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Old 14th Sep 2021, 10:20 am   #1
dazzlevision
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Default Thorn DC43 deck valve mono tape recorders - bias setting and head markings queries

Can anyone shed any light on the meaning of the coloured dots and stripes shown on the photo of the rear of the record/play head fitted in these Thorn group DC43 (three speed, four track) tape deck, mono valved tape recorders?

In addition, does anyone know the significance of the “205” figure handwritten on the top of the R/P head’s metal cover?

In the photo showing the Philips “beehive” trimmer capacitor (C10), there is a small axial Polystyrene capacitor (C11) near it (they are connected in parallel). Sometimes, during production, another Polystyrene capacitor was soldered in parallel with C11, on the copper side of the PCB. C10 and C11 in parallel feed the 55kHz HF bias oscillator signal to the RP head.

The various service manuals for this range of models do state that the value of C11 is “Select On Test” and can vary from 18pF to 50pF, in order to get the correct record-playback frequency response at 3.75 ips (whilst keeping the RP head’s bias current within the 200-300uA range).

However, the Ultra 6212 that I’m currently working on has two 50pF capacitors in parallel fitted on the component side, but none on the copper side (almost certainly fitted during factory set up). This makes C11 100pF, which is well outside the service manual range of values for C11.

The apparent consequence of this, is an inability to achieve the correct 3.75ips frequency response by adjusting C10, whilst keeping the bias current within 200-300uA. It also seems to be causing a problem with the record level meter, as when both record and play keys are depressed*, the meter has a steady “dc component” added, which appears to be due to the higher bias current (when compared to a “normal” DC43 model that I also have).

* The bias oscillator is not powered up with HT until the play key is also depressed.
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Old 14th Sep 2021, 12:36 pm   #2
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Default Re: Thorn DC43 deck valve mono tape recorders - bias setting and head markings querie

Yes does sound a little strange that the factory SOT (Select On Test) value was set to 100pF when the manual states a range of 18 - 50pF, do the SOT values on other machines of this range that you have worked on normally have C11 in the 18-50pF range ?

David
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Old 14th Sep 2021, 12:40 pm   #3
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Default Re: Thorn DC43 deck valve mono tape recorders - bias setting and head markings querie

I gather it was difficult to manufacture heads to have identical characteristics especially as it related to record bias. (Head wear also changed a head's electrical characteristic). Maybe the markings on the heads represented the inductance of each newly manufactured head, making it easier on the production line to set record bias without actually having to make time consuming multiple record and play tests at different trimmer (bias current) settings.

Maybe too in the factory Thorn used two different sources of tape heads which each had different electrical characteristics, requiring different value series capacitance?

Last edited by TIMTAPE; 14th Sep 2021 at 12:58 pm.
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Old 14th Sep 2021, 1:40 pm   #4
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Default Re: Thorn DC43 deck valve mono tape recorders - bias setting and head markings querie

Have you checked the Record Equalisation Adjustment (L7) as per the procedure in the manual ?

David
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Old 14th Sep 2021, 1:57 pm   #5
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Default Re: Thorn DC43 deck valve mono tape recorders - bias setting and head markings querie

This old tape machine would have been designed for the relatively low bias tapes of the day. Probably easiest to set up using such a matching tape. Maybe the different capacitor value was designed for later higher bias tapes which were just coming on to the market.

But often an older machine's oscillator doesnt have the power to fully bias (or erase) later tape types.
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Old 14th Sep 2021, 2:14 pm   #6
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Default Re: Thorn DC43 deck valve mono tape recorders - bias setting and head markings querie

Looks like 225 on this deck:

https://www.vintage-radio.net/forum/...3&d=1529317518

Lawrence.
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Old 14th Sep 2021, 2:18 pm   #7
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Default Re: Thorn DC43 deck valve mono tape recorders - bias setting and head markings querie

Quote:
Originally Posted by ms660 View Post
Looks like 225 on this deck:

https://www.vintage-radio.net/forum/...3&d=1529317518

Lawrence.
That photo looks very familiar - was it one of my posts?
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Old 14th Sep 2021, 2:20 pm   #8
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Default Re: Thorn DC43 deck valve mono tape recorders - bias setting and head markings querie

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Originally Posted by DMcMahon View Post
Have you checked the Record Equalisation Adjustment (L7) as per the procedure in the manual ?

David
Yes, it was spot on.

I've also checked the adjustment for the record level meter "calibration".
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Old 14th Sep 2021, 2:21 pm   #9
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Default Re: Thorn DC43 deck valve mono tape recorders - bias setting and head markings querie

Re: Post#7, Yes:

https://www.vintage-radio.net/forum/...d.php?t=147478

Lawrence.
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Old 14th Sep 2021, 2:22 pm   #10
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Default Re: Thorn DC43 deck valve mono tape recorders - bias setting and head markings querie

Quote:
Originally Posted by TIMTAPE View Post
I gather it was difficult to manufacture heads to have identical characteristics especially as it related to record bias. (Head wear also changed a head's electrical characteristic). Maybe the markings on the heads represented the inductance of each newly manufactured head, making it easier on the production line to set record bias without actually having to make time consuming multiple record and play tests at different trimmer (bias current) settings.

Maybe too in the factory Thorn used two different sources of tape heads which each had different electrical characteristics, requiring different value series capacitance?
I have worked on quite a few of these DC43 models and the R/P heads have all looked the same, with the same coloured markings on the rear.

The Thorn service manuals for these models all have the same part number for the R/P head and there are no changes to the bias level/frequency response checking and adjustment procedures.
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Old 14th Sep 2021, 2:28 pm   #11
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Default Re: Thorn DC43 deck valve mono tape recorders - bias setting and head markings querie

Quote:
Originally Posted by TIMTAPE View Post
This old tape machine would have been designed for the relatively low bias tapes of the day. Probably easiest to set up using such a matching tape. Maybe the different capacitor value was designed for later higher bias tapes which were just coming on to the market.

But often an older machine's oscillator doesnt have the power to fully bias (or erase) later tape types.
Yes, I have considered that, and I am confident that these models (made during the 1965 to 1969 era) were all set up with the contemporary "brown oxide" tape type - not "high output/low noise" "grey oxide" types.

I have made test recordings on the traditional (brown oxide) and newer (grey oxide) types of tape. Neither will set up correctly. I would expect that with the grey type, but not the brown type.

I have a Ferguson 3230, using the same innards and bought new in 1967. It was supplied with BASF (brown oxide) tape, which I still have. They have also been supplied (from new) with EMI and Scotch tapes. They also came with a useful "tape handbook" issued by the tape manufacturer - doubtless to elicit future sales of tape!

Last edited by dazzlevision; 14th Sep 2021 at 2:45 pm.
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Old 14th Sep 2021, 4:53 pm   #12
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Default Re: Thorn DC43 deck valve mono tape recorders - bias setting and head markings querie

Quote:
Originally Posted by ms660 View Post
Looks like 225 on this deck:

https://www.vintage-radio.net/forum/...3&d=1529317518

Lawrence.
And looks like 205 on Post 1 attachment.

David
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Old 14th Sep 2021, 5:52 pm   #13
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Default Re: Thorn DC43 deck valve mono tape recorders - bias setting and head markings querie

Hi!

Recommended head h.f. bias current in microamps perhaps?

Chris Williams
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Old 14th Sep 2021, 6:15 pm   #14
dazzlevision
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Default Re: Thorn DC43 deck valve mono tape recorders - bias setting and head markings querie

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Hi!

Recommended head h.f. bias current in microamps perhaps?

Chris Williams
Yes, it could be (but it is not present on all sets).
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Old 14th Sep 2021, 9:00 pm   #15
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Default Re: Thorn DC43 deck valve mono tape recorders - bias setting and head markings querie

I'm not clear what is the actual audio issue. Does the recorded tape seem under biased, eg too much highs and weak and distorted lows compared to
good recordings?
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Old 16th Sep 2021, 11:46 am   #16
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Default Re: Thorn DC43 deck valve mono tape recorders - bias setting and head markings querie

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Originally Posted by TIMTAPE View Post
I'm not clear what is the actual audio issue. Does the recorded tape seem under biased, eg too much highs and weak and distorted lows compared to
good recordings?
The issue (apart from the erroneous record level meter reading error problem) is that the frequency response test recordings I have made, as per the manufacturer's service manual procedure, show a very out of spec higher frequencies response - well outside the +/- 3dB wrt 1kHz reference level. Tested at 3.75 ips (as per the service manual procedure). This is using a 1960s EMI "brown" oxide tape, of the type the recorder was designed to use.

I have tried altering (reducing) the value of the "select on test" fixed capacitor (C11) to vary the R/P head's bias current in order to achieve the desired higher frequencies response (within the specified bias current range of 200-300uA). It does improve slightly, but it's still well away from correct result.

I then double checked the (record mode) HF boost setting of L7, to peak at 18kHz (at 7.5 ips) and it is correct. The HF boost at the other two speeds also peaks at 14kHz and 7 kHz (3.75 and 1.875 ips respectively).

I am confident the mechanical alignment of the R/P head is correct - the top of the tape aligns with the upper wear mark on the face of the head and the azimuth was set using a test tape (recorded on a new Tandberg 10XD - a long time ago!).

So, although the R/P head wear doesn't look severe (and other DC43 machines give correct frequency response results, with comparable head wear), I shall have to try another R/P head. Luckily, I have an NOS one, obtained from Thorn Consumer Electronics' service department in the 1970s.

The machine plays back pre-recorded tapes well (to my ears).

Any other suggestions as to the cause of the poor higher frequencies response fault would be welcome!

I'd also like to establish the manufacturer of the heads (I think Marriott) and the meaning of the coloured markings on the rear.
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Old 16th Sep 2021, 1:34 pm   #17
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Default Re: Thorn DC43 deck valve mono tape recorders - bias setting and head markings querie

I suspect many if not most people these days dont bother (if they ever did) to do a full test of frequency response, noise, distortion etc, being content if the recorder sounds pleasant to their ears. A full, objective test regime is much more rigorous and revealing of machine and tape faults.

Is the tape path well demagnetised?

Are the test tones recorded at a low enough recording level so as not to saturate the tape in the highs (which highs of course are already boosted /pre emphasised)?

If the tape head has a wear groove, readjusting azimuth can literally lift the tape off the head gap, greatly reducing the high frequency response especially. Azimuth should normally only be changed on a head with no appreciable wear, especially with no ridges or shoulders at the edges.

Is the pressure pad (if there is one) in excellent condition and pressing the tape centrally and intimately onto the head face, specifically onto the head gap? Decks which use a pressure pad often eventually create a nasty flat or even concave wear pattern on the head face. The resultant recessed head gap is not always obvious except under strong magnification.

The ideal head face contour is usually convex. A "spacing loss" here of only microns can drastically reduce high frequency response, which can also be caused by just a very thin film of stubborn dirt or grime on the R/P head.

Last edited by TIMTAPE; 16th Sep 2021 at 1:53 pm.
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Old 16th Sep 2021, 2:10 pm   #18
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Default Re: Thorn DC43 deck valve mono tape recorders - bias setting and head markings querie

Quote:
Originally Posted by TIMTAPE View Post
I suspect many if not most people these days dont bother (if they ever did) to do a full test of frequency response, noise, distortion etc, being content if the recorder sounds pleasant to their ears. A full, objective test regime is much more rigorous and revealing of machine and tape faults.

Is the tape path well demagnetised?

Are the test tones recorded at a low enough recording level so as not to saturate the tape in the highs (which highs of course are already boosted /pre emphasised)?

If the tape head has a wear groove, readjusting azimuth can literally lift the tape off the head gap, greatly reducing the high frequency response especially. Azimuth should normally only be changed on a head with no appreciable wear, especially with no ridges or shoulders at the edges.

Is the pressure pad (if there is one) in excellent condition and pressing the tape centrally and intimately onto the head face, specifically onto the head gap? Decks which use a pressure pad often eventually create a nasty flat or even concave wear pattern on the head face. The resultant recessed head gap is not always obvious except under strong magnification.

The ideal head face contour is usually convex. A "spacing loss" here of only microns can drastically reduce high frequency response, which can also be caused by just a very thin film of stubborn dirt or grime on the R/P head.
I have demagnetised all metallic items on the tape path, including the heads (with my Ferrograph defluxer). I also clean the tape path before making the FR recordings with cotton buds and IPA.

I agree that the full FR test is likely to highlight shortcomings in the recorder, but in this case, it is far from acceptable. The 1kHz reference tone was set to 20dB below the 0dB indication mark on the record level meter. All input test tones to the recorder were kept at a constant level. The (highest frequency used) 14kHz tone registered 0dB on the record level meter when recording the tones, so none were at an excessive level that would cause over distortion on playback.

There is wear to the face of the R/P head and a slight "step" can be felt when running my fingernail across it in the vertical direction.

As the locking paint on the heads of the two height/azimuth adjusting screws had been disturbed (either by the "phantom twiddler" or someone had in the past fitted a replacement head), I thought it wise to readjust using my test tape.

The pressure pad is clean and "fluffy" and does press the tape flat against the R/P head.

I've retrieved my NOS R/P head and will fit it when time permits and repeat the FR tests, after first setting the height and azimuth.
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Old 16th Sep 2021, 3:05 pm   #19
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Default Re: Thorn DC43 deck valve mono tape recorders - bias setting and head markings querie

It might be worth recording test tones at an even lower level, say another 10db lower and see if the highs come up re 1 kHz. If at too high a record level there can be an almost runaway situation where the highs act as bias and self erase severely. So an increase in highs at record can means less highs on playback. An inversion.
An unworn head will normally require a higher bias setting than a worn one for the same response.

Last edited by TIMTAPE; 16th Sep 2021 at 3:23 pm.
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Old 16th Sep 2021, 5:07 pm   #20
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Default Re: Thorn DC43 deck valve mono tape recorders - bias setting and head markings querie

Quote:
Originally Posted by TIMTAPE View Post
It might be worth recording test tones at an even lower level, say another 10db lower and see if the highs come up re 1 kHz. If at too high a record level there can be an almost runaway situation where the highs act as bias and self erase severely. So an increase in highs at record can means less highs on playback. An inversion.
An unworn head will normally require a higher bias setting than a worn one for the same response.
I have tried recording at an even lower level, but the HF playback level was much the same - well down.

I'll report back when the NOS R/P head is installed.
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