Beomaster 8000: Exchanging the Opamps in the Signal Path and Test

After updating the uProcessor board in the Beomaster 8000 that I am working on right now, it was decided to also update the opamps while the unit was in service position. We recently noticed that the opamps in the signal path of the Beomaster 8000 can degrade resulting in increased distortions (THD), i.e. it is a good idea to also replace the opamps when the boards are out for restoration. Here we go:

This shows the control panel PCB before the upgrade:

Most of the 8-pin ICs on this board are signal path opamps. This shows the board with new socketed LF535 opamps installed:

On to the preamp/input board. I forgot to take a picture of the original condition of the board. Here are a couple shots of the board after replacing the opamps with socketed LF353 units (except the phono input, which was replaced with a low noise LM833 type):

A detail photo of the phono pre-amp section:

After implanting the boards I characterized the performance of the unit with my QA400 audio analyzer. The bandwidth curve yielded the spec -1 dBV drop between 100 and 20,000Hz, and the total harmonic distortion (THD) values at volume 5.0 (just below clipping) were 0.008% on both channels, which is consistent with other Beomaster 8000s we measured. See here for a detailed discussion of such measurements. So far so good...the unit went on into our living room to see if the performance of this Beomaster 8000 is consistent in day-to-day operation.

Beomaster 8000: New Signal Path Opamps and Amplifier Performance Characterization with a QA400 Audio Analyzer

Inspired by Sonavor's recent effort to characterize the amplifier performance of a Beomaster 8000 after replacing all signal path opamps, my Australian customer asked that the same would be done to his 8000 before sending it back (after upgrading its circuit with a muting function for the FM section while the Phono input is selected to eliminate crosstalk).

And I am glad he asked. It turns out that it is a great idea to replace the opamps when performing a full restoration of the Beomaster 8000. My measurements yielded a 14 dBV improvement of the THD (Total Harmonic Distortion) performance on the left channel after implementing new opamps.

This shows the preamplifier and input selector board with the original opamps in place:

and after replacing them with new LF353 units (except the phono input, which was replaced with a low noise LM833 type):

I used IC sockets to spare the opamps the stress of soldering. Probably not necessary, but then why not. I did the same for the control panel PCB. This shows it with new LF353s in place:

After putting the control panel back together I performed a series of measurements to see if the amplifier performance would measure up to the values given in the service manual. It turned out that such a comparison is difficult to make due to the inherently different measurement methodology used in the 80s and with the instrumentation that is available today. But I think the measurements below show that this 8000 is now in good shape.

Let the fun begin:

I use a Quant Asylum QA400 audio analyzer for such measurements. Essentially this device yields a Fast Fourier Transform (FFT) of the audio signal that is put into it, and it can perform some measurements (i.e. calculations on the FFT spectrum) based on that data. The measurements include power, total harmonic distortion, signal-to-noise, and frequency response. 

Let's have a look at the output spectrum measured with a 0 dBV 1 kHz signal at the Tape 1 input of the Beomaster:

The dBV values essentially give the amplitude ('level') of the signal relative to a standard 1Vrms signal V0. Since level=20*log(V/V0), a signal change of 20dBV corresponds to a 10x change of the amplitude. As an example, if the measured amplitude of the 1kHz signal at the output of the amplifier is 10 Vrms with an input signal of 1 Vrms from the waveform generator, then level=20*log(10/1)=20*1=20 dBV. If the output signal were 100 Vrms, the level gain would be 40 dBV and so on.

Looking at the spectrum above, you probably wondered why the 1 kHz peak is only at about -8 dBV, while the amplifier operated at a volume setting of 5.0, which is close to the maximum output amplitude it can muster. The reason is that the measurement was performed via a voltage divider that was connected as load at the output of the amplifier. The divider was built from two 4 Ohm 50W power resistors and a 0.1 Ohm 3W resistor in series. This shows the setup:

Since the Beomaster is able to produce 100W output power into an 8 Ohm load, the resistors need to be mounted on a heat sink. I used a RF amplifier can that I had laying around from another project. The Y-shaped red wires that are soldered to the small 0.1 Ohm resistor connect to the BNC jack on the right side of the RF can, which then is connected to the QA400 input. The resistor chain is connected on the left to the speaker jack of the Beomaster. This setup guarantees that the QA400 input never gets more than about 1 Vpp. But it also means that the output amplitude that the QA400 'sees' is only 1/81th of the actual amplitude that is applied across the 8.1 Ohms. 

Using the above level=20*log(V/V0) formula, we can calculate that the level difference due to the voltage divider is about -38dBV (20*log(1/81)=-38.17 dBV). This means that the 1 kHz peak in the above graph would be at ~-8+38dBV=~+30dBV. I could have shifted the peaks in the graph, but since all measurements that are performed are inherently differences between two levels, this shift really does not matter, i.e. in the following all levels are just as they came out of the QA400, i.e. -38dBV lower than the real signals.

So what do we see in the above spectra? Mainly the THD spectrum to the left of the main 1 kHz peak and some noise. We can see that the 1st harmonic at 2 kHz is about -84 dBV weaker than the main peak. According to the above calculation, this means that the amplitude of that distortion is about 10^4 (= 10,000) times weaker than the main signal. Well below what a human ear could notice. The -84 dBV value is close to the THD measurement result of the QA400, which came in at about -82 dBV (=0.008%). The measurement is a bit worse than the 84 dBV value determined from the graph since there are higher order THD peaks that add to the total distortion. This was the only measurement result that changed before and after replacing the opamps in the signal path. My initial measurements of the left channel yielded a measly -68 dBV THD value, and after the opamp exchange this was improved to -82 dBV. So I think it is a great idea to replace all opamps when the boards are upgraded with new capacitors, just to be on the safe side.

So how does this value compare to the THD value from the Beomaster 8000 service manual? The value stated there is "< 0.05%". So we could be happy and say: "Wow this 8000 is almost 10x better than the value in the manual!". Not so fast, I would think, since the manual states that the measurements were performed according to the "IHF A-202" standard. At this point I do not know what this means, i.e. we need to postpone this comparison with the stated values. But I think we can confidently say that this Beomaster is performing reasonably well and is probably within the original specifications.

Another interesting measurement to perform on an amplifier is its signal to noise (SNR) ratio. This essentially gives us a number that qualifies how much stronger the signal is relative to the noise ("hiss") of the amplifier. This measurement is a bit more difficult to do and understand since we are comparing a defined signal peak amplitude (or power) with a diffuse noise background that is composed of a continuum of frequencies spanning the entire audible range and beyond.

I played a bit with the QA400 settings and it turned out that the SNR measurement is strongly dependent on the number of samples used for the FFT transformation of the input signal. I was able to "change" the SNR value from 65 dBV to 88 dBV simply by changing the FFT resolution from 8196 samples to 65535 samples. This means that depending on the setting the SNR value changed by a factor of 10, one full magnitude. So what is happening? A bit of reading up on the internet and a semi-cryptic response from the Quant Asylum tech support suggested that at lower FFT resolutions the main 1 kHz peak spreads out over several 'frequency bins', thereby lifting the spectral power outside the 1 kHz line, which is misunderstood by the FFT algorithm as part of the noise. This means that at higher FFT resolutions this 'frequency spill out' becomes less pronounced. So we can assume that the higher value is closer to the true value, even though the true value may be even better. Unfortunately the QA400 can only go to 65535 samples, i.e. we would need to find better equipment for an answer. 

Wondering about this topic, I performed an experiment that would allow me circumventing the FFT based calculation of the SNR. I measured the signal power of the spectrum at various FFT resolutions and it turned out that the PWR value is not significantly dependent on the resolution. This makes sense since PWR measures the power of the entire spectrum, i.e. integrates over the entire frequency range. So resolution is of limited importance, as long as the signal peak is still 'caught' in one of the sampled frequency bins.

This realization enables a basic SNR measurement: Measure the PWR value with the signal present at the amplifier input and then ground the input and measure again. Subtract the two values and the SNR is obtained.

My measurements yielded on both channels -8 dBV with 1 kHz signal (at Volume setting 5.0) and -98 dBV with the input grounded (also at Volume 5.0). The difference is 90 dBV, a bit better than the best 88 dBV value measured via FFT analysis. This compares to a ">77 dB" stated in the service manual. Again, we do not really know at this point how the 77 dB value was measured, but I think we are on the safe side and can conclude that this Beomaster 8000 is operating on spec.

Another interesting measurement is the frequency response ("FR"). The QA400 does this measurement by sending a square pulse into the input of the amplifier and then measuring the response at the output. Since a square pulse contains all wavelengths, the FFT of the response yields a true spectrum of the amplifier FR. I verified this by manually measuring the transmission for a few frequencies and the FR curves were exactly matched, i.e. I think we can believe this measurement as it comes out of the QA400. This shows the FR spectrum measured on both channels:

I cut the spectrum off at 1 kHz since there was some 60 Hz noise, and the FR drop at 20 kHz seems typically referenced to 1 kHz. So what we see from the graph is that there is a 1 dBV drop from 1 kHz to 20 kHz. The manual prescribes 0.5 dBV, but of course, again, we do not know how this was measured etc...A difference of 0.5 dBV corresponds to a signal ratio of 10^(0.5/20)=1.06, i.e. the signal at 20 kHz is 6% smaller than it should be. Not very dramatic, and most likely this discrepancy is a result of the different measurement methodologies that were applied in the 80s.

Since everything was hooked up, I decided to measure the FRs for the various filters the 8000 has. This shows the spectra:

The spectra show the FR for filter 1, 2 and both active, for bass and treble sliders set to minimum and maximum, and the flat (filter button "off") response in direct comparison. We can see nicely that the treble and bass sliders allow a ~±10 dBV change of the higher and lower frequency ranges, and that the filters cut off around 7 kHz and 10 kHz at  as prescribed in the manual.

So in summary, I think we can say that this Beomaster is in excellent condition and that everything works as it should.

Beomaster 8000 Circuit Upgrade: FM Audio Muting During Use of Phono Input

After reviving the Beomaster 8000 that came for a visit from Australia with new processor crystals, it was time to do some additional work on it. My customer was annoyed by the crosstalk between FM audio and the phono input while listening to vinyls records, and a solution to alleviate this issue was requested.

Phono inputs are especially susceptible to crosstalk since they are about 100 times more sensitive than regular high level line inputs. The result is that when the phono input is selected (with a player attached or grounded inputs) the FM audio signal can be heard weakly in the background (if a station is dialed in). And that is of course not very Beolovely. The reason for this behavior is a design flaw in the Beomaster circuit: When another input than FM is selected and previously a station was tuned in the FM tuner stays on and continues blasting its signal out to the amplifier board, even though one wants to listen to a record via the phono input.

This diagram shows the solution I developed after a bit of head scratching and downloading the data sheet for the CA3189E IF stage integrated circuit.

The CA3189E chip is responsible for shifting the audio modulation of the FM signal that is received in the front end of the tuner to a lower frequency and amplifying it before the audio signal is then extracted in the detector. The CA3189E was designed with radio applications in mind that do not have a microcontroller at their disposal. The Beomaster mutes the FM audio while tuning between stations to prevent the speakers from emitting loud static noise. It does that by extracting signal strength information from the CA3189E via pins 13 (TP22) and 12, and feeding it into the microcontroller after processing in some additional circuitry. The microcontroller then decides when to ramp the output volume down using the main volume attenuators. 

Alternatively, this chip can do this on its own via its audio mute input (pin 5). The CA3189E data sheet contains a circuit that shows how to do this if you are interested. In the Beomaster 8000 design this pin is simply grounded via R37 to permanently turn the signal output on. 

The circuit on the data sheet in combination with some measurements of the signal strength related output of pin 12 (it seems to be close to 0V between stations and at about 5V with a strong FM station dialed in) suggested that pin 5 could be controlled directly via the microcontroller signal that is fed into the two AD10/278 input selector chips on the preamplifier board. The AD10/278s have three inputs for selecting FM, Tape 2 and Phono. When one selects one of these inputs on the control panel of the Beomaster 8000, the corresponding input goes high (~4.8V), activating that particular signal path on the chips. So my idea was to simply connect the Phono selector line to the audio mute input of the CA3189E via R37 after severing the ground connection of R37 by unsoldering the resistor on the ground facing end. That way, whenever Phono is selected the FM audio muting circuit is automatically activated muting any FM audio in the system that could crosstalk with the phono input.

This shows the practical implementation of this scheme. I elected to make the AD10/278 connector directly at the ribbon cable coming from the microcontroller board. That way the additional connection can be removed from the amplifier board when it needs to be taken out.

I soldered the jumper wire to the plug contacts

and then modified the black cap with a bit of Dremeling to fit over the jumper wire:

Then I wrapped the plug with some electrical tape to keep the jumper wire from moving so it will not break off. This shows the plug plugged in next to one of the AD10/278:

I connected the other end of the jumper to the disconnected end of the R37 resistor after securing it with some shrink tubing to the capacitor located next to it. The solder spot was secured by some additional white shrink tubing:

And then it was time to turn the Beomaster back on. And it seems to work smoothly. FM still works and when Phono is selected there is no more FM audio superimposed. Beolovely!

Beomaster 8000: Input Selector Trouble - Tuner Audio Mixed with Phono or TP1 Inputs (Pt. 2 - Conclusion)

I thought this was fixed! But it wasn't! Last year I reported about a strange intermittent issue with a Beomaster 8000 that I was restoring: When switching the input to Phono it sometimes mixed in the tuner signal that both turntable signal and tuner signal were audible at the same time...a pretty useless situation. My investigation back then determined that there was a discontinuity around P82 on the uProcessor board, which occasionally caused the input switches on the preamp board to malfunction. P82 carries the control signals for the two electronic multiplexers that control which input is routed into the preamp-circuit of the Beomaster. I implemented a bunch of jumpers, shown here:

and the problem appeared to have gone away.

As it turned out recently, this was an illusion as the Beomaster again showed this issue when I tried to play a lovely 1950s vinyl by Cal Tjader of which I bought a significant number recently. 

My initial thought was that maybe the ribbon cable that connects the input selector pins of the uProcessor to the preamp board had an issue, and so I ordered a custom manufactured 5-lead ribbon with appropriate 0.1" headers and put it in. Here is an impression of the end plugged into the uProcessor board:

And a photo of the preamp end:

While this new cable made a great connection and looked very pretty, it did not cure the problem. But at least another potential root cause was ruled out.

Back to the bench it went. I opened it up (again) and wiggled P82, and there it was (again)...depending on the pressure on the brand new connector the input behaved correctly or malfunctioned. I took the board out and had a closer look. All the jumpers were in good shape, so the problem had to lie somewhere else. However, all the traces checked out o.k. (again) and so I was a bit at a loss.

I put the board back in and played the Beomaster for a while scratching my head. Then it occurred to me that such problems are often related to ground connection issues. I prepared a jumper cable from the preamp board to the uProcessor board. Then I induced the issue and subsequently made a separate ground connection between the boards with the jumper. And, eureka, the input out of a sudden behaved normally. With the jumper in place I was not able to produce the issue again. I took the board out again and inspected it more. And I finally understood how the grounding plane is setup on the uProcessor board. I realized that this board is actually a double sided PCB, even though it has no etched traces on the back side. However, they used the continuous Cu layer as a ground plane from which several connections are made through the board to the traces. This is hard to see since the surface just looks a uniform green and there is no obvious mention in the manual where they show the PCB layout. After this it became clear to me how the ground connection to the preamp board was made. This shows the pertinent part of the board:

C67 connects through to the patterned side of the board and so makes the ground connection that connects to pin 2 of P82. The picture shown on top shows the backside of this area. I had soldered a jumper in from the C67 leg to Pin 2 of the header to bridge potential cracks in the trace connecting the two. What I did not consider was that the problem was rather caused by a 'cold' solder joint on the other side causing an intermittent contact. I resoldered the area and then the problem went away. Another demonstration that most control issues in the 8000 are caused by problematic solder joints on the uProcessor board.

Back to testing it for a bit longer, but I think it may finally be ready for its journey to its new owner in the UK.

Beomaster 8000: Input Selector Trouble - Tuner Audio Mixed with Phono or TP1 Inputs

*************************Achtung: There is a follow up to this post. **********************

Oh well...I ran the Beomaster 8000 that I finished up recently for some time in my Beosystem 8000 setup and all was well, but then it developed a strange issue: When switching from FM tuner to Phono the phono signal was audible, but the tuner signal was still there in full strength. So instead of switching from one input source to another the Beomaster decided to mix them together instead! So back to the bench it went!

After I opened it up I discovered that I could provoke the issue by wiggling P82 on the uprocessor board. P82 transmits the microcontroller's wishes to the input switches on the preamp board. So this made immediately sense: The FM input did not get turned off when opening up the Tape 1 or Phono inputs due to an intermittent connection (the audio input switches on the preamp board are basically gates that open and close depending on high/low control signals from the processor).

First I thought it was a plug issue, and I cleaned the contacts a second time and bent the header bins a bit to give everything a bit more mechanical intimacy, but the problem persisted. So I came to the conclusion that the issue must be something more serious, like cracked traces or bad solder joints. This photo shows the original condition of the P82 header from the solder side:

Close optical inspection did not reveal anything abnormal, but I was able to demonstrate that at least Pin 1 had an issue since I was able to 'disconnect' it from the component it is connected with by flexing the board a bit. So I put in a magnet wire jumper and put the board back in. After firing the Beomaster up, well, no change...o.k. I took the board back out and this time I jumpered all five solder points to their next neighbors:

Magnet wire is really very helpful for this type of fix, since the polyurethane coating burns off when heated with the solder iron. This allows to make safe wire connections in tight spaces.

After this fix, the problem seemed to be gone. So I put the Beomaster back together...let's see if it is stable now! I usually test these babies for a few weeks until I send them back to their owners. There are just too many components and interconnections that can develop problems, and often they are intermittent. The only way to make reasonably sure that the experience on the customer end is a happy one is to give them a good testing under normal operation conditions. But I sure hope that this was the last post about this particular Beomaster 8000...;-)!

Beomaster 8000: Step Four - Rebuilding the FM Tuner, Preamplifier and Control Panel PCBs

After completing the power supply restoration of the Beomaster 8000 that I am rebuilding right now, it was time to do the remaining boards. While it is relatively rare to find a dead capacitor on the FM tuner, preamplifier and control panel PCBs (they run relatively cool), it is a great idea to replace them anyway while one is in there. That brings everything on the same 'capacitor clock' (for another 30 years countdown...;-). It is also a great idea to re-solder all the wire to board headers since they tend to develop bad solder joints over time. Here are a few impressions of my progress:

This shows the board under the control panel switches and sliders in its original condition:

And after replacing the capacitors:

On to the preamplifier board:

In original condition:

and after rebuilding it:

The FM tuner has two boards, the front end and the detector part:

This shows them after exchanging the capacitors:

On to the display and uComputer boards!

Beomaster 8000: Inputs Not Working

A Beomaster 8000 that I rebuilt last year arrived a few days ago for trouble shooting. The owner told me that he suddenly lost all the inputs. No matter what input button was pressed nothing was audible, while the display would show the right input etc...This indicated to me that the uProcessor was working properly. So I opened it up with the plan to look into the chain of command between the uProcessor and the input selector chips. Since both channels of any input did not work, I hoped I could rule out a failure of the input selector chips. 

Once I had the unit open, I turned it on. Everything worked. That is of course frustrating if the problem is intermittent. I started methodically wiggling the connections between the uProcessor and the input chips. Nothing came up, everything was stable. 

Upon further reflection (while listening to some nice NPR jazz on the frustratingly working Beomaster) I determined that an interruption of the power supply of the preamp board would have a similar effect. So I checked the connections between the power supply board and the preamp. And indeed, when wiggling P46 that carries the power rails to the preamp board, I was able to get the inputs to turn off occasionally. So there was a bad contact somewhere. Here is a picture of the area:

The colorfully wired vertical plug is the one in question. This area is also prone to cracked traces on the circuit board due to the multitude of connectors that torque the board in that corner. A bit of a design weakness.

I took the board out and inspected for cracks and the like, but was not able to find anything obvious. So I resoldered all the headers and cleaned them and put the board back in hoping that a hairline crack in one of the header pin solder points was the root cause. After reconnecting all plugs, I fired the Beomaster up and wiggled the plugs in that area again, and nothing happened anymore. Seems everything is stable now. So it seems this issue may be fixed...but of course with intermittent stuff, one never really knows until one knows.

Beomaster 8000: The Mystery of Swapped Tape 1 and 2 In- and Outputs and a Broken Damper Arm (Sad Story)

A Beomaster 8000 that I recently restored showed a strange phenomenon when I hooked it up to my Beogram 8000 and Beocord 9000 for in-situ testing: When I pressed start on the Beocord, the Beomaster came on with the Tape 1 input selected as it should due to the Beolink connection between the two. However, there was no sound from the tape (and no recording either as I found out shortly after). However, I was able to hear the playback signal very faintly when I cranked the Beomaster up to full volume. This indicated that there was something fundamentally wrong with the Tape inputs. The plot thickened when I hooked my iPad up to the Tape 1 input and only got sound out of the receiver when selecting Tape 2. Then I connected it to the Tape 2 input, and, you guessed it, I needed to select Tape 1 to be able to listen to it.

It appeared that the inputs were swapped...very strange!

The Beomaster went back to the bench, and I opened it up. Then I traced the signals to plug P3 on PCB3 (preamp). And indeed both in- and output leads were swapped on this plug, effectively connecting Tape 1 to the Tape 2 connections on ICs 202/102, while Tape 2 was connected to IC4.

I removed the terminals from the plug housing and replaced them in the correct order. I made a schematic for future reference:

After this I thought everything was fine, and I tested the inputs. And indeed, now I plugged in the source to Tape 1, and it played when selecting Tape 1 as input. The same for Tape 2. Great! (I thought!). But then I played with the 'input level presets' (potentiometers accessible from the outside of the Beomaster that allow to attenuate the inputs if a signal source is too strong). This yielded another surprise: Now the Tape 2 presets affected the Tape 1 input, and the Tape 1 presets affected the Tape 2 input...very strange! But of course much better than the original situation where I was not able to use my Beocord properly. Still strange!

After scratching my head a bit, I remembered that some Beomasters come with a different connector box, where the tape RCA connectors are connected to the Tape 1 input instead of Tape 2. At that point it dawned on me what probably happened to this Beomaster 8000 of unknown ebay provenance:

The previous owner swapped the original input box that had the RCAs on Tape 1 with an input box  from an other Beomaster that has the RCAs on Tape 2, and it seems that these two configurations have Tape 1 and Tape 2 swapped on P3 that connects to the circuit board.

Here is a pic that compares the two boxes and shows their P3 terminal arrangement:

Note the different arrangement of the Tape in and out RCAs on top and the input level presets labeled below. Tape 1 and 2 are swapped. At this point it escapes me why B&O would have changed the P3 pinout...there is really no reason, they could just have connected the RCAs to Tape 1, relabeled the box and been done with it.

It is interesting to note that all Beomaster 8000 circuit diagrams that I so far have seen show the "standard configuration", but there is a significant percentage of Beomasters that have the "RCA on Tape 1 configuration"...

Anyway, this Beomaster is working now as it should (as long as one remembers which input level presets to use) and my Beocord works happily with it.

Some bad news: While putting the left side of the Beomaster back together after rearranging the inputs, a 'petit malheur' happened: When I bolted the control panel lid back onto place, and closed it a loud crack occurred, and then the lid was undamped. first I thought the linkage broke, as it has before, but no, a more spectacular problem arose:

The damper arm cracked into two pieces:

This picture shows the arm fragment still attached to the linkage with the broken off part:

Here is a shot of the two cracked parts assembled with the damper:

And the broken arm separately:

I guess it is time to fire up a 3D printer...;-). Small parts like these are an interesting printing challenge to get the proper fit and stability. Stuff for another blog entry.