Beomaster 8000 Indicator Bulb Replacement with LEDs

My Beomaster 8000 was on the work bench again! The manual tuning indicator stopped working. Of course this is not acceptable! Upon opening it up a quick test determines that the bulb (L2) burned out. Here is a pic of the bulb and housing:

Of course I did not have the prescribed 6V 80mA bulb...I also like the idea of using LEDs for their much better lifetime and cooler operation etc...So I decided to replace the bulb with a LED. Well, what should have been a 30 min operation turned into a 3hrs experiment. First I put in one red (that is what I had laying around) LED in with a 200 Ohm resistor to set the current at less than 20 mA. This worked, but after putting on the red plastic cover it became clear that the illumination was too directed, and half of the to be illuminated area was darker. O.k. I took the LED back out and replaced it with two red ones in series and a 8.6 Ohm resistor to limit the current. This worked much better:

So far so good! Mission accomplished! Or rather not! I put the Beomaster back together, and then plugged it in again for a last test before bringing it back into the living room. Then I realized that the LEDs were still on on a low level after switching from manual to automatic tuning. Previously I only tested the LEDs by turning the Beomaster on and off, which worked fine. But just switching to automatic tuning while on did not completely turn off the LEDs!! Annoying! Why? Well, a look into the circuit diagram shows that there is a 220 Ohm resistor (R36) from the collector of the IC2 Darlington to ground, which will generate a weak current through the lamp even if the Darlington is off:

I really wonder why they put the resistor there. The only thing that comes to mind is to 'pre-heat' the bulb a bit to make it turn on faster. Anyway, this resistor still permitted a weak current through the LEDs, enough to turn them on weakly. I removed the resistor, and then everything was fine. Happy Beomaster 8000 again! Here is a pic without resistor:

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

Beomaster 8000 Tuner Repair: Exchange of Prescaler U264B

***********************this is a follow-up to this post****************************

Yesterday the U264B prescaler ICs arrived from Germany and the UK. I bought two batches of Telefunken NOS ICs from to make sure I would have something that actually works. It turned out that both batches worked. I guess now I have enough U264Bs to last me for the next 40 years. This does feel good...;-).

I unsoldered the old chip (series TFK 208). Before:

The new one I put in using a DIP8 socket. I wanted to test all the U264Bs I received, and this enabled a fast swapping of the chips. Here are the pics of the empty socket and with a new old TFK 225 installed:

I also got some series 234 and they also worked. This puts the question to rest whether the series number matters...it does not. The tuner works very nicely now. However, I wonder why this IC died. Maybe it is time to consider putting some TVS diodes on the power rails to prevent over-voltages.

On a general note, I always run my B&O on uninterruptible power supplies to keep them safe from the all to often occurring brown outs and voltage spikes in our 3rd world style power grid.

One more comment about the defect U264B: When I measured the voltage at pin 1, the meter showed only about 1.2V instead of the value of 3.3 in the circuit diagram. The replacement chips showed a value of 3.25V. Maybe this can be used to identify a broken chip if one is far from an oscilloscope.

Beomaster 8000 Frequency Counter Feedback Issue - Dead Prescaler TEF U264B (?)

I was just about getting bored about Beomaster 8000 #4...it seemed it only had 'standard issues', which is nice as a soldering exercise, but nothing beats the 'detecting' if there is a new previously unknown problem! After fixing the displays and the output stages everything seemed great, until I turned on the tuner. While the frequency rotary encoder properly dialed in the desired frequency, the tuner itself did not really care about this setting. Basically, all I got was white noise no matter what frequency I selected. Oh well...a quick measurement of the tuning voltage yielded that it was either at the low end or at the high end but never in between...then after some playing around I noticed that when I set the freq close to 91.7 MHz I could hear some radio stations 'zooming' through.

Strange! Monitoring the tuning voltage showed that if I set the freq slightly above 91.7 the tuner would more slowly ramp the freq to 108 MHz, and when I set the freq slightly below 91.7 it would reverse course and go down to 87 MHz. In fact, after a while I was able to actually dial in stations with some stability by just very slightly changing the frequency above and below 91.7 and thereby driving the tuner slowly up or down.

Reading the "Product Description" about the tuning mechanism revealed that the microprocessor rams the tuning voltage up or down depending on whether the dialed in frequency is higher or lower than the actual tuner frequency. This is used to stabilize the tuner, but also to change the frequency. Such a mechanism requires feedback. In this case this feedback is provided from the tuner board via plug P84 to the u-controller board (#9). Here is the relevant circuit:

The orange tags are my measurement points (see below). After the transformer L2 the signal ( arrives at the preamplifier input of the prescaler chip U264B. I do not have a fast enough oscilloscope for a proper 100MHz measurement, but by 50MHz Rigol gave me a blurry 10mV sine wave at Pt.1. A better way to detect the presence of the signal was to use a signal analyzer. I hooked its 50 Ohm input up via a 10x oscilloscope probe to reduce the load on the signal. This is what I got:

The U264B chip is necessary to divide the 100MHz range tuning frequency down to something the slow 2 MHz microprocessor (IC4) can actually count. It also amplifies the signal with its preamplifier. After the 1/64 prescaler the frequency is in the 1.5MHz range. This signal comes out at pin 7 of the U264B (Pt.2). This signal now has a ~600mV amplitude, but a ~4.3V offset. Here is a oscilloscope shot measured on fully functioning Beomaster 8000 #2 (on the presently open Beomaster #4 this signal was absent suggesting that the U264B is fried. Judging from the above described behavior it also suggests that IC4 is programmed to 'think' that an absent frequency feedback signal represents 91.7 MHz):

The PNP transistor TR5 has its emitter hooked up to 5V, i.e. the 4.3 to 5V output signal of the U264B is able to turn this transistor on and off. This results in an amplification of the signal at the collector of TR5, and now we have a standard 5V square wave signal, perfect for the flipflop CD4013. Here is an oscilloscope shot from the collector:

This signal is fed into the flipflops in the CD4013, which are in series to affect a further 4x reduction of the frequency, i.e. now the signal is about 400kHz. This is how the CD4013 output looks like:

This is the signal that is sent to pin 21 of IC4, the frequency counter feedback input.
Now I am waiting for some NOS U264B chips from Germany where someone is selling off his hobby kit. Lucky me! I will report back once I put in a new chip. I hope this will fix it.

******************This post discusses the solution of the issue.**********************

Beomaster 8000 Display Repair

Another Beomaster 8000 display repair marathon. I must have soldered about 280 SMD LEDs by now into these displays (it takes about 70 LEDs and this is my fourth display...;-). I basically repeated what I described in: http://beolover.blogspot.com/2012/09/beomaster-8000-display-repair-hopefully.html
Here is a pic of the outcome. I put them into my display test breadboards and ran them at 2V (~0.53 Amp):

All segments are present. It is a good idea to do a mechanical test while the LEDs are out in the open. I give them a pretty good push and pull with a toothpick in all directions to make sure they have good contact. Since there is no solder mask the solder has a tendency to distribute on the pads, and it can be difficult to create a decent solder bump at the contact pads. Inspection with a magnifying glass is also a good idea to make sure they are aligned and centered precisely. If they are not it is difficult to get the bezels back on in a way that they block the light laterally (if the bezels are not set well and there are gaps, then light enters adjacent segments and in a dark room this can be seen...not acceptable for the beolover!). Now I will give them a 'burn in' over night and then mount the covers back on. this Beomaster 8000 is hopefully soon ready for a test in the living room!

Beomaster 8000 Display Issues

Today I looked into the display issue on Beomaster 8000 #4 (see http://beolover.blogspot.com/2013/09/beomaster-8000-first-inspection.html). The initial suspicion that there is something wrong with the communication between the microprocessors and the displays turned out to be wrong...the missing displays segments are just broken, like in most Beomaster 8000s when they come from ebay. I extracted the balance display and inserted it into my display test fixture and biased it with ~1.7V. Only four segments showed up for the party:

It is interesting to note that there are two 7 segment display decoders (IC1 and IC2 on the processor board (=#9)). IC1 addresses both balance and volume displays, while IC2 drives the frequency and the input displays. Which one of the two displays is on depends on the phase signal, which turns them on alternatingly at a frequency that is imperceptible by the human eye. It follows that the segment drivers are both o.k. if one of the two attached displays is showing a coherent data display (even with some display segments missing). The same is valid for the phase signals. Oh well: On to another display repair session (check out: http://beolover.blogspot.com/2012/09/beomaster-8000-display-repair-hopefully.html). I just ordered the SMD LEDs...for a consistent brightness across all four displays, I will have to do them all.

Beomaster 8000 Output Transistor Replacement

Today I checked out the output stages of my 4th Beomaster 8000. The blown fuse indicated some trouble there. Indeed a visual inspection of the two output amps revealed two darkened R236/7 power resistors, a telltale sign that the output burned out at some point. The right channel seemed to look pristine. I removed the heat sinks with the output transistors.

To make sure I used my multimeter to measure the resistances in the 6 output transistors of both channels.

On the right I measured high resistance values (several 100k) in the emitter-collector circuit, while on the left the resistance was less than 1 Ohm, indicating a full short circuit between the power rails.

Next I rebuilt the left output board with new electrolytic caps and a 11 turn 100Ohm trimmer for the quiet current adjustment (see post http://beolover.blogspot.com/2011/09/output-stages-testrecap.html for details on this procedure). Then I replaced the 6 output transistors (3x TIP141 and 3x TIP146). This is one of the more painful Beomaster 8000 repair procedures since one needs to extract the old transistors and then squeeze the new ones in under the springs with some heat sink compound, which always turns into a mess. I recommend to use vinyl gloves for that. Here are the dead transistors after extraction:

Interesting to see that they were made in Italy...the good old days. The new ones that went in were from Malaysia.
The next step was firing up the left channel with bench power supplies (see again http://beolover.blogspot.com/2011/09/output-stages-testrecap.html for details). By the way, there is no problem with first turning up the +15V supply, and then slowly ramping the ±54 supplies to their max voltage, while watching the current (I usually keep the current limiters on the supplies close to turnoff to make sure that nothing adverse happens during this test)

The currents into the board (at 18mV across R236/7) were a bit higher this time:

+54V ---> 0.15amp
-54V --->0.16 amp
+15V ---> smaller than 0.01amp

Not sure why this difference...at any rate it seems the output is working again properly. After 30min the temperature on the heat sink was just slightly above ambient...like it should be.

Beomaster 8000 - First Inspection

I bought a 4th Beomaster 8000 recently on ebay. It was listed with the usual "nothing happens when I plug it in, probably the fuse needs replacement" statements, which deterred many bidders, i.e. I got it fairly cheap. Externally it is pretty good, making it a worthwhile (my wife appears to have a different definition of 'worthwhile'...;-) project.

Today I finally got around to looking into it. A first look at the fuses on board 7 indeed revealed a blown 10A fuse (F2). F3 was o.k. This immediately suggested that probably one of the output stages burned out due to a non-contact quiet current trimmer.

I opened the Beomaster up and disconnected the power to the output amplifiers (red and black leads that connect to the boards via tabs) securing the plugs with some tape to prevent them from making accidental contact when the power is on. Then I replaced the fuse with the proper 10A quick acting type and plugged the Beomaster in. A good sign: the decimal point LED in the volume display came on, while nothing else happened.

Then I tapped one of the radio preset buttons. The Beomaster came alive, but showed strange displays:

The volume came on at 6.0, i.e. full throttle. I reset it to 2.0 with the volume store button, which seemed to work with the usual blinking of the display during the storing process. We will see if this fix is permanent or if there is some problem. The frequency display showed a local station, i.e. the radio might be o.k. Turning the rotary encoders showed that these two displays are o.k. w/o missing segments...a rarity. The other two displays showed garbage. Usually one would think that some segments are dead. However the perfect performance of the other two displays suggests that there is a deeper lying problem needing further investigation. Let the fun begin!

Beomaster 8000 Suddenly Cuts Output Stages Power (Continued)

Again my Beomaster 8000 cut out! I opened it up again, and a quick tracking of the pin 16 voltage revealed that IC4 still did not connect properly to the relay driver. It turned out that the via I thought I had fixed yesterday was still intermittent. The bad contact was on the top side of the via, not at the bottom where I resoldered it. The pic shows the three vias for pins 14-16 from the top after removal of the top part of the shield can.

I resoldered the vias from the top side and now the Beomaster seems to run stable. Note that there is a tendency of the solder to be pulled down below when soldering the via from top on a flat laying board, i.e. the solder bump below gets larger, while from the top it appears that the solder 'vanishes'. This means that the soldering should be relatively quick, just long enough that the bump reflows properly.

Beomaster 8000 Suddenly Cuts Output Stages Power

It has been some time that I had to post something noteworthy of my B&O efforts. I guess this means that all my Beos are in a happily restored state. However, like with classic cars, one needs at least two of each no matter how perfectly restored they are. This showed last night when we indulged in one of the new Longmire episodes and suddenly in the middle of the show the Beomaster 8000 output died. A quick on/off cycle indicated that the ramp up relays on the main transformer did not switch anymore: The Beomaster was quiet like a lamb, but still turned the display on and off in an orderly way. This suggested that there was an issue with the communication between the microcontrollers and the relays. I switched out the Beomaster with one of my other units and we continued watching.

Today I opened the Beomaster up. After plugging it in and turning it on, the relays clicked like never anything happened. An intermittent problem...don't we love them?? Well, on the other hand this suggested immediately that there was a simple contact issue somewhere in the path between the responsible microcontroller (IC4) and Relay 2 on board 7. I slightly bent the microcontroller board back and forth, and promptly the relays cut out again. I followed the path from the plug to the microcontroller with my multimeter (IC4 pin #16 should pull the line to the relays down to ground when the Beomaster 8000 is on). What I found was that the via that connects the trace from pin #16 through the board right above the pin of the microcontroller had become an intermittent contact. The photo shows the three vias (in the center of the pic between "9" and "C97"). Please, note that getting there requires removal of the bottom part of the shield can that limits the RF emissions from the processors - this can be done by carefully inserting a screw driver in various positions around the can and turning its blade, thereby prying the lid off without bending it.

I had a similar problem before with the first Beomaster 8000 I restored. That one would out of a sudden go into stand-by, and not wake up anymore. In that case the pin #14 via was the culprit. I think the lesson here is that a complete restoration of a Beomaster 8000 should include the preventive measure of resoldering these vias.

Fixing was quick: I put small dabs of flux paste on the vias and then simply heated them until liquid with my soldering iron (at 380C). This restored the Beomaster to its original reliability and it is now happily back in our living room.