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Old 10th Jun 2019, 11:31 am   #1
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Join Date: Aug 2006
Location: Madrid, Spain / Wirral, UK
Posts: 5,859
Default Tandberg series 12

A recent junk shop find with a problem on Ch.2. Basically the level is too low.

- On playback of known good tapes, there is rather weak audio, perhaps a quarter of the level of ch.1
- The output stage for ch 2 seems okay. If you set ch. 2 to 'amp' (this feeds ch 1 tape replay to ch. 2's output stage for mono playback) the audio comes through fine on that side. However, doing the same for ch. 1 gives low volume, suggesting weak tape-derived signal. Ch 1 of course replays its own off -tape signal perfectly.

- When fed with an input, the meter for ch 2 reacts as expected, the volume/rec level varies it properly,but very little signal makes it to the tape during actual recording. A very faint recording does happen though. Erasure is fine.

- Touching the head wires also reveals that the audio buzz is weaker on ch.2. Presumably the actual head is alright, as the problem would seem to be downstream

I have cleaned the heads and they are spotless. Also the rec/amp switches have been thoroughly cleaned. There is no intermittent operation when jiggled as can sometimes happen with, say, the series 15.

What I am now trying to identify are possible causes common to both record and playback. Between the head and the output stages there are:

-EQ amp
-Rec preamp
-Playback preamp
-Vol control.
-Rec /amp switch

To me it seems that the EQ amp (and in particular the area around Q109) might be the only item that could affect performance in both modes. I intend to check the 27v supply to this stage.

This s the first time I have worked on this model, so thought I would mention all this in the hope of any tips from anyone with more experience of this unit. Hope I am not barking up the wrong tree!
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Last edited by ben; 10th Jun 2019 at 11:39 am.
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