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In recording mode is fluctuations on both channels , especially at frequencies of 1-10 kHz. The problem is only when recording, mechanics and playback are normal.
My be frequency self-interference ? When I turn on fluorescent lamp over table (PCB) fluctuating increase !
Lamp has magnetic ballast? Perhaps an heterodyne between 50/60Hz and any other harmonic and the bias oscillator feeding erasing an record head.
The lamp has a switching power supply, but without a lamp there are also fluctuations in the recording mode and they are decrease with an increasing frequency above 8 - 10 kHz from the signal generator.
Where are you measuring the troble? Directly across recording head? Do you have another head? Try get one and couple it to the magnetic gap s it would be a secondary of transformer. For cassette decks I had built a cassette with the tape and rolls removed and inside a small head than can act as a reading head.
Measuring the trouble on LINE OUT. In this deck three heads. Can you show schematic of your measuring cassette ?
I bought this cassette. How to use it in a recording mode with or without an amplifier. I don't see any signal ?
Attachments
Hook it to an oscilloscope using the highest gain available. The signal is really low, perhaps 1mV or so. Perhaps the cassette with the test head needs to be adjusted (azimuth) to get proper coupling with DUT's head. Else the head in the player may be faulty: shorted or open.