Maestro EP3 T70-T2 Bias Oscillator Question

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Maestro EP3 T70-T2 Bias Oscillator Question

Postby pmkipp » 15 Jan 2020, 18:48

I have been rebuilding an old EP3 and have hit a snag, mostly from the "where do I find this?" corner.

Someone before me had wired in a new power cable and must have done it backwards and blown both bridge rectifiers so nothing worked at all. Replaced the rectifier for the solenoid/shut off switch and the motor and solenoid worked, but no power was getting to the board. Replaced the main input rectifier (and the big can caps) and the light came on and there was some playback of old recordings, but no erasing/recording. On the schematic I have (attached) there is supposed to be 37volts at the input for the recording head, but there is zero volts there. The problem seems to be with the transformer marked T2 - it is a Nortronics T70-T2 transformer/bias oscillator. 26volts goes in at both lug 1 and 3, but nothing comes out at lug 5. How do I test this to be sure that is the problem. And if it is, is there a reasonable alternative to this part which seems to be no longer available?

Thanks

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Re: Maestro EP3 T70-T2 Bias Oscillator Question

Postby deltafred » 16 Jan 2020, 10:19

Just checking that you realise that the output on pin 5 should be AC (with a frequency of around 40 or 50kHz) not DC.

First I would check that the footswitch jack contacts are making when no footswitch is plugged in.
Next check Q7, Q8, then the components around them.
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Re: Maestro EP3 T70-T2 Bias Oscillator Question

Postby pmkipp » 17 Jan 2020, 02:10

Thanks - is the voltage at 1 and 3 also AC, or is it DC?
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Re: Maestro EP3 T70-T2 Bias Oscillator Question

Postby pmkipp » 17 Jan 2020, 20:09

AC voltage coming out of the T70T2 is 28 volts at pin 5 and 120 volts at pin 4. Resistance at erase head is 40ohms and at the record head is 70 ohms. Playback jack is not shorted. The 28 volts go through a trimpot on the way to the record head, but it has to be all the way off to let the whole 28 volts through.

It also seems the input signal goes nowhere. The buffer/preamp all tests good, but the signal doesn't even get to the transistor.

What a mess...
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Re: Maestro EP3 T70-T2 Bias Oscillator Question

Postby deltafred » 18 Jan 2020, 14:07

Good that the bias oscillator is working.

You don't need the full oscillator voltage on the record head, you need just enough to overcome the magnetic hysteresis of the record head, hence the preset to adjust it. (It's about 40 years since I worked on tape decks and my memory is a little hazy but I think this is correct.)

pmkipp wrote:It also seems the input signal goes nowhere. The buffer/preamp all tests good, but the signal doesn't even get to the transistor.


Are you saying that the input signal isn't reaching Q5 gate?
How did you test the buffer/preamp?

If the amplified input signal is not appearing on Q5 drain then Q5 might be faulty
or there is a short on the PCB
or the cap or resistor that go to ground have gone short circuit (220pf , 1M) (Unlikely that the 1M would go short circuit)
or there is a broken copper track
or the cap or resistor that are in series with the input ore open circuit (.047, 100K).

(Internet fault diagnosis is highly dependant upon accurate and unambiguous information!)
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