Beolink 5000: Glueing the LCD Screen Back In

The Beomaster 4000 that I recently outfitted with a toroid transformer came with another Beolove challenge piggybacked into the box: A broken Beolink 5000 remote control, which had a detached LCD screen. Unfortunately this Beolink 5000 was beyond repair since the LCD screen had mechanical damage at the contacts end, probably due to a fall of the display on a hard floor or similar. 

We decided to not give up but rather try to procure a potentially fixable unit from ebay. Luckily, a few weeks ago a 'lot' of three Beolinks became available with the promise that one of them was actually still in fully working condition. My customer won the auction and had the units shipped directly to my bench. Unfortunately, it turned out that the LCD screen of the working unit also was already loose,

but it had not detached yet from its ribbon cable. Luckily, the seller had the presence to tightly wrap this unit so the screen stayed where it was during shipping and it did come alive when I put batteries into the unit:

I decided to develop a fix for glueing the display back into the remote body without breaking the very flimsy plastic foil ribbon connector that connects the LCD display to the circuit board. In fact this particular design feature seems to do in 99% of the Beolinks since it simply rips off when the screen detaches and falls out due to ageing glue. The glue seems to be the same foamy stuff they used for attaching the aluminum cover to the Beogram 8000/8002 turntables, i.e. it is no surprise that the LCD screens also pop out at this point in time.

Anyway, the task was to not kill the ribbon connector, while handling everything and cleaning the old glue out etc...It was beneficial that I had the two other units with completely detached screens to practice on. I used them to develop a 'tool set' that would allow me to do the repair with minimal mechanical stress on the ribbon connector. 

Here we go:

I opened the unit up:

Then I carefully pressed the LCD screen out of its frame and moved the PCB and screen a bit to the left to release the board from its tabs on the right. Once the two parts were liberated I supported them with a custom designed 'brace' that would stabilize them in their relative alignment:

I applied the matching top part:

And flipped the entire 'assembly' over to the right:

Now I removed the board-to-board ribbon cable from the display control board:

The two tabs left and right of the ribbon can be pulled out and then the cable comes out:

This liberated the display part allowing me to go to work on removing the glue from the display frame:

The next step was to remove the glue from the LCD. The orange part of the brace now served as support during this procedure. I had designed a screw hole into left side of the orange part that allowed me to bolt down the circuit board to make sure there was no movement between LCD and PCB to protect the ribbon:

After cleaning off the glue it was time to install some industrial strength double sided tape in the frame. I had developed two more 3D printed parts that would allow me to apply the tape and then cut it to size with precision to not impair the translucency of the screen:

and with tape applied:

Then I cut the 'window' out of the tape layers using a razor blade and a matching 3D printed part that fit exactly into the display bay from the top:

This was the end result:

after removing the tape backing

Then I reconnected the display board and flipped it over with the braces in place:

After carefully pulling out the red brace part, I pressed the display into its bay, making sure that it was aligned properly:

And then came the moment of truth! Did the ribbon cable survive this stressful operation? After putting the keypad back on and inserting the three batteries it turned out that the operation was successful. The unit came alive again!

The next step is to figure out how to repair completely detached screens...another day, more Beolove!

Beocord 5000 (4715/4716): Test Drive

A happy day! The second Beocord 5000 (4715/4716) is back together and is playing happily with its friends, the Beomaster 6000 4-Channel and the Beogram 4002. I decided that the soundtrack of 'Alfie' by Sonny Rollins would be a nice celebratory first recording:

Alfie is definitely one of my favorite Michael Caine movies, and the sound track is just awesome! Here are some impressions of the recording session:

Beocord 5000 (4715/4716): Polishing the Plexiglass Cover

My current Beocord 5000 (4715/4716) restoration is coming to an end. Today, I polished the plexiglass cover, which came out fairly nicely. I followed the procedure outlined earlier. Here is a picture of the outcome (it still 'wears' the protective tape to shield the back part of the panel from the polishing process):

Beocord 5000 (4715/4716): Calibrating Recording Current and Bias

Happy that the Beocord 5000 (4715/4716) tape mechanism seemed to work, I made a first test recording using my waveform generator. When playing back the tape, I realized that one of the channels was off by about 3 dB on the Peak Program Meter (PPM). This meant that the calibration of the deck was off. Not a surprise, considering the age. I guess this also meant that I was just lucky with the first Beocord 5000 that I rebuilt for my Beolab 6000 set-up, which performs admirably without doing a thorough calibration. Anyway, I felt that the first step towards a solid calibration would be to get a reliable 'frequency response' measurement in place. Frustratingly, none of my bench equipment offered anything useful in this direction. I looked around for free software that would let me do such measurements with my computer (the audio range is quite unchallenging to modern sound cards in PCs).

I was not able to find anything open source that would enable such measurements without too much hassle. But I finally found the 'audioTester', which was written by an enthusiast (Ulrich Müller) in Germany. He offers an evaluation shareware version that allows to do measurements for 10 min before a restart becomes necessary. I downloaded this version onto my old MacBook Pro that I use as my go-to Windows 7 PC in Bootcamp mode whenever I need to use PC-only software. The installation went well and the built in sound card of the MacBook seems adequate for the task. After a bit of experimentation (the owners manual is a bit scant) I was able to make a non-synchronized sweep-measurement to determine the frequency response. It needed to be a non-synchronized measurement since the 2-head Beocord does not offer a monitor function for listening to the recording during the recording process. During a non-synchronized measurement the software determines the frequency of the sound and plots the signal level relative to the measured frequency. This means that for a response measurement, one needs to record a sine wave sweep across the entire spectrum (I did a 60s logarithmic sweep from 10-20kHz), then rewind the tape and during playback the measurement is done. I tested the mechanism by directly feeding the 60s sweep from my waveform generator into the audioTester. The measurement yielded a constant level across the entire spectrum, indicating that the signal path through the MacBook sound card was linear.

After this I did a baseline measurement of the Beocord at 0dB. This requires to adjust the signal level for recording in a way that the Dolby B ICs put out 740mV(RMS) signal at their pin 7. This can be achieved by setting the waveform generator to an amplitude of about 50mV(pp) and using the recording level potentiometer to adjust the recording level to 740mV(RMS) at pin 7 on both ICs for left and right signal path. Dolby should be switched off during calibration.

Here is a shot from my oscilloscope of the 0dB signal at pin 7 at 333kHz (the pp voltage of 2.3V is 0.2V too high due to noise). The RMS voltage is shown at the bottom right corner of the screen.:

Oscilloscope probe at pin 7 of the Dolby B IC for the right channel: 

After recording the sweep, the payback yielded this frequency response curve:

I was still in the learning process with the 'audioTester, which is the reason that the curves only start at 100Hz, but it is obvious that for high frequencies there is a several dB difference between the channels. This explains the discrepancy on the PPMs during playback.

So I set out to do a calibration. I followed the procedure outlined in the service manual. This procedure first adjusts the recording current that playback and recording signals are the same at pin 7 at 333Hz. This adjustment is done for a medium signal level of 200mV(RMS) at pins 7 of the Dolby ICs. The recording current is adjusted with the trimmers 1R99/47 for CrO2 tapes (I used a TDK SA 90). 

So the first step is to adjust the Dolby B output to 220mV(RMS):

The procedure to get the right adjustment is simply to make a recording, play it back and monitor the signal at pins 7. If the signal is too high turn the respective trimmer (L or R channel, and tape type)clockwise, if it is too low, counter clockwise. Repeat until about 200mV are achieved during playback. The trimmer adjustment is pretty sensitive, i.e. small steps (~5 degrees) are advised. On this pic I am adjusting the left channel during the calibration for a Fe2O3 tape:

Once the recording current is adjusted properly, it is time to do the 'bias' adjustments. This adjustment sets the amplitude of the 92kHz bias signal that is fed into the erase head. This signal needs to be of perfect amplitude that high frequencies can be recorded properly. The amplitude of the bias signal is adjusted with trimmer capacitors that tune the resonance frequency of a pickup coil that takes the signal up from the bias oscillator.

The procedure for this adjustment is as follows: Set input signal to 333Hz and 22mV(RMS), adjust record level potentiometers that you get 740mV(RMS) on pin 7 of the Dolby ICs. Now reduce the amplitude of the input signal by a factor 20 (-26dB) to 1.1mV (RMS). If the PPMs are calibrated right, only the lowest lamp should be on for each channel. Now set the input signal to 15kHz and adjust the bias trimmers for both channels (C70/72 for CrO2, and C71/73 for Fe2O3) that during playback the same -26 dB signal is seen at pin 7  (37mV RMS) as during recording. Here is a picture when I adjusted the right CrO2 trimmer:

Unfortunately (if one does not have the special tool that fits into the trimmers from the solder side of the PCB - I might 3D print myself one someday...;-), one needs to lift the preamplifier PCB every time an adjustment is made, while the recording requires the board in place that the recording switch can be activated by the solenoid, and that the board is properly grounded. Hence, this is a bit an annoying process. Also the trimmers are very sensitive, i.e. there are a few adjustment cycles to go through until the bias is adjusted properly. 

After the calibration was done for both tape types I measured the frequency response again with the audioTester. Here are the results (I compiled them into Excel graphs for both tape types):

It is remarkable that there is little difference between the two tape types. This is probably a testament to the quality of modern Fe2O3 tape materials. As to be expected the low level (-20dB for CrO2 and -26dB for Fe2O2) are much better at high frequencies than the 0dB curves and reach smoothly to 15kHz. This difference between low and high levels is a common trait of all tape recorders. It is also nice to see that the Dolby B system does not seem to introduce significant distortions. The curves are almost indistinguishable from the non-Dolby curves.

It is interesting to compare these curves with some curves of high quality 3-head decks that are posted online. These curves show a surprising wide range of curve shapes, some even with oscillatory features in them. It seems that the Beocord 5000 holds its own, especially when considering that the measured 3-head decks are all much more 'younger', and that all have a monitor function, i.e. the response was measured during the actual recording process, which optimizes tape positioning etc...

Beocord 5000 (4715/4716): Calibrating the Peak Program Meters (VU Meters)

I put the Beocord 5000 (4715/4716) back together. It seems to work now. I made a recording using a signal my waveform generator, and it seems the playback is undistorted across the frequency range and at a reasonable level. So I started to do some measurements to check if the deck is within spec. Before getting into measurements like frequency response etc...I thought it would make sense to start with calibrating the "Peak Program Meters" (PPM) as the meters are proudly called in the service manual. Seems fast electronic audio level meters were something new in consumer units back then in 1978...I studied the service manual to get some advice for their calibration. The manual suggests to directly feed 300 Hz from the waveform generator into the input of the Dolby B ICs, and then do the adjustment of the PPMs.

I chose a slightly different path and simply fed a 300Hz 25 mV amplitude (not pp) signal into the inputs of the Beocord, switched the deck into 'record' (paused), and then used the recording level meters to adjust each of the channels to get the prescribed 740 mV RMS at the outputs of the Dolby B ICs (pin 7). This can be measured with a multimeter switched to AC. This will give the RMS voltage of the AC signal. Once the 740 mV RMS are at the output, the trimmers for the PPMs can be adjusted for 0 dB (first red light).

Here is a pic of the part of the Operation Control PCB (#2) where the trimmers are located:

The two larger trimmers are for the 0 dB adjustment. Instructed by the service manual, I adjusted them until the 0 dB lamps just lit up. The next step was to do the -25 dB adjustment. This calibrates the slope of the meter. This adjustment requires to lower the output signal at the Dolby B ICs to -20 dB relative to the 0 dB 740mV RMS signal. -20 dB corresponds to a factor 10 of the amplitude of the signal, i.e. I adjusted the record level sliders to get 74 mV amplitudes at the Dolby B outputs. The service manual prescribes to adjust the -25 dB trimmers in a way that the -25 dB lamps just light up. Not sure why they recommend to get the -25 dB lamps to light up, and not the -20 dB ones, considering that the signal level was lowered by 20 dB...anyway, I decided to stick with the service manual, and did the adjustment. Then I did the 0 dB adjustment one more time (as suggested by the manual). After this adjustment I can now trust the meters, which sets the foundation for a successful frequency response measurement.

Beocord 5000 (4715/4716): Recapping the Power Supply, Motor Control, Electronic Switch and Operational Control Boards

And the recap continues! Today I did the remaining electrolytic capacitors of the Beocord 5000 (4715/4716) that sits on my bench right now. Always feels great when all the electrolytics are new! Piece of mind!

The remaining boards were the power supply including reservoir caps and the AC motor cap, motor control, electronic switch and operational control boards:

This is a shot of the Motor Control board that controls the take up motor. Only one Tantalum capacitor to replace:

After replacement:

Here is the Electronic Switch board that manages the keyboard and houses the counter and tape transport monitor. A picture before the recap:

And after:

On to the Operational Control PCB. Before:

and after:

Below is a picture of the two recording volume sliders underneath the Operational Control board. I just love this type of B&O 1970s solution. Basically an analog version of digitally simulated sliders on a touch screen...;-). The two red strips are pushed into a white lit compartment underneath the plexiglass cover. This gives the illusion of a linear indicator that gradually changes from white to red as the sliders are pushed to higher volume. The toothed racks drive the potentiometers on the Operational control board that actually take care of the recording volume adjustment:

And finally the pictures of the power supply. Only one electrolytic capacitor directly on the board. Before:

and after:

The power supply has several external reservoir capacitors. This is the 1 uF capacitor on the +12V regulator. Before:

and after:

And the main reservoir capacitors of the power supply together with the AC motor cap. Before:

And after. The two small back-to-back polar 33uF capacitors replace the original bipolar 16uF capacitor on the capstan AC motor:

I guess now it is time to put everything back together for a test! Exciting!

Beocord 5000 (4715/4716): Recapping the Preamplifier and the Radio Amplifier Boards

Time to recap the Beocord 5000! Lots of tantalum capacitors in these units. I usually replace all tantalum caps with modern quality Japanese electrolytic caps. Tantalum capacitors can catch fire if they go with a short circuit due to the strongly exothermal reaction that Ta performs with oxygen, i.e. it is a good idea to replace them all at this age (~35 years). In the 70's Ta capacitors were used due to their much smaller size compared to same vintage aluminum based electrolytic capacitors. Today's manufacturing technologies allowed the Al capacitors to catch up in the size game, i.e. modern Al cans can have a similar footprint as the 1970s Ta capacitors. Therefore, there is no reason anymore to replace Ta capacitors with Ta models.

Here we go: A picture of the Preamplifier board before recap. All the small blue dots are Ta caps...:

And here after exchanging them - looks much more boring due to the black color theme of most modern electrolytic capacitors. I would love if they came up with more colorful components again!

This is a photo of the 'Radio Amplifier' board before recap:

And after:

Tomorrow, I will do the remaining boards and the reservoir caps of the power supply.