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:

link

Small LEDs and 0603 LED visible:

link

Now working:

link

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):

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

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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?

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.