Beogram 4004 Type 5526: RIAA preamp board hiccup and fix on Beogram 4004 Turntable #1 from Dallas

In the last post of the Beogram 4004 Turntable #1 from Dallas I installed Beolover's custom RIAA preamplifier board he designed for the Beogram 4002 turntable.

I left off in that post with the Beogram 4004 ready to check that the original functionality of the modified output board still worked. In other words, the normal mode where the phono cable is connected to an amplifier that provides the RIAA preamp.

That test worked perfectly so it was on to try out the new RIAA preamp mode of the Beogram 4004.
I switched the two connectors from the original module 8 connectors P8 and P9 to the two connectors on the Beolover RIAA preamp board.

Now I would be able to connect the Beogram phono cable to an amplifier's auxiliary, tape or CD source input connectors and listen to the Beogram.

I was surprised when I tried that and the Beogram 4004 would not respond to any of the control panel buttons.

Thinking about it for a few moments I realized that while the Beogram output board on the Beogram 4002 and 4004 have the same P8 and P9 connectors, the BG4004 has an extra connector and circuitry for its remote control capability with a Beomaster 2400.

Here is a diagram of the Beogram 4004 output board and its additional remote control circuit.

The part of the diagram below the dashed red line is the phono output (with muting relay) that does not involve the remote control. The Beolover RIAA preamp board uses those two connectors to perform its function. Switching the P8 and P9 connectors between the original module 8 output board and the RIAA preamp board allows switching between the original, no RIAA output and the RIAA amplified output from the Beogram.

With the additional remote control support circuitry in the Beogram 4004 some of the P8 inputs are needed in the remote control circuit and work with the Beogram control panel via the additional P10 connector on the output board.

When I move the P8 and P9 connectors from the original output board to the RIAA preamp board the original output board loses its 30V, 21V and system ground wiring. Those are not only necessary for the remote control circuit but also for the control panel switches in the Beogram 4004.

That is the reason the Beogram 4004 acts dead when the P8 and P9 are configured for RIAA preamp mode.

The fix turned out to be simple. When the P8 connector is moved from the original board to the Beolover RIAA preamp I must have jumper wires keep those key pins (30V, 21V and system ground) between the boards connected.

Here is a diagram of the output board with the RIAA preamp board after the jumpers were added.

I retested the Beogram 4004 with the Beolover RIAA preamp board again by connecting it to the Tape 1 input DIN jack on my Beomaster 6000.

It works awesome!

This is a great mod for a Beogram 4002/4004 turntable. With this modification all of the record play bases are covered. In a vintage audio system where the amplifiers were designed with specific phono inputs you can use the Beogram with the output connectors in their original position. When you need to connect to a modern amplifier that has no phono record play support you can set the Beogram to use the RIAA board.

I will definitely want to do this same mod on one of my own Beogram 4002 or 4004 turntables.
Time to put this cabinet back together and get the Beogram ready for delivery to its owner.

Beogram 4004 Type 5526: Last minute mod on the Beogram 4004 Turntable #1 from Dallas

The Beogram 4004 turntable number one (of two) from Texas was ready to return home to its owner when a special modification was requested. The owner has a modern integrated Bang & Olufsen system from the late nineties (Beocenter 2500) that is now the intended receiver for this Beogram. That presents a problem because the modern systems do not come with a built-in RIAA preamplifier which is required to prepare raw phono cartridge signals for amplification. The resurgence in the popularity of vinyl records has meant a whole bunch of amplifier systems out there without support for phonographs.

There are quite a few third party solutions of course. Those involve a small RIAA preamplifier box between the turntable and the modern amplifier. That works quite nicely but there is a better way when it comes to a Beogram 4002 or 4004 turntable.

Beolover has designed and built a custom Beogram 4002/4004 RIAA amplifier board that connects right into the Beogram (on the existing output board).

The beauty of this design is two-fold.

1. The modification is internal to the Beogram so it doesn't require an external box and additional cables. The original phono cable is used as-is.

2. The modification retains the original connections so the Beogram can still be used with amplifiers that have their own RIAA preamplifier.

The installation of the Beolover RIAA preamp is quite easy.  Here is the sequence.

In most cases the installation is just to remove the phono output muting relay and solder in the custom RIAA preamp board. For this case I also had to remove the grounding option switch I had installed earlier in the project.

The grounding option is not necessary when using the RIAA preamp connectors. If the owner wants to change the receiving amplifier for this Beogram later, to one that has an RIAA preamp, then the grounding option is still nice to have.  The switch I had installed before would be in the way so it has been replaced with a simple two-pin jumper between signal ground and system ground.

Here is the Beogram 4004 output board with the new RIAA preamp mod re-installed.
This picture shows the board connected up for use as the original Beogram....without any RIAA preamplifier.  I want to test that the original functionality works first.  Then I will switch it to use the Beolover RIAA preamp connectors and test the Beogram in that configuration.

If any other Beogram 4002/4004 owners would like to try this preamplifier for their Beogram just contact Beolover at beolover@gmail.com.

Beogram 4004 Type 5526: Reassembly, Service Manual Checks and Record Play

Merry Christmas!

The Beogram 4004 turntable from Texas plays its first record since its restoration.

The original (and only) owner of this beautiful Beogram 4004 turntable was an avid jazz lover so as homage to him I felt the first record played on the restored turntable should be a jazz record. I am fond of the trumpet and I like Freddie Hubbard so there you go. The red vinyl reissue of Red Clay also makes for a cool photo.

Now to backtrack to how I got here.

In the previous post I had completed the electric work checks and I ran some functional tests of the Beogram's analog control system. Everything was working perfectly. Although the control panel button board didn't exhibit any problems this is a good time to clean the switch contacts.

The fixed contact looks really good while the contact that engages with it shows some tarnish. That is to be expected. I removed all of the fixed contacts so I could clean all of the surfaces with a fiberglass brush and coat the surfaces with Deoxit.

The next step was to reassembly the control board and get all of the loose pieces reinstalled in the Beogram cabinet for some service manual checks.

The first service manual adjustment was the turntable platter height. It turned out to just be an adjustment check as this turntable was still at its perfect adjustment set by the Bang & Olufsen factory. Pretty amazing when you think about this being 2018 and this Beogram was made in 1978.
Note that this adjustment is made with the platter belt removed.

It is worth mentioning that in some of the pictures the lighting shows there are finger-wear marks on the control buttons of this Beogram. That means this was a well used turntable. Even more remarkable at how pristine this Beogram is. It didn't just sit around for show.

I put the platter belt back on and performed the platter speed adjustments next. This Beogram had its DC platter motor restored by Beolover. Combined with new electrical components (capacitors and speed trimmers) in the controller board, the speed trimmers need to be dialed in to set the platter speeds correctly for 33⅓ and 45 RPM.

On the next service manual adjustments I removed the platter belt again to check the tonearm lowering limit, tonearm length, tracking force calibration and the record tracking.

Everything was right on except the tracking force. I had to recalibrate the tracking force knob but it was no problem.

The linear tracking sensor of the Beogram is checked and adjusted by manually rotating the platter with a test record while the tonearm is engaged for play. The revolutions of the platter are counted as you observe the tangential tracking motor advance the arm assembly. I haven't found a good way to document that with still photos so here pictures of the the post adjustment check. In this case, like most of the other adjustments, the tangential tracking was already perfect.

This tracking check with the platter belt back on checks that the tracking control system keeps the stylus centered with the tonearm as stylus moves in the test record grooves.

Satisfied that the Beogram is adjusted properly for record play I finished reassembling the Beogram cabinet. That involved the last adjustments which are mainly cosmetic. The floating chassis has to be adjusted so the platter is centered in the deck opening and the platter surface is even with the Beogram deck.

That brings us up to date with the first record play test.

Before reinstalling the Beogram 4004 dust cover I will test play the Beogram connected to one of my stereo systems for a few days to make sure I didn't miss anything and exercise all of the new parts.

Beogram 4004 Type 5526: Finishing up the electrical work and testing the results

I left off the first of two Texas Beogram 4004 restorations with the main (controller) board and the output board recapped. There was another electrical task I wanted to complete before reinstalling and testing the results.

We like to (insist really) that the 33⅓ RPM and 45 RPM indicator lamps be replaced with Beolover LED replacement assemblies. The reason for that is those lamps are part of the platter speed control circuit and our speed stability testing shows these LED replacement modules provide a measurable improvement to the platter motor performance stability.

Here is the original RPM indicator module removed and the installation of the replacement Beolover RPM indicator assemblies.

All of the electrical pieces are ready so it is time to reassemble things back in the Beogram cabinet.

One of my earlier posts showed that the suspension locking bushings on this Beogram were deteriorated to the point of crumbling away. The next pictures show the installation of the Beolover 3D printed replacement bushings.

I am not fully reinstalling the circuit boards yet. Just connected up so I can test power and basic functionality of the electrical work.

It is very common for these Beogram 4002/4004 units to work electrically after the work on the boards. Since this Beogram 4004 is in such pristine condition I fully expected that to be the case here.
To my surprise there were a couple of problems. The On automatic scanning for a record did not work. Neither did the Off automatic return or did cueing up and down. The manual scanning worked and the platter motor worked.

Looking at the sensor lamps in the Beogram I could see that the fixed arm lamp was working along with the tracking sensor lamp. The new RPM indicator lamps worked but the carriage position lamp did not. I figured the burned out carriage position lamp was the problem.

I have had to replace these before so I grabbed my usual amber LED replacement part.
Here is the failed, original carriage position lamp.

...and here is the replacement.

The new lamp works as expected and I adjusted the voltage on the anode side per the service manual to +5VDC when a clear part of the position slide scale is between the lamp and sensor.
The signal at the anode side of the LED looks correct when the tonearm carriage moves across the sensor.

Unfortunately the auto scanning and cueing functions still fail.  That means there is some control function problem.

These Beogram 4002/4004 turntables from the seventies still look modern today. The way they operate will cause most unfamiliar observers to think this turntable is controlled by some computer chip of some kind. They would be wrong of course. The controller in this turntable is an analog control system. Optical sensors and microswitches feed the analog control circuits where transistor switches manage the logic. Pretty cool.

Looking at the circuit diagram for the start, stop and scanning I looked for where the fault could be.

The transistor switches I suspected are highlighted in red. 1TR12 and 1TR13 are supposed to latch for their respective forward (ON) and reverse (OFF) scanning. I desoldered and checked them with my transistor tester and they appeared to be good. I went ahead and tested 1TR11, 1TR14 and 1TR15. They also measured good. Next I tried 1TR8...also apparently good.

After a lot more probing and measuring I was at a loss to what the problem was. I finally decided that it had to be something involving those transistors or their connection. While the service manual says the transistors there are all BC237B transistors the board actually used BC547B transistors. I decided to go ahead and replace all six transistors...and the Beogram control logic started working correctly.

Interesting that the faulty transistor(s) measure good out of circuit but I have seen that before. It is something to keep in mind when diagnosing a problem.

Now I have a working Beogram 4004. I can move on to doing the service manual checks then play some records.

Beogram 4004 Type 5526: Recapping the boards

There are just two boards to replace capacitors on in the Beogram 4004. In addition to the capacitors there are two relays that can eventually fail so I always replace those with a modern sealed relay assembly supplied by Beolover. The relay on the main board is for platter speed selection (33⅓ RPM & 45 RPM).  The relay on the output board is for the phono cartridge muting. There are two trimmer resistors on the main board that I also replace. Those are the platter speed adjustment trimmers. The replacement trimmers are sealed, multi-turn type trimmer resistors.

Here is the before picture of the main board.

Here is the main board after the updates. The 1C10 capacitor was changed from its original 0.33uF value to 10uF. The reason for that is due to the rebuild testing of the DC platter motor. When Beolover rebuilds the motors he tests them for stability using the different 1C10 capacitor values that Bang & Olufsen used in these turntable controllers. On the tests for this rebuilt motor Beolover found the 10uF value to be the best pairing.

Now for the output board. Here is the before shot.

...and the after shot. Note that I added a small switch to the output connector to allow shorting signal  ground to system ground. That shouldn't be necessary when connecting the Beogram 4004 to a Bang & Olufsen amplifier via the DIN plug. However, when connecting to a non-B&O preamplifier there can sometimes be a hum in the signal. The Bang & Olufsen provided adapter cables for DIN to RCA plug type hookups included an external grounding wire for this purpose. The switch makes it convenient to just set the grounding as necessary inside without an extra wire.

There is one reservoir capacitor in the Beogram 4004 cabinet that needs replacing. I replace that capacitor with another Beolover custom part. The Beolover part is designed to mount perfectly in the original clamp.

Here is the new reservoir capacitor installed. It fits perfect and looks great.