Defiant T12 Recording problem

[AndiiT]

Hi, I am in the process of restoring a Defiant T12 reel to reel machine and have a question about an issue I have when recording.
When the machine is in the record mode any signal presented to the input socket(s) can still be heard faintly from the speaker; I can't understand how this should be happening as the pentode section of the ECL86 is connected to act as the erase/bias oscillator when in record mode.

Looking at the circuitry the output transformer is in series with the oscillator coil and I wondered if the pick up could be leakage via the bias coupling capacitor.
Does anyone else have one of these machines and would be able to confirm if I should be able to hear any signal from the speaker whilst in record mode, or should I be looking for a fault?


Regards and thanks

Andrew

[lambretta]

Hi Andrew I have recently acquired one of these machines if you read my previous post [fidelity t1/2 tape deck]you will see I have a very similar problem. you seem to have a greater knowledge of electronics than me so I might be picking your brains. I have been trying to post some photos on the site but cant get either my phone or our tablet to send them to this PC to forward them on
regards Alan

[Welsh Anorak]

Hi Andrew
I suspect that your set is similar to a 'furniture store special' I'm doing at the moment. And, yes, I get a faint sound when recording. As you say, it shouldn't be possible and I suspect you are right, or else there might just be some form of capacitive coupling. Whatere it is - hi-fi it aint!
Glyn

[AndiiT]

Hi,
Thanks for your responses. As I already mentioned, the only way that I can see how the speaker would operate in record mode is by capacitive coupling from the bias feed capacitor and the way the circuit is designed with the output transformer in series with the oscillator coil.

I did consider performing an "unofficial modification" which involved connecting the output transformer in such a fashion that it was not in series with the osc. coil but the primary side of the coil is tapped in such a way that I would have to modify that part of the circuit too and may cause more problems than it's worth in doing so.

Agreed these machines aren't exactly Hi-Fi but it's nice to know that there are at least two others exhibiting the same behaviour

Andrew

[quiberon]

Does the deck have separate record level and playback level controls?

If so it is possible that could be to enable you to monitor the signal going onto the tape.

[lambretta]

HI its me again. When I had one of these machines in the 60s the record level indicator [magic eye] only came on in the record mode . On this machine it comes on as soon as you turn on the power so is this another indication of a fault ? On this machine the volume control becomes the recording level control in the recording mode

[AndiiT]

Originally Posted by lambretta

……..[magic eye] only came on in the record mode….

The T12 circuitry is arranged so that the Magic eye operates in record or playback mode; the only machines that I have ever encountered where the magic eye is only lit in the record mode are the Fidelity Playmaster and Braemar (basically a Playmaster in posher/updated livery) models.


Regards

Andrew

[julie_m]

With a 2-head recorder, you can't do "proper" monitoring of the recorded signal anyway, so the pentode section of the ECL86 (there's usually an ECC83 and an ECL86, an EM-something magic eye and a full-wave metal rectifier) does double duty as the erase and bias oscillator.

A bit of signal leakage into the speaker probably wasn't seen as problematic, especially if it would not be loud enough to cause a howlround.

[lambretta]

Hi Andrew/ Julie the machine I had in the 60s was a braemar and there was no audio leakage when in record . On this one the leakage is high enough to give serious feed back more like a PA than a tape recorder. I think my machine has developed several faulty components unfortunately I don't have the ability to detect which ones but I am ok with a soldering iron and would be able to change them if I knew which ones they were. In fact I would love the challenge of getting this machine up and running again ALAN

[AndiiT]

Hi,
There certainly wouldn't be any audio on the Braemar in record mode as I have owned both Fidelity Playmaster and Braemar (basically a Playmaster in updated livery) models in the past and the design is such that it would not be possible.

Back to to the T12….giving this some thought the circuitry may have been arranged deliberately so that there was a slight leakage of the recorded signal to the speaker in the record mode due to the fact that the machine has a superimpose facility so perhaps the circuit designer thought that being able to monitor the input signal would be a useful feature.

As has already been mentioned these machines are hardly "Hi-fi" and perhaps the circuit designer thought that a small amount of leakage was acceptable, although from what Alan (lambaretta) is saying about his machine the feedback sounds excessive, which is not the case with my own T12.

I have managed to enclose a shot of the associated circuitry where R17 is the feed to the record head and C13 is the bias coupling capacitor in case anyone who has already contributed wants to take a look and see if they agree with my thoughts regarding the "fault" (or not as the case may be)

Regards

Andrew