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Old 15th Sep 2020, 8:40 am   #95
DMcMahon
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Join Date: Apr 2019
Location: Worthing, West Sussex, UK.
Posts: 6,587
Default Re: Uher CR 240 AV Cassette Recorder

Still getting quite an intermittent playback signal on the left channel at MP3, not yet able to prove if due to the signal being intermittent at the output of the playback amp or intermittent at the Dolby board, every time I monitor those places the signal comes good before I can monitor.

Plugged stereo headphones into the 1/4" headphones jack to make sure there is no output signal present (when scope shows good signal at MP3), there is no audible signal there at all.

Scoped the intermittent good left channel signal to the input of the IC2 amplifier chip (pin 7), this is good and controllable by the volume control. But there is zero output at the output pin 12. IC 2 drives the loudspeaker and headphones, IC 3 does the right channel.

IC 2 & IC 3 are TBA 820P which are a 14 pin DIL package, these are long superseded by 8 pin version TBA 820M. Can still purchase the 14 pin TBA 820 as NOS but some of them are silly premium prices.

Have got a somewhat intermittent signal at MP4 on the right channel but not the music playback signal, but a 50Hz (20mS) sinewave, maybe a mains hum signal, have not yet traced its origin. Like the left channel the 50Hz signal arrives at the input of the amplifier (IC 3) and is controllable by the volume control, but there is zero signal at the output of the amplifier.

Possibly looks like the two TBA 820 amplifiers are dead but before I fully condemn them, need to check the various capacitors (including Electrolytics and Tantalums) around them in case their outputs are being pulled down.

Trying to find a 7 or 8 pin DIN plug so can check the line outputs at the Radio and Autoradio DIN sockets (the 7 pin plug will probably also fit the 8 pin socket actually).
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