By popular request (really, I got quite a few emails about this!...;-), I finally completed my Beogram DC motor restoration video! It demon...
Saturday, January 31, 2015
This entry is a follow-up to my recent posts about my Beogram 4002 (5513) remote control (check out the YouTube video). I recently drew up a plastic 'tray' that insulates the receiver board inside the turntable from the solder points on the main circuit board (my temporary approach to tape a plastic foil to the bottom of the PCB was not very beolovely in my opinion...;-). Yesterday, I received the part. I designed the tray to make a press-fit with the circuit board, so there are no bolts needed, it simply clicks into place. Here are a few impressions:
Tray and board before assembly:
The three rectangular recesses in the tray give the solder points of the through-hole parts some space.
I am planning to design one more part, which will allow the bolting of the receiver board to the main PCB of the turntable utilizing the original screw that is underneath the receiver. More about this in the next update.
Friday, January 30, 2015
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:
Sunday, January 25, 2015
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):
Wednesday, January 21, 2015
This is a follow up to my last blog entry about restoring the damping of the Beomaster 8000 volume wheel rotation. Often, the volume encoder looses all its damping over time, which can create an unpleasant situation when someone uninitiated to the issue accidentally cranks up the volume to 6.0 because the volume wheel does not stop spinning.
My latest solution involves a 3D printed 'paddle wheel' that press-fits on the volume encoder shaft. Together with a more heavy duty high viscosity damping grease, the Nyogel 767A a very smooth purely viscous damping effect can be achieved. I made a short video about the new part and its installation:
Monday, January 19, 2015
I finally received the 3D printed reservoir capacitor adapters for the Beomaster 8000 that I am currently rebuilding. So I set out to install the new capacitors as final step of the full recap of this unit. Here is a picture of the Makerbot II printed adapters together with the new 10000uF capacitors (105C power supply capacitors made by Nippon United Chemi-Con EKMH630VSN103MA50M):
These adapters give the modern, 35mm x 50mm capacitors the proper size to fill out the compartments of the Beomaster. For installing new reservoir capacitors the Beomaster needs to be dismantled substantially:
Once access is enabled, the procedure is fairly simple: Unsolder the old caps, and put the new ones in. Here are the pics of the right channel. Before:
And the left channel. Before:
And this concludes the recap of this unit!
While I waited for the 3D printed adapters, I fixed the broken out left hinge of the control panel cover. Luckily the fragment did not get lost...I glued the broken out piece back into place with epoxy glue. So far this seems to hold up fairly well...only time will tell if this really worked and whether it will survive shipping to the UK. Here is a picture of the glued-in fragment:
Sunday, January 18, 2015
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...
Monday, January 12, 2015
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.
Saturday, January 10, 2015
I redesigned the circuit board for the remote control receiver that I recently built for my Beogram 4002 (5513), and had a few manufactured by a professional PCB manufacturer. The boards came today, so I had to populate one of them right away and give it a spin in the Beogram...;-). Exciting!! I always love to see a new circuit board come together.
The main design changes were to put all components on the top side to ease the assembly process, and to move the ISP port out of the way of the fixture that holds the top aluminum panel. I also removed the reset button from the design and gave the keypad connector some more room. Furthermore, the location of the header that allows to directly plug the board into the keypad socket now protrudes from the main board area. This makes it much easier to plug it in, since one can now see next to the header pins and direct them into the proper female jacks on the keypad socket. This was difficult with the prototype. Here are a couple pictures of the assembled board. Front:
The large hole is for access to the screw that holds the main PCB of the Beogram. I am planning to give the receiver a 3D printed 'tray' to fix it in place with that screw, and to insulate it from the solder points of the board below. For now, this insulation is provided by a cut from a overhead transparency taped to the bottom of the board - not too pretty, but works. After populating the board the Atmega328p microcontroller needed to be programmed. I hooked the board up to a bench supply and connected the AVRISP MkII programmer and gave it some brain:
Now it was time to install the board. Here is a photo of the board plugged into the socket of the keypad connector. The connector itself is plugged into the socket on the board. This preserves the keyboard functionality.
This looks pretty nice now!...Perfect location of the IR receiver cable, too. I am looking forward to having the printed tray. This should make it fully plug-and-play. After reinstalling the panels the Beogram functioned identical to the prototype (as recently demonstrated in this YouTube video).
Thursday, January 8, 2015
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:
Here is the Electronic Switch board that manages the keyboard and houses the counter and tape transport monitor. A picture before the recap:
On to the Operational Control PCB. Before:
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:
The power supply has several external reservoir capacitors. This is the 1 uF capacitor on the +12V regulator. Before:
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!
Wednesday, January 7, 2015
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:
Tomorrow, I will do the remaining boards and the reservoir caps of the power supply.
I continued recapping the Beomaster 8000 that sits on my bench right now. I rebuilt the filter and tone control PCB that sits underneath the control panel, the two tuner PCBs, and the preamplifier board. As usual, I used quality Japanese 105C capacitors. I also cleaned all board connectors with a fiberglass brush to remove oxides, and then I coated them with DeoxIT D100L to protect them from future decay. Here are some pictures of the boards before and after:
Filter and tone control PCB before:
Tuner boards before:
and a detail shot:
And the preamplifier board before:
and a detail shot:
On to the main reservoir caps for the outputs!