Telefunken Magnetophon 75 - head resistance - UK Vintage Radio Repair and Restoration Discussion Forum

oldgeezer · 12th Oct 2015, 4:49 pm

I dug this one out of the attic with some old tapes. (I've only played with Radios up till now)

They turned out to be recordings made by my Uncle in 1964 and although very quiet and crackly I could hear them enough to know I'd like to listen to them all.

Unfortunately the tape recorder gave up before I got too far and having put it on the bench, I find the resistance of the playback head is 25Meg ohms (+-). (and that may just be my fingers!)

Does this mean the head is O/C and I need a new one? Are they obtainable? Should I look for another tape recorder on the well known auction site and give up on this one?

Many thanks
Keith

Ted Kendall · 12th Oct 2015, 6:31 pm

If you're getting Megohms, either the head was open circuit to start with or you've fried it with the Avo! As a general point, it's not good practice to put DC through a head winding as you'll magentise the core at best. If you have a de-gausser handy, this is less of a problem, but you do run the risk of sending it o/c.

Telefunken 75 heads were made by an Italian concern, if memory serves, and they do sometimes turn up on auction sites. The 75 was pretty decent as a performer, so it's worth some trouble to get it going - would you believe servo back-tension on a 5" spool domestic?

TIMTAPE · 12th Oct 2015, 9:12 pm

In my experience record/play heads going open circuit is rare. If you can hear some signal off tape it's usually best to first look for other causes such as a dirty head face, poor tape to head contact, mechanical misalignment, magnetisation etc.

oldgeezer · 12th Oct 2015, 9:50 pm

Well, I did hear some audio from the tape - but now nothing at all. I will investigate other possibilities too though, as the microphone on record doesn't register now either. But I do get hum when I put my screwdriver on the grid of the first audio valve. The voltages are all correct on the valve and I'm wondering if I should start changing some of the capacitors.

oldgeezer · 15th Oct 2015, 3:32 pm

So, I've changed some capacitors that looked bad, but to no avail.

I've removed the record/playback head now and it measures around 300Meg ohms resistance out of circuit. Can that possibly be right??

julie_m · 15th Oct 2015, 6:03 pm

I reckon it's open-circuit. But you probably haven't ruined it with your meter.

It's simply not true that the battery in a multimeter will burn out the windings of a tape head. A multimeter shouldn't put out more current -- 50 μA for a 20 kΩ/V meter e.g. AVO 8, 1 mA for a 1 kΩ / V meter e.g. AVO 7 -- than the machine does when it's recording, and it is not for long anyway. There is still a good reason not to use one for that purpose anyway, though, as you run the risk of permanently magnetising the core -- the symptom of that is, your tapes get a little quieter every time you listen to them, as the recording is progressively destroyed. (Guess how I know about that?) And that misunderstanding probably is where the myth came from. A good long blast on RECORD ought to remove any remaining magnetism, or you could use a demagnetiser. A digital meter will not put out enough current to leave any remaining magnetism.

oldgeezer · 15th Oct 2015, 7:58 pm

Many thanks.

I suspected it from early on in my fault finding.

Now to find a spare.....!

DOFFERY · 16th Oct 2015, 12:30 pm

Old geezer,
If it is the three pin plug in type I think I have a couple of these in stock, impedance about 270 ohms.

Let me know . by pm if you have any query re. the head.

Cheers.

Colin.

oldgeezer · 16th Oct 2015, 10:26 pm

Hi Colin,
I've taken a couple of photos of the head and where it is mounted.
I cant pm the photos so I've shown them here.
if you have a head that may do the trick I would be very pleased and happy to purchase one.
please let me know
thanks
Keith

DOFFERY · 17th Oct 2015, 10:54 am

Keith, that looks like a Bogen head not a Telefunken , I can only surmise this replaced a faulty Telefunken head in the past. !!
Your machine is a 75 or the later 75 /15 ? I have many items for the range.

It looks like someone has replaced the head mount & used the Bogen.

However if you require a head , wire ended I also have those, the head mount on the original really was for the 3 pin Telefunken head as I mentioned.

Let me know.

Colin

oldgeezer · 17th Oct 2015, 12:23 pm

Hi Colin,
Many thanks.
A photo of the head housing to try shed a little light on what may fit it.....

DOFFERY · 17th Oct 2015, 2:48 pm

I have never come across that head mounting in my decades of tape repairs etc. Alas not knowing the impedance of the head(any number on the head?) difficult to know the best. However I have a Bogen head, mono half track plus a complete Telefunken head case with an original Telefunken head for the 75/15, it was fitted to both variants.

The problem is with the head mount you have! Could the original head/surround as I have be fitted ?

Do you want to try both items, the heads are perfect, new & testing OK but differ in impedance!

Let me know what you think . OK ?

Colin.

oldgeezer · 22nd Oct 2015, 10:34 pm

New Telefunken head installed - it looks like the old one was replaced with a Bogen but it all looks original... odd!

Anyway - machine worked but very quiet and almost impossible to hear - so I changed most of the interstage capacitors. C3, C8, C12, C15.

BINGO! Now working perfectly, although I suspect other capacitors may need changing. I just wanted to make it play.

Many thanks Colin and others with hints.
Keith

12th Oct 2015, 4:49 pm	#1
oldgeezer Pentode Join Date: Oct 2014 Location: St Albans, Hertfordshire, UK. Posts: 135	Telefunken Magnetophon 75 - head resistance I dug this one out of the attic with some old tapes. (I've only played with Radios up till now) They turned out to be recordings made by my Uncle in 1964 and although very quiet and crackly I could hear them enough to know I'd like to listen to them all. Unfortunately the tape recorder gave up before I got too far and having put it on the bench, I find the resistance of the playback head is 25Meg ohms (+-). (and that may just be my fingers!) Does this mean the head is O/C and I need a new one? Are they obtainable? Should I look for another tape recorder on the well known auction site and give up on this one? Many thanks Keith

12th Oct 2015, 6:31 pm	#2
Ted Kendall Dekatron Join Date: Jul 2008 Location: Kington, Herefordshire, UK. Posts: 3,670	Re: Telefunken Magnetophon 75 - head resistance If you're getting Megohms, either the head was open circuit to start with or you've fried it with the Avo! As a general point, it's not good practice to put DC through a head winding as you'll magentise the core at best. If you have a de-gausser handy, this is less of a problem, but you do run the risk of sending it o/c. Telefunken 75 heads were made by an Italian concern, if memory serves, and they do sometimes turn up on auction sites. The 75 was pretty decent as a performer, so it's worth some trouble to get it going - would you believe servo back-tension on a 5" spool domestic?

12th Oct 2015, 9:12 pm	#3
TIMTAPE Octode Join Date: Jul 2006 Location: Perth, Western Australia Posts: 1,969	Re: Telefunken Magnetophon 75 - head resistance In my experience record/play heads going open circuit is rare. If you can hear some signal off tape it's usually best to first look for other causes such as a dirty head face, poor tape to head contact, mechanical misalignment, magnetisation etc.

12th Oct 2015, 9:50 pm	#4
oldgeezer Pentode Join Date: Oct 2014 Location: St Albans, Hertfordshire, UK. Posts: 135	Re: Telefunken Magnetophon 75 - head resistance Well, I did hear some audio from the tape - but now nothing at all. I will investigate other possibilities too though, as the microphone on record doesn't register now either. But I do get hum when I put my screwdriver on the grid of the first audio valve. The voltages are all correct on the valve and I'm wondering if I should start changing some of the capacitors.

15th Oct 2015, 3:32 pm	#5
oldgeezer Pentode Join Date: Oct 2014 Location: St Albans, Hertfordshire, UK. Posts: 135	Re: Telefunken Magnetophon 75 - head resistance So, I've changed some capacitors that looked bad, but to no avail. I've removed the record/playback head now and it measures around 300Meg ohms resistance out of circuit. Can that possibly be right??

15th Oct 2015, 6:03 pm	#6
julie_m Dekatron Join Date: May 2008 Location: Derby, UK. Posts: 7,735	Re: Telefunken Magnetophon 75 - head resistance I reckon it's open-circuit. But you probably haven't ruined it with your meter. It's simply not true that the battery in a multimeter will burn out the windings of a tape head. A multimeter shouldn't put out more current -- 50 μA for a 20 kΩ/V meter e.g. AVO 8, 1 mA for a 1 kΩ / V meter e.g. AVO 7 -- than the machine does when it's recording, and it is not for long anyway. There is still a good reason not to use one for that purpose anyway, though, as you run the risk of permanently magnetising the core -- the symptom of that is, your tapes get a little quieter every time you listen to them, as the recording is progressively destroyed. (Guess how I know about that?) And that misunderstanding probably is where the myth came from. A good long blast on RECORD ought to remove any remaining magnetism, or you could use a demagnetiser. A digital meter will not put out enough current to leave any remaining magnetism. __________________ If I have seen further than others, it is because I was standing on a pile of failed experiments.

15th Oct 2015, 7:58 pm	#7
oldgeezer Pentode Join Date: Oct 2014 Location: St Albans, Hertfordshire, UK. Posts: 135	Re: Telefunken Magnetophon 75 - head resistance Many thanks. I suspected it from early on in my fault finding. Now to find a spare.....!

16th Oct 2015, 12:30 pm	#8
DOFFERY Retired Dormant Member Join Date: Aug 2004 Location: Blackpool, Lancashire, UK. Posts: 1,488	Re: Telefunken Magnetophon 75 - head resistance Old geezer, If it is the three pin plug in type I think I have a couple of these in stock, impedance about 270 ohms. Let me know . by pm if you have any query re. the head. Cheers. Colin. Last edited by DOFFERY; 16th Oct 2015 at 12:49 pm.

16th Oct 2015, 10:26 pm	#9
oldgeezer Pentode Join Date: Oct 2014 Location: St Albans, Hertfordshire, UK. Posts: 135	Re: Telefunken Magnetophon 75 - head resistance Hi Colin, I've taken a couple of photos of the head and where it is mounted. I cant pm the photos so I've shown them here. if you have a head that may do the trick I would be very pleased and happy to purchase one. please let me know thanks Keith Attached Thumbnails

17th Oct 2015, 10:54 am	#10
DOFFERY Retired Dormant Member Join Date: Aug 2004 Location: Blackpool, Lancashire, UK. Posts: 1,488	Re: Telefunken Magnetophon 75 - head resistance Keith, that looks like a Bogen head not a Telefunken , I can only surmise this replaced a faulty Telefunken head in the past. !! Your machine is a 75 or the later 75 /15 ? I have many items for the range. It looks like someone has replaced the head mount & used the Bogen. However if you require a head , wire ended I also have those, the head mount on the original really was for the 3 pin Telefunken head as I mentioned. Let me know. Colin

17th Oct 2015, 12:23 pm	#11
oldgeezer Pentode Join Date: Oct 2014 Location: St Albans, Hertfordshire, UK. Posts: 135	Re: Telefunken Magnetophon 75 - head resistance Hi Colin, Many thanks. A photo of the head housing to try shed a little light on what may fit it..... Attached Thumbnails

17th Oct 2015, 2:48 pm	#12
DOFFERY Retired Dormant Member Join Date: Aug 2004 Location: Blackpool, Lancashire, UK. Posts: 1,488	Re: Telefunken Magnetophon 75 - head resistance I have never come across that head mounting in my decades of tape repairs etc. Alas not knowing the impedance of the head(any number on the head?) difficult to know the best. However I have a Bogen head, mono half track plus a complete Telefunken head case with an original Telefunken head for the 75/15, it was fitted to both variants. The problem is with the head mount you have! Could the original head/surround as I have be fitted ? Do you want to try both items, the heads are perfect, new & testing OK but differ in impedance! Let me know what you think . OK ? Colin.

22nd Oct 2015, 10:34 pm	#13
oldgeezer Pentode Join Date: Oct 2014 Location: St Albans, Hertfordshire, UK. Posts: 135	Re: Telefunken Magnetophon 75 - head resistance New Telefunken head installed - it looks like the old one was replaced with a Bogen but it all looks original... odd! Anyway - machine worked but very quiet and almost impossible to hear - so I changed most of the interstage capacitors. C3, C8, C12, C15. BINGO! Now working perfectly, although I suspect other capacitors may need changing. I just wanted to make it play. Many thanks Colin and others with hints. Keith