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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 Feb 2018, 3:13 pm   #1
mole42uk
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Default Ferrograph 7 head resistance?

Hi, I just did a search but haven't got an answer.

What should be the DC resistance of a Ferrograph Series 7 BE28Z replay head (¼ track stereo). That's the square heads with screws through from the top.

The one I have reads 70Ω for each channel which seems a bit low. The amplifiers are working but there's no output from the tape.
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Old 9th Feb 2018, 3:38 pm   #2
julie_m
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Default Re: Ferrograph 7 head resistance?

That sounds about right for a replay head on a solid-state machine. (Valves like voltage, transistors like current.)

Note that if you used an analogue meter to measure the resistance of a tape head, you will need to demagnetise it afterwards; otherwise, your tapes will get gradually quieter and quieter with every listen .....
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Old 9th Feb 2018, 3:59 pm   #3
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Default Re: Ferrograph 7 head resistance?

Thanks Julie. I keep one of the old Wearite demagnetisers quite close to the workbench!
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Old 9th Feb 2018, 7:23 pm   #4
BillDWVA
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Default Re: Ferrograph 7 head resistance?

Hi Richard, I’ve just measured a spare half-track stereo head, BP16Z, and it’s dc resistance is 43 ohms.
Are you sure the amplifiers are OK? What model of Series 7 have you got?

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Old 10th Feb 2018, 10:41 am   #5
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Default Re: Ferrograph 7 head resistance?

Thanks Bill, that's reassuring.
This machine is a 704 - ¼ track 3-speed without power amplifiers or speakers.

The replay amps respond normally to a finger on the input pin but one of them is microphonic (in other words, there's a sound from the output if I tap anywhere on the PCB) so there's a fault somewhere.

I tracked down another fault yesterday, I'll be checking today. The record head has the 100kc/s bias signal, but no audio. I think that the Record switch SW108 is not opening so the output go the Record amplifiers is grounded.
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Old 10th Feb 2018, 7:33 pm   #6
BillDWVA
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Default Re: Ferrograph 7 head resistance?

Hi Richard, I have the exact same model, quarter track Ferrograph Series 7, and had to chase a few problems with the first stage replay amp boards mostly low output and noise, with help from forum members here some years ago.

I have just checked its head line-up and it has numbers FE28 - Erase; FR28Z - Record and FP28Z - Playback handwritten onto the head can labels. The spare head I measured yesterday is from a Logic 7.

I have a Super Seven type 7052D on the bench at the moment and it’s playback amps are intermittently noisy but wiggling the Equalisation selector switch can make the problem go away or worse so that is where I’m going to look first; for dirty switch contacts.

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Old 10th Feb 2018, 8:05 pm   #7
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Default Re: Ferrograph 7 head resistance?

I suspect that the head label got displaced, but 28z seems right for this model. I’ve been frustrated today, the input signal gets through the various amplifiers but not onto the tape.

I never liked Series 7 Ferrographs. I think this one knows.
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Old 13th Feb 2018, 4:21 pm   #8
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Default Re: Ferrograph 7 head resistance?

Richard,

Am I correct in remembering that Ferrograph use phono plugs to connect the head assembly to the electronics in the Series 7?

I once had to deal with a machine that would not record onto the tape. It turned out to be a dry jointed phono plug connection to the record head. Now I do appreciate that you have measured a low resistance for your head - but where did you make the contacts with your multi-meter?
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Old 13th Feb 2018, 5:13 pm   #9
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Default Re: Ferrograph 7 head resistance?

That's a point! I measured the resistance at the back of the head but I remember thinking that the phono plugs might have seen better days.
The replay heads are, of course, small crimped connectors to pins on the board so they aren't a problem.
Thanks for the tip, I'll have a look tomorrow, the machine is hiding under the bench at the moment.
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Old 15th Feb 2018, 12:36 pm   #10
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Default Re: Ferrograph 7 head resistance?

I was full of enthusiasm about checking the phono plugs until I realised that I'd already proved that they were okay by measuring the 100kc/s erase and bias signals on the respective heads. :-(

Today I'll be testing with a better oscilloscope since I only had my little portable DSO Nano the other day.
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Old 15th Feb 2018, 12:49 pm   #11
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Default Re: Ferrograph 7 head resistance?

Bear in mind that when measuring with a scope across the record head the bias swamps the display and it can be very difficult to see the audio unless you stop / disconnect the bias oscillator.
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