I just checked the heads with a scope. There's a very very clean and strong bias oscillation in record mode in both channels.
I find that the audio getting into the head is just extremely weak and distorted so I'm going to try finding the fault and will post what it is when I find it.
The idea of a superimpose switch seems to fit quite well. When I go into this mode, the record head is biased and the blanking head is turned off.
Will post more later.
I find that the audio getting into the head is just extremely weak and distorted so I'm going to try finding the fault and will post what it is when I find it.
The idea of a superimpose switch seems to fit quite well. When I go into this mode, the record head is biased and the blanking head is turned off.
Will post more later.
Well, I checked the tube stages with the scope and found that there's no bias at all in any of the stages. The cathode and grid voltages are all zero. Something's fishy here.
I'll proceed to check if there's any plate current and see why the tubes are not dropping a voltage on the cathode resistors.
I'll proceed to check if there's any plate current and see why the tubes are not dropping a voltage on the cathode resistors.
Bias should be set to 78khz for the TK35 and the bias voltage should be set to 220mV across a 200 ohm resistor. I would have thought that an old tape, depending on how it has been stored of course, should still be able to record something, even if the 'top' response is perhaps muffled because of deterioration in the oxide (I have tapes from the '50s which still work OK). Make sure that the heads are clean, but I would have thought you would have got something, regardless how well used the heads are - besides, the erase heads will also have had a lot of use and they still work.
One trick I 've seen is to connect the erase heads for recording to see if a signal is getting through, but I don't think you are at that stage yet.
Because the TK35 is a three speed model the circuit has extra equilisation that will not be present in your machine.
One trick I 've seen is to connect the erase heads for recording to see if a signal is getting through, but I don't think you are at that stage yet.
Because the TK35 is a three speed model the circuit has extra equilisation that will not be present in your machine.
Yes, it is interesting that it will still erase.
I'll set the bias as you say and see if that changes anything.
Do you mean to hook the record lines to the erase head to see if it puts any audio on the tape?? Seems like a good idea.
I have put a scope on the record head during recording and I see a bias signal modulated with audio. The signal is a clean sine wave and the audio on it is also quite clean and undistorted.
I'll first calibrate the bias and if that doesn't work, I'll try recording with the erase heads.
Also, I know that demagnetizing the heads can help. I tried that with my electromagnet at 60Hz and at about 120Hz but it didn't make a difference for record, however, the playing is much better. Is there a good frequency to degauss the head at, or does it not matter?
Thanks for all the help so far guys.
-- Duo.
I'll set the bias as you say and see if that changes anything.
Do you mean to hook the record lines to the erase head to see if it puts any audio on the tape?? Seems like a good idea.
I have put a scope on the record head during recording and I see a bias signal modulated with audio. The signal is a clean sine wave and the audio on it is also quite clean and undistorted.
I'll first calibrate the bias and if that doesn't work, I'll try recording with the erase heads.
Also, I know that demagnetizing the heads can help. I tried that with my electromagnet at 60Hz and at about 120Hz but it didn't make a difference for record, however, the playing is much better. Is there a good frequency to degauss the head at, or does it not matter?
Thanks for all the help so far guys.
-- Duo.
One other thing that's bothering me is the fact that the original owner had recorded lots on it and it worked fine. Nothing was touched since then and now I have it but it won't work.
I cleaned every single moving contact, every pot, every switch, rotor, tube pins, all of it. I polished the heads, re-aligned them, calibrated the audio stages, tested the tubes, put in new capacitors, checked the resistors, de-magnetized the heads, oiled the motor and every bearing in the unit, tightened all the screws, calibrated the relays and tested them, tested every gawdarn part in the whole darn machine.
There's not much left that could be wrong. The only things that could be causing it now are misadjusted bias, or a totally dead head.
If it's the head, which it probably is, does anybody here have heads for one of these machines? I really want to get it working so I'll definitely pay for a new head if I can find one.
hmmm.... This thing has got to be one of the most complicated repairs I've ever done in my life. lol
I cleaned every single moving contact, every pot, every switch, rotor, tube pins, all of it. I polished the heads, re-aligned them, calibrated the audio stages, tested the tubes, put in new capacitors, checked the resistors, de-magnetized the heads, oiled the motor and every bearing in the unit, tightened all the screws, calibrated the relays and tested them, tested every gawdarn part in the whole darn machine.
There's not much left that could be wrong. The only things that could be causing it now are misadjusted bias, or a totally dead head.
If it's the head, which it probably is, does anybody here have heads for one of these machines? I really want to get it working so I'll definitely pay for a new head if I can find one.
hmmm.... This thing has got to be one of the most complicated repairs I've ever done in my life. lol
Duo,
Magnetised heads are unlikely to cause the problem you are experiencing. If a head can playback it can record. The reason is that on playback the head-gap sets the smallest wavelength that can be replayed, but not the shortest wavelength recorded, since it is the trailing edge of the gap that leaves the magnetic print.
Tape machines can be complex to fault-find. Think yourself lucky you don't have to fix old broadcast open reel 1" and 2" machines like me. But at least I get paid
Keep at it....
Cheers,
Magnetised heads are unlikely to cause the problem you are experiencing. If a head can playback it can record. The reason is that on playback the head-gap sets the smallest wavelength that can be replayed, but not the shortest wavelength recorded, since it is the trailing edge of the gap that leaves the magnetic print.
Tape machines can be complex to fault-find. Think yourself lucky you don't have to fix old broadcast open reel 1" and 2" machines like me. But at least I get paid
Keep at it....
Cheers,
Duo,
An easy way to check the the heads for sure is to apply a signal from another source to them while the set is in record - take a signal from the anode of a final stage before the output tube in an amp via a cap and connect it to one side of the head, the other to chassis earth. Leave the existing connections as well, so the recorder can supply the bias.
The recording won't be brilliant because of the lack of equilisation but it will confirm if the heads work. If you simply get faint distortion on the tape then the heads are not receiving the bias at the amplitude or magnitude they require.
I remember as a lad having an old Philips which had the same problem - no record although everything I checked seemed to work OK and bias was good. Eventually I connected an extra lead from the pre-amp output via a seperate switch to the head to make sure I could get a signal on record!
An easy way to check the the heads for sure is to apply a signal from another source to them while the set is in record - take a signal from the anode of a final stage before the output tube in an amp via a cap and connect it to one side of the head, the other to chassis earth. Leave the existing connections as well, so the recorder can supply the bias.
The recording won't be brilliant because of the lack of equilisation but it will confirm if the heads work. If you simply get faint distortion on the tape then the heads are not receiving the bias at the amplitude or magnitude they require.
I remember as a lad having an old Philips which had the same problem - no record although everything I checked seemed to work OK and bias was good. Eventually I connected an extra lead from the pre-amp output via a seperate switch to the head to make sure I could get a signal on record!
Well, this player is as a learning experience, an addition to my collection of junk, and I also want to use it as well.
I checked the bias frequency, it's 62.7Khz, should that work? Or should I push it up some?
I tried putting audio from a different source into the head and I get nothing, therefore, I think that the bias is bad.
I checked the bias frequency, it's 62.7Khz, should that work? Or should I push it up some?
I tried putting audio from a different source into the head and I get nothing, therefore, I think that the bias is bad.
How about a reel machine?
In general, the bias is about 10x the audio voltage. Older machines have poorer heads (wider gaps and poorer magnetic materials). If you have room for it, a Studer A80 can now be picked up (alright, carefully lifted by two strong blokes) for <£600. the Studer is a (semiconductor) mastering quality machine well worthy of your attentions. Ideally, you would like the flats on the heads to be <3mm, but even 4mm is equalisable to 15kHz.
In general, the bias is about 10x the audio voltage. Older machines have poorer heads (wider gaps and poorer magnetic materials). If you have room for it, a Studer A80 can now be picked up (alright, carefully lifted by two strong blokes) for <£600. the Studer is a (semiconductor) mastering quality machine well worthy of your attentions. Ideally, you would like the flats on the heads to be <3mm, but even 4mm is equalisable to 15kHz.
Duo said:
If the bias is working you should be getting something regardless of the exact frequency - depending on the model the early Grundigs used anything between 40khz - 78khz. You might have to consider running another 'bias line' from the oscillator to the head, controlled by a seperate switch. As mentioned earlier, the bias voltage should be around 220mV for a TK35, the audio drive being considerably smaller.
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