Quote:
Originally Posted by saxmaniac
Thanks, yes the trimmers C1 and C2 are mounted right by the heads. I can't get a reading with the scope directly across the 100 ofm resistor as on side of the scope input is earthed and will simply short the bias to earth. I thought of removing the earth but thought that that would add so much spare capacity as to make any readings meaningless. I measured bias at erase head and got a large perfect sine wave. At the RP head with the scope between the head input and earth, there's a very distorted spiky waveform but the trimmers have no effect on the amplitude, just pull the frequency around a bit.
The symptoms are that the machine works fine on erase and playback, but recording speech results in distorted highs as described last by Tim.So letters S and T are distorted and worse on the lower tape speeds. I'm struggling to think of an alternative method of adjusting the bias. I can't see how there's a fault other than adjustment as there's nothing much in that particular part of the circuit? Thinking of hanging some sort of pick up coil near the RP head and get some sort of reading that you doesn't load the circuit, and alter the level by trial and error, but thoughts from more experienced people here would be most welcome!
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How/where are you connecting the 100 ohm resistor ?
I see in the instructions it states to connect the resistor in series with the record/playback head and ground. From the schematic it does not look like there is a direct ground connection to these heads, so not sure how the resistor is supposed to be connected ? If there is a ground connection then connecting the scope should not make a difference as long as the earthed side of the scope lead is connected to the earth connection.
I assume you do not have a suitable VTVM or FET VOM both of which would have isolated inputs.