Beomaster 2400 Type 2902: Problems found during performance testing

This Beomaster 2400 had breezed through the early burn-in tests and listening to it during that time sounded like any other restored Beomaster 1900/2400 amplifier. To really compare the performance however we like to take some THD (total harmonic distortion) tests and look at a frequency response plot.

The early THD tests showed nice values at close to 6 Watts of output into 8Ω dummy speaker loads.

As I continued measuring the performance while increasing power the output began to deteriorate to where I was getting out of spec THD prior to reaching the rated 20W power output. Actually, my Beomaster 2400 Type 2902 claims 25W continuous power into an 8Ω load. The Type 2901 service manual claims 20W continuous power into an 8Ω load.

I stopped and rechecked the no-load current settings.

Those values look good so I did another physical check.

I could see some cracks in the emitter resistors. Pulling them out and measuring them didn't reveal any problem with their value though but I changed them anyway. Because the Beomaster still plays decently this is a case where it would be helpful for a hard over failure.  As it is I will have to start replacing and retesting parts in this Beomaster output amplifier. Pulling out transistors and measuring them while they are not under load will likely not reveal a bad transistor.

This will take a bit of time and I already scheduled a week or two off so I will pick this amplifier back up later.

Beogram 4004 Type 5526: Second Beogram 4004 from Texas - Shipping to new owner

After this second, original owner, Beogram 4004 turntable restoration the owner decided to put it up for sale here on the Beolover Blog.

I am pleased to say that this beautiful Beogram 4004 now has a new owner.  This post shows how to prepare and pack the Beogram 4004 for shipping.

We have had quite a lot of experience now in packing and shipping various Bang & Olufsen components. Our methods have tweaked here and there. Turntables are the most critical to pack safely. Mechanical parts must be secured and of course the turntable itself must be protected.

As tempting as it is to secure the turntable for shipping without removing the phono cartridge it is still best to remove it.

The removal and installation of a Beogram MMC phono cartridge is not too difficult.  I always make sure the stylus guard is in place when handling the cartridge.

In this first picture of the cartridge removal process I show how I secure the tonearm with my fingers so it won't move when pulling the cartridge off.

With the other hand I grasp the phono cartridge and stylus guard to pull the cartridge from the tonearm connector. It will resist coming off as the fit should be tight. Do not wiggle the cartridge horizontally. It comes off straight and slightly down. Don't force it.

Here you can see the connection points of the MMC cartridge and tonearm connector.

The Beogram 400x turntables have a floating chassis for the platter and tonearm assembly.  There is a chassis lockdown procedure to secure the floating chassis for shipping (or even transporting to a friend's house :-) ).  In the case of the Beogram 4004 turntable there are three floating chassing lockdown screws. They are accessible from inside the cabinet and from outside the cabinet. The normal case for an owner is to use the outside lockdown screws.

The screws are on the underside of the Beogram 4004 cabinet and you DO NOT want to turn the turntable upside down to get to them.

I hang the edge I am tightening or loosening over the edge of a table and operate the screw from underneath.

This photo shows the three Beogarm 4004 lockdown screws as seen from underneath (hanging over my workbench table). The lockdown screws do not need but a couple of turns to loosen them. Gently turn the screw driver until it won't turn but do not apply a lot of torque. Don't over tighten the screws.

Now that the phono cartridge is safely removed and the chassis is locked down for transport the packing can begin.

My first step on this turntable packing was to put a protective plastic film (like you find on new products) over the outside of the Beogram dust cover. I didn't photograph that process but the film just lays over the dust cover.

After that I remove the Beogram top platter. It is another one of those things that is tempting to leave in place but it is better to removed it and pack it separately.  Why take any chances?

I cut and place a foam insert where the top platter was and a couple of strips of foam padding over the keypad and speed display.

Then I cut to fit, a piece of polyethylene foam to secure the remaining space between the dust cover and inside of the Beomaster.  I wrapped the polyethylene foam with the same plastic film I put on the outer surface of the dust cover.  Although the foam is pretty soft I worry that movement during shipping could result in swirl scratches on the underside of the dust cover. The film should prevent that.

Now I can close the dust cover and secure the main parts of the Beomaster.

I don't like to use any tape with adhesive if possible so I wrap the Beomaster dust cover down on the cabinet with packing wrap.  After that I like placing the audio component in a plastic bag to protect against any moisture. I also hate the way the electrical cord and phono cable can dangle and the plugs potentially scratch something.  So I wrap them in foam.

The next step is wrapping the Beogram lengthwise and crosswise with egg crate type soft foam. The foam is secured with plastic wrap.

This Beogram 4004 is now ready to go into the shipping box.

Over the last few years I have found that it is better to use as small a shipping box as possible. Too large of a box becomes difficult for shipping carriers to handle and increases the chance of a drop.

With the Beogram securely tightened down and padded the job of the box is for rigid support and protection from handling mishaps (bumps and small drops).

I use double-walled cardboard shipping boxes and glue the bottom flaps together.  After that I glue polyethylene foam sheets to the bottom and the four walls.

The wrapped Beogram 4004 fits right into the foam lined area.

This is where the Beogram 4004 top platter gets packed. I like to use a vinyl record shipping box to hold the platter.  I also add some small polyethylene pieces on the side of the Beogram and slide the box with the phono cartridge in there.

The final steps are to add the last polyethylene foam sheet for the lid and then secure the shipping box.

This turntable is now on its way to a new home.

Beomaster 8000 - Workbench Unit - Investigating FM source audio in the Phono source channel

In this post I wanted to revisit Beolover's fix for reported FM audio crosstalk in the Phono channels.
I actually have been checking for that fault in my Beomaster 8000 restorations and with these new source switching IC device replacements. Unfortunately I had been unsuccessful in replicating the problem.

To review the problem...
Make sure the turntable connected to the Beomaster 8000 does not have a vinyl record ready to play.
When the Beomaster 8000 is switched on from Standby to Phono the Beomaster 8000 volume can be cranked up without hearing any music from the Phono music source.  That is good and is what is expected.

Next switch the Beomaster selected source to an FM station (preferably a strong station). Note the music that is selected. Now switch back to the Phono source (again with no record playing). Turn the Beomaster 8000 volume up and you can begin hearing (faintly) the FM station that you were just on bleeding into the Phono channel.  That is the problem.

As I mentioned, I had not been able to create this problem until this weekend when I moved my workshop Beomaster 8000 unit to a different bench and connected up a powered Terc FM antenna to the radio antenna input. This produced the strongest FM signal I had seen on the Beomaster. Retesting the FM crosstalk problem...I now had reproduced it.

Interestingly this test setup is with my new analog switch devices installed in the Beomaster 8000 preamplifier board. The preamplifier board also has TI OPA2134PA OpAmp devices for all of the analog sources.

Up until now I had reasoned that any audio crosstalk must be occurring in the analog switches the Beomaster 8000 preamp board uses. That made the most sense. However, this preamp board has the new multiplexer switching devices that have very good audio specs and low crosstalk ratings. I thought that should take care of any audio source crosstalk.

It turns out that it doesn't and that is quite surprising...and I do not understand the source of the problem at this point.

An acceptable work around for someone not wanting to make any wiring changes is to always put the Beomaster in Standby mode prior to selecting the Phono source. That will keep the FM tuner in the Beomaster muted while using the Phono source.

We want our cake and eat it too though so I decided to implement Beolover's fix for this problem.

I performed the same wiring modification Beolover came up with but I implemented the change differently.  I wanted to be able to easily switch from the original wiring to the modified wiring.  That option will allow me to keep investigating the problem.

My implementation is to put female jumper connectors on the two wiring routes related to this problem and fix.  Beolover's fix is to run the FM board audio mute control line to the preamplifier board phono source selection line. When Phono is selected on the Beomaster control panel the phono source selection line will tell the FM board to mute the FM audio. When the phono source is deselected the FM control will unmute the FM audio.

Here is the modified wiring for this on the FM board. The picture shows the FM audio mute control line connected to the default path the Beomaster 8000 originally takes.

The yellow wire from the FM board 1R37 resistor can be disconnected from the default ground path and connected to the Preamp board added connector (also on a yellow wire).

This path routes the FM audio mute control line to the preamp board Phono source selection control line.

Testing this modification in the workbench Beomaster 8000 unit I verified that I get no FM audio at all in the Phono channel with I have the FM audio mute control line connected to the preamp board.  I also verified that moving the FM audio mute control line back to the default position re-introduces the FM crosstalk problem.

Beomaster 2400 Type 2902: Reassembly and new display masks

The Beomaster 2400 has been playing music again in my workshop and I haven't even finished reassembling it.  To complete the reassembly I need new display masks for the tone controls (bass, treble and balance position indication).  Those parts are no longer available from Bang & Olufsen so I have to make replacements. I have done that before on the Beomaster 1900.

Here is the Beomaster 2400 with just the new bass display mask remaining to reinstall. The treble and balance indicator masks are already in place.

Interestingly this Beomaster 2400 serial number uses the same display mask pattern as the Beomaster 1900 units. That is interesting because on later serial number Beomaster 2400 units, but the same type: T2902, the mask pattern changed.

Here is the later mask pattern next to the one this Beomaster came with.

The early mask pattern used a clear square to indicate the current control position with vertical lines displayed on either side. The later pattern blacked out everything except the selected position which now has vertical lines. I guess they had a lot of discussion in the design department over which pattern they liked :-).

It is hard to capture the detail of the display masks but I was able to change the speed and sensitivity on the camera to get this close up of the new masks installed.

With the display indicator masks back in place I could reinstall the back plate and tone control deck.

As with a lot of the Beomaster 1900 and 2400 units this one arrived at the shop with missing and broken feet. Luckily that is another part supplied by Martin Olsen and I installed four new feet on this Beomaster.

The Beomaster is now ready for functional testing. I will start that a little later. For now I will leave the receiver playing an FM station as part of my ongoing burn-in testing.

It looks great and sounds great...and I haven't finished cleaning and polishing the cabinet yet.

Beomaster 2400 Type 2902: Lamp replacement and voltage checks

This Beomaster 2400 has reached the final stages of the restoration.  I replaced the indicator lamps and performed the first voltage checks on the receiver today.

On the Beomaster I replace the lamps that are only for illuminating a display function with LED devices. There are five lamps that are part of the circuit that they display the state of.  Those lamps must remain incandescent type bulbs. I typically get my incandescent lamps from Martin Olsen as I know he will source me the correct type.

Here is the before picture of the Beomaster 2400 volume indicator lamps.

....and here are the replacements. These are two that must be incandescent type bulbs.

Next are the tone control lamps (Bass, Treble and Balance).  These are lamps that can really benefit by changing to LED type bulbs. The sliding indicator masks on this Beomaster unit are deteriorated from the heat of the original incandescent bulbs.

The replacement LED lamps give off a lot less heat and should not damage the replacement indicator masks I am going to install.

Here are the original lamps.

Here are the replacement LED assemblies I created. Note that the polarity has to be correct on the LED lamps.

The LED lamps must have a current limiting resistor.

The source selection display board is the last board to replace lamps on. Here is the original configuration.

The far left bulb and the two incandescent bulbs on the far right must remain incandescent type bulbs.
The rest will be changed to LED devices. The red LED for Standby indication doesn't need to be changed if it still works.

Time to test the lamp replacements.

Standby works

The Standby lamp is a good indication that +15VDC is good on this Beomaster.  Time to check the power supply voltages.

Here is the +15VDC reservoir capacitor. The voltage here looks good. It is prior to +15VDC voltage regulation.

Selecting a source (i.e. FM5) I see that I have +15V so I performed that service manual adjustment.

While the Beomaster is still open where I can (barely) get to the left and right channel no-load current adjustment pots I performed those adjustments.

It is too difficult to place probes across the emitter resistor from the component side of the board while it is installed so I attach my probe wires on the trace side.

Now I can easily measure the voltage across one of the left and right channel emitter resistors for the no-load current adjustments.

Both channels dialed in easily. Well...relatively easy.  The trim pots for this adjustment are not the easiest to reach.

Note that I had the polarity of the probes reversed on the right channel measurement. It doesn't matter for this adjustment check. I am only concerned that I set the voltage on the meter to 12mV.

I also checked the ±31V rail voltages for the output amplifier. They measured good...probe polarity is important here :-).

The final service manual voltage checks I performed were the tuning voltages. There is one adjustment for FM1 and another for FM5.

So far so good on the reassembly and electrical checks.

I will finish reassembling this Beomaster then take it over to the test bench for some functional testing. I did perform a sneak peak by connecting up an FM antenna and an iPod Nano to the Tape source and listened to music via the headphones jack. The Beomaster 2400 is playing music again. Tone controls all work so a nice milestone has been reached on this project.

I will leave the Beomaster 2400 playing an FM station while hooked up to a pair of Beovox S75 speakers I keep in my workshop.  While it is busy doing that I can work on assembling the new display indicator masks for the bass, treble and balance controls.