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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 9th Jan 2021, 4:29 pm   #121
DMcMahon
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

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Originally Posted by TerryB44 View Post
Thanks David, I used a 1k resistor across the terminals which is near enough the resistance of the head. I may just try one of the heads as an exercise - I might even be able to get it close enough to the tape to see if there is any audio present!
I assume you got no hum with 1k resistor ?
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Old 9th Jan 2021, 4:38 pm   #122
TerryB44
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Well, after all that it's definitely the head! I know, we told you so, but wiring in the head got rid of the hum and holding the head against the tape worked a treat, sound back in all its glory! Thanks to all who've helped with this.
All I need to do now is find a suitable head or I could I suppose at a push use this head after removing the old one from its mounting plate - I think its held on with two small screws, but it is a bit worn, very worn in fact! It looks to be a full track head as the gap is placed centrally but I guess there may be enough azimuth adjustment to position the head in the right place - hopefully!

Last edited by TerryB44; 9th Jan 2021 at 4:43 pm.
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Old 9th Jan 2021, 4:40 pm   #123
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

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Originally Posted by DMcMahon View Post
Quote:
Originally Posted by TerryB44 View Post
Thanks David, I used a 1k resistor across the terminals which is near enough the resistance of the head. I may just try one of the heads as an exercise - I might even be able to get it close enough to the tape to see if there is any audio present!
I assume you got no hum with 1k resistor ?
No I didn't David.
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Old 9th Jan 2021, 4:47 pm   #124
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

So is the head you are talking about from the Copycat, could it be easily mounted at least to do some proper recording/playback checks ?

David
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Old 9th Jan 2021, 4:53 pm   #125
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

I'm not sure how these heads work. It could probably be mounted easily enough but can it record as well as play? In the case of a combined record playback head is it the circuitry that changes it from one to the other, or is it the head itself that is different? (Showing my ignorance here!) The heads in the tape echo I've got are all the same as far as I know with one that records and three that playback.
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Old 9th Jan 2021, 5:24 pm   #126
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

The Truvox head has 2 small screws which go through the baseplate and through the housing and screw into the head to position it correctly. The holes in the head I've got are much further back in the head and are too big anyway, so it would mean fixing it with glue or something and it be difficult to get the correct alignment on the mounting plate, plus I lose my tape echo machine functionality! So, I'll have to try and source something a little bit more compatible. Do any of the ones you have David look anything similar? See picture below.
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Old 9th Jan 2021, 9:24 pm   #127
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

No I do not think I have anything like this/suitable.

I know very little about heads, believe Playback and Record heads are engineered differently, how a combined Play/Record head actually technically works I am unsure.

David
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Old 9th Jan 2021, 10:00 pm   #128
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

The main difference between record and replay heads, when these are purpose-designed, is the gap length. The gap on a record head has to be long enough for the flux generated across the gap to penetrate the full depth of the tape oxide - the precise gap length is of less significance, as recording actually takes place slightly beyond the trailing edge of the gap, as the tape leaves the bias field. The gap on a playback head is critical to quality of reproduction, as it has to be long enough to capture sufficient flux to give a decent signal-to-noise ratio, but short enough for the extinction frequency (when the recorded wavelength equals the gap length) to be above the highest frequency of interest.

Record heads have a higher inductance than play heads - more turns, in other words, to ensure the tape can be saturated, whilst the number of turns on a play head has to be balanced against other losses.

In domestic machines, where the coating thickness of the tape is thinner than professional tapes, it is possible to make one head perform both functions adequately.

Last edited by Ted Kendall; 9th Jan 2021 at 10:06 pm.
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Old 9th Jan 2021, 11:18 pm   #129
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

In case it's useful in your search for replacement heads, these are the specifications for the heads given by HW Hellyer in his review of the Series 80 Truvox's -

“Head details are as follows: Record/Playback head inductance is 1.25mH (-track) and 850mH on either section (-track). Impedances are 7.8K (-track) and 5.5K (-track). Resistances: 1.1K (-track)and 1.1K (-track). Erase head inductance is 1.4mH (-track) and 80H (-track). Record signal current is 70A for the -track head and 50A for the-track head, and the required bias voltages for these are 120V and 75V respectively. Bias and erase frequency is 55 Kc/s and erase voltage is 45V (-track) and 12V (-track). The gap length is 0.00014in., and the crosstalk figure stated for the two-gapped head is better than 55dB.”

Mike

[EDIT - looking at these, I wonder if their proof-reader missed a few things - e.g -the inductances of the half-track and quarter-track playback windings seem wildly out of proportion - perhaps it should be 850H.]

Last edited by Boulevardier; 9th Jan 2021 at 11:28 pm.
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Old 9th Jan 2021, 11:27 pm   #130
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Hi Ted, thanks for that excellent tutorial. You have explained in a couple of paragraphs everything I need to know about record/playback heads. I now realise that if I can't source a compatible head I'll have to settle for just having a machine to playback on, which is fine as that was really the point of trying to fix this machine in the first place. Cheers!
It speaks volumes for the technology that good recordings from that era, to my ears at least, have a quality that no digital recording ever has, which perhaps explains why a lot of artistes are going back to tape technology. They can sample as much as they like but even at CD 44000 sample rate there are still gaps. Analogue is a continuous waveform, no gaps, and maybe it's the gaps that make the difference! That's my theory anyway!
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Old 11th Jan 2021, 3:45 pm   #131
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

I've fixed the copicat head temporarily with the help of several very small blobs of Blu-tac which at least allows me to playback. The head is not very good quality either but it will do until something better comes along. It was playing a touch slow though, half a tone to be precise, which turned out to be the erase head pressure pad. A little tweak with the pliers and it's now back at normal speed so I can play along on my guitar in the right key now!
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Old 11th Jan 2021, 9:04 pm   #132
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Well done Terry.

David
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Old 11th Jan 2021, 9:35 pm   #133
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

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Originally Posted by Boulevardier View Post
In case it's useful in your search for replacement heads, these are the specifications for the heads given by HW Hellyer in his review of the Series 80 Truvox's -

Head details are as follows: Record/Playback head inductance is 1.25mH (-track) and 850mH on either section (-track). Impedances are 7.8K (-track) and 5.5K (-track). Resistances: 1.1K (-track)and 1.1K (-track). Erase head inductance is 1.4mH (-track) and 80H (-track). Record signal current is 70A for the -track head and 50A for the-track head, and the required bias voltages for these are 120V and 75V respectively. Bias and erase frequency is 55 Kc/s and erase voltage is 45V (-track) and 12V (-track). The gap length is 0.00014in., and the crosstalk figure stated for the two-gapped head is better than 55dB.

Mike

[EDIT - looking at these, I wonder if their proof-reader missed a few things - e.g -the inductances of the half-track and quarter-track playback windings seem wildly out of proportion - perhaps it should be 850H.]
All those figures seem wildly low to me. 80mH is a fairly normal medium impedance replay head for transistor amplifiers, a suitable head for valve amplifiers without a transformer would be much higher.

Similarly a record head would be typically much higher than your figures especially with such a relatively low bias frequency.
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Old 12th Jan 2021, 10:58 am   #134
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

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All those figures seem wildly low to me. 80mH is a fairly normal medium impedance replay head for transistor amplifiers, a suitable head for valve amplifiers without a transformer would be much higher.

Similarly a record head would be typically much higher than your figures especially with such a relatively low bias frequency.
I agree that there’s something weird about Hellyer’s specs, Barry. Out of interest and for comparison, I looked up the specs for the Bogen UK202B R/P heads, which seem to work well with both valve and IC input stages (that is, until they go o/c!). These specs also come from Hellyer, although the inductance agrees with that given in JR Stuart’s WW articles on his “High-quality Tape Recorder” (though Stuart mysteriously gives the head’s inductance as “120mH at 1KHz” [ - eh..?!]) --

(From: “Tape Recorder” magazine,
September 1967, article ‘Tape
Recorder Service’, HW Hellyer)

Recording head UK202, 2/2 track
Inductance – 120mH
Impedance at 1kHz – 780Ω
DC resistance – 75Ω
Bias at 100kHz – 23V at 1mA
Recording current (3-75 ips) - 120μA
Output at 2kHz (3.75 ips) – 2.4mV

Bogen did make a separate Playback head with different characterisitics, but Stuart specified the UK202 for both R and P.

Mike
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Old 12th Jan 2021, 4:19 pm   #135
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

My Hellyer's shows 1.25H inductance for the Truvox D82 2 track combined Play/Record Head.

David
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Old 12th Jan 2021, 5:03 pm   #136
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

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Originally Posted by DMcMahon View Post
My Hellyer's shows 1.25H inductance for the Truvox D82 2 track combined Play/Record Head.

David
That sort of sounds more consistent with the 4-track value. I got the 1.25mH from the 1965 Hellyer link you posted in #45.

Mike
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Old 12th Jan 2021, 5:12 pm   #137
DMcMahon
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Looks like a typo error in that 1965 magazine article.

My 1.25H reference is from the 1965 Hellyer's servicing book.

David
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Old 12th Jan 2021, 6:04 pm   #138
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

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Originally Posted by DMcMahon View Post
My Hellyer's shows 1.25H inductance for the Truvox D82 2 track combined Play/Record Head.

David
That seem plausible.

The record / play head on the (valve) EMI TR50 measures390mH which is getting on for the same ballpark.
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Old 12th Jan 2021, 6:05 pm   #139
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

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Originally Posted by DMcMahon View Post
Looks like a typo error in that 1965 magazine article.

My 1.25H reference is from the 1965 Hellyer's servicing book.

David
Even so, that 1250mH seems huge when compared with the Bogen's 120mH.

Also, the "120mH at 1Kc/s" given by Stuart baffles me.. Surely inductance is just inductance and doesn't vary with frequency?

Mike

EDIT - crossed post with Barry
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Old 12th Jan 2021, 9:28 pm   #140
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Measured inductance will change with frequency as the characteristics of the core material vary with frequency.

Leevers Rich measured their head inductances at 10Kc (Khz), I don`t know how much that affected the reading compared with measuring at 1Kc but 1 and 10 were the options on the Marconi TF1313A that was used.
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