Beocord 5000 (4715/4716): Installing Rebuilt Rubber Pinch Rollers

Extensive tests of the Beocord 5000 (4715/4716) that I rebuilt recently (links to the relevant blog entries) revealed that occasionally, under certain conditions (air humidity in the 60% range and/or low room temperature below ~65F), the leading capstan would stop advancing the tape, with the unwanted consequence that the feed-in capstan would push the tape out into the space between the erase head and the feed-in pinch roller. Luckily, the auto-stop feature of the Beocord reacts fast enough to catch this happening before the tape gets damaged. One of these events is demonstrated at the beginning of the video below. 

Experimentation revealed that pressing the leading pinch roller slightly towards its capstan with a finger solved the issue. Cleaning of the roller and capstan did not help much. And since the spring that controls the roller pressure looked pristine and its adjustment nut showed an intact B&O applied red paint spot, I came to the conclusion that most likely the rubber of the pinch roller had hardened, reducing the friction, causing occasional loss of transport. It is important to note here that the tape is transported forward by the friction between the roller and the tape, and not by friction between the capstan and the tape. The capstan only drives the roller via contact above and below the tape.

I searched a bit around on the web trying to find replacement pinch rollers for the Beocord, but to no avail. The issue is that the rollers have a fairly 'rare' diameter of 10.8 mm, which apparently was not used by any of the major tape mechanism manufacturers. It seems that most pinch rollers that were used back then were 1/2 inch. But surfing around on some 'tape heads' web sites and message boards soon yielded the name Terry Witt, who appears to be the only person on our planet who professionally rebuilds classic tape pinch rollers. I read some positive reviews about his work, and so I decided to get in touch. After a short email forth and back, I decided to extract the rollers of another Beocord that I had in my closet and sent them to him for rebuild. I was very pleased by the results! Here is a picture of the rebuilt rollers together with his business card:

The main challenge with the roller rebuild is to extract them from the Beocord's roller arms. Essentially, one needs to use a vise or press to push out the pins on which they run. Nothing for the faint of heart, but definitely doable. I made a YouTube video about the process:

I tested the deck by now for a few hours after inserting the rebuilt rollers, and everything seems to work great now. But I will only believe it after another couple weeks of testing...one realizes that mechanical issues are where the challenges lie with these old vintage units!

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.

Beocord 5000 (4715/4716): Exchanging the Belts

Once the tape mechanism was extracted from the Beocord 5000 (4715) that I am currently rebuilding it was time to replace the limp decaying original belts. I pretty much followed the procedure that I am showing in my YouTube video:

I improved the procedure in one point, however: I realized after doing it the first time that the alignment of the eject lever relative to the tape mechanism is crucial to get the proper action point when pressing the eject key once the unit is completely reassembled. Last time it took me quite a while to get this properly readjusted, since I only marked the position of part #256 ("lever") with a pencil, which was not precise enough. This time, I put a couple dabs of white paint where the part connects to the drive mechanism. Here is a picture:

This resulted in a much more accurate reassembly once the belts were in.

Here are a couple pictures of the newly installed belts:

I did the bottom belts first:

Here is a shot of the belt that drives the spindles and the free-wheeling mechanism. I also put a new light bulb in there - the old one looked pretty black, indicating little remaining lifetime:

After I put the drive back together and plugged it in, it turned out that the take up spindle would only occasionally feel like doing its job, usually after a making a clicking noise. It turned out that the retaining ring that holds the spindle assembly together (part #173 on the schematic below) was not seated properly. This caused the axle to have to much play at the top (top part of sequence #168), which resulted in random disconnects from the second part from the top in #168. Since this part engages with the cassette tape, this caused the tape not being wound anymore during play. A crumpled tape was the result. Luckily, I expected such issues, and used an old unwanted tape that has already a bunch of crumply regions...;-).

Here are a couple pictures of the part taken when I extracted it for inspection:

The small wheel at the bottom drives the counter, and is below the center plate of the mechanism. The above picture shows how I extracted the part after removing the retaining ring that holds the counter wheel in place, and the entire mechanism together. 

After I put everything back together a second time, the mechanism started to work normally. On to the recap of this unit.

Beocord 5000 (4715/4716): Extraction of Drive Mechanism

Experience makes life easier! This is my second Beocord 5000 restoration, and so I further streamlined my 'tape mechanism extraction' procedure that I outlined in my earlier post about replacing the belts in these units where I also posted a YouTube video about this process. The first step is to open the unit up. This time I took the plexiglass cover off since it needs to be polished anyway. This makes it much easier to extract the tape mechanism. I also lifted the preamplifier and radio amplifier boards up, since this unit will receive a full recap. Here is a picture of the 'unfolded' unit:

Inspired by earlier comments by 'chartz' of Beoworld.org that one can leave the solenoid in the enclosure when extracting the mechanism I realized that one does not even need to loosen the solenoid fixture if one takes out the screw that holds the bottom cover of the drive in place before taking the mechanism out (I wondered last time why there are three access holes in the front of the enclosure, but did not realize the purpose at that point - live and learn...;-).

That way one does not loose any of the alignment, except for the height of the tape mechanism. This issue was tackled by putting some small dabs of white paint on the two screws that hold the mechanism up relative to the solenoid bracket. Here is a picture of the right screw, and the screw that holds the bottom cover (that is the one that sits in the square cutout of the solenoid bracket):

When I took this picture, I was still of the opinion that I needed to move the solenoid a bit forward to get the tape mechanism out, so I marked the position of the bracket relative to the enclosure at the bottom, too. But as said above, this is not necessary. All that is needed is to mark the position of the screws that hold the mechanism to the solenoid bracket (the slightly rusty screw near the center of the photo). 

So, in summary, all one needs to do to get the drive out is to take these three screws out at the front, unplug the plug that connects the heads. After driving the counter screw at the back all the way out the drive can be lifted up and unplugged from the connector at the back and taken out (this pic shows the solenoid bracket loosened - I was still in the learning process...;-):

The next step is replacing the belts.

Beocord 5000 (4715/4716): Polishing of Plexiglass Panel

I started working on another Beocord 5000 (4715/4716) restoration. It appears that cassette tapes are making their comeback to catch up with vinyl...;-). What better way to spend a Saturday afternoon than with making a mixtape for a loved one! I miss this way of showing affection...these days, all we do is swap Spotify playlists...not much Labor of Love in that! 

In this spirit I started out with polishing the badly scratched panel of a spare Beocord 5000 as a practice run for the better-condition panel that will go with the Beocord once I am done with its internal restoration. I felt I needed a practice run, since I worried about the coating layer that is on the plexiglass panels of these B&O units. It appears they felt that this coating would improve the shininess of the surface, since it appears that it does not change the tint of the panel. After I polished it, the tint appeared very similar despite the missing coating.

Here is a picture of the heavy damage of this panel:

I essentially followed the polishing procedure that I applied to the panels of my Beolab 4500 speakers. See here and here for a detailed description of this process.

The main difference with the Beocord 5000 panel occurred during the first sanding step with 220 grit dry paper to get rid of the heavy scratches. When I started sanding, immediately the coating disintegrated into tiny shreds and I observed this wool-like appearance:

After I worked through this step everything was as usual and I progressed through the ever finer sandpapers of the Micromesh polishing kit. A pretty good workout! After I went through all the sanding steps I followed up with their polishing solution, and then I ended up with a pretty nice result:

There are still a few minor scratches that are visible to detailed scrutiny, but I think this panel would now look very nice in the usual setting in a living room etc...It is very difficult to get them absolutely perfect since the slightest oversight in one of the polishing steps requires to go back to this step and repeat the entire procedure from that point on.

Beocord 5000 (4715/4716): Fader Fixed and Putting it Back Together

I looked into the Fader of the Beocord 5000 (4715) that is (was, hopefully) on my bench. It turned out that the contacts of the Fade In and Fade Out control buttons were strongly corroded. They simply did not make contact. I pulled a strip of paper coated with DeoxIT D100 through the contact pads, while pressing the buttons, and this cleared things up. The paper had nice black streaks of them, indicating a fair amount of oxide removal. I did the same with the other buttons, giving this deck an as-new user feel. Swift response whenever a button is lightly pressed...very nice!

This encouraged me to put the deck back together. But first I needed to re-glue the veneer on one of the corners of the wood panels. I used some wood glue that I carefully inserted in between the veneer and the particle board using a thin needle. This allowed me to avoid bending the veneer to much. Here are a couple of pics of the procedure. I used snippets of transparency foils to prevent accidentally glueing on the cardboard pads that I used to protect the wood from damage by the carpenter clamps.

Once the corner was stabilized I put the wood panels back on the BC5000, and connected it to the Beomaster 6000 for a first tryout with the Beogram 4004. I taped Water Babies by Miles Davis, one of my all-time favorites of my vinyl collection. This went very well. There is almost no difference between the recording and the original. Also, the 'infamous' double capstan mechanism does not seem to have a problem...no eaten tapes so far, and the recordings seem to be very consistent. Here are a couple impressions of the kit in action:

And a somewhat shaky GIF of the entire set-up while taping (it is pretty long and a linear sweep is pretty difficult with a cell phone...;-):