Dansette Consort 3 Tape Recorder

[crackle]

Hi
I bought a Dansette Consort 3 tape recorder over the weekend. I don’t normally get too involved in tape machines but this was too much of a bargain to miss.
I have serviced and freed up the seized mechanical parts of the BSR tape deck, and this is now working fine, even with its original belts, and all controls move smoothly. FF and RW were the main trouble and this turned out to be seized pivot bearing for the spool mechanism. They needed a fair bit of heat to unseize them to remove for cleaning.

There is a problem with the record function.
I have tested the machine with a pre-recorded tape and the output seemed to be good.
However when I recorded from a radio via the line input socket the recording was quiet and distorted. (the distortion could have been caused by too much signal)
My knowledge of the workings and the principles that tape recorders electric’s use is a bit limited.
Please can any one help and explain how the erase mechanism works, is this a high frequency oscillator or is DC used to magnetise the erase head.
Is the audio signal “modulated” with another signal before being fed to the record/play head.
Any suggestions as to why the recording function is very quiet and distorted would be most helpful.
Thanks
Mike

[julie_m]

Are you sure you had the gain turned up enough?

On most reel-to-reel recorders, the volume control is used to set the recording level, and the level meter is actually a "magic eye" indicator tube. The level is right when the green areas just meet on the loudest sounds.

Also, if you haven't done so yet then you need to replace the coupling capacitor to the grid of the pentode section of the output valve. If this part is faulty, it can trigger a nasty cascade failure.

[llama]

The erase and recording bias in most machines of that era are based on the power valve running as an oscillator in the region of 50kHz. That valve (EL84?) drives the speaker in playback but forms the oscillator during record. Being relatively high power, it drives the erase head with 50kHz, thoroughly wiping the track on the tape. A small sample of the 50kHz is _added_ to the record signal (not modulating it). The level of this "small sample" would be set such that the high frequencies are just not self-erased.
Graham

[crackle]

Hi
Refering to the radio servicing sheet for the consort 4.
Am I right in assuming in record mode the EL84 is supplying the signal for the erase head.
And that V1B supplies the final signal for the record head.
I thought there was some sort of bias applied to the record head in tape recorders but I cant see how or where this comes from.
There is a trim pot on the circuit board, I wonder what this is for.

AJS, I have not changed the "capacitor" as yet as they are all polyprop or mustard types and I thought these are mostly OK. I will check to see if there is any positive voltage on the grid of the EL84
Thanks in anticipation of a little help.
Mike

[peteratnith]

Hi Mike It does sound as if the bias/erase oscillator isnt working. As has been said, the output stage is usually switched to be an oscillator on record. Its worth checking the grid coupling cap on playback, i.e. check that theres no voltage on G1. If thats OK, switch to record and look at G1 again, this time there should be a negative voltage there if its oscillating OK. You can read the bias/erase voltage at the erase head and at the rec/play head. use the AC range on your meter. I cant remember exact voltages (long time ago) but its quite high, about 50v AC.The pot on the board is probably the bias level pre set. Just a thought, does it erase?. If it does, the oscillator must be working and if the pot you refer to is dodgy no bias will reach the record head, giving weak distorted playback of the recording you just made. Keep us informed

Peter

[crackle]

Hi Graham
Thanks your explanation is most useful, I must have been typing my next post when you posted your reply.
Peter
Now I come to think of it I believe there was some of the original recording "under" the new distorted recording, so it makes some sense to look at the oscillation of the EL84.
As the EL84 is working on playback it could be something to do with the pre-set pot.
On the circuit for the Consort 4, which is the nearest I can find to the consort 3, which is also 4 track I cant see a preset anywhere on the circuit.
Please can you give me a clue where in the circuit the bias adjust pot might normally go.
I will check out a few things tomorrow night and report back. It may just be the bias adjust pot has a dirty track (it is a skeleton type) and it needs a bit of back and forth cleaning and a small squirt.
Thanks for your help.
Mike

[crackle]

Hi
Is this where the bias resistor is in the circuit?
Thanks
Mike

[peteratnith]

Hi Mike, Should have explained "bias" earlier.
There are 2 types of bias in use in a recorder.
1) the DC bias used to control the DC current in the output stage (EL84). This is the resistor in the EL84 cathode cct. It can be upset by leakage of the coupling cap feeding G1 (pin2)

2) AC bias used on record, generated by the EL84 acting as an oscillator. Failure of this gives weak distorted recordings. This AC, about 50Khz is used for 2 things. It feeds the erase head to erase earlier recordings, and is also mixed with the audio to be recorded. Both these AC signals can be measured easily at the erase and rec/pb heads using a multimeter on its AC range (try 100v). On the BSR deck, remove the whole top cover to see the back of the heads, I dont think the small triangular cover comes off itself. You have found the preset which is probably the AC bias pot, try measuring AC there, on record, you should see some AC, on all legs of the pot. You dont need a tape on the machine during your tests but dont run it on record for long periods, say 5 minutes, because if the oscillator is running, the erase head will run hot due to not having the cooling effect of tape running over it. Not a huge issue but now you know why the erase head can get hot! . Keep us posted

Peter

[crackle]

Hi Peter
Thanks for your reply Peter. Am I right in thinking of T1 forming the 50khz tuned circuit with C10 and the EL84, when in record mode.
I have found it very difficult to follow the audio path from V1B to the record head. There are track switches which have not been shown clearly in the diagram which I have.
Please can you give me a clue, is the audio fed from V1B to the record head and mixes with the AC bias from V2 through C13 and R12.
Thanks Mike

[peteratnith]

Hi Mike Yes the 50Khz tuned cct. is the secondary of T1 and C10.
The 50 Khz is fed out via C19 and probably feeds the erase head directly, or via switching if its a 4 track machine. C19 also feeds 50Khz to the r/p head, again via switching, via the pot you found.
The record audio will come from the previous valves anode, via a coupling cap. and a resistor of about 100K to 200K ohms, to the record head. The resistor is to give constant record current through the inductive record head. It also prevents too much 50 Khz leaking back into the record amp. Do you have the complete diagram?

Regards Peter

[crackle]

Hi Peter
I have the circuit for the Consort 4 but not the consort 3. I dont have any other information either such as voltages printed board layout.
It would be useful if I had a bit more info. Do you know if the information on the consort 4 on VRSD has more information. If this had the same circuit board I would pay and down load it.
Thanks
Mike

[peteratnith]

Hi Mike, I cant find much on your actual machine but if you google Fidelity tape recorder, you will find a site selling manuals including the Fidelity playmaster. Right click the circuit and save it as a jpg. Its typical of many machines of this era, the oscillator/output stage is very similar to yours. it uses 1 ECC83 1 EL84 and a magic eye. I looked at it to find the value of the record series resistor for recording, they use 270K where I said 100 to 200K Quite close!

Peter

[crackle]

Hi Peter
I have tested and there is no voltage on the erase head. I have changed a mustard type capacitor in the oscillator circuit and what looks like a big fat tantalem in the cathode circuit of V1B, both these looked like they had a crack in them, but they measured in excess of 20M ohm.,
Still no joy, I have also changed the valves to no effect.
I am going to have to put this to one side for now as I have to continue changing the caps on my sons Ashdown amp. (in the hope of fixing the intermittant fault that happens when the amp gets hot)
Thanks for your help it has been most useful in helping me to understand a little more about tape recorders.
I will be on the look out for a circuit and test information for this one and if I come across it I will have another go at fixing the record oscillator.
Thanks
Mike

[AndiiT]

Hi,
In had a similar fault with an Elizabethan LZ29 some years ago.

There should be an electrolytic capacitor of around 32uF connected between the Screen grid of the EL84 and chassis/ground, this could be masquerading as part of the HT smoothing and be contained in a "multi" can along with a couple of other caps.

It's purpose is to decouple the AC signal from the screen grid when the valve is acting as an amplifier.
Before delving in to the Bias/Erase oscillator circuitry in any depth it may be worth replacing the aforementioned electrolytic.

Regards

Andrew

[crackle]

Hi Andrew
are you referring to the capacitor connected to pin 9 (G2) of the EL84
Thanks for your sugestion
Mike


Edit
I have just isolated C9 which is connected to pin 9 and replaced with a new 32mf cap and there is no change still no oscilator
Thanks
Mike

[peteratnith]

Hi Mike, It would be good if we could establish whether the EL84 is oscillating or not. Anode voltage should be about 200 or so DC but there should be at least 200 volts AC also. What kind of meter are you using?.
Grid 1, pin 2 should have a negative DC voltage if its oscillating. If its definately not oscetc. try measuring the resistance of osc. coil secondary, its value should be in the service manual. If it reads 0ohms, suspect the parallel cap. Try disconnecting C19. If its shorted it will stop the osc. C11 cant be short as that would short the EL84 anode to earth via the osc. coil. Try a small cap. around 500pf across it to prove it. Look at the switch contact that connects G1 (pin 2) to the osc. coil. OC here would definately stop the osc. It could be shorted turns on the Osc. coil but I cant remember ever seing that in many years.

Dont Give up!

Peter

[crackle]

hi Peter
I will go through your sugestions. I dont like giving up but I have another project on the go as well.
will let you know
Thanks
Mike

[crackle]

Hello Peter
I have had some success with the oscillator circuit.
I have checked every component around the EL84 and drawn a new circuit.
The variable resistor R16 20k had a small indent where the wiper has been sitting for all these years.
After cleaning the track on it I set the wiper position back to where the indent was, checked continuity and put it back in the circuit. I have been thinking all along that the problem must be a faulty component elsewhere, it may still be a faulty component but I cant find it.
I got out the oscilloscope and connected this up to the erase head and found that if I adjusted the trim pot to the non earthy end then I got a high frequency oscillation. It is only about 20 v peak to peak but it seems sufficient to now operate the erase head and the quality of the recording is not distorted, (well only if you keep the levels down and don’t over do the record level.)

Please can you take a look at the revised circuit I have drawn and give any comments.
Do you think the oscillator will work better if I replaced R17 and returned the value to 56K
I have marked all the values which differ from the Consort4 circuit with a red circle.

I have kept all the values as originally found when I got the recorder, and I should add that the circuit did not look like it had been tampered with (prior to me getting it.)

Thanks
Mike

[crackle]

Just a quick update. I put a 56k resistor in place for R17 and this improved the point at which oscillation occurred more with the trim pot in the centre. However the P2P voltage was not improved so I left the pot at the non earthy end for max voltage.