Beomaster 4400 - Australia Project - Wrapping Things Up

This Beomaster 4400 from Australia passed its final bench testing today.

I can also report that musically the Beomaster 4400 sounds great.

Here are a couple of pictures of my first listening tests on the bench.

I will install the Beogram 4400 cabinet cover next.  Then move the receiver to a listening room where I can spend more time checking out all of its features.


Prior to this listening test, I ran my usual harmonic distortion and frequency response tests on the Beomaster 4400.
I use a dummy speaker load of 8 ohms and use a 1 KHz, 0.316 VRMS test input sine wave into the Tape 1 inputs (L & R).

I run the harmonic distortion (THD) and frequency response tests with the Beomaster 4400 at its maximum rated output.  For an 8 ohm speaker that is 50 Watts.  I actually tested it at 55 Watts in this case.

Here are the measurement results.

The signal drop off above 22 KHz on the frequency response measurement is due to the measurement device (QuantAsylum QA401 Audio Analyzer) ending its test run.

The THD measurements at 55 Watts are nicely below the Beomaster 4400 rated value of 0.1%.

Those measurement results are what I have been getting from my Beomaster 4400 restorations so they are good measurements to make and compare to.

To get to this final bit of the Beomaster 4400 restoration testing I spent some extra time installing some power protection features to the receiver.

On all of my Beomaster 4400 restorations I have been installing an arc suppression device to protect the Beomaster 4400 power switch.  The Beomaster 4400 On/Off switch switches full power across its contacts and I have seen one Beomaster 4400 where those contacts were pitted so bad the switch had to be replaced.

My previous Beomaster 4400 restorations were all Type 2419 versions though.  They only required a single arc suppression device.  The following schematic shows the Type 2417 and 2419 power switch differences.

The On/Off switch of the Type 2417 routes both the hot and neutral AC wires to the Beomaster 4400 transformer while on the Type 2419, only one of the AC wires runs through the switch.

On the Type 2419 version I only have to put one arc suppression device across the switch contacts because the two sets of contacts are connected together for the one AC wire they handle.

The Type 2417 uses a set of contacts for both AC wires (hot and neutral) so I added arc suppression on each contact.

Here is my installation of the arc suppression for the Type 2417.  I had to build a 3D printed support to mount them inside the Beomaster 4400 cabinet.  The black mounting screws for the assembly are plastic.

Another power protection install I did was to install a bleeder resistor on each of the two ±35 V rail reservoir capacitors.  The bleeder resistors insure that both 10,000uF capacitors discharge together and relatively evenly when power is turned off.  This also provides a bit of safety when the Beomaster 4400 is opened up for service.  The two 10,000uF capacitors will be at zero volts in that case.

For the bleeder resistors across 10,000uF capacitors 0C8 and 0C9 I chose 5100Ω.

The final power protection installation I did was to build and install a soft start circuit for the ±35 VDC rails.  A lot of amplifiers do this as the soft start circuit slows down the in-rush of current to the reservoir capacitors when power is turned on.  Limiting the in-rush current will put less stress on the transformer, fuses, diode bridge and reservoir capacitors.

The way it works is when the Beomaster 4400 power switch engages, the ±35 V rail voltages go through a pair of 22Ω resistors instead of directly to the 10,000uF capacitors.  That slows down the charging up of those two capacitors.  After about 1.7 seconds, a relay switches out the two 22Ω resistors so the remaining 5 to 10 VDC of rail voltage directly charges up the reservoir capacitors.

Here is a picture that shows the 10,000uF reservoir capacitors charging up with and without the soft start circuit.

A nice side effect of the soft start circuit is that it greatly reduces any "thump" sound through the speakers when the Beomaster 4400 turns on.  However, the reason I added the soft start circuit was for long term protection of the power components.

Here is the soft start circuit installation that I implemented.  I made a small 3D printed mount for the circuit board so I could easily attach it to the inside of the Beomaster 4400 cabinet.

The soft start circuit is powered by the Beomaster 4400 22V power supply.
The inputs to the soft start are the ±35 VDC from the Beomaster 4400 bridge rectifier.
The outputs from the soft start go to the the two 10,000uF capacitors (0C8 and 0C9).

Note: For a Type 2419 Beomaster 4400 I remove the two auxiliary AC plugs that the Type 2417 does not have.  Removing the two AC plugs provides room for the soft start circuit and I created a 3D printed plate to cover the rectangular cabinet cutouts for those plugs.

Now for some more listening to this Beomaster 4400 before packing it for shipment back to Australia.

Beomaster 4400 - Australia Project - Reassembly and Power Check

Since the last update I finished the capacitor replacement tasks on this Beomaster 4400.

In addition to that, I replaced four incandescent lamps for the front panel. Those were the Overload Indicator, Power On and the two Tuning Indicators.

I cleaned the front panel along with applying some Deoxit to the four slide controls - Volume, Bass, Treble and Balance.

I also reattached the Beomaster 4400 heatsink assemblies with new SIL Pad thermal conductive material on the power transistors.

Here are the before and after photos of the IF Section PCB (PC 2).

Next is the Stereo Decoder and Indicator Circuit PCB (PC 3).

Last is the Pre-Amplifier PCB (PC4).

The next few photos show the indicator lamp replacement in the Beomaster 4400.
The front panel indicator lamps consist of a plastic base with contacts for the power wire and ground, a slot for mounting the lamp fixture (onto a bayonet on the front panel) and a socket for a 60ma-80ma lamp.

It takes a bit of patience and care to replace these lamps.

Here is the Overload Indicator lamp.

Here is the Power On lamp.

Here are the two Tuning Indicator lamps.  The flexible wire that connects power to the two lamp fixtures makes removal and insertion a little tricky.

The front panel controls cleaned up good after the Deoxit treatment on the slider controls.

The next two pictures show the Beomaster 4400 with it's circuit boards re-installed.
The photos show the SIL Pad thermal conductive material behind the transistors mounted to the heatsinks.

This is the point during the restoration where I can test that the Beomaster 4400 has electrical power.

I first plugged the Beomaster 4400 to my Dim Bulb with Variac tester.
That allows me to check that there isn't anything shorted that will draw a bunch of power and damage other electrical components.

With the Beomaster 4400 plugged into the tester, I turn the Beomaster on, then start applying AC with the variac.  I monitor the current draw of the Beomaster as well as the light bulb of the tester to see if any problems begin to appear.  That allows me to stop and shut power down if I see the beginnings of a problem.

The Beomaster 4400 powered up clean without any issues.

Assured that no shorts exist I plugged the Beomaster 4400 into a normal AC socket.

My first step was to check the power supply voltages.  I measured the +15 VDC voltage regulator output, the +35 VDC supply, the two +24 VDC, +33 VDC, -12 VDC supplies and the +-35 VDC rail voltages for the power amplifier.

Here is the +15 VDC check

Transistors 5TR3 and 5TR4 regulate +24 VDC to the Right channel tone controls and Left channel tone controls respectively.

There is a 35 VDC regulated voltage to the IF Section (PC 2).

After the the Beomaster 4400 volume and tone control circuits there is a Bessel Filter circuit just prior to the output amplifier.  That circuit uses a -12 VDC reference voltage.

The output amplifier circuits have rail voltages of +-35 VDC.

The last voltage check I made was the 33 VDC regulator circuit on the preamplifier board.

The voltage checks were all good so I performed the No-Load Current Adjustment for the Left and Right channels.  The service manual says to adjust the respective channel trimmers for 10mV to 15mV across each channel's emitter resistors.  For now I set them to 10mV.

This Beomaster 4400 is ready for some performance testing.  I will setup some of those tests for the next post.

Beomaster 4400 Type 2417 - Australia Project - Electronic Restoration Progress Update

After doing the initial assessment of the Beomaster 4400 Type 2417 I started in on recapping the main PCB (PCB 5).

Here is PCB 5 before recapping.

Here is PCB 5 after recapping.

I removed the two 10,000 uF reservoir capacitors for the +-35 VDC rails of the output amplifier.

While I was working with the power supply components I removed and replaced the three bridge rectifiers in the Beomaster 4400.

I replaced the large bridge rectifier attached to the side of the transformer first.

Next, I replaced the two bridge rectifiers at the other end of the Beomaster 4400 PCB 5 board.
I also replaced the 15 V Regulator that mounts to the side of the chassis.

Here are the new +-35 rail reservoir capacitors.

So far I have removed and replaced 39 electrolytic capacitors in this Beomaster 4400.

For next time I will recap the Beomaster 4400 audio input board and the two FM boards to complete the last bit of recapping tasks.
I will also replace the thermal conductor insulators on the output amplifier transistors that mount to the Beomaster 4400 heat sinks. 

Beomaster 4400 Type 2417 - Australia Project - Initial Checks

Here is a really nice example of a Beomaster 4400 Type 2417 receiver.

This one has been sitting patiently in my backlog of Bang & Olufsen restoration projects for a while.

It is the first Type 2417 unit I have seen.  All of my other Beomaster 4400 restorations have been Type 2419 units.

The Beomaster 4400 Type 2419 receivers have a transformer for the US market.  Their transformer is for 117 V, 60 Hz and has two switched AC outlets in the back for connecting other audio components.  I never liked having those switched AC outlets as it means more current through the Beomaster 4400 receiver's power switch when it is turned on.

The Beomaster 4400 Type 2417 does not have any additional AC outlets and it has a voltage selector switch so the transformer can be configured for markets that have AC voltages of 110 V to 240 V. 
Much nicer.

Like so many of the Beomaster 4400 receiver cabinets, the four rubber feet are worn down and there is corrosion around where they mount to the bottom, metal plate.  I will have to replace those and clean off the rust.

The procedure to open the top of the Beomaster 4400 cabinet for access to the internal components is pretty easy.
Two screws need to be loosened in order to move two cabinet latches out of the way.
Four screws have to be removed to allow the wooden cabinet to slide back and up for removal.
A couple of those screws are partially hidden by Beomaster 4400 heat sinks.

With the cabinet opened up I am pleased to see that the internal components look like they are the original factory components.

This photo shows the space between the Beomaster 4400 voltage selection and the transformer.
In the Type 2419 units, this space is crammed with the wiring for the two switched AC outlets.

Normally I jump straight to cleaning and replacing the electrolytic capacitors.

This time I decide to try some basic testing on the Beomaster 4400.

The first thing I did was to check the no-load bias current per the service manual.
Both channels (Left & Right) were low so I adjusted them to about 10mVrms across their respective emitter resistors.

Next, I set up my audio tester.
As in other Beolover Blog posts where I test my restored Beomaster amplifiers, I am using my two 8.1 ohms dummy loads for the speakers.

These dummy loads are made up of a 4 ohms resistor + a 0.08 ohms sensing resistor + a 2 ohms resistor + a second 2 ohms resistor.  In series these measure 8.1 ohms.

The different resistor segments allow me choices of where to place my differential measurement probes to measure the power/frequency/distortion of the output amplifier.  My QuantAsylum QA401 Audio Analyzer can only handle a maximum of 26 dBV (50 W across an 8 ohms load) so the resistor segments let me choose a measurement point that is friendlier to the analyzer. 

In the case where I was going to measure 50 Watts out of the amplifier across my 8.1 ohms dummy load, I would place my differential probes across the 2 ohms sensing resistor of my dummy load.
The maximum dBV across the 2 ohms resistor for the 50 Watts output would be around 14 dBV.

On this pre-testing of the Beomaster 4400 Type 2417 output amplifier, I don't expect to get to a very high output.

The output of the Audio Analyzer uses two single-ended signals for the Left channel and the Right channel.  I typically connect them to the Tape 1 or Tape 2 source input.  The most common test signal I use is a 1kHz sine wave of 0.316 Vrms.

This photo shows the test connections underneath the Beomaster 4400 cabinet.

For my initial testing of the Beomaster 4400 Type 2417 I started with the volume at zero and observed the distortion measurement as I increased the output across the dummy load resistors to 1 W.

The THD measurement was surprisingly good at 1 Watt output from the amplifier.

Things went bad for the Right channel when the output amplifier went just past 7 Watts.

Here is what the workbench looks like during the audio analyzer testing.

So a bit of pre-restoration testing just to have as a reference when the restoration is complete.

My next task is to begin moving/removing some of the Beomaster 4400 internal components to make way for the electrolytic capacitor replacement.  I will also change the trimmer resistors in the amplifier section and rework the thermal insulation of the transistors on the Beomaster 4400 heat sinks.

There are also some transistors on the main PCB of the Beomaster 4400 that get really hot.  I will look at attaching some thermal protection to them as well.

As I do on all of the Beomaster 4400 restorations I work on, I will install an arc suppression device for the power switch.