featured some measurements before from his QA400. It is a nice little bench analyzer that uses a PC to do all of its analysis. The connection to the computer is via a USB cable and software on the PC sets up the tests performed. Up until now I have only done low voltage level testing with this type of setup because the inputs to sound card devices and this QA400 are limited to the ±5 volt range. Connecting to the power amplifier outputs was not doable. I picked up some differential voltage probes by the QA400 manufacturer that will allow me to measure higher voltages. For this Beomaster source-to-speaker test I will use a couple of fixed 8 ohm loads to simulate speakers and connect the differential probes to the dummy speaker connectors.

The probe is powered by batteries and has a BNC connector to run a coax cable to the QA400 input terminal. For some reason I could not get my test setup working as I wanted with both differential probes connected at the same time (one for the right channel and one for the left channel). With both probes connected I see some undesirable noise at 60 Hz from my house line voltage. So far swapping cables and probes doesn't solve the problem. The only thing that works is using one of the probes on one channel at a time. Then there isn't any 60 Hz noise present. So I performed my tests one channel at a time switching the speaker connection to the Beomaster. The measurement results look pretty good. I won't post all of the frequencies I checked (400Hz, 1kHz, 5kHz, 10kHz, 15kHz) but here is the 1kHz test on the left channel. I tested with the analyzer set to unweighted and A-weighted.

Here is a 5kHz test on the right channel.

Viewing the distortion numbers and sound to noise level readings at a few select frequencies was nice to see. This Beomaster sounds really good but I like to see some sort of measurable result besides my own ears.

Now it is back inside to see what is wrong with the remote control receiver.

Measurements with a Spectrum Analyzer - What Does the dBm Readout Really Mean?

Last year I was given a broken Beomaster 3000-2, and I recently decided to fix it up. It turned out that the tuner has some problems. So I started learning abut FM stereo transmission and RF in general. At some point I started wondering about doing proper radio frequency (RF) measurements and characterizations etc.... I have access to a spectrum analyzer and after playing with it for a bit I was wondering what the 'mysterious' dBm units mean it uses to display its peaks. Well, a visit to wikipedia told me that this stands for dB relative to a power of 1 mW. Great! So what does this mean? Let's look at the formula:

dBm = 10 log (measured power/ 1 mW)

This formula means that dBm gives us the power of the signal relative to a standard 1 mW signal. Why not 1W?...convention. In fact, there is also dBW, which is power relative to, you guessed it, 1W!

So far so good...but what does it really mean? What confused me was "where is this 'measured power' that is compared to 1mW actually measured"? Let's imagine for a moment that we are a Spectrum Analyzer. Sitting inside our enclosure, we see the world through the connector on the front panel, i.e. the measured power must be measured inside the enclosure! not anywhere in the 'device under test' (DUT) where the analyzer is connected to! This is where the 'input impedance' of the analyzer comes into play. On the input of my analyzer it says '50 Ohm'. What this means is that we can basically think of the entire expensive analyzer as a 50 Ohm resistor connected between the input and ground. In other words, when the analyzer is connected to the DUT, then we actually load the DUT output with a 50 Ohm load.

Now it gets pretty straight forward: The measured power in the analyzer is the power dissipated in the 50 Ohm resistor. In other words, if we know the RMS voltage on the input of the analyzer then we can calculate the power in the resistor, and understand the dBm readout.

To validate this concept, I did a simple experiment: I hooked the 50 Ohm output of my waveform generator directly into the 50 Ohm input of the spectrum analyzer. I also hooked my oscilloscope into the output of the waveform generator via a BNC T-adapter. This is the circuit:

The 50 Ohm, 1 MOhm resistors and the 15 pF capacitor are the internal impedances of the generator, analyzer and oscilloscope. The oscilloscope also has a capacitive impedance, which is 15 pF.

(BTW: this great little schematic was made with iCircuit for iPad...a fantastic way to spend $10! Very easy to play with circuits to understand electronics concepts. The kicker: It does real time simulation while you play with circuit components - give it a try!)

This circuit basically illustrates that the voltage (amplitude) of the signal from the waveform generator is basically divided in half by the 50/50 voltage divider formed by the output impedance of the generator and the input impedance of the analyzer. How about the 1MOhm/15 pF impedance of the oscilloscope? Surely the 1 MOhm does not factor in much, but how about the capacitor? My experiment runs at 10.7 MHz, i.e. if we calculate the impedance of the capacitor via 1/(2*pi*f*C) we get 992 Ohm for this frequency. This means the cap can (at 10.7 MHz) be viewed as a 992 Ohm resistor in parallel with the 50 Ohm impedance of the analyzer. Calculating the resulting total resistance we get 47.6 Ohm, i.e. a change of about 5%. Not too impressive, but one definitely sees a small change in the dBm reading of the analyzer after hooking up the oscilloscope (it was about 0.2 dBm).

My goal in the experiment was to verify that the analyzer indeed gives me a 0 dBm readout if the power dissipated in the 50 Ohm impedance of the analyzer is 1 mW.

This photo shows the readout of the spectrum analyzer after playing with the waveform generator amplitude to achieve an exact 0.00 dBm peak value of the 10.7 MHz peak.:

Here is the waveform generator screen:

This means that a 0.64V amplitude apparently delivers 1 mW (0 dBm) into the analyzer 50 Ohm impedance. If the above circuit holds, we should get half this amplitude (0.32V) at the input of the analyzer. A look at the oscilloscope shows this:

The Rigol DS 1052 apparently measures amplitude across the entire ±wave form. It shows a value of 699 mV, corresponding to a true signal amplitude of 0.35 V, i.e. a bit more than half of the 0.64V shown on the function generator. The 'bit more' is probably a measurement problem. It cannot be explained by the oscilloscope impedance, which should have reduced the reading a bit. Pretty close, though! Lets calculate the power using the presumed 50% (0.32V) amplitude at the analyzer input:

The corresponding RMS amplitude is 0.32V/sqrt(2)=0.226V. Using P=V^2/R to calculate the power, we get: P=0.058/50 W = 0.00102 W = 1.02 mW. Pretty close! The 'mystery' of the spectrum analyzer readout seems to be solved!

While reading up on spectrum analyzers, I learned that they actually work pretty much like a FM tuner. Like a tuner, they work best at a certain input power level. It appears that actually attenuating the signal with a calibrated BNC attenuation 'pad' to a -20dB level or so may be a good idea to protect the fairly sensitive inputs of these devices as long as one has a strong enough signal. This is a bit like using a 1:10 oscilloscope probe as long as your voltages are not too low....this protects the oscilloscope input from too high voltages. For further reading: There is a great (but also rather long) introduction to spectrum analyzers from Agilent. It is posted here: http://cp.literature.agilent.com/litweb/pdf/5952-0292.pdf