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 Type 2419: Personal Project : Main PCB Recap, New Reservoir Capacitors, New Trimmer Resistors

It may look low profile and small but recapping a Beomaster 4400 is quite a big desoldering and soldering task.

I finished replacing all of the electrolytic capacitors and trimmer resistors on the Beomaster 4400 main board. I also installed an arc suppression device like I installed on the previous Beomaster 4400 restoration project. The purpose of that device is to provide extra protection to the Beomaster power switch contacts.

Here is the Beomaster 4400 after the recapping (the before photos are in the previous post)

Here are some closer shots of the inside of the Beomaster 4400 after the recap.

and here is a closer look at the reservoir capacitors and the arc suppression device.

With the main board and chassis completed I can move on to the three FM boards and the preamplifier board.

Beomaster 4400 Type 2419 - Power on testing: Part 2

The Beomaster 4400 is finally reassembled for a second round of power on testing.  In the previous post I had to replace the power on/off switch because the original switch was badly burned.  The new switch is a rebuilt switch that I tested in the last post.

While planning the reassembly of this Beomaster I looked at various options for preventing deterioration of the power on/off switch contacts during switching. I know there is arcing going on when the mechanical switch makes the line connection to the Beomaster power transformer. The original switch was totally burned up and the replacement switch also showed signs of arcing on its contacts. I cleaned and smoothed the contacts the best I could and I don't want another one of these switches to burn up.

A number of audio manufacturers used RC snubber devices and small safety capacitors across switch contacts to reduce the arcing when the switch is operated. In my investigation of those devices I am not convinced they are as effective as I would like them to be.  I finally found a device specifically for arc suppression across contacts. It is called the NOsparc GGXAC1F480.  It is designed for use in protecting the contacts of relays on HVAC units so I can see where someone would think it is overkill for this power on/off switch case ;-).  Never the less I was impressed by the Arc Suppression Technologies video that showed how effective their arc suppression device is. I called the company up and spoke with one of their engineers to find out if their product would work in my Beomaster 4400 on/off switch application. He confirmed that it would work so that is the route I took.

The NOSparc device is pretty good sized as seen in the photos below. I found a good mounting spot on the bracket that holds the reservoir capacitors then tapped a couple of M3 holes in the bracket and mounted the NOSparc device right there.

Here is the NOSparc device connected to the Beomaster 4400 power switch. I attached small connection terminals to all of the wires that connect to the switch. I also used a larger gauge jumper wire between the switch contacts from what was there originally.

I reapplied line voltage to the Beomaster in steps like I did the first time...a variac to slowly increase line voltage while monitoring current and a dim bulb tester.

No issues applying the power that way so I removed the dim bulb tester from the circuit and just connect the Beomaster directly to a line outlet.

The Beomaster turns on and I was able to check voltages.
Here are the rail measurements again.

I also checked the 24V input to Beomaster 4400 +15V regulator.  The voltage is good and the measurement stays at 24V this time.

Confident in the results so far I adjusted the no-load current for the output amplifiers to 14mV across the emitter resistors (on both the left and right channel).

After that I connected an iPod Nano to the Beomaster 4400 Tape 1 source input and listed to it via headphones. Finally...good sound is emitting from this receiver again.

Now for a bit of bad news :-(.  While I was testing voltages on the Beomaster 4400 the power switch started to act up again and would not always switch on.  It has no problem switching off but the power on function is proving to be unreliable. I can tell this is a mechanical problem so I see myself replacing this Beomaster 4400 power switch at least one more time.

I can continue to get this receiver to come on so I will be able to continue testing the Beomaster 4400 performance. Now to start looking for another power switch.