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: Input Selector Trouble - Tuner Audio Mixed with Phono or TP1 Inputs (Pt. 2 - Conclusion)

I thought this was fixed! But it wasn't! Last year I reported about a strange intermittent issue with a Beomaster 8000 that I was restoring: When switching the input to Phono it sometimes mixed in the tuner signal that both turntable signal and tuner signal were audible at the same time...a pretty useless situation. My investigation back then determined that there was a discontinuity around P82 on the uProcessor board, which occasionally caused the input switches on the preamp board to malfunction. P82 carries the control signals for the two electronic multiplexers that control which input is routed into the preamp-circuit of the Beomaster. I implemented a bunch of jumpers, shown here:

and the problem appeared to have gone away.

As it turned out recently, this was an illusion as the Beomaster again showed this issue when I tried to play a lovely 1950s vinyl by Cal Tjader of which I bought a significant number recently. 

My initial thought was that maybe the ribbon cable that connects the input selector pins of the uProcessor to the preamp board had an issue, and so I ordered a custom manufactured 5-lead ribbon with appropriate 0.1" headers and put it in. Here is an impression of the end plugged into the uProcessor board:

And a photo of the preamp end:

While this new cable made a great connection and looked very pretty, it did not cure the problem. But at least another potential root cause was ruled out.

Back to the bench it went. I opened it up (again) and wiggled P82, and there it was (again)...depending on the pressure on the brand new connector the input behaved correctly or malfunctioned. I took the board out and had a closer look. All the jumpers were in good shape, so the problem had to lie somewhere else. However, all the traces checked out o.k. (again) and so I was a bit at a loss.

I put the board back in and played the Beomaster for a while scratching my head. Then it occurred to me that such problems are often related to ground connection issues. I prepared a jumper cable from the preamp board to the uProcessor board. Then I induced the issue and subsequently made a separate ground connection between the boards with the jumper. And, eureka, the input out of a sudden behaved normally. With the jumper in place I was not able to produce the issue again. I took the board out again and inspected it more. And I finally understood how the grounding plane is setup on the uProcessor board. I realized that this board is actually a double sided PCB, even though it has no etched traces on the back side. However, they used the continuous Cu layer as a ground plane from which several connections are made through the board to the traces. This is hard to see since the surface just looks a uniform green and there is no obvious mention in the manual where they show the PCB layout. After this it became clear to me how the ground connection to the preamp board was made. This shows the pertinent part of the board:

C67 connects through to the patterned side of the board and so makes the ground connection that connects to pin 2 of P82. The picture shown on top shows the backside of this area. I had soldered a jumper in from the C67 leg to Pin 2 of the header to bridge potential cracks in the trace connecting the two. What I did not consider was that the problem was rather caused by a 'cold' solder joint on the other side causing an intermittent contact. I resoldered the area and then the problem went away. Another demonstration that most control issues in the 8000 are caused by problematic solder joints on the uProcessor board.

Back to testing it for a bit longer, but I think it may finally be ready for its journey to its new owner in the UK.

Beomaster 8000: Input Selector Trouble - Tuner Audio Mixed with Phono or TP1 Inputs

*************************Achtung: There is a follow up to this post. **********************

Oh well...I ran the Beomaster 8000 that I finished up recently for some time in my Beosystem 8000 setup and all was well, but then it developed a strange issue: When switching from FM tuner to Phono the phono signal was audible, but the tuner signal was still there in full strength. So instead of switching from one input source to another the Beomaster decided to mix them together instead! So back to the bench it went!

After I opened it up I discovered that I could provoke the issue by wiggling P82 on the uprocessor board. P82 transmits the microcontroller's wishes to the input switches on the preamp board. So this made immediately sense: The FM input did not get turned off when opening up the Tape 1 or Phono inputs due to an intermittent connection (the audio input switches on the preamp board are basically gates that open and close depending on high/low control signals from the processor).

First I thought it was a plug issue, and I cleaned the contacts a second time and bent the header bins a bit to give everything a bit more mechanical intimacy, but the problem persisted. So I came to the conclusion that the issue must be something more serious, like cracked traces or bad solder joints. This photo shows the original condition of the P82 header from the solder side:

Close optical inspection did not reveal anything abnormal, but I was able to demonstrate that at least Pin 1 had an issue since I was able to 'disconnect' it from the component it is connected with by flexing the board a bit. So I put in a magnet wire jumper and put the board back in. After firing the Beomaster up, well, no change...o.k. I took the board back out and this time I jumpered all five solder points to their next neighbors:

Magnet wire is really very helpful for this type of fix, since the polyurethane coating burns off when heated with the solder iron. This allows to make safe wire connections in tight spaces.

After this fix, the problem seemed to be gone. So I put the Beomaster back together...let's see if it is stable now! I usually test these babies for a few weeks until I send them back to their owners. There are just too many components and interconnections that can develop problems, and often they are intermittent. The only way to make reasonably sure that the experience on the customer end is a happy one is to give them a good testing under normal operation conditions. But I sure hope that this was the last post about this particular Beomaster 8000...;-)!

Beomaster 8000: Step Four - Rebuilding the FM Tuner, Preamplifier and Control Panel PCBs

After completing the power supply restoration of the Beomaster 8000 that I am rebuilding right now, it was time to do the remaining boards. While it is relatively rare to find a dead capacitor on the FM tuner, preamplifier and control panel PCBs (they run relatively cool), it is a great idea to replace them anyway while one is in there. That brings everything on the same 'capacitor clock' (for another 30 years countdown...;-). It is also a great idea to re-solder all the wire to board headers since they tend to develop bad solder joints over time. Here are a few impressions of my progress:

This shows the board under the control panel switches and sliders in its original condition:

And after replacing the capacitors:

On to the preamplifier board:

In original condition:

and after rebuilding it:

The FM tuner has two boards, the front end and the detector part:

This shows them after exchanging the capacitors:

On to the display and uComputer boards!

Beomaster 8000: Display Restoration with SMD LEDs

After I replaced the indicator light bulbs of the Beomaster 8000 that I am restoring right now I did the 7-segment displays. The 7-segment displays are essential for the signature looks of the 8000 and dead segments are at best annoying. I usually recommend to rebuild the displays even if they are still o.k. since the death of individual segments is virtually unavoidable considering the age of the units and the potential rigors of shipping. The original LEDs in these displays are not encapsulated, i.e. the bonded contact wires are not protected against vibration and thermal challenges etc.... Therefore, the usual failure mode is loss of contact at the bonding locations. Final failure is often preceded by intermittence as these wire bonds. This Beomaster had a few segments in this stage, which may or may not explain the claim by the seller that the displays were in good working condition. I posted many entries in the past about my restoration process, a summary can be found on my dedicated Beomaster 8000 page. The pictures shown here are posted to document the work done on this particular Beomaster 8000.

This shows the display board after I extracted it:

This shows the displays after unsoldering them. It is mandatory to use a desolder gun for removing them to prevent damage to the fragile pre-FR4 age PCB:

Then I liberated the displays from their plastic mounts,

and opened them up:

Then I removed the original LEDs and then soldered SMD LEDs into place. Then it was time for my 24hrs test where I power the boards from a test-jig that I set up with a couple solders breadboards:

Then I put the covers back on and tested them again to make sure all segments survived the procedure:

Then it was time to solder the displays back in. Here you can see them mounted back on their plastic mounts:

And back on the PCB: 

And fired up together with the newly rebuilt indicator lights (unfortunately I forgot to use a FM preset, so the source display is dark in this picture). The volume is a 6.0 to get the clipping indicator to light up:

On to the uProcessor board.

Beomaster 8000: Recapping the Tuner Boards

The final boards to be recapped in the Beomaster 8000 that I am currently restoring were the FM tuner boards. Here are some impressions of this effort. This shows the boards with the original capacitors:

And here after replacing all electrolytic capacitors with 105C grade Japanese units:

With these boards it is especially important to clean the contacts between the boards and coat them with DeoxIT. I had several cases in the past where one channel did not work on the tuner, which could directly be traced to a contact issue between these two boards. After I put everything back together I tested the Beomaster and everything works fine now!

This concludes the recapping of this Beomaster. On to the displays and the uProcessor boards. On into the heart of the dragon...;-)

Beomaster 4400 (2419): Missing Preset Cover Replacement

It seems the Beomaster 4400 (2419) that I recently restored runs stable. I have it connected to my Beogram 4002 and I also tested it with the Beocord 9000 that I am working on right now, and everything functions very nicely. The final task was to replace the missing preset cover. There must be a big heap of these lost covers somewhere on this planet, but as long as we do not know where this Preset Cover Shangri-La is, getting authentic reproductions (injection moulded as the originals) from 'Dillen' (beoworld.org) is a great option. He recently sent me a few, and I installed one. Fits very well, maybe a bit tough to remove, but that will keep it (hopefully) away from their hiding place...;-). Here is a pretty shot of the cover:

Time to double box this Beomaster 4400 and send it back to its owner. That is the hard part of Beolove! Constant separation trauma...love can be pain...;-)

Beomaster 4400 (2419): Tuner Repaired

The Beomaster 4400 that I am rebuilding right now seems to be running on all cylinders again. But a little more work was needed: 

After restoring the 15V rail the tuner came alive, but with a very weak output, while a standard 1Vpp signal piped into the tape input yielded a normal output response. So the problem was definitely not in the amp section. So I poked around on the IF board (#2). First I suspected a broken filter crystal, but the three turned out to be o.k. But while I was messing around out of a sudden the tuner started blasting full volume, indicating another broken trace/bad contact issue. I cleaned the contacts of the two board-to-board connectors and plugged it back in...no change. So I re-soldered all the contacts of the plugs and I also replaced the two electrolytic capacitor on this board with quality Japanese 105C types while I had it out. I also put in new headers and jacks for the signal meter connectors. They looked strangely corroded with some white coating. Here is a picture of the rebuilt board:

After I plugged everything back together the tuner came to life at full volume, but only on the presets. The main FM dial still did not work. This indicated some more contact issues, since the FM dial is nothing but another tuning voltage delivering trimmer in the context of the circuit diagram. Just looks more important...;-). I finally figured out that the culprit was a crack in the switches PCB (#6):

This was a pretty big one. It separated the 4.7V end of the main dial 100k pot from the voltage source, and so the pot was not able to change the tuning voltage from the 26V on the other end, i.e. the tuner was permanently set to 108 MHz independently of the pot setting.

I installed a jumper (grey) to bridge the crack (the grey/white wire goes to the pot):

This brought the FM dial back to life. Then I epoxied the PCB back together using a couple PCB snippets that I had laying around from designing Beogram 4000 scale illumination LED boards:

Here is a nice shot of the tuning indicators after the repair. Both tuning lights work again, and the stereo indicator is on. Beautiful! Beolove!

One the two capacitors that I replaced on the IF board was completely dead. It showed the characteristic cracks in the can, and measured 25nF instead of 4.7uF. This makes this Beomaster 4400 a definite candidate for a recap:

Beomaster 8000: Recapping the Preamplifier and FM Tuner PCBs

I continued the recap of the current Beomaster 8000 restoration project. Yesterday, I did the preamplifier and FM tuner PCBs. Not much spectacular to report, just a bunch of new 105C grade Japanese capacitors replacing the aged and partially cracked 1980s types. Here are a few impressions:

Preamplifier board before:

And after replacement of the capacitors:

And a detail shot:

Tuner PCBs before:

And after. It is important to clean the contacts between the boards. I always coat them also with some DeoxIT D100 to prevent further oxidation down the road. If one does not do this there is a good chance that the tuner may tune out on one or both channels since the signals travel through these connectors. This picture shows the boards after the recap:

And a detail shot:

On to the filter and tone control board, the last PCB that needs new capacitors. Then it will be time to put the Beomaster back together and test the 3D printed latch! Exciting!