Beomaster 2400 (Type 2902): Repair of Permanently Illuminated FM Stereo Indicator

After repairing the weak FM reception issue of a Beomaster 2400 that I am currently restoring, I had to accept that the unit had yet another issue: The FM stereo indicator was permanently on whether an FM station was dialed in or not, or even whether FM was selected at all. The indicator was on all the time! If everything works well, the green indicator only comes on when good FM reception is achieved.

A glance into the circuit diagram revealed that the stereo light is directly controlled by pin 6 of the monolithic MC1310 FM Stereo Demodulator IC that extracts the audio signal from the FM broadcast signal.

This left little room for ambiguity: This clearly meant that the bulb driver integrated into the IC had failed. The good news here is that this IC was pretty popular in the 1970s and 80s and so it can still be found as NOS. I ordered a bunch from a vintage IC vendor in China. I received the package a couple weeks later. 

I put the Beomaster back on the bench. This shows the original MC1310 soldered in place:

It is implemented in a strange way with the pins bent in an alternating way. This shows the solder side of the board with the pins already unsoldered:

I removed the IC and it looked like a little cockroach with its bent pins:

Buying NOS ICs is always a bit risky, and I expected having to do a bit of triage to find a properly functioning IC among the ones I received. So I desired having an IC socket implanted that would allow me to insert a few of the chips until I found one that would work properly. I had a few 14-pin IC sockets in my stash with long pins. This shows one of them next to the original IC:

I cut and bent the pins to match the pattern of the original IC. It turned out that the best way to achieve a good fit is to try inserting the socket from the solder side first to make sure the pin pattern is matching the solder points:

This shows the socket with the pins bent into a machining pattern:

I inserted the socket from the component side and soldered it into place.

Then I thoroughly cleaned the pins of one of the NOS ICs, coated them with De-oxit, and stuck it into the socket:

I plugged the Beomaster in and fired it up. The stereo light was off! Yes!! But my euphoria subsided quickly: It turned out that only one or two stations across the dial would achieve stereo reception. So something was wrong. Normally, most local stations in Albuquerque will come in with a nice, strong stereo reception on a Beomaster 2400!. 

The most likely culprit was the VCO (Voltage Controlled Oscillator) frequency. The 1310P uses a Phase-Locked Loop (PLL) to lock onto the 19kHz pilot tone. If the VCO frequency of the 1310 is off, it becomes impossible for the PLL to achieve a lock. I measured the VCO signal on 2TP4 with my oscilloscope:

 

The trace yielded a frequency of 19.8 kHz, i.e. a bit too high. The frequency of the VCO can be adjusted by tuning 2C24 as shown here:

Turning 2C24 a bit resulted in a clean 19kHz signal:

And this fixed the issue! Now the stereo light behaved as it should: Strong stations bring it on and weak stations do not. It looks like the tuner department is finally sorted out! This project may finally come to a successful end! Stay tuned for the full restoration report!

Beomaster 2400 (Type 2902): Repair of Weak Tuner Reception

I am currently restoring a Beomaster 2400 (Type 2902) for a customer from Virginia. After doing all the standard restoration tasks (replacing all the electrolytic capacitors, implanting new quiescent trimmers and installation of the Beolover LED Replacements for Beomaster 1900 and 2400 Indicator Light Bulbs I fired the unit up and it worked beautifully with a Beogram 4002 plugged in.

After enjoying a couple of records I noticed I got only static from the FM tuner no matter where I set the dial. Normally, the 2400 produces a pretty strong stereo signal from local stations without even plugging in an antenna. I plugged in an antenna and now I was able to pull in some stations, but with a loud hum in-between them and in poor sound quality. So something was definitely wrong with this tuner!

I pulled out the circuit diagram and started scratching my head. I measured the voltages around the JFET transistors in the front end cascode. The circuit diagram prescribes 3.6V at the gate of 1TR1 but I got only about 440mV. Way too low!

This shows the relevant circuit diagram section (click on it to see a magnified version):

The "1" section is the tuner 'front end' that is contained in the shield can next to the antenna connector of the Beogram. TR1/TR2 are its cascode amplifier stage. This design essentially results in a very high input impedance 'transconductance' amplifier that also has a low capacitance to be able to work at high frequencies. The gate of 1TR1 is connected via two resistors 330k and 1M to the emitter of 2TR1 in the IF stage of the tuner (yellow wire that goes into the box). This connection is essentially the Automatic Gain Control (AGC), which changes the gain of the cascode stage to keep the signal at a similar level for strong as well as weak stations to prevent overloading of the IF stage. So the 3.6V shown in the circuit diagram is for a specific signal strength coming in at the antenna port, "500uV with pilot signal" as specified in the manual and will vary with the signal strength of the station that the tuner is set to.

After measuring the too low gate voltage I initially thought the cascode transistors had a problem (they can easily get damaged since they are not protected from static charges coming in through the antenna port). So I ordered a few BF256 JFETs that can still be found new and that have similar parameters like the original TIS88A that have been long obsolete. But installing them brought no change. The gate voltage remained at 440mV.

This made me think that 2TR1 in the IF stage on the main PCB ("2") may have an issue since its emitter essentially defines the gate voltage of 1TR1 (yellow wire that goes into the front end can). I measured the DC voltages around it, and indeed I found only about 500mV at the emitter instead of the prescribed 4.6V in the circuit diagram.

This transistor is originally a BF240 which is also obsolete these days. The good news is that it can be replaced with still available MPSH10 types. I put in a new MPSH10

and this fixed the problem. Now the tuner pulled in local stations perfectly without antenna and the loud hum between stations was gone, too! I was now able to measure voltages between 2.1V and 4.1V at the 1TR1 gate depending on what stations I tuned in and whether I had the antenna plugged in or not. The lower voltage corresponded to stronger stations and the higher to weaker ones. The red voltages in the above diagram were measured with the BF256 and MPSH10 in place. 

There is one remaining issue with this Beomaster 2400: The tuner stereo light is permanently on, even when Phono or Tape is selected. Since the light is directly connected to pin 6 of 2IC2 via a resistor I am afraid the IC has an issue. Luckily the MC1310 is a popular monolithic FM stereo demodulator used in many tuner designs of the 1970s and 80s, and it can still be found at NOS dealers. I ordered a few. Stay tuned! (pun intended!...;-).

Beomaster 8000 - Workbench Unit - Investigating FM source audio in the Phono source channel

In this post I wanted to revisit Beolover's fix for reported FM audio crosstalk in the Phono channels.
I actually have been checking for that fault in my Beomaster 8000 restorations and with these new source switching IC device replacements. Unfortunately I had been unsuccessful in replicating the problem.

To review the problem...
Make sure the turntable connected to the Beomaster 8000 does not have a vinyl record ready to play.
When the Beomaster 8000 is switched on from Standby to Phono the Beomaster 8000 volume can be cranked up without hearing any music from the Phono music source.  That is good and is what is expected.

Next switch the Beomaster selected source to an FM station (preferably a strong station). Note the music that is selected. Now switch back to the Phono source (again with no record playing). Turn the Beomaster 8000 volume up and you can begin hearing (faintly) the FM station that you were just on bleeding into the Phono channel.  That is the problem.

As I mentioned, I had not been able to create this problem until this weekend when I moved my workshop Beomaster 8000 unit to a different bench and connected up a powered Terc FM antenna to the radio antenna input. This produced the strongest FM signal I had seen on the Beomaster. Retesting the FM crosstalk problem...I now had reproduced it.

Interestingly this test setup is with my new analog switch devices installed in the Beomaster 8000 preamplifier board. The preamplifier board also has TI OPA2134PA OpAmp devices for all of the analog sources.

Up until now I had reasoned that any audio crosstalk must be occurring in the analog switches the Beomaster 8000 preamp board uses. That made the most sense. However, this preamp board has the new multiplexer switching devices that have very good audio specs and low crosstalk ratings. I thought that should take care of any audio source crosstalk.

It turns out that it doesn't and that is quite surprising...and I do not understand the source of the problem at this point.

An acceptable work around for someone not wanting to make any wiring changes is to always put the Beomaster in Standby mode prior to selecting the Phono source. That will keep the FM tuner in the Beomaster muted while using the Phono source.

We want our cake and eat it too though so I decided to implement Beolover's fix for this problem.

I performed the same wiring modification Beolover came up with but I implemented the change differently.  I wanted to be able to easily switch from the original wiring to the modified wiring.  That option will allow me to keep investigating the problem.

My implementation is to put female jumper connectors on the two wiring routes related to this problem and fix.  Beolover's fix is to run the FM board audio mute control line to the preamplifier board phono source selection line. When Phono is selected on the Beomaster control panel the phono source selection line will tell the FM board to mute the FM audio. When the phono source is deselected the FM control will unmute the FM audio.

Here is the modified wiring for this on the FM board. The picture shows the FM audio mute control line connected to the default path the Beomaster 8000 originally takes.

The yellow wire from the FM board 1R37 resistor can be disconnected from the default ground path and connected to the Preamp board added connector (also on a yellow wire).

This path routes the FM audio mute control line to the preamp board Phono source selection control line.

Testing this modification in the workbench Beomaster 8000 unit I verified that I get no FM audio at all in the Phono channel with I have the FM audio mute control line connected to the preamp board.  I also verified that moving the FM audio mute control line back to the default position re-introduces the FM crosstalk problem.

Canada Beomaster 8000: FM and FM Interface Boards

The Beomaster 8000 FM section is on two boards, the FM board and FM Interface board. The restoration task on these two boards is primarily cleaning, replacing capacitors and reflowing solder joints on the board connectors.

Here are the two boards as they were received.

I removed the components that are being replaced then cleaned the boards.

I didn't run into anything dramatic on these two boards which is how I like it. Several of the removed capacitors were out of tolerance.

I replaced the capacitors 2.2uF and under with WIMA MKS capacitors. For values higher I replaced using 105°C rated electrolytic capacitors.

Here are the completed boards ready to be reinstalled in the Beomaster.

Beomaster 8000: Reassembly for Testing

I can finally start putting this Beomaster back together where it can be put through some functional tests. There is a final component that needs attending to however and that is the door damper for the lid that covers the Filter & Tone Control panel.

The Beomaster 6000, 8000 and Beocord units from this period share a door damper component that keeps a spring loaded lid from flying open. The damper applies some resistance to the spring so the door opening is more controlled.

Here is the damper that came out of this Beomaster 8000 unit. You can see that damping grease has started to leak out. I have seen units where the damping devices are still sealed and don't leak but I have also seen a bunch that are leaking. My guess that the environment they were stored in plays a part in how they have survived.

Here is the damping component opened up.

Ready for the new damping grease.

Here is the reassembled damper installed back in its place in the Beomaster.

It is easiest to reattach the door to the damping component with the Filter & Tone Control board removed. Now that the door is working again I can continue with the rest of the Beomaster reassembly.

During this phase of the project I test play the Beomaster every chance I get. The more actual use of the restored receiver the better. I noticed that the FM tuner didn't sound right and looking at the Signal-Multipath meter in the photo above there should be an illuminated red "MPX" lit up. It is dark in the photo. When the Beomaster 8000 is tuned to an FM stereo station the MPX lamp lights up to indicate it is decoding stereo FM. That is what my ears were trying to tell me. The FM tuner was tuning stations but they were only playing mono.

That means lifting the Filter & Tone Control panel to do a little trouble-shooting of the FM board. Since the problem seemed to involve just the FM stereo functionality I concentrated on the things that just relate to that.

I made some minor adjustments to the 1R45 (Stereo Decoder - oscillator) and 1R41 (Opening of Stereo Decoder) trimmer resistors. Doing that got the MPX LEDs in the Signal-Multipath meter to illuminate on an FM stereo channel. Unfortunately I was still only hearing mono from the Beomaster.

That left the Stereo Decoder chip (1IC3) as the next suspect. It is responsible for decoding the stereo signal.  Fortunately I had a spare TCA 4500A chip in stock.

That did the trick. The FM stereo channels now decode properly and the tuner sounds good again.
Here is the fixed FM tuner. The red arrow points out the illuminating MPX light.

Now to finish closing this unit up and begin the final testing.

Beomaster 8000: Power Test Update

The +15V regulator is now functioning correctly on this Beomaster.

The next thing to do is recheck the no-load current and the DC offset adjustments again.

I did both channels and no problems with those adjustments. I didn't expect any since I had already tested these output amplifiers by themselves earlier.

I usually do a first listening test using the Beomaster TP2 source once I am confident the amplifiers are ready. This unit fired up and played just as beautifully as I expected.

I performed the same check on TP1 and it played great too.

After an initial scare where the FM tuner wasn't producing any sound, it turned out the problem was only a faulty connector not making contact. I fixed that and the FM tuner also works great. The signal meter and tuning display indicate the FM board is picking up stations.

A good second test overall. The power supply tests all pass now and I was able to successfully recheck the output amplifier settings. Best of all I got to hear a nice preview of this Beomaster playing music again.

The next steps are the obvious display segment repair and I will do the preventative update of the microcomputer oscillators.

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.