Ferrograph 631 tape head

[djrm]

Greetings everyone.
Last year I bought an old Ferrograph 631 mono tape recorder. It plays back some old tapes I have already, however it does not record.
The amplifier is ok, the recording level meter shows a good signal and the AC signal and bias output voltages measures fine.
The manual does not describe the operation of the tape head, it looks unusual as it has two windings although it is only a mono head.
I'm wondering how to diagnose if the signals are getting into the head (and tape) properly.
The head on these is quite low impedance I think, can I measure the current with a series resitor perhaps.
I have no idea what the current should be though.
I dont know what could be wrong, can a head fail so it doesnt record any yet still plays back?
Perhaps it is a simple switching problem but I havnt found a dodgy contact yet.
Any ideas about how to find the problem gratefully received.
Kind regards, David.

   

[Mike Watterson]

If the same head is used for playback and record, and it plays, it's not the head.
Use a scope to trace the signal from input towards the head. Note that the head in record may have 35 KHz to 85 KHz bias, I don't know what frequency is used. If the bias was missing on record it would just be distorted. If the oscillator was completely dead you'd have no erase and STILL distorted recording.

Or trace from head back towards the input.

How is the head wired? Is it really a single head or two heads side by side (to allow monitoring during record)?

Also if the "DC" resistance of the windings is OK, the head is likely doing something, heads can be worn or dirty, but that's obvious on playback with a single head system.

I have some old 1/4" tape heads, but that doesn't sound like the problem unless recording uses a separate head or separate poles in the same body.

[pwdrive]

According to a schematic I'm looking at the Rec/PB head winding has a tap for the bias feed, the whole of the winding is used on both Rec and PB, I would first check the switch contacts (SW3)

If the bias is getting to the head I would then normally kill the bias so it's easier to trace and evaluate the record signal level.

Lawrence.

[djrm]

Thanks for the advice. There is bias on the head, I do not know the signal level which should be present on the head though.
If the bias is simply fed into a tap in the head windings it should be straight forward. I was wondering if it was a completely seperate winding with some fancy goings-on.
n.b. This is a two head machine, with seperate erase head and combined record and playback head.
The erase is working. the head resistance is in the order of 10s of ohms iirc, quite low but I think this is correct.
Kind regards, David.

[Mike Watterson]

So it's not the head as it plays back.
Somewhere between audio feeding meter and head. Maybe a switch, bad joint, faulty part only used on record, impossible to say. Just trace signal and see where it gets lost. Mostly likely when it's a switched single head for rec/play that it's a bad connection or switch contact. Check switches in Rec position and machine off with a meter?

[djrm]

What I didn't tell you was that it didnt play back very well. it was hard to tell because the tapes I have are four track stereo and all I hear is a garbled backwards sound.

Another reason it didnt play back too well was because of the layer of rust and dirt on the head, I had cleaned it with IPA when I got the recorder but it needed a soft wire brush on the dremill to make it shiny. 

I've spent hours tracing and c hecking the circuit through but it records now - just needed cleaning, doh. Thanks for the useful suggestions anyway.

bias coil resistance 4.0R, 5mH
signal coil resistance 47R, 110mH
Aparentley you need to degaus after using a meter to make such measurements.
I have a tape head demagnetizer from years back fortunatley.
Perhaps a bit of a polish with some rouge next.

Kind regards, David.

[djrm]

After a long walk this morning, today I have completed the repairs on the tape recorder, the one outstanding problem was traced to a poor contact on the record/playback switch. Aparently the input to the first stage amplifier from the tape head shoud be shorted to ground in the record position, it isnt usedas the mic input goes to the second stage when the mic is inserted into the jack socket. What was happening was that if no input jack was connected then the amplifier would oscilate at high frequency and motoboat too. I had changed and padded out most of the capacitors before finding the real problem being that the input wasnt being shorted, an application of electrolube and a tweak to the switch contacts has cured it so I removed the padding capacitors and put the screws back.

Then it was time to look through my collection of old tapes, most are from my old Sony stereo '80s recorder which I dont have any more, some play ok if there is nothing on the other side of the tape. One in particular of some jazz music recorded off the radio in about 1990 plays very well. I have the output connected to the KT66 valve amplifier and lowther horn LS. Most interesting was a small tape I found when clearing out the house of my Father in law after he passed. It was recoded in a local church and featured him singing in a choir back in 1960. I played it to Jenny, my wife who also sings and she joined in with the excepts from Handles mesiah. It was probably recorded before she was born - how vintage is that.

Stay safe, David.