I have a problem, the TDA1380 is probably defective.
Digital Play A only works if pin 47 (TDA1380) is connected to ground with 1 MOhm. RDMUX shows all 8 channel signals, with 1-4 being slightly weaker. Digital Play B does not work. Only channels 5-8 are present as signals. Channels 1-3 appear depending on the position of the external 1-MOhm potentiometer. Channel 4 does not appear when Play B.
All 1µF capacitors at the signal inputs from the head are OK. All head resistors measure the same. The DC offset at the inputs is correct at 3V. INMFL (p47) has 1.2V. VBIAS has 1.4V, AGC has 1V. The AB switch has 3.3V alternately, depending on the play direction. The capacitors and resistors in the FB area (INMFL, INMFR, MFL1AB, MFR1AB) have also been replaced.
Analogue playback sounds like a cardboard disc in the spokes of a bicycle wheel. This normalises when 1 MOhm is applied to pin 47 or the signal level is low. Analogue playback possible on both sides.
Q101 has been resoldered.
Of course, the head, capstan and rubber rollers were meticulously cleaned with isopropanol.
Did I overlook anything during troubleshooting?
Where can I get a TDA1380?
So, quick update. Cause identified within the film head.
Normally, you check the segments in the head one by one. Everything is fine.
The measurements on the (feedback) circuit on the TDA1380 after replacing all capacitors were also fine. The imbalance in the measured values occurred when the flexible cable was connected to the board.
This was due to a shortcut (15 ohms) within the head between J101 pin 25/26 and segment TB4+ / TB4-.
Thorough cleaning of the film head with isopropanol did not help.
The only solution: apply a rising DC voltage until the shortcut ‘burns away’. - That worked.
Although the shortcut is now 12 kOhm, all 8 channels in the RDMUX are working again.
Play A and B are now working again – for the time being.
This method is very drastic, but without any chance of getting replacement parts, it was the last resort. 
You are the very first person I’ve ever heard that repaired their head this way. Really clever thinking 
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Yes, what else could I do? It was the only logical solution. Getting a replacement film head is extremely difficult, if not impossible. I have been a hi-fi technician since 1991 (B&O, Onkyo, Denon, etc.). - Sometimes you have to resort to unconventional solutions. 
I wouldn’t call it repaired, but it works again. The thing had been sitting in a box for 30 years. It’s so nice to be able to listen to those old tracks from the 90s on the device again.
Well, even our @drdcc did not come up with this one.
I would say this is a brilliant way getting it working. We all learned something new.
That is what this forum is all about. Welcome to the forum @Laserzicke