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Old 27th Apr 2017, 1:22 pm   #1
Eichberger
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Default Setting bias on Tandberg 12

I hope someone can offer a little advice.

I have recently bought a Tandberg 12 21 in fairly good nick and am trying to get it working as well as possible.

When playing back some recordings I made on it there is a very significant boost of the higher frequencies. Otherwise the sound is very good though (really really good!)

I am guessing this would be due to the bias signal and I have identified the trimming capacitors which adjust the bias but I am nervous about touching them on such an old machine. So can anyone advise:

- Do you think the problem is bias or is there another possible culprit?
- Are 50 year old trimming capacitors likely to break if you turn them?
- Should I just use trial and error or is there a good method for adjusting?

Thanks.
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Old 28th Apr 2017, 8:17 am   #2
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Default Re: Setting bias on Tandberg 12

I see you haven't received a response so I'll make a start.

Whilst it is true that excessive bias can wipe out higher frequencies, I wouldn't personally go there as a first step. Do you have any known good recordings from other sources?

Now another set of questions! Do you have the circuit diagram ("schematic")? Can you read them? (I'm guessing "yes" from the tone of your initial posting). What test equipment do you have?

If this is a multi-speed machine the problem may simply be one of the EQ networks so worth checking the other speeds (which?) and report back to us.

Others, please chip-in!
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Old 28th Apr 2017, 6:22 pm   #3
Eichberger
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Default Re: Setting bias on Tandberg 12

Hi Llama, thanks for replying.

I do have the circuit diagrams and can understand them to some extent. I have an electronics degree but that was a long time ago and I was not a very diligent student! My test equipment only runs to a multimeter I'm afraid and no pre-recorded tapes.

I see what you mean about the EQ circuit. Will test later on and report back.
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Old 28th Apr 2017, 10:41 pm   #4
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Default Re: Setting bias on Tandberg 12

If both the left and right (or 1 and 2) channels suffer from the same treble boost, it is unlikely that the same fault would occur in the EQ circuits of both channels. I would think it more likely that the bias was set up for another type of tape than you are using, i.e. one that didn't require so much bias.

The trimmer capacitors are fairly sturdy in these machines, so there should be no issues when trying to adjust them.

The correct procedure is something along the lines of recording two test tones at say 1 kHz and 10 kHz, play them back, and observe their relative strengths on an oscilloscope or VTVM. If the higher frequency is too pronounced, increase the bias, otherwise decrease it, repeat until the two tones have the same level, with perhaps a slight 1-2 dB drop on the higher one (slight overbias). Do this at the highest tape speed.

Another method is to record white noise (e.g. interstation noise on an FM receiver (with the Mute function switched off if there is one), and play it back, if it sounds brighter than the original, increase the bias, otherwise decrease it, repeat until satisfied. It's difficult to set a specific amount of overbias this way, but on the other hand you can get in the right ballpark without any special equipment or instruments.
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Old 30th Apr 2017, 7:52 pm   #5
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Default Re: Setting bias on Tandberg 12

Thanks for your input ricard, that is really helpful.

The treble boost does seem to be both channels. I should be able to do those tests using my synthesisers to produce the test tones/white noise.

So if I understand you correctly I need to increase the bias. Do you know if I should turn the trimmers clockwise or anti-clockwise? Also how far do they move? Do they have the 290 degree movement of preset variable resistors?
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Old 1st May 2017, 8:32 am   #6
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Default Re: Setting bias on Tandberg 12

A synthesiser may be an excellent tool, I would check that it has a uniform frequency response at the frequencies involved, so you don't end up adjusting the bias to compensate for deficiencies in the frequency response of the synth!

Going down the route of synthesisers, one way to check the complete frequency response if you have an oscilloscope available, is to set up sine wave swept by a reasonably fast sawtooth LFO over the audible frequency range. If you look at the resulting waveform on the oscilloscope with the sweep set to the same time as the LFO period, you'll basically see the frequency response directly on the screen. (It's hard to set the frequencies to be exactly the same, so the image will essentially wander about at best).

The trimmers used don't have any end stops, basically you can turn them forever, as all your doing is moving a semicircular plate in relation to another one, like in this diagram:
http://www.murata.com/~/media/webren...006.ashx?la=en
(from http://www.murata.com/~/media/webren...006.ashx?la=en)

That means it's also impossible to know which way to turn to increase the bias. Again, with an oscilloscope or VTVM you can measure the bias on the record/playback head while you're turning to at least see if it is increasing or decreasing. Make a note (or mark) of the original position so you can get back to a known state if for some reason you can't seem to adjust it properly.

Another thing: there are tapes which require a lot of bias, which might be more than the Tandberg can handle successfully. I have a Tandberg 9100X which is a mid-70's model, and when I bought it it had been set up for some high bias AGFA tape. While it did manage to record fine, the machine couldn't erase this particular tape properly, I don't know if it was because the high coercivity tape was hard to erase, or because the high bias required meant there wasn't enough current for the erase head to successfully wipe the tape. For that machine I switched to Maxell UD (which I was told was the tape that the machine was designed for) and set the bias accordingly, and it has been fine ever since. The 12 is about a decade earlier so I would think it was designed with some contemporary BASF tape in mind.

Last edited by ricard; 1st May 2017 at 8:39 am. Reason: Added discussion on tape types.
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Old 2nd May 2017, 6:52 pm   #7
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Default Re: Setting bias on Tandberg 12

Thanks again everyone for your input.

I finally got a bit of time today to do a some testing. It became clear very quickly that the bias trimming is not the issue. The amount of treble boost is far greater than that created by the bias control.

lt looks like you might have been right llama about the EQ circuit. I am just in the process of doing some plots at the different speeds using a software spectrum analyser. Will post the results when it's done.
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Old 2nd May 2017, 10:27 pm   #8
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Default Re: Setting bias on Tandberg 12

OK so I've done some more tests and might have been wrong in my last post.

The attachments are screen shots of plots from a spectrum analyser VST program on my Mac.

I recorded white noise on one channel (EQ'ed to make it's spectrum as flat as possible) and slowly swept the bias control round 360 degrees. I have then measured the payback signal and found the positions with the highest and lowest treble boost.

The test signal is shown on the 'input' plot and the min and max positions are shown on the 'output 7.5 ips max bias' and 'output 7.5 ips min bias' plots.

I did a single plot at 3.75 ips for comparison. Also checked the right channel was exhibiting the same behaviour (it was).

Even at maximum bias there is still a 10 db treble boost at 10kHz. I am using SM911 tape which is described as standard bias. Is this a case of too little bias on the machine or is there another problem?
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Old 3rd May 2017, 12:21 am   #9
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Default Re: Setting bias on Tandberg 12

Even though it is "standard bias" the SM911 tape is probably still a lot "higher energy" than tapes made in the day of the Tandberg and I'd expect there to be an excess of treble on playback. To be optimised for a modern tape the machine probably would need changes to the record EQ as well.
I suspect the machine would be better matched to an LPR35 tape.

Also the response shouldnt be tailing off above 10kHz at least at 7.5ips. It should be healthy up to 15kHz or higher. For standard frequency response testing the record level should be well below 0 on the record meter, probably -20db especially at the slower speeds, or there will be an overload of high frequencies which are already artificially boosted above the lower frequencies.
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Old 3rd May 2017, 10:30 am   #10
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Default Re: Setting bias on Tandberg 12

I'd be inclined to get hold of a reel of something like Scotch 203, BASF LP35LH or even LGS35 and see how the thing performs on tape for which it was designed. Then you will at least be starting from a known position. The new generation of tapes which arrived in the seventies, such as Agfa PEM 368, combined better HF performance with a higher bias requirement, so some modification may be necessary to use them. Best to know it's working as intended before diving in, I think.
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Old 3rd May 2017, 12:52 pm   #11
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Default Re: Setting bias on Tandberg 12

As previously mentioned you have to adjust the record EQ, because all tapes mentioned may be are "unknown" to your RR of the 50s...
My suggestion: All people dealing with tape recorders know an easy way to adjust the bias. Record an 1kHz signal at 0VU reading and turn the bias control slowly until you get the maximum output level during playback. This setting is very close to the bias of the tape type you are using. After that adjust the recording EQ at 10kHz signal at -20VU for a flat playback frequency response and readjust the bias at 10kHz signal at -20VU. Do that several times until you get the target result. At last check if you record an 1kHz signal at 0VU you get the same level at playback if not adjust the record level trimmer to achieve that. Do not touch the reproduce amplifier's settings because if you do that then you will need a test tape to readjust.
BTW, do you know at what magnetic tape flux the reproduce amplifier has been set up?
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Old 3rd May 2017, 1:09 pm   #12
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Default Re: Setting bias on Tandberg 12

Re the above I think I was in error to mention LPR35 which is just a thinner base (1 mil) version of SM911. Both are probably unsuitable for your older Tandberg. The tapes Ted mentions sound more suitable. I think the Tandberg also uses a felt pressure pad and generally it works better with non back coated tapes.
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Old 3rd May 2017, 1:35 pm   #13
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Default Re: Setting bias on Tandberg 12

Quote:
Originally Posted by mitajohn View Post
As previously mentioned you have to adjust the record EQ, because all tapes mentioned may be are "unknown" to your RR of the 50s...
The Tandberg 12 is more of a mid-60s machine than 50s (launched in 1966 I believe, so well into the 60s)... still designed way before those high bias tapes though of course.
Quote:
My suggestion: All people dealing with tape recorders know an easy way to adjust the bias. Record an 1kHz signal at 0VU reading and turn the bias control slowly until you get the maximum output level during playback. This setting is very close to the bias of the tape type you are using. After that adjust the recording EQ at 10kHz signal at -20VU for a flat playback frequency response and readjust the bias at 10kHz signal at -20VU. Do that several times until you get the target result. At last check if you record an 1kHz signal at 0VU you get the same level at playback if not adjust the record level trimmer to achieve that.
I don't think this Tandberg has a recording EQ adjustment, nor one for playback, being a domestic machine of its time. I must say I've never used the method you describe, but then again I've only dabbled in amateur machines.

I think that the cuplrit is that contemporary tape should be used as a lot of more modern tapes have too high bias requirements. In the early 60s Tandberg manuals said something like "American tape of high quality should be used" which I think would have been Scotch 203 or something like that. I think they shifted to BASF at some point as their standard tape, but that might have been in the early 70s.

Tim: yes the 12 (as all single motor Tandbergs) uses pressure pads, although unusually the (sole) pad is on the erase head rather than on the record/playback head, reportedly to minimize modulation noise.
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Old 3rd May 2017, 2:48 pm   #14
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Default Re: Setting bias on Tandberg 12

Quote:
your RR of the 50s...
Sorry, that was a typo. Yes well into 60s...

Quote:
I don't think this Tandberg has a recording EQ adjustment, nor one for playback
Yes it does, as I remember, although nothing fancy, some coils for adjusting high frequency EQ both on playback and record amps. I have lost my SM.
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Old 3rd May 2017, 8:01 pm   #15
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Default Re: Setting bias on Tandberg 12

Thanks for the input everyone. This is all really interesting. I have attached another plot (please ignore the lower line) using an old tape I found. No idea what make it is but it's on an EMI spool. The boost is much lower.

I can't see any Scotch 203 available in Europe. There are some BASF LP35LH reels on eBay so I'll try and get one of them.

Quote:
Originally Posted by mitajohn View Post
Quote:
I don't think this Tandberg has a recording EQ adjustment, nor one for playback
Yes it does, as I remember, although nothing fancy, some coils for adjusting high frequency EQ both on playback and record amps. I have lost my SM.
Yes, there is a reference to these in the service manual thought no description of where to find them or what to do with them!

Quote:
Originally Posted by TIMTAPE View Post
the response shouldnt be tailing off above 10kHz at least at 7.5ips. It should be healthy up to 15kHz or higher. For standard frequency response testing the record level should be well below 0 on the record meter, probably -20db especially at the slower speeds, or there will be an overload of high frequencies which are already artificially boosted above the lower frequencies.

This is definitely a problem. The attached plot was with the input 10dB less than the crossover point on the magic eye meters. I did another at 20dB less but both still have the same trailling off from 10kHz. I am puzzled as to what would cause this identically on both channels. The manual says it should be 30-20,000 c/s(±2dB 40- 16,000 c/s) at 7.5 i.p.s. though as you said Ted Kendall I need to check with the right tape first.
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Old 3rd May 2017, 10:49 pm   #16
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Default Re: Setting bias on Tandberg 12

For best high frequency response the tape needs to be in intimate contact with the head gap. The term for when it's not is "spacing loss" and can be due to foreign matter buildup on the head face, wear on the headface or misalignment so the tape doesnt contact the headface properly - or a combination of these!
It only takes a tiny amount of spacing loss for the very high frequencies to fall away.
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Old 4th May 2017, 9:43 am   #17
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Default Re: Setting bias on Tandberg 12

Spot on. I had a look at the head and noticed the lateral alignment is off. Must have moved when I was adjusting the height and azimuth. Will adjust (once I figure out how!) and test again. Thanks so much for your help TIMTAPE
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Old 4th May 2017, 10:52 am   #18
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Default Re: Setting bias on Tandberg 12

Quote:
I had a look at the head and noticed the lateral alignment is off. Must have moved when I was adjusting the height and azimuth. Will adjust (once I figure out how!) and test again.
That's a task. If you don't have the skills and the proper gear it's a bit tricky business. Remember though that your r2r has one head for rec/play so some misalignment is not too much critical. What is really matters, is the state of the head: If it's worn down and the head gap has opened up, it means loss of high frequencies, as well as its magnetization.

Before you had been taken the measurements had you set MPX filter OFF?

BTW, would you please post the link from where you got your SM?
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Last edited by mitajohn; 4th May 2017 at 11:02 am.
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Old 4th May 2017, 4:29 pm   #19
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Default Re: Setting bias on Tandberg 12

Quote:
Originally Posted by mitajohn View Post
Quote:
I don't think this Tandberg has a recording EQ adjustment, nor one for playback
Yes it does, as I remember, although nothing fancy, some coils for adjusting high frequency EQ both on playback and record amps. I have lost my SM.
The service manual can be found here:

http://sportsbil.com/tandberg/12-servicemanual.PDF

And you're right, there is an adjustment coil in the equalizer curcuits, however, it seems that it affects both record and playback, essentially providing the peak in the treble response in the record and playback equalization curves on pages 26 and 27 in the service manual. Section 5.7.14 says to adjust the coils for a resonant peak at 18 kHz at 7 1/2 ips.

Personally, I'd be wary of adjusting this beyond what the service manual says because a) it affects both record and playback and b) it doesn't affect the amount of treble lift, but rather the frequency. But perhaps I'm just being shy.
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Old 4th May 2017, 5:50 pm   #20
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Default Re: Setting bias on Tandberg 12

Quote:
Originally Posted by mitajohn View Post
Thats a task. If you don't have the skills and the proper gear its a bit tricky business. Remember though that your r2r has one head for rec/play so some misalignment it is not too much critical. What is really matters, is the state of the head: If its worn down and the head gap has opened up, it means loss of high frequencies, as well as its magnetization.
Photo of the head is attached. Do you think that is the culprit?
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