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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

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Old 20th Mar 2020, 11:09 pm   #1
EF80TVVALVE
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Default Hitachi SDT-2480 Tape deck recording fault

Hi all

I've been working on this Hitachi music centre. I've done a lot of work to the tape deck on it and now I have it playing well and with a replacement belt the mechanism is now working well too. The playback works fine but I'm having trouble with the recording circuit. This evening after I replaced the belt I took a look thinking maybe the bias pots need adjusting after all of the work undertaken but when measuring the record bias voltage it shown that there is infact a fault.

I'm getting a high voltage (initially over 1 volt) that slowly reduces down to 0 on the test point, almost as if I'm seeing a capacitor discharge. According to the manual it should be 7mV but I'm not getting a stable voltage and it just reduces down to 0 before jumping up to 50mV or so and discharging again. I did turn the bias pots just to see if they made any difference but they have no effect on this at all.

I don't usually repair cassette decks so I'm unsure where is best to start looking. Can anyone suggest where would be a good place to start? I've attached some photos of the circuit and manual below.
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Old 20th Mar 2020, 11:21 pm   #2
EF80TVVALVE
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Default Re: Hitachi SDT-2480 Tape deck recording fault

Just answered my own question here, an earthing problem was causing it. I now have a stable voltage of 2.3mv on both sides but I can't adjust for any more. I'll start checking resistors but if anyone can suggest anything in the meantime then that will be great!
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