Beomaster 8000 Type 1903 - Arizona Restoration Project - A Couple More PCB Restoration Tasks

After the previous post I couldn't stop thinking about the Beomaster 8000 frequency response results.  The output was a little beyond the expected -0.5 dB at the two ends of the 20Hz to 20,000Hz frequency range.  Since the preamplifier board and volume control/filters board of this Beomaster had never been restored, I decided to go ahead and update those two PCBs.

Here is the before and after restoration of the Beomaster 8000 Preamplifier PCB.
The op amp devices and electrolytic capacitors on this board are all in the audio signal path.

I replaced the electrolytic and tantalum capacitors with nonpolarized audio capacitors.  
The op amps were replaced with TI OPA2134 op amps. They are more expensive than the original TL072 and LF353 op amps but the OPA2134 op amp has a very good reputation for usage in audio applications.

The Beomaster 8000 audio signal path leaves the Preamp PCB and goes to the Volume Control/Filters PCB.  The Volume Control circuit is first and the Filters circuit is second.  However, in the Beomaster 8000, the audio filtering (including Bass and Treble) is bypassed by default.  The Filters button inside the metal control panel lid engages and disengages the filtering.

The performance measurements I have been making have been with the filters bypassed.

Here are the before and after photos of the Beomaster 8000 Volume Control and Filters board.

Here is the Beomaster 8000 Volume Control and Filters board after being restored.
Like the op amps in the signal path of the Preamplifier PCB, I used the TI OPA2134 op amps here as well.

Now for the frequency response measurements again.

Quite surprising to me, there doesn't appear to be much improvement.  I was expecting a more noticeable result.

At 20Hz the signal is down just over 0.5 dB.  At 20,000Hz the signal is down about  0.8 dB on the Left Channel and just over 0.5 dB on the Right Channel.

As an FYI, I did adjust the Tape 1 input pots to make sure the Left and Right channel test signals are as identical as possible.

I decided that I needed to look at the frequency response measured at different points in the signal path.

First, just the preamplifier section.
I still kept the input signal source on the Beomaster 8000 Tape 1 source input.
For the output measurement though, I pulled the jumper bars between the Beomaster 8000 External Filter input and output RCA jacks.  This would show me the audio test signal only at the preamplifier stage.

My test cables picked up some noise from the line 60Hz but the frequency response is still very flat.
The output at 20Hz is only down about 0.02 dB and the output at 20,000Hz is only down about 0.04dB.

Next is the Beomaster 8000 Output Amplifier.
I used the Output Amplifier P39 connector for the audio input signal and I measured the output across the 8 ohm, dummy speaker loads.

My test setup encountered much less noise with this measurement.

The Output Amplifier frequency response measurement was also nice and flat.
At 20Hz the output is only down about 0.2dB while at 20,000Hz the output is only down about 0.05dB.

The Preamplifier and Output Amplifier boards checked out very flat as I would expect.

That means the Volume Control and Filters board must be the affecting the frequency response measurement I get with the end to end test.

Here is the Left Channel frequency response with the audio input at P27 of the Volume Control and Filters PCB and the output measured at P24.

Again, my test setup picks up the line 60Hz but you can see how much different the shape of the frequency response is.

I hadn't looked at just measuring the Volume Control and Filters board by itself before so I spent a little extra time.  Because the capacitors and op amps are all new and known good components, I wondered about the AD7110 (logdac) IC device itself which performs the Beomaster 8000 volume control functions.

Instead of jumping in and replacing the volume control devices (there is an AD7110 IC for each channel), I decided to take advantage of having a couple of spare Volume Control and Filter boards available.

Both of my other (restored) Volume Control and Filter boards yielded very similar frequency response waveforms.  It would appear that is a characteristic of this volume control design...but I will need to measure more Beomaster 8000 units to confirm or refute that.

For now, this as far as I will take this experiment.  The frequency response results I am getting are very close to the -0.5dB range specified in the Beomaster 8000 technical data sheet.  More importantly, the music output of this Beomaster 8000 sounds very good as things currently stand.  So I will start putting components back into place inside the Beomaster 8000 cabinet and do some final listening tests before shipping this receiver back.  

I have several more Beomaster 8000 units to restore so I will definitely be repeating these measurements for comparison.

Beomaster 8000: Exchanging the Opamps in the Signal Path and Test

After updating the uProcessor board in the Beomaster 8000 that I am working on right now, it was decided to also update the opamps while the unit was in service position. We recently noticed that the opamps in the signal path of the Beomaster 8000 can degrade resulting in increased distortions (THD), i.e. it is a good idea to also replace the opamps when the boards are out for restoration. Here we go:

This shows the control panel PCB before the upgrade:

Most of the 8-pin ICs on this board are signal path opamps. This shows the board with new socketed LF535 opamps installed:

On to the preamp/input board. I forgot to take a picture of the original condition of the board. Here are a couple shots of the board after replacing the opamps with socketed LF353 units (except the phono input, which was replaced with a low noise LM833 type):

A detail photo of the phono pre-amp section:

After implanting the boards I characterized the performance of the unit with my QA400 audio analyzer. The bandwidth curve yielded the spec -1 dBV drop between 100 and 20,000Hz, and the total harmonic distortion (THD) values at volume 5.0 (just below clipping) were 0.008% on both channels, which is consistent with other Beomaster 8000s we measured. See here for a detailed discussion of such measurements. So far so good...the unit went on into our living room to see if the performance of this Beomaster 8000 is consistent in day-to-day operation.

Texas Beomaster 8000: Preamplifier, Filter & Tone Controls

I finished the rework of the preamplifier board along with the filter & tone control board out of the Beomaster 8000 receiver from Texas. The task was pretty much the same as for the Beomaster 8000 from Canada. The electrolytic capacitors were replaced with new capacitors. The LF353N (or TL 072CP) opamps removed and new LF353N opamps inserted. The tone control sliders and source input volume trimmers were flushed out and cleaned with Deoxit. All of the board connector solder joints were reflowed with solder to insure against any weird problems with hairline cracks in solder joints. It is better to be safe than sorry. Especially with audio equipment this heavy.

The two boards look clean and new again. Soon I will be at the point I can give them a try.

Here is the filter & tone control board before the restoration.

Here are the components I am replacing removed.

There were no real surprises on this board except an area on the trace side that had some suspicious looking solder joints around IC102 and IC202. They just didn't look good to me.

I desoldered the bad looking joints and resoldered them.

After the resoldering I discovered one bad area on the board where most of the trace had broken away.  For that one I wrapped a small wire around the pin that connects to the bad trace pad and soldered everything so it is solid again.

That does it for the filter & tone control board.

Now for the preamplifier board.  Here is how the board came.

This is the preamplifier board after removing components and cleaning. I used Deoxit in the source volume trimmers.

This preamplifier board had TL 072CP instead of LF353N opamps. Both are listed as interchangeable devices in the service manual.

The board cleaned up well and I installed 8-pin sockets for the new LF353N opamps.

Here is the completed preamplifier board.

Canada Beomaster 8000: Filter & Tone Control Board

The next component for restoration on the Beomaster 8000 from Canada is the filter & tone control board. The audio signal of the selected audio source goes from the preamplifier board to the filter & tone control board where the bass, treble and filter circuitry lives. This board also has the Beomaster volume control which sets the volume level of the receiver output amplifiers according to control signals from the microcomputer board. The microcomputer board also mutes and restores the volume level when the selected source is changed. Another circuit on the filter & tone control board is for the FM signal strength meter.

Here is the board before the restoration work.

The trace side of the board has screws for securing the filter & tone control plastic panel. This board was missing one mounting screw so I will supply one when I reassemble the board.

Here is the board with the control panel removed. The restoration will involve reflowing the solder connections of the board connectors, replacing the electrolytic capacitors, replacing the LF353N opamps and cleaning the bass/treble slider controls.

Here is the board after cleaning, reflowing, recapping and new LF353N devices.

There is an adjustment procedure in the service manual regarding the left and right channel DC offset for the Beomaster 8000 volume control circuit. If the offset voltage is too high it will result in a static or clicking noise in the speakers when the volume is changing (up or down).  The two trimmers on the filter and tone control board are for adjusting out that offset. There are some associated fixed resistors next to the trimmers that can be disconnected as needed to aid in the offset trimming. The board above has two of those resistors (one in each channel) disconnected. Once the Beomaster restoration is complete I will go through the volume DC offset procedure and readjust the trimmers as necessary. I have found they usually do require some sort of adjusting.

Beomaster 8000: Finishing up the capacitor replacement

This Beomaster is getting closer to a test run. I completed the capacitor replacement on all of the boards now. There were no real surprises there. I did find a higher than normal number of capacitors that were out of tolerance and quite a few on the border of being out of tolerance. In addition to the capacitor replacement I also replaced some opamp devices and I reflowed the solder joints on all of the board connectors.

Here are the FM and FM Interface boards.

Next, the preamplifier board.

I had seen quite a bit of dirt, grime and rust on this Beomaster in the previous blog posts so I was wary of possible corrosion on the circuit boards. You can see that the six input level trimmers have some sort of grime on the plastic housing.

Sure enough, several of these trimmer knobs would barely turn. I used two types of Deoxit spray on the trimmers. Each trimmer has three slots that expose the inner workings and I was able to flush the grime and get them working smoothly again.

Along with the capacitor replacement there are seven audio opamp devices that get replaced. Beolover just posted a nice description of why we replace these particular opamps. Here is the preamplifier board with the capacitors removed and new eight pin sockets ready for new LF353N opamps.

...and here is the completed preamplifier board.

The Filter & Tone Controls board also has some audio opamps in the signal path that get replaced. There are also a few capacitors to replace. The tone control sliders and switches also received Deoxit treatment so they operate smoothly and have clean contacts.

As I did on the preamplifier board I added sockets for the new opamps. The sockets are nice because it makes it safer to install the new opamps (no solder heat gets applied to the actual device). Having the socket also makes it easy to switch out a different opamp in the future should that become necessary.

The display and microcomputer boards have only two capacitors each (that get replaced). The display board will be revisited later to change out the display segment LEDs and to replace the four indicator lamps.

Here are the two replacement capacitors on the display board.

This is also just a first visit to the microcomputer board. I will come back to it and replace the two oscillators later.

The 22uF (9C85) capacitor on the left has been replaced before. The original capacitor leads were cut and the replacement capacitor soldered to the remaining leads (still soldered to the board). That isn't necessarily bad but I prefer removing the capacitor and properly soldering in a new one. 

An important thing on this microcomputer board is that some of the component leads must be soldered to both sides of the board. The negative lead of 9C85 is one of those type of components.

The capacitor replacements on this Beomaster 8000 are now all done. I can reinstall the preamplifier, FM, FM Interface, Filter & Tone Controls boards in the Beomaster chassis now and start connecting their cables.