Cassette Playback Reference Tap for B & 0 Playback Level Check?

[Chris55000]

Almost all Cassette Deck Service manuals state you should use a special test cassette, "TEAC MTT–112" is often quoted – for the B & O "Beocoder 1900" I am repairing for my friend, the S.M. quotes "Pegel 333Hz 6780035" but it doesnt state what level standard this is recorded at!

Can anyone suggest a level cassette I can buy that I don't need to find three figures in postage and import charges for please?

Failing that, is there another means of setting the playback reference by using an a.f. Oscillator?

Chris Williams

[knobtwiddler]

I would try emailing Alex @ A.N.T Audio. He has been selling some test tapes recently. Be very, very careful with Ebay tapes (9/10 are junk). There is a guy in Germany who sells a set of 6 that are good, but not quite as good as the Teac / Abex ones that cost a fortune. PM me if you want further info about how to get them from him. They were about 60 Euros last time I checked. The tape itself is perfect, but he isn't able to get shells as good as the expensive OEM tapes.

[TIMTAPE]

On cassette decks it's especially the frequencies much higher than say 300 to 400 Hz, such as 10 kHz or higher which go out, and the higher the frequency the more likely it will be incorrect. 333 Hz happens to be in the range least susceptible to error, which I guess is why it was chosen as a reference.

It's a bit of a fiddle but on many pre recorded cassettes made for the major record labels from say the 80's is often a set of calibration tones just before the actual programme starts on side 1 and again at the end of side 2. I believe these tones were used inhouse by the duplication plants to check quality control of their product. They're incredibly short duration (a few seconds in total) but if you're able to make a good digital recording of them and digitally slow down playback they can be used as not only a reference for mid tones but also up to 18 or 20 kHz, the highest tones of course being less reliable. But normally the 300 to 400 Hz range should be pretty close I think.

Being a full range set of tones, the ones I've checked are recorded at what seems to be 20 db below the Dolby reference of 200 nWbm.

A problem with the very short duration of the tones is they are pretty much useless for setting head azimuth for the high frequencies but since the program was recorded by the same deck and head, that programme can be used as an azimuth reference and some people do use such pre recorded tapes as an azimuth reference if nothing else because they seem to have been found generally reliable. Having been recorded on "open reel" they are independent of cassette shell vagaries.

These pre recorded tapes were recorded at up to 100 times normal speed on serious Studer open reel decks specially modified to carry large reels of cassette tape. These cassettes are worth so little on the market now, so the calibration tones on them can be a low cost, high quality calibration reference for those who dont mind a bit of extra work.

Unfortunately there's pretty much no alternative to using a calibrated playback tape as reference.

[Marconi_MPT4]

B&O service cassettes were purchased as a kit of six tapes which included one for pegel (level). All were approximately 25 meters length recorded on chrome tape, which I think might have been BASF or one of the main European manufacturers. Most B&O tape machines from the 70's into early 80's were calibrated to use IEC standard cassettes.

Some old (c.1980) training course notes suggest the pegel frequency of 333Hz is accurate to ±1% with tape flux density level quoted as 25mM/mm (?) ±0.5%. Time constant 1590μs, 70μs. Head gap position (azimuth) within 90° ±2'

Hope that helps.
Rich

[mitajohn]

Here's a pic from the relevant cassette. Its a chrome tape and the Time constant is 1590μs, 70μs. Some time later the B&O turn to the new IEC Time constants for cassettes and to 315Hz/250nWb/m ref level.

[Mooly]

I'm going to say that provided the levels have not been previously altered then there is no real reason to suspect they are incorrect. Its purely an electronic adjustment to match the head fitted and there is no reason for the gain of the replay amplifiers to alter.

I would proceed like this...

Azimuth can be out due to wear of the tape path and head but if your friend has recorded material to keep then this should be left alone even if it is misaligned.

If you have a good selection of decent pre-recorded tapes then check that the play back image is 'central' and not one sided when using an average of a few tapes.

Having said the azimuth should not be altered you can record your own 'azimuth tape' to allow you to alter the azimuth setting to get the best result on pre recorded tape (for level checks and subjective Dolby tracking) and then use the azimuth tape to set it back where it was.

Listening is best checked on headphones.

If the playback levels are OK and you are happy then the preset record levels and bias should be set to give identical results between L and R channels on a record/playback test. This should be performed with a generator and scope and always followed by careful listening to compare source and recording. It is important to ensure both channels are set identically.

[mitajohn]

Originally Posted by Chris55000

.... B & O "Beocoder 1900" Chris Williams

What is the type of the deck?

[cmjones01]

According to this page in a B&O manual:
https://www.manualslib.com/manual/11...1.html?page=58
The 6780035 test tape is at a level of 250nWb/m (or pWb/mm as they specify, which is exactly the same thing). That's a bit higher than most other test tapes. I've got an ANT Audio U04103-70A which is recorded at 200nWb/m, or "Dolby level". The Teac MTT-150 tape is also at this level, but other tapes are at different ones (147nWb/m and 185nWb/m are popular choices). This mostly reflects the difference in where different manufacturers (and even different models from the same manufacturer) put the '0dB' mark on their meters.

In any case, once you've got a tape at a known level, you can calculate the level you should be seeing at the test point described in the manual. For example, if your tape is at 200nWb/m and the manual says to adjust for 620mV, do your adjustment for 620*sqrt(200/250) = 550mV. That's the example given in the B&O manual. I'm not completely sure why that square root is in there, but I suspect that the tape magnetism in nWb/m actually determines the power transferred to the head, not the voltage. Those more experienced in tape matters can correct me!

Chris

[mitajohn]

Originally Posted by cmjones01

According to this page in a B&O manual:
https://www.manualslib.com/manual/11...1.html?page=58 Chris

Is this the type in question? This is the Beosound Century!


We are looking for the Beocord 1900 type 2643! Isn't it?

[TIMTAPE]

Originally Posted by Mooly

I'm going to say that provided the levels have not been previously altered then there is no real reason to suspect they are incorrect. Its purely an electronic adjustment to match the head fitted and there is no reason for the gain of the replay amplifiers to alter.

I agree. If the trimmers are still at their original and correct factory settings, which they often are, left and right trim pot wiper angles will match each other visually very closely and there's normally no reason to change them. Further if a reference tape does show the levels are out, twiddling the trimmers won't fix the problem. The source of the problem is deeper and needs to be diagnosed and repaired.

[cmjones01]

Originally Posted by mitajohn

Is this the type in question? This is the Beosound Century!
We are looking for the Beocord 1900 type 2643! Isn't it?

Yes, the manual I quoted is almost certainly for the wrong model. I didn't even check. It's not relevant in this case. What's important is that the manual showed the magnetic recording level on that particular test tape - 250nWb/m. That's what we need to know in order to work out how to align the Beocord 1900 with a different test tape.

Chris

[mitajohn]

Originally Posted by TIMTAPE

I agree. If the trimmers are still at their original and correct factory settings, which they often are, left and right trim pot wiper angles will match each other visually very closely and there's normally no reason to change them. Further if a reference tape does show the levels are out, twiddling the trimmers won't fix the problem. The source of the problem is deeper and needs to be diagnosed and repaired.

I agree too. If the deck is going to be used only for playback then a good clean up, lubrication, belt replacement and head demagnetization is a good solution. By the way as it is written in the SM the EQ Time constants, at least for CrO2 tapes, are the new ones (3180μs/70μs), so playback would be OK.

[cmjones01]

Originally Posted by mitajohn

I agree too. If the deck is going to be used only for playback then a good clean up, lubrication, belt replacement and head demagnetization is a good solution. By the way as it is written in the SM the EQ Time constants, at least for CrO2 tapes, are the new ones (3180μs/70μs), so playback would be OK.

I'd add that it is worth checking the playback calibration level if there's any intent to play back Dolby-encoded tapes. They will sound bad if the level's wrong (especially if it's too low, which it usually is). Many decks don't need it adjusting, as you rightly say, but some do: I have a Denon DR-M22 here which, though apparently low mileage, has suffered from drift in the values of some preset pots which affected both the playback and record levels. On the other hand, I have other decks (a 1980 Akai springs to mind) which are older and in poorer condition and yet are still spot-on.

Chris

[TIMTAPE]

Originally Posted by cmjones01

I'd add that it is worth checking the playback calibration level if there's any intent to play back Dolby-encoded tapes. They will sound bad if the level's wrong (especially if it's too low, which it usually is). Many decks don't need it adjusting, as you rightly say, but some do: I have a Denon DR-M22 here which, though apparently low mileage, has suffered from drift in the values of some preset pots which affected both the playback and record levels. On the other hand, I have other decks (a 1980 Akai springs to mind) which are older and in poorer condition and yet are still spot-on.

Chris

Well even with the 300 - 400 Hz levels adjusted spot on is no guarantee a given Dolby encoded recording will track (decode) well. There are multiple other variables which can be out, including whether the original Dolby encoded recording was or is still within spec itself. The Achilles Heel is usually the highs being out, whether on the recording, in the playback or in both.

[Ted Kendall]

Originally Posted by TIMTAPE

Well even with the 300 - 400 Hz levels adjusted spot on is no guarantee a given Dolby encoded recording will track (decode) well. There are multiple other variables which can be out, including whether the original Dolby encoded recording was or is still within spec itself. The Achilles Heel is usually the highs being out, whether on the recording, in the playback or in both.

Hence the Play Trim controls on NAD cassette decks. Accurate Dolby B decoding isn't a given these days, especially when playing old tapes.

[TIMTAPE]

Originally Posted by Ted Kendall

Hence the Play Trim controls on NAD cassette decks. Accurate Dolby B decoding isn't a given these days, especially when playing old tapes.

Yes and it wasn't a given back then!

[Ted Kendall]

No, indeed - Ken Gundry of Dolby Laboratories said in a radio programme in the 70s that the modern cassette deck was "a fluke" - meaning that if everything was just so, it worked very well, but that everything had to be just so. It was said in the context of dealer service not being as good as it should be.