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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 16th Sep 2021, 7:35 pm   #21
DMcMahon
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Default Re: Thorn DC43 deck valve mono tape recorders - bias setting and head markings querie

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I'd also like to establish the manufacturer of the heads (I think Marriott) and the meaning of the coloured markings on the rear.
Have previously seen various colour markings on various heads across several/many different machines, never been able to discover what they might signify, do not know if there is likely to be any universal/standard meaning.

David
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Old 16th Sep 2021, 8:00 pm   #22
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Default Re: Thorn DC43 deck valve mono tape recorders - bias setting and head markings querie

If they are Marriot I might have known the person responsible for the markings if they were made by them, used to pass by their premises on my way to and from work in Falmouth (thumbnail shows the building they were once in)

Some Marriot heads here (mag. page A46 & A47):

https://worldradiohistory.com/UK/Wir...ld-1969-05.pdf

Lawrence.
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Old 16th Sep 2021, 8:23 pm   #23
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Default Re: Thorn DC43 deck valve mono tape recorders - bias setting and head markings querie

Thanks for the reference. A company which manufactures tape heads is hard to find these days.
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Old 16th Sep 2021, 8:30 pm   #24
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Default Re: Thorn DC43 deck valve mono tape recorders - bias setting and head markings querie

I think that some folks from Phi Magnetronics worked there as well.

Lawrence.
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Old 19th Sep 2021, 4:48 pm   #25
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Default Re: Thorn DC43 deck valve mono tape recorders - bias setting and head markings querie

I've fitted the new (unused) R/P head and done the head to tape positional alignment. I've noticed that this head has a slightly different combination of coloured dots and horizontal line on the rear.

There is a favourable "Tape Recorder Service" review (by H W Hellyer) of this series of Thorn DC43 deck models, starting on page 244 of the June 1967 issue of Tape Recorder magazine: https://worldradiohistory.com/Archiv...nd-1967-06.pdf

The July 1967 issue has a favourable equipment review (by A Tutchings*) of the Ferguson 3214: https://worldradiohistory.com/Archiv...nd-1967-07.pdf (page 298). "To sum up then: The 3214 is an excellent example of a mono quarter track recorder in the middle price range, with better than usual sound quality."
The frequency response plots will also be a useful benchmark during my tests now the new head is fitted.

* Almost certainly the manufacturer of the "Thorn Height & Azimuth Test Tape" referred to in the (electrical side) service manuals for this series of DC43 deck models.

Last edited by dazzlevision; 19th Sep 2021 at 5:02 pm.
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Old 19th Sep 2021, 10:40 pm   #26
TIMTAPE
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Default Re: Thorn DC43 deck valve mono tape recorders - bias setting and head markings querie

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...The frequency response plots will also be a useful benchmark during my tests now the new head is fitted...
Yes they should be a good reference. Notice Tutchings first tests just the playback response, probably using a calibrated tape. This was always recommended. We have to confirm playback response is within spec. Only then can we test record performance using the decks own playback as the reference.

Last edited by TIMTAPE; 19th Sep 2021 at 10:49 pm.
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Old 20th Sep 2021, 1:48 am   #27
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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 dazzlevision View Post
...The frequency response plots will also be a useful benchmark during my tests now the new head is fitted...
Yes they should be a good reference. Notice Tutchings first tests just the playback response, probably using a highly accurate calibration tape.
Confirming playback response is within spec was always recommended. Only then, using the deck's own playback as the reference can we measure record performance.

I have proper expensive calibration tapes from manufacturers, but to basically check the high frequencies off the tape are actually getting through, I've long used another method. I use a blank, unrecorded tape and use its background hiss as source. It's a quiet hiss so the deck's playback electronics cant afford to be too noisy. On a good deck we should be able to hear/measure/display on a screen, this tape hiss above the deck's own preamplifier background noise.

Last edited by TIMTAPE; 20th Sep 2021 at 2:05 am.
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Old 20th Sep 2021, 6:41 pm   #28
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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
Quote:
Originally Posted by dazzlevision View Post
...The frequency response plots will also be a useful benchmark during my tests now the new head is fitted...
Yes they should be a good reference. Notice Tutchings first tests just the playback response, probably using a highly accurate calibration tape.
Confirming playback response is within spec was always recommended. Only then, using the deck's own playback as the reference can we measure record performance.

I've long used another method. I use a blank, unrecorded tape and use its background hiss as source. It's a quiet hiss so the deck's playback electronics can't afford to be too noisy. On a good deck we should be able to hear/measure/display on a screen, this tape hiss above the deck's own preamplifier background noise.
Unfortunately, I don't have any playback response test tapes or a spectrum analyser for (unrecorded) off-tape noise measurements.
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Old 20th Sep 2021, 8:39 pm   #29
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Default Re: Thorn DC43 deck valve mono tape recorders - bias setting and head markings querie

There is a spectrogram analyzer in the free audio editing programme Audacity.
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Old 21st Sep 2021, 4:48 am   #30
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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
There is a spectrogram analyzer in the free audio editing programme Audacity.
My Mac desktop doesn't have a "line in" socket and only a built-in microphone, so I can't easily use Audacity. So, I think I'll persevere with trying to get the record-playback frequency response within the tolerance specified in the service manual, now that a new R/P head is fitted and physically aligned. I'm confident the electronics side is correct.
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Old 21st Sep 2021, 7:49 am   #31
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Default Re: Thorn DC43 deck valve mono tape recorders - bias setting and head markings querie

It's relatively easy and inexpensive. The home desktop these days can handle audio recording without a problem as it could 20 years ago. A simple USB stereo audio interface and the matching software is all that's needed.
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Old 2nd Oct 2021, 12:46 pm   #32
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Default Re: Thorn DC43 deck valve mono tape recorders - bias setting and head markings querie

Update:

I have just noticed that the cause of erratic frequency response tests at the 3.75 ips tape speed is due to an intermittent failure of the deck mechanism to change speed correctly!

When the speed selection knob is moved from 7.5 ips to 3.75 ips, it sometimes moves the idler wheel (that couples drive from the stepped motor pulley to the flywheel) into the wrong pulley step (i.e. 1.875 ips)!

No wonder the HF response is very poor at 14kHz (the 1.875 ips -3dB cutoff frequency is 7kHz!).

I think this is due to the die cast part of the speed change mechanism moving either up or down its blackened steel shaft (see photo).

I was so engrossed with the electrical side, that I didn't notice the tape was running slow when 3.75 ips was selected. Doh!!

I'll sort that out first and then check the bias current and frequency response figures at 3.75 ips (as per the service manual procedure).
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Old 2nd Oct 2021, 10:52 pm   #33
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Default Re: Thorn DC43 deck valve mono tape recorders - bias setting and head markings querie

Good that you have found something that explains the poor frequency response results, hopefully you can rectify the speed control and then more normal results at 3 3/4 ips.

David
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Old 3rd Oct 2021, 8:32 am   #34
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Default Re: Thorn DC43 deck valve mono tape recorders - bias setting and head markings querie

When looking into why the deck's speed change mechanism was erratic, I noticed that one of the four blackened steel spacers that the motor sub-deck mounts on wasn't engaging correctly with the motor sub-chassis, meaning it was not quite parallel with the main deck chassis.

These four support spacers have (at their free ends) a short reduced diameter section that fits into the four holes in the sub-deck (see photos).

I managed to get the errant spacer to engage correctly with the sub-deck's hole and refitted the sub-deck to the main chassis. This seems to have done the trick and the speed change mechanism now works reliably.

The stepped area of the white plastic part engages with the short stub at the end of the grey die cast part, which then moves the idler wheel up or down to the correct position for engaging with the stepped motor pulley.

I should point out that I haven't stripped the Ultra's tape deck right down in order to do the above work - the photos are of a Thorn DC432 deck (stereo version of the DC43 deck) that I dismantled for parts (as well as having a lot of photos showing how it is actually assembled - for future reference!).
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Old 11th Oct 2021, 5:34 pm   #35
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Default Re: Thorn DC43 deck valve mono tape recorders - bias setting and head markings querie

I'm making some progress. I have a BASF calibration tape (which has recorded spot frequencies) on it and have determined that the playback levels when using this are OK. I've also readjusted the azimuth setting of the R/P head with this tape.

Even so, I cannot get a satisfactory frequency response at 3.75 ips and previous recordings are not fully erased.

So, I have fitted a spare tape head platform (also containing the tape guide posts - see the third photo in post #1) into this recorder, which has undisturbed R/P head height/azimuth adjusting screws (i.e. the factory applied locking paint is intact).

I measured the ac volts at the erase head and the R/P head bias current (across a 1% 100 Ohm resistor) in both the original and replacement head/tape guides units and they are as close as makes no difference (the coloured markings on both R/P heads are also identical).

I can now make new recordings, which erase fully previous the recording. However, although I can get the 3.75 ips record/playback response correct (within +/- 3dB) on what appears to be a 1960s EMI tape, if I use other branded tapes from that era, they give significantly different results (i.e. not within 3dB at 14kHz, w.r.t. the 1kHz reference level).
That's probably why it says in the recorder’s operating instructions "For best results, use the type of tape supplied with the recorder"!

In addition, with a variety of tape brands, there is always excessive HF boost at 7.5 ips (after setting up the recorder at 3.75 ips).

After the problem with erratic tape speed selection, I have been very careful to make sure the tape speed is correct when making all test recordings.

I’ve also checked that all of the frequency response determining electronic components (at the three tape speeds) are in spec and that the switching of these is also correct.

I’m now wondering if this is normal for these DC43 deck recorders. I do have another one and I’m going to make some comparative tests using that, to see if this is typical or whether the Ultra 6212 does still have a fault somewhere…..
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Old 13th Oct 2021, 5:26 pm   #36
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Default Re: Thorn DC43 deck valve mono tape recorders - bias setting and head markings querie

I now have a complete spare chassis from a Marconiphone 4238 on the bench (this, and the Ferguson 3238, were the last two models to be released that used the DC43 deck and valve electronics). The 4238 chassis is dated late 1968 (according to the date codes on some electrolytic capacitors and the ITT Selenium bridge rectifier) and must be one of the last ever production run.

I’ve been careful to use the same reel of EMI “brown” (1960s) tape on both decks, in order to get an accurate comparison between the two.

The coloured dots and lines on the rear of the R/P head are the same as on the Ultra 6212 head.

After powering up, I measured the output of the 55kHz HF bias oscillator stage (ECC82) at the point which feeds bias to the record/play head via a parallel pair of a fixed and a variable capacitor (to set the correct R/P head bias current).

The level was considerably lower (140Vpk-pk) than that measured on the Ultra 6212 chassis, so I tried a replacement ECC82, which didn’t work at all. After replacing the original valve, it still wouldn’t work! This was due to a manufacturing defect.
Pin 2 of the B9A valveholder was much shorter than it should be and only just got whetted in the solder bath at the factory. Stressing the valveholder during valve removal broke this rather tenuous connection. A new valveholder restored oscillation and with a new Mullard ECC82, the bias voltage was much higher (240Vpk-pk).

I then inserted a 1% 100 Ohm resistor in the earthy side of the R/P head’s track 1 wiring, in order to check the bias current, which was 23mV rms (the service manual states the acceptable range is 20–30mV).

A frequency response test recording was made at 3.75 ips, with a 1kHz reference, plus 10k, 12k and 14k tones. 14kHz is the upper limit of the claimed +/-3dB response at this speed.
The (normalised about 1kHz) results were: 0, -4, -3 and (at 14kHz), there was an audible tone (in addition to a diminished 14kHz sinewave on the ‘scope) when played back, which seems to indicate some sort of intermodulation effect during the recording process. I had the same effect on the Ultra 6212. This result seems acceptable as far as the response check is concerned.

I then measured the values of several (carbon composition) fixed resistors in the treble boost circuitry used in record mode. They were all well above the upper tolerance limits. I replaced all of them, as they may well have had an adverse effect on the response.
The new 3.75 ips results were: 0. -6, -6 and the same audible tone at 14kHz.

I also made some test recordings at 7.5 ips (both before and after resistor changes), at 1, 10, 14 and 18kHz (18kHz being the upper +/- 3dB limit at this speed).
The “before” results were: 0, +1, +8, +8.
The “after” results were: 0, -3, +3, +3.

So, a bit of a mixed bag. The results at 3.75 ips are out of spec, but those for 7.5 ips are in spec (and no audible playback tone at the 7.5 ips top test frequency either, unlike at 3.75 ips).

The specifications listed in service manuals for these DC43 tape deck models state a “frequency range”, with no +/- dB limits given. So, with some visible head wear on the sets I’ve been working on and the ageing of certain passive electronic components and the valves, the most recent results that I’ve obtained are acceptable.

I’ll put my Ultra 6212 back on the bench and fit a known good ECC82 bias oscillator valve, to see what effect that might have on erasure and frequency response performance (in case the one fitted is out of spec).

I’d be interested to hear from anyone who can explain the mechanism that causes the audible range playback tone at a recorded spot frequency of 14kHz at 3.75 ips.
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Old 13th Oct 2021, 8:26 pm   #37
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Default Re: Thorn DC43 deck valve mono tape recorders - bias setting and head markings querie

Good work and very interesting results, any actual noticeable audible difference post resistor changes ?

I assume the tone you hear from the 14kHz recording is not actually 14kHz but some lower frequency artifact, my hearing would not hear a 14kHz tone, I cannot explain its generation though.

David
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Old 13th Oct 2021, 10:13 pm   #38
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Default Re: Thorn DC43 deck valve mono tape recorders - bias setting and head markings querie

If there are no db tolerances stated for the record/playback frequency response, then we have no firm guidance for testing.

14 kHz at 3.75ips and recorded 20 db down on a 1960 tape is probably pushing it.

I think the old design rule for bias frequency was that it should be 4 to 5 times the highest frequency to be recorded. This was to avoid harmonics of the highest frequency beating with the bias signal. Your Thorn has only a 55 kHz bias frequency, just over 4 times 14 kHz. By comparison a Revox A77 deck in the same era used a bias frequency of 120 kHz.

If you havent already done it it might be worth checking that the bias traps are correctly adjusted.
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Old 14th Oct 2021, 7:29 am   #39
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Default Re: Thorn DC43 deck valve mono tape recorders - bias setting and head markings querie

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Good work and very interesting results, any actual noticeable audible difference post resistor changes ?

I assume the tone you hear from the 14kHz recording is not actually 14kHz but some lower frequency artifact, my hearing would not hear a 14kHz tone, I cannot explain its generation though.

David
I will be making some test recordings from CD, to subjectively assess sound quality. However, my hearing only extends to 10kHz!

TIMTAPE offers an explanation for the audio frequency tone (lower that 10kHz, of course and visible on my 'scope) heard when playing back a single 14kHz recorded tone and I would agree that harmonics beating with the 55kHz bias frequency is the most likely cause.

I doubt replacing the resistors in the higher frequencies boost feedback path will have a significant audible effect - I just wanted to eliminate them from my investigations.
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Old 14th Oct 2021, 7:44 am   #40
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Default Re: Thorn DC43 deck valve mono tape recorders - bias setting and head markings querie

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If there are no db tolerances stated for the record/playback frequency response, then we have no firm guidance for testing.

14 kHz at 3.75ips and recorded 20 db down on a 1960 tape is probably pushing it.

I think the old design rule for bias frequency was that it should be 4 to 5 times the highest frequency to be recorded. This was to avoid harmonics of the highest frequency beating with the bias signal. Your Thorn has only a 55 kHz bias frequency, just over 4 times 14 kHz. By comparison a Revox A77 deck in the same era used a bias frequency of 120 kHz.

If you havent already done it it might be worth checking that the bias traps are correctly adjusted.
The service manual does state that, when checking the record to playback frequency response, the level at 14kHz should be within 3dB of the 1kHz reference.

I'm sure you are correct in saying that the audible tone is a result of harmonics of the 14kHz tone being recorded, beating with the 55kHz bias signal. For example 4x14 = 56 and 56-55 = 1kHz. I'll try to measure the frequency of the tone, but I can only do that in the time domain, on my scope. I don't have a spectrum analyser.

However, this effect would surely have been present when these recorders were being tested in the factory, so would give a confusing result at 14kHz....? It would depend on what sort of test gear they used to measure the playback levels.

There are no bias traps (tuned circuits) in the circuitry of these models.

When I have access to my scanner, I'll post the service manual's bias setting/frequency response test procedure and a circuit extract.
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