Hello,

I would like to ask for help with my new Beomaster 5000. I have some issues concerning radio section of the unit.

Problems:

• Most important problem - no stereo FM reception. FM in mono mode plays extremely well, sensitivity is very high and quality beats almost everything I have. But all other tuners on a just a few centimeters of wire can tune in to the local stations in stereo mode - Beomaster event with a good antenna can't. I started troubleshooting the problem by looking at TCA4500A - stereo decoder. I tried adjusting the 5K pot for frequency oscillator but with no luck (returned to the first value). I also measured the signal from CPU switching the decoder to mono/stereo (pin 9) - it changes between 70mV and about 2.5V when selecting FM MONO on the front panel (but the STEREO indicator never appears) - it seems that the signal from CPU is correct. The stereo-indicator output pin (number 7) doesn't change at all. I also tried to measure frequency with multimeter on the pin 15 - I get about 150-190 kHz, I don't know if that's correct. The supply to the IC seems to be ok, about 11,3V. TCA4500A does not get hot - I read that it is a typical symptom of damage. Currently I have no idea what should I check next.
  

• No MANUAL tuning indicator light. I can switch to MANUAL mode on the front panel and then tune manually, but it is not visible as MANUAL text on the display. Is this indicator a LED?

What I did so far:

• full recap of preamp board
• full recap of amp/PSU board (including big 10000uf cans and 4700uf cap)
• new 100ohm cermet trimmers and adjust of idle current
• adjust of +5V voltage
  
• new caps for CPU module + new 3V battery
  
• new caps for IR transceiver module
• partially new caps  on the tuner section (changed 2u2 near TCA4500A + 100uf near TCA4500A + almost all 10uf)
• new 5K cermet trimmer for generator (if this term is correct) of TCA4500A
  

If you have any idea what might be wrong or any hints for me - please help :) I have a good multimeter and oscilloscope at home so I can measure anything that you might consider important.

Best regards,

Marcin

I'll start from the easier question...

The MANUAL indicator is indeed a LED and it is driven by the MM5450 chip (pin 36) on the amplifier board. That chip is working as you'd see a lot more problems without it, but it could have a dead output. More likely you have a broken solder at the wiring connectors carrying the signal or on the circuit boards.

That same chip drives the stereo on/off control of the tuner board, but that indeed seems to be working correctly (it should go low for stereo).

I'm not an expert on tuners and the full adjustment is far too complex to go through in words (you should have the service manual and an FM stereo test generator), but I have a couple of observations:

Your frequency measurement on pin 15 seems low - the oscillator should run at 228 kHz - but it's possible that your multimeter disturbs it. The right test point is at pin 11, where you should find 19 kHz. That 5k trimmer (R82) is for adjusting the oscillator. Personally, I don't trust the frequency scales on multimeters too much...

The adjustment procedure tells to solder a 220k resistor from TP9 (base of TR7) to +15V while doing this; apparently that either turns the decoder on full stereo or full mono, I couldn't find a proper data sheet.

Also, to rule out that the problem isn't already upstream of the stereo decoder, you could put your oscilloscope at the decoder input (pin 1) and confirm that you at least find the 19 kHz pilot tone coming in. It can be hard to see while it's mixed in with audio and the damn stations never shut up for a single second

Ok, small update.

MANUAL indicator issue cured. One of 3 LEDs for MANUAL light was bad. They are extremely small, difficult (or nearly impossible) to solder manually and probably not available. But I had a couple of red LEDs in package 0603. After scratching off the transparent paint there was no problem to solder one of them to the pads. It was a little bit too bright so I put a black mark with CD permanent marker on it and now it is perfect :)

LEDs are under the white diffusion plastic box:

Small LEDs and 0603 LED visible:

Now working:

Regarding stereo problem - I checked the frequencies:

- on PIN15 I saw a frequency of 150kHz, but I easily regulated it with 5K to about 228kHz

- on PIN11 the frequency with biggest amplitude is much too small (50Hz?) - but probably I did too fast and was not enough careful looking at something around 19kHz

The signal on PIN11 (period of 20ms as 4divx5ms/div):

I also checked the signal on Pin1 of TCA4500A. This is how the input looks like when tuned to stereo station with good reception:

I don't really know whether the constant frequency at about 20kHz (one div is 0.5ms) is the one we're looking for (pilot signal). This constant freq stayed there for a couple of minutes and was not depending on type of programme (voice/music).

The thing that makes me wonder what is wrong is also voltage at pin9 - the mono/stereo switch. The datasheed says that threshold is about 16mV, but on the line coming from MM5450 I never get anything lower than 70mV. May it be the fault of MM5450?

Datasheet of TCA4500A

Well, so for now I still suspect MM5450 IC. Today I noticed that volume on one of channels is not controlled well, on the change 28 to 30 it becomes quieter and so changes the input voltage to TDA1074 (IC potentiometer). Then, at about 38 there is a big jump 'up'. I measured all the resistors on both channels but they're OK and of identical value. So the last thing that may be bad is MM5450 - at least a couple of ouputs.

Best regards,

Marcin

Good work so far! I forgot the display is a module with those horrible early surface mount LEDs.

The volume problem indeed seems like it could be related to the MM5450. You should check the actual outputs of the chip, they will count up & down in binary along with the volume - pins 14-18 for R channel, 9-13 for L (and obviously equally in both channels when balance is at center).

That chip is still available and not too expensive, so you might replace it just to get it off the list of suspects, but I don't believe the stereo problem is its fault. The 16 mV threshold on the data sheet refers to the pilot tone level at the input, not the stereo control; 70 mV is certainly low enough for pin 9.

The signal at pin 1 seems OK to me, but I'm wondering about the high level of 50 Hz in the other pic, that can't be normal.

I found the same datasheet, but to me it seems to be missing a lot of pages that describe the actual operation. There's a small chance I have the actual data book at work.