Unusual Mono Tape head - UK Vintage Radio Repair and Restoration Discussion Forum

sp10mk11 · 4th Sep 2016, 5:27 pm

Hello I have a Silver Radio/Cassette from the early 1970s, the transport is an ITT type except with latching keys built on to it and an auto stop bar

The head has 3 pins, there seems to be a central pin that measures 77 ohms to one pin and 112 ohms to the other, so am I right in assuming that one side is for play back and the other for record, this does have a separate erase head just incase you are thinking that it does not.
the switching is done by the Record/Play switch, I would like to replace this head but I have nothing like it, plenty of stereo heads and mono heads with only two connections.

Anyone any ideas or advice.

Thanks
Gary

sp10mk11 · 4th Sep 2016, 7:17 pm

I have been looking at some ITT cassette recorder schemetics and the Model SL-75 seems to have the same type of head, in as far as it has 3 connections and similar circuitry ( I am presuming that Silver copied it).

Gary

ricard · 5th Sep 2016, 2:47 pm

I've seen this on Philips recorders as well. I could be completely wrong, but my guess is that you want as many turns as possible in playback to get as high a voltage as possible from the head, but in record mode you want fewer turns to so you that the bias voltage can be lower - especially important in battery-operated equipment.

Refugee · 5th Sep 2016, 3:21 pm

I read this thread last night and it got me thinking.
Without looking at what all the wires connect to I will have to just guess that the 77 ohm coil has the bias on it and the 112 ohm winding carries the audio in record mode.
The 112 ohm winding obviously connects to the preamp in playback mode.

5th Sep 2016, 4:39 pm

A self oscillating bias used with a PM (permanant magnet) erase?

sp10mk11 · 5th Sep 2016, 5:21 pm

Hello Chaps thanks for your comments I was thinking a long the same lines, I might try and use a standard head and alter the switching on the record/replay switch, and fiddle about to get it to play and record well

The whole system runs on 7.5 volts DC but i can get upto about 12.7 from the bias oscillator what that will drop to with another head do not know......yet
The main problem is the Record/replay head is worn, no amount of cleaning demagnetising polishing will bring the top end up much beyond about 6K

merlinmaxwell
This tape deck has an AC powered erase head

Again Thanks
Gary

Maarten · 6th Sep 2016, 2:21 am

Quote:

Originally Posted by sp10mk11

I have been looking at some ITT cassette recorder schemetics and the Model SL-75 seems to have the same type of head, in as far as it has 3 connections and similar circuitry ( I am presuming that Silver copied it).

Probably the other way around given the Japanese transistor in the schematic, or there was no copying involved at all (Silver might have been the OEM, or ITT and Silver used the same OEM).

sp10mk11 · 6th Sep 2016, 11:58 am

Hello Maarten
Yes they are both very similar, the one shown is the ITT head circuit, could possibly find the same sort of circuit on other makes of the time on reflection.
Gary

wd40addict · 6th Sep 2016, 1:05 pm

My Grundig C410 had the same style of head, and back in the day it had to be changed due to wear...

For playback the whole winding is used: one end is grounded, other end connects to the base of the PB amp, with the centre connected to a 15k resistor to gnd. On record bias goes to one end, record signal to the centre and the other end is grounded.

Presumably the point of all this is to maximise head output on playback, whilst not needing a large voltage swing for recording. Effectively combining a high and low Z head in one unit.

sp10mk11 · 7th Sep 2016, 3:07 pm

Problem Solved !
Hmm shall I blame one small AG13 battery that runs a TDK Head Demag unit or shall I blame my self for being a silly old sod that doesn't always pay attention to what he is doing and failing to see the demag was not working properly
I wanted to try out a tape head that sort of similar resistance, so had it in my hand and put the demag on it and pushed the head forward, just a feint red glow came from the LED..........no don't tell me the battery after about 10 years is dying......................argh, so of course I thought of the original that had lost response and sensitivity, so out with trusty old Ross tape recorder head de-mag wand, gave the ITT head a good going over put it back in and hey ho a top end (of sorts) and recording sensitivity returned, bias back from 9.5 AC volts to 4.9 AC volts for TDK AD tape.
I am getting doddery I know and these sort of mistakes occur in other jobs some times, I have about 2 hours a day when I am in what I call "up Time" anything more and I am slowly getting tired and weary, but I am a sod for pushing on regardless but occasionally it has cost me dear in terms of errors.
So chaps thanks very much for your time and input on this one, pilot error I am afraid

Gary

ricard · 7th Sep 2016, 6:38 pm

I think we've all made mistakes of that type from time to time, we take something for granted and feel ashamed when we realize that we'd completely overlooked something.

Great that you've got it going. I'm a bit confused about the bias voltage, would the bias voltage acreoss the head really be dependent on any residual DC magnetization? Indeed, the application of AC bias would in itself tend to demagnetize the head to start with.

sp10mk11 · 7th Sep 2016, 8:05 pm

Ricard
sorry I didn't explain that well, when the head was not working properly I had to turn the bias up to 9v to get a reasonable signal on the tape, but at the same time this was over biasing the head so the sound was compressed and flat.
Gary

ricard · 9th Sep 2016, 2:17 pm

Ok, thanks for the explanation, that makes sense then.

4th Sep 2016, 5:27 pm	#1
sp10mk11 Octode Join Date: Feb 2009 Location: Cornwall, UK. Posts: 1,275	Unusual Mono Tape head Hello I have a Silver Radio/Cassette from the early 1970s, the transport is an ITT type except with latching keys built on to it and an auto stop bar The head has 3 pins, there seems to be a central pin that measures 77 ohms to one pin and 112 ohms to the other, so am I right in assuming that one side is for play back and the other for record, this does have a separate erase head just incase you are thinking that it does not. the switching is done by the Record/Play switch, I would like to replace this head but I have nothing like it, plenty of stereo heads and mono heads with only two connections. Anyone any ideas or advice. Thanks Gary

4th Sep 2016, 7:17 pm	#2
sp10mk11 Octode Join Date: Feb 2009 Location: Cornwall, UK. Posts: 1,275	Re: Unusual Mono Tape head I have been looking at some ITT cassette recorder schemetics and the Model SL-75 seems to have the same type of head, in as far as it has 3 connections and similar circuitry ( I am presuming that Silver copied it). Gary Attached Thumbnails

5th Sep 2016, 2:47 pm	#3
ricard Octode Join Date: Mar 2006 Location: Lund, Sweden Posts: 1,632	Re: Unusual Mono Tape head I've seen this on Philips recorders as well. I could be completely wrong, but my guess is that you want as many turns as possible in playback to get as high a voltage as possible from the head, but in record mode you want fewer turns to so you that the bias voltage can be lower - especially important in battery-operated equipment.

5th Sep 2016, 3:21 pm	#4
Refugee Dekatron Join Date: Apr 2012 Location: Worksop, Nottinghamshire, UK. Posts: 5,554	Re: Unusual Mono Tape head I read this thread last night and it got me thinking. Without looking at what all the wires connect to I will have to just guess that the 77 ohm coil has the bias on it and the 112 ohm winding carries the audio in record mode. The 112 ohm winding obviously connects to the preamp in playback mode.

5th Sep 2016, 4:39 pm	#5
Guest Guest Posts: n/a	Re: Unusual Mono Tape head A self oscillating bias used with a PM (permanant magnet) erase?

5th Sep 2016, 5:21 pm	#6
sp10mk11 Octode Join Date: Feb 2009 Location: Cornwall, UK. Posts: 1,275	Re: Unusual Mono Tape head Hello Chaps thanks for your comments I was thinking a long the same lines, I might try and use a standard head and alter the switching on the record/replay switch, and fiddle about to get it to play and record well The whole system runs on 7.5 volts DC but i can get upto about 12.7 from the bias oscillator what that will drop to with another head do not know......yet The main problem is the Record/replay head is worn, no amount of cleaning demagnetising polishing will bring the top end up much beyond about 6K merlinmaxwell This tape deck has an AC powered erase head Again Thanks Gary

6th Sep 2016, 11:58 am	#8
sp10mk11 Octode Join Date: Feb 2009 Location: Cornwall, UK. Posts: 1,275	Re: Unusual Mono Tape head Hello Maarten Yes they are both very similar, the one shown is the ITT head circuit, could possibly find the same sort of circuit on other makes of the time on reflection. Gary

6th Sep 2016, 1:05 pm	#9
wd40addict Octode Join Date: May 2009 Location: Stevenage, Herts. UK. Posts: 1,518	Re: Unusual Mono Tape head My Grundig C410 had the same style of head, and back in the day it had to be changed due to wear... For playback the whole winding is used: one end is grounded, other end connects to the base of the PB amp, with the centre connected to a 15k resistor to gnd. On record bias goes to one end, record signal to the centre and the other end is grounded. Presumably the point of all this is to maximise head output on playback, whilst not needing a large voltage swing for recording. Effectively combining a high and low Z head in one unit.

7th Sep 2016, 3:07 pm	#10
sp10mk11 Octode Join Date: Feb 2009 Location: Cornwall, UK. Posts: 1,275	Re: Unusual Mono Tape head Problem Solved ! Hmm shall I blame one small AG13 battery that runs a TDK Head Demag unit or shall I blame my self for being a silly old sod that doesn't always pay attention to what he is doing and failing to see the demag was not working properly I wanted to try out a tape head that sort of similar resistance, so had it in my hand and put the demag on it and pushed the head forward, just a feint red glow came from the LED..........no don't tell me the battery after about 10 years is dying......................argh, so of course I thought of the original that had lost response and sensitivity, so out with trusty old Ross tape recorder head de-mag wand, gave the ITT head a good going over put it back in and hey ho a top end (of sorts) and recording sensitivity returned, bias back from 9.5 AC volts to 4.9 AC volts for TDK AD tape. I am getting doddery I know and these sort of mistakes occur in other jobs some times, I have about 2 hours a day when I am in what I call "up Time" anything more and I am slowly getting tired and weary, but I am a sod for pushing on regardless but occasionally it has cost me dear in terms of errors. So chaps thanks very much for your time and input on this one, pilot error I am afraid Gary

7th Sep 2016, 6:38 pm	#11
ricard Octode Join Date: Mar 2006 Location: Lund, Sweden Posts: 1,632	Re: Unusual Mono Tape head I think we've all made mistakes of that type from time to time, we take something for granted and feel ashamed when we realize that we'd completely overlooked something. Great that you've got it going. I'm a bit confused about the bias voltage, would the bias voltage acreoss the head really be dependent on any residual DC magnetization? Indeed, the application of AC bias would in itself tend to demagnetize the head to start with.

7th Sep 2016, 8:05 pm	#12
sp10mk11 Octode Join Date: Feb 2009 Location: Cornwall, UK. Posts: 1,275	Re: Unusual Mono Tape head Ricard sorry I didn't explain that well, when the head was not working properly I had to turn the bias up to 9v to get a reasonable signal on the tape, but at the same time this was over biasing the head so the sound was compressed and flat. Gary

9th Sep 2016, 2:17 pm	#13
ricard Octode Join Date: Mar 2006 Location: Lund, Sweden Posts: 1,632	Re: Unusual Mono Tape head Ok, thanks for the explanation, that makes sense then.