Beomaster 1900 Type 2904: Reassembly and Testing

Continuing from the last post, I performed two of the Beomaster 1900 Service Manual adjustments.
The No-load Current Test (12mVdc across the output amplifier emitter resistors).

...and the P1 and P5 Tuning Voltage adjustments.

The Beomaster was now ready for reassembly and some audio testing.

The first step was to re-install the plastic masks for the Bass, Treble and Balance controls.

After that, the button panel is installed.

The button panel installation is a little tricky. There are 10 touch control buttons.  Each button has a metal spring connecting the panel button to a corresponding metal post soldered onto the main board.
When installing the button panel you have to make sure all of the button springs mate with their metal circuit board posts.  

My technique is to get most of the spring contacts to mate...8 or 9 out of 10 is good.

For the spring contacts that do not successfully mate, they can be corrected using a prong tool from underneath frame.

Here is an example of one of the springs not mating.

The picture on the right shows the corrected spring contact mated with the post.

I screwed down the button panel and powered the Beomaster 1900 on to check if, indeed, all ten buttons worked.

They do.

Now for the installation of the program panel and test play some music through the Tape input.

I used both speaker outputs of the Beomaster 1900 during this initial audio testing.
A pair of Beovox MC120.2 speakers on Speaker 1 and a pair of Beovox S-55 speakers on Speaker 2.

They sounded very good so I finished up the re-installation, installing the bottom cabinet plate.

I hooked up an FM antenna and did some listening tests on the tuner.  P1 through P5 all tuned successfully and also sound very good.

While a successful listening test carries a lot of weight in judging the success of a restoration, I like to do a basic harmonic distortion test at full power and a 1000 Hz sine wave input (on the Tape input).
I also like performing a 20 Hz to 20KHz frequency response test at full power (again, using the Tape input as the source).

For these audio measurements I use a QuantAsylum QA401 Audio Analyzer.  There is a newer QA403 available now but the QA401 is still very good and I continue to use it.

I always mention that my audio tests cannot be directly correlated with what is printed in the service manual (or B&O published specs) as the equipment in the mid-seventies was quite a bit different.
Also, many of the specs are a little vague regarding how, specifically, they were performed.

For that reason, I try to do my tests the same way each time using my audio analyzer, then compare my results to previous results I measured on other restoration projects.

My speaker loads for the audio tests are fixed resistors that provide an 8.08 ohm speaker load for each channel (Left and Right).

8.08 ohms because each 8 ohm load resistor assembly is made up of one 4 ohm load in series with a 0.08 ohm, sense load and two 2 ohm loads.

The reason for that is because the QA401 audio analyzer has a limit of 28 Vp-p (or 19.8 Vrms).
19.8 Vrms = 26 dBV which is the form of the input value QuantAsylum uses in stating the amount of input.

At full power of 20W output across 8 ohms, the Beomaster 1900 maximum output my QA401 should see (across the dummy load resistor assembly) is 22 dBV.  So I can place my QA401 differential measurement probes across the full 8.1 ohm dummy load safely.

If I really want to be at a safe measurement difference I could put my QA401 probes across one of the other sections of the dummy load (i.e. across the 4 ohm resistor, the 2 ohm resistor or the 0.08 ohm resistor).  However, the best results should come from using the highest output level I can safely use.

Here are my distortion measurements at maximum rated output power (or as close as I could get) across the 8.1 ohm dummy load. (The Tape input source was a 1KHz sine wave at -12dBV).  

Left THD: 0.038%

Right THD: 0.062%

The volume control for this Beomaster 1900 version is only adjustable in sixteen steps.  Later serial number units (serial number 1826011 and on) increased the volume selection steps to 128.  The output power is the same power but the volume adjustment is much finer.

Because of the coarser, sixteen step volume control, I played around with the volume control value and my source sine wave generator amplitude to achieve an output that was close to the maximum rated power for this amplifier. 

I am pleased with the harmonic distortion measurement results.  Although, as you can see, the Left channel is producing 20% more power than the Right channel.

I measured the audio test signal through the Beomaster 1900 pre-amplifier to its voltage amplifier stage in the output amplifier.

At the input to the output amplifier (Test Points TP201 and TP302 for the Left and Right channels respectively), the Right channel is about 10% lower than the Left channel.
When the signal is at the collector of transistors TR212 (Left channel) and TR312 (Right channel), the difference in the channels is 12%.

I will do some more investigating on the difference in output level between the Left and Right channels.
It appears that most of the difference occurs in the output amplifier.

Meanwhile, I continued on to a frequency response measurement at maximum power. 
I ended up with a little more volume for the measurement and the Left channel output was 5 Watts over its rated value.
The measurements are within the expected value for this amplifier but again, the Left channel output is higher than the Right channel (by 23%).

Beogram 8002 Type 5633: Wrapping the project up

While I started work on the cosmetic part of the Beogram 8002 restoration I left the turntable functional components connected to my workshop Beomaster 8000 (also in a partially disassembled state) for listening to.

My wall-mounted shelving is extremely strong and playing records never skipped once during my listening tests.

Meanwhile, out in another workshop, I started to work on cleaning up the cabinet parts for this Beogram 8002.  Other than the usual double-sided tape deterioration and loose panels, this Beogram 8002 is in very nice shape.  

However, the deteriorated, original B&O tape found its way all over the cabinet, even on the dust cover.  I imagine that some of that occurred during shipping of the Beogram 8002 here.

I used a bunch of Goo-Gone to make the clean up easier.

That product works really well on old adhesive and makes it easier to scrape the old adhesive off.
It still takes some manual effort with cleaning cloths to get the surfaces down to just the bare material again.

Here is what the removed adhesive looks like after it was removed.

Here are some more of the cabinet pieces after cleaning.

I did a little bit of polishing on the dust cover just to get it new looking again. It didn't have any significant scratches and is really nice over all.

I took the cabinet parts back into my electronics workshop to finish assembling the cabinet pieces.

Here are the new 3M VHB (0.8 mm) double-sided tape pieces attached to the cabinet for remounting the aluminum trim piece.

In preparation for the tonearm compartment lid damping I applied some industrial damping grease.
Before installing the heaving turntable components is a good time to apply this damping grease.

I will apply some more Nyogel 767A damping grease to the deck lid hinge lever when I remount it to the cabinet.

Next, I installed the functional turntable components (floating chassis, transformer, circuit boards and control panel.

After testing that the turntable components were still working one more time I closed up the cabinet, attached the metal deck lid on the 3M VHB double-sided tape and attached the three floating chassis suspension clips.

The next step was to reattach the dust cover and deck lid hinge with the long, metal hinge rod I removed back when I began this project.

The next sequence of photos show the reattachment of the metal deck lid.  I do it at this stage so I can use the dust cover as a guide for the metal deck lid alignment on its hinge.

With the dust cover and metal deck lid reattached to the cabinet via the metal hinge rod, I connected up the dust cover lid damping mechanism.

Here is the Beogram 8002 reassembled.

...and back playing records in my office.

After a bit of listening I will pack this Beogram for shipment back to Idaho.

Beogram 8000 Type 5613: Finishing up

Since the previous post on this Beogram 8000 project, I have been enjoying playing records on this Beogram turntable while I looked for a replacement dust cover and back plate.

I was able to find the two pieces I needed for the dust cover repair but the dust cover needed a little bit of polishing to be an acceptable replacement.

The replacement dust cover isn't without a few blemishes but it polished up nicely without requiring a full dust cover restoration.

Here is the back plate I found to replace the broken, original back plate.

My preferred way of removing and installing a Beogram 8000/8000 dust cover assembly is with the hinge bar removed.  I find it much easier to install the dust cover and tonearm compartment lid without being attached to the back plate.

Here is the back plate attached to the Beogram 8000 cabinet.

Now the dust cover can be attached and the metal hinge rod inserted to hold it in place with the cabinet.

I stop the hinge rod as it reaches the tonearm compartment lid hinge.
For the metal tonearm compartment lid I still had to reattach the lid to the hinge.
I prefer to do that with the dust cover and lid hinge in place so I can align the metal door when I attach it.

Another task here is to replace the metal door hinge damping grease.  
Here are photos of the new damping grease and the metal door hinge attached to the cabinet.

To attach the metal lid to the hinge I use the same type of VHB tape that I used for the deck panel.

Using the dust cover, the side of the cabinet and the control panel as visual guides, I fit the metal lid to the VHB.

The fit is perfect and the new damping grease works great.

The final steps are to attach the spring damping mechanism for the dust cover.

Here are the pieces for the dust cover damping mechanism.
There is a leaf spring and a hinged bracket that fits into the back of the cabinet and into the hooks of the metal receiving bracket epoxied to the underside of the dust cover.

These components along with the back plate are common areas where these Beogram 800x turntables get broken.  I will mention it again, the back plate should really not be removed if it is still properly attached to the back of a Beogram 800x cabinet.
The most common problem of the dust cover damping not working is that the original, double-sided tape for the metal receiving bracket has detached from the underside of the dust cover.  That causes the dust cover lowering and raising mechanics to fail.
If that happens the damping hinge mechanism needs to be disconnected from the metal receiving bracket first...then the dust cover removed (by pulling the metal rod for the piano type hinge out).

The fat end of the leaf spring fits under the tension set screw (shown with the Allen wrench above) while the narrow end of the spring fits into the hinged bracket.

The flat, plastic part of the bracket faces the front and the top of the hinged bracket fit into the hooks of the metal, receiving bracket (located under the dust cover).

It seems a bit odd the first time you work with this damping mechanism but it isn't really difficult.
I suggest starting out by making sure the hex tension screw is opened up to where it is easy to fit the leaf spring underneath.  That will also have the least tension on the spring when the hinged bracket is fit into its position.

When the components are attached as in the above photo, the Allen wrench is used to tighten and apply tension to the spring damping of the dust cover.

The service manual says to adjust the tension so when lifting the lid it will automatically seek its top position when the lid opening reaches about 32 cm.
The tension adjustment should also result in the lid lowering operation automatically closing when the lid opening reaches 12 cm.

Here is the fully open position.

The last piece to fit in the cabinet is the flat, plastic trim piece that hides the hinge damping mechanism.

That completes this project and clears space on my workbenches for the next group of Beogram turntables.