Beomaster 2400: Packing up the Beomaster for shipment

The Beomaster 2400 played continuously for about forty-eight hours without any problem. I exercised all of the source inputs and even made a test recording from a Beogram 8002 to a Beocord 9000.

Now it is time to properly pack the unit up and ship it home.

The following pictures show how to pack the Beomaster 2400 for safe shipping.

It is a good idea to use a plastic bag to protect from any packing material scratching of the audio component and to act as a moisture barrier.

I like to use 1 1/2 inch eggcrate foam for protecting the Beomaster and secure it with non-adhesive packing wrap).

For the shipping box I glue the bottom flaps together with wood glue then glue polyethylene padding to the floor and walls of the box.

I place the wrapped Beomaster into the padded shipping box and use more eggcrate foam for the ends and the middle of the Beomaster .

Now I can place the polyethylene lid over the packed Beomaster and seal up the box.

This Beomaster 2400 is ready to go.

Beomaster 2400: Finishing up with the listening tests

Time to wrap up this Beomaster 2400 restoration project. It is reassembled and all of the testing is done. I have had this receiver in one of my listening test rooms for a while. It has performed great connected up to a Beocord 9000, Beogram 8002 and a pair of Beovox M100.2.  Those speakers are rather large for this slim, little receiver but in a ten foot by eleven foot room it works out fine.

This Beomaster is ready for another forty years of music enjoyment.

A few more hours of listening and I will pack it up and ship it home.

Beomaster 2400 Type 2902: Retesting performance measurements after the right channel rework

My break from the workbench is over. Time to resume and wrap up this Beomaster 2400 restoration.

I left off with the discovery that the right channel output amplifier was distorting.

In resuming work on this Beomaster I removed and tested all of the right channel output amplifier transistors out of circuit. It wasn't any surprise that they all tested good since the right channel was working at low power levels. That being the case I decided to replace all of those transistors in the right channel along with the two emitter resistors. I also wasn't happy with the single-turn trimmer resistors I had installed for the no-load current adjustment. The single-turn resistors are fine but I prefer a multi-turn resistor for that particular adjustment. It is easier for me to dial in the correct value and feels more precise.

Here are pictures of the transistors that were replaced and the new trimmers.

The new multi-turn trimmers made re-adjusting the no-load current adjustment a breeze.

Now it was time for checking the left and right output amplifier channels THD again.
This type of measurement poses a problem regarding my stating that the THD for this amplifier is exactly per the service manual published technical data.

For one thing, the published audio specs stated by various manufacturers are a little vague. They often quote DIN specs that they followed but details of those specifications are difficult to find. Even then, the measurement equipment is different now.

My methodology is to use the manufacturer specs as a guide but measure all of my amplifiers using my own same test setup and same measurement devices. That gives me a set of data I know the source of and can use for comparison.

I have shown this before but here is my test setup.

I use a fixed, dummy speaker load of 8.08Ω on each channel. The 0.08Ω part of the load is a series sense resistor that I can connect my audio analyzer probes to for making power signal measurements with. The audio analyzers have limits on the AC voltage they can handle. While my QuantAsylum QA401 analyzer does have built in attenuators I still need to be careful.

The test connections go to my signal generator, QA401 analyzer, multimeters, oscilloscope, dummy load restistors and the amplifier under test (BM2400 in this case).

I use the multimeters to verify the AC signal the amplifier is outputting to the load resistors.
The Beomaster 2400 doesn't have a volume control that you can tell the precise volume level you are on. It does have three preset volume levels you can select from on start up though so I usually use those.  Then I use the amplitude adjustment of the test signal generator and check the amplifier output with the multimeter to dial in a precise measurment output. The oscilloscope also aids in monitoring the actual input signal and amplifier output signal.
The QA401 audio analyzer measures the THD.

Here is a quick look at the Beomaster 2400 service manual technical data.

I am only interested in the rated power output and the distortion data.
Bang & Olufsen states here that they used DIN 45 500 for their measurement specification. I don't have those specs and I'm not sure they would really help me with the test equipment I have on hand.

So I will use the technical data that says this amplifier should be able to output 20W into 8Ω speaker loads. It should do that at a THD value close to what the THD is at lower output power.  My dummy loads are fixed resistor loads and that is fine as I use those on all of my tests. Plus I am collecting my own measurement data for apple to apple comparisons.

The service manual stated harmonic distortion spec of 1000Hz, 50mW : <0.07% seems kind of odd.
I am reading that to mean a 1000Hz test sine wave input and the Beomaster output set to only 50mW  across an 8Ω load. That is only about 0.6Vrms.

I decided to go ahead and make that measurement anyway but I still will do my own THD measurement at the full 20W output.

Here is the 50mW THD measurement for the left and right channels of the Beomaster 2400.

Both channels are in the 0.035% range which is well below 0.07%.
So if that is what the published spec meant then great.

For my own full power test at 20W of amplifier output across the 8Ω loads I get this -

This is what I wanted to see. The left and right channels are both performing close to each other and at a respectable THD of around 0.04% (at full rated power).
The Beomaster 2400 heatsinks do really warm up at this output level and normal listening level for the amplifier is probably down around 1W to 5W.

One last note about just doing the THD measurements here. Most of the time restorers rely on listening to known, good music sources through known good speakers to check the result of an audio restoration.  That is fine and an important test to pass.  I just feel more comfortable in having a repeatable instrument measurement test as a means to check that there isn't something hidden going on and that both channels measure close to what my other restorations have been. It paid off in this case because this Beomaster sounded fine in my initial listening tests after the recap but the higher power testing revealed a problem.

This amplifier is ready to move on to the final functional tests.
Those are where I will connect up a Beogram and Beocord then play the Beomaster 2400 for several days continuously (most of the time using FM as the source).

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.

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.