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Vintage Tape (Audio), Cassette, Wire and Magnetic Disc Recorders and Players Open-reel tape recorders, cassette recorders, 8-track players etc.

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Old 4th Apr 2020, 6:12 pm   #1
saxmaniac
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Default Tandberg model 15 bias adjustment

Hi folks
I've a problem with sibilance on speech recording on this. I'm trying to check bias level with just the aid of an oscilloscope.
It may not be possible to follow the procedure in the manual as the oscilloscope inputs isn't isolated from earth
I was wondering whether anyone has an alternative method such as a measurement at the rp head, and I could do with advice on correct level. Also, is the level adjustment made by the trimmers adjacent to the heads? Tweaking these seems to alter the frequency but not the amplitude
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Old 5th Apr 2020, 7:51 pm   #2
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Default Re: Tandberg model 15 bias adjustment

I do not know this model but by looking at the schematic.

The bias frequency adjustment is C221 (150 - 750pF) and the bias amplitude (adjusted for current by measuring voltage across a resistor in the procedure) adjustments are (if 4 track) C1 and C2 (10 - 60pF).

The pre-amp board layout drawing shows C221 fitted on the board, as far as I can see C1 and C2 are not fitted to the board, so assume that the trimmers you reference by the heads must be C1 and C2, would possibly expect these capacitive trimmers to be physically smaller than C221, because they have smaller capacitance value.

Have you tried scope connection without its earth lead connected ?
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Old 6th Apr 2020, 6:08 am   #3
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Default Re: Tandberg model 15 bias adjustment

Quote:
Originally Posted by saxmaniac View Post
Hi folks
I've a problem with sibilance on speech recording on this. I'm trying to check bias level with just the aid of an oscilloscope.
It may not be possible to follow the procedure in the manual as the oscilloscope inputs isn't isolated from earth
I was wondering whether anyone has an alternative method such as a measurement at the rp head, and I could do with advice on correct level. Also, is the level adjustment made by the trimmers adjacent to the heads? Tweaking these seems to alter the frequency but not the amplitude
It depends on what you mean by sibilance. Too much highs re lows? Or distorted highs like spitting?
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Old 6th Apr 2020, 6:57 am   #4
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Default Re: Tandberg model 15 bias adjustment

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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Old 6th Apr 2020, 8:02 am   #5
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Default Re: Tandberg model 15 bias adjustment

At the slower speeds, the record amp boosts the highs more to compensate for the greater slow speed losses, so distortion of the highs happens more easily. At 15/16 IPS or slower the highs distort easily.

What happens if you back off the record level or feed it a less treble heavy signal?

Underbias first causes distortion and weakness of the lows, and only later the highs. Overbias first reduces the highs but has a milder reducing of the lows.

At slower speeds such as 1 7/8 IPS and slower, intimate tape to head contact becomes really important for the highs, both on record and playback. Maybe tape to head contact is poor, in which case good performance (within the specs) will not be possible.
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Old 6th Apr 2020, 9:09 am   #6
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Default Re: Tandberg model 15 bias adjustment

You could just do it the old-fashioned way, although it is a bit long-winded without a separate play head. If you increase the bias from minimum, the output from tape will rise to a peak and then start to fall again. The point you want is roughly where a 1kHz tone drops 1dB "over the hump". Without a monitor head, you need to make a series of short tests with the trimmer turned a little way up each time - pencil and paper and the counter are useful here - and note the setting which gives the right answer. Don't put too much level in whilst doing this test, as the tape may saturate and obscure the hump.
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Old 6th Apr 2020, 10:26 am   #7
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Default Re: Tandberg model 15 bias adjustment

Quote:
Originally Posted by saxmaniac View Post
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!
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.
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Old 6th Apr 2020, 10:29 am   #8
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Default Re: Tandberg model 15 bias adjustment

Check first of all that the EQ switch is actuated by the speed change. I had one with a broken plastic linkage here. See also condition of R229, R232 (150R). If they are altered in value, you will get symptoms you describe and/or weaker erasure.

There were two different factory EQ settings on this model. Above s/n 251 5502, setup was for low noise tape. You will get sibilance due to underbiasing if you use newer formulation tapes on recorders with s/n below that number. What is yours?

I have also noticed some sibilance on speech at lower speeds if one is not very careful with the recording level on these units! Back off the gain a bit on some test recordings and see if that improves matters.

The bias oscillator frequency is unlikely to have shifted so I would leave that trimmer on the main board alone. C1 and C2 behind the heads can be tweaked as described above.

Attached info from p33 of the service manual (pdf)
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Old 6th Apr 2020, 11:31 am   #9
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Default Re: Tandberg model 15 bias adjustment

In the manual and schematic I'm looking at the 100 Ohm resistor for the bias adjustment etc is connected in series with the ground end of the heads.

In that schematic that's the right hand connection to CH1 head and the right and connection to CH2 head, in other words between those connections and wiper contacts 12 and 16 respectively on the track selector switch, the common ground connection for both heads on record is via connection 22 on the preamplifier board and the record/playback switch when it's set to record, don't forget to convert from peak or peak to peak down to RMS if using a 'scope to set levels, if that's any help.

Lawrence.

Last edited by ms660; 6th Apr 2020 at 11:50 am. Reason: clarification
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Old 6th Apr 2020, 11:54 am   #10
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Default Re: Tandberg model 15 bias adjustment

Hi Lawrence,

Yes can now see the earth connection (not helped by the schematic being split) in Record position though connection 22 up to channel 2 wiper 16 (switched to Extra position) and through to channel 1 wiper 12 (switched to Duo or Norm positions).

David
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Old 6th Apr 2020, 6:08 pm   #11
saxmaniac
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Default Re: Tandberg model 15 bias adjustment

I had connected the resistor in series with the non ground end of the head. My manual is in Norwegian and I misunderstood that point. I'll have another go that way so it should be possible to use the scope (I don't have an alternative means of measuring)
Ben, it's got a label stating set up for low noise tape but the sibilance is very bad on 3 3/4 on a variety of different tapes. 7 1/2 is fine and 17/8 I don't expect perfection anyway! The eq switch is mechanically fine but I'll check those resistors you mentioned anyway. Thanks again for all your responses, I won't be lightening quick doing this as I'm one of those so called key workers so not totally at leisure!
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Old 7th Apr 2020, 12:48 am   #12
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Default Re: Tandberg model 15 bias adjustment

I'm not sure there is a problem with the machine. Tape always struggled with heavily recorded high frequencies at slower tape speeds.

Is it possible the speech signal you have been recording is weak in the bass and rich in the treble? On such a signal the record meter often under reads, especially significant at low tape speeds. For frequency response testing, service manuals normally specified a recording level of -20db, specifically to avoid saturating the tape with high frequencies.

Again, what happens when you lower the record level?

Last edited by TIMTAPE; 7th Apr 2020 at 1:18 am.
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Old 7th Apr 2020, 7:35 pm   #13
saxmaniac
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Default Re: Tandberg model 15 bias adjustment

Well folks thanks to your help I've achieved total success! The main problem was me not connecting the 100 ohm resistor in series with the ground connection. Got good reading with scope, the bias was a touch high on both channels, adjusted down and machine works fine even on 1 7/8, no sibilance even with a toppy signal at high level. So thanks again
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Old 7th Apr 2020, 7:58 pm   #14
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Default Re: Tandberg model 15 bias adjustment

That is fantastic news, well done !
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Old 7th Apr 2020, 10:14 pm   #15
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Default Re: Tandberg model 15 bias adjustment

That's great news but reducing bias a little normally increases treble a little.
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Old 8th Apr 2020, 6:47 am   #16
saxmaniac
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Default Re: Tandberg model 15 bias adjustment

I think I had lost the original setting by rocking the trimmers slightly, so it's possible the original problem was slightly low bias
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Old 8th Apr 2020, 7:40 am   #17
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Default Re: Tandberg model 15 bias adjustment

Yes those circular capacitor trimmers don't help. You don't know whether it's increased or decreased until you change it.

Low bias kind of makes sense. It increases the treble (by not erasing it as much) but that can also create a rather brittle sound because it magnifies dropouts due to tape imperfections, dirt on the tape etc. The normal bias current somewhat overbiases the highs which actually helps with this problem. In record, as the tape momentarily lifts slightly away from the head due to dirt getting in the way, this reduces the actual bias to the surface of the tape, which increases the treble a little. It's not perfect but it helps a little.
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Old 8th Apr 2020, 6:05 pm   #18
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Default Re: Tandberg model 15 bias adjustment

When I'm setting up the bias on these machines, I usually momentarily connect a 'scope to the recording head while turning the bias trimmers, not to see the absolute level, but to get an idea if I'm actually increasing or decreasing the bias.

If you have another recorder you can set them up side be side, and use the second one as a playback machine for the same tape, i.e. you place the tape on the left hand reel holder of the left machine (the one you want to set the bias on), thread it through both machines, and place the take-up reel on the right hand machine. Setting the second machine to playback you then get a three-head arrangement, albeit with a large distance between the record and playback heads!

One thing though, is that the right hand machine must run at exactly the same speed, or slightly faster, or there will be an ever increasing loop forming between the machines.
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Old 8th Apr 2020, 9:39 pm   #19
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Default Re: Tandberg model 15 bias adjustment

Ah the convenience of 3 head machines! A stereo machine isn't too bad but the little Fostex A8 8 trackers are a right pain. Two heads only. To get them well set up for the built-in Dolby C it's lots of tape shuttling back and forth, and then it's the same thing seven more times...
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