| Duo |
Hi everyone.
I recently obtained a very nice Grundig reel to reel tube tape recorder. It plays okay but I really want to use it for recording.
There are a number of buttons on the front. I know how to set it between mono and stereo, and pick the track sets for mono. Also, I know then when I press a little knob labeled R/S a little indicator tube shows the signal input from my source which is variable via the volume knob on the faceplate.
There are three buttons on the bottom right hand side of the unit. One is labeled like a backwards 'Q', the other a microphone, and the other similar to an omega symbol. It seems that the microphone and omega buttons select different recording inputs but I don't know what the backwards Q one is.
The R/S button also turns clockwise and goes down. If I turn it clockwise with an input, a signal shows on the tube display but with different characteristics.
If I hold the R/S button down and then press play, the unit seems to lock into some sort of record mode, but I get nothing on the tape when I play back.
This problem has really got me stumped. I tested all the tubes and made sure that the power supply and wiring are functional.
The unit looks very similar to the TK line of Grundig players but it came out of a console somwhere in the fiftes or slightly earlier.
Please help me, I can't seem to figure this darn machine out!
I really want to record!!
-- Duo |
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| dhaen |
Duo,
The backward "Q" was a normally used to describe a pick-up or phono source.
The Omega is the microphone input.
If I think of more, I'll post it.
E&OE :)
Cheers, |
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| dhaen |
The Grundigs of this era used some Wima caps that looked like big Orange Drops (but not orange) which became hydroscopic.
If you disconnect each and measure, you may find they have resistance of a few meg ohms.
Some tape machines had the ability to superimpose onto an existing recording. This was done by switching off the erase head, reducing the bias and signal to the record head.
I wonder if the S/R is superimpose/record - just a guess.
Does the machine play back existing recordings?
Can it erase an existing recording?
Cheers, |
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| JOE DIRT® |
| is the omega symbol not for a headphone>???...I`m guessing |
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| dhaen |
The omega is definately mic.
The reversed Q is line-in.
A third symbol (which is like Q rotated 90 deg CCW) is phono pickup.
This input was for a crystal or ceramic cartridge. These cartridges were high output, sometimes line level, and if loaded by 1 meg, would need no EQ.
More useless information where that came from...:)
Cheers |
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| Duo |
Hmmm very interesting.
The recorder will erase a tape if I try getting it into the so called record mode.
The superimpose option seems logical to me by how it's acting when I go into that mode.
The machine will play existing recordings and it also seems to get the input I feed it. The problem is that the input isn't getting onto the tape. It will erase an existing tape just fine, in fact, quite well, so I'm sure the erase head is working. Just, it seems almost as if the amplifier isn't sending the record signal to the head.
Also, coulf these hydroscopic caps be causing the record problem?
I'll definately check their resistance, maybe even with my megger.
I guess a few megs might throw some bias around a bit, who knows. |
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| dhaen |
The fact that it erases is good news. It means thet the bias oscillator is working. The fault is therefore in the record-amplifier.
Costs were often cut by making some of the circuitry common to record and playback. This meant there was more than a little switching involved. Check the switches that activate when you change from playback to record. They are probably quite dirty, and could be corroded. It is unwise to try to individually clean or retension contacts. Best to try to blow away dust, and if you have some, apply a little switch-cleaner spray, then repeatedly operate the switches.
Good luck ;) |
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| bournville |
Is this recorder a TK60 by any chance? The TK60 was the first stereo recorder released by Grundig over here (the UK) in the early '60s.
If it is, or a similar model, it may have two power valves in the bias oscillator stage - Grundig sometimes used an EL95 for generating the erase voltage, and a seperate EL95 for providing the bias voltage. If there is a fault with the latter stage then the recorder will erase alright, but without the bias voltage recording will not be possible, you might get very faint distorted sound at best when you try to record. |
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| dhaen |
Bournville,
You remember these better than me, I think :)
I hadn't realised / remembered the dual EL95's in this model.
Duo said it came out of a console, so it probably hasn't got a normal model number.
Duo,
A picture might help, as might the valve lineup.
Cheers, |
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| Duo |
Okay, the tube compliment is:
2X ECC81
2X EF86
1X EL95
Here are some pictures. |
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| Duo |
| This is the indicator that seems to show up in record mode. |
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| Duo |
| These are the record option buttons. |
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| Duo |
| That look exactly like the on I have. I'll bet that's what it is, a TK 35! |
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| Duo |
| The only difference is the track selector buttons on the right hand side, in which place is a knob on the actual TK35. |
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| Duo |
| Does anybody know which tube is used for the recording amplifier??? |
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| bournville |
...The problem is, the TK35 (which electrically the same as the TK30 but with the three speed deck) is a mono recorder, with 1 x EF86, 1 x ECC81, 1 x EL95, 1 x EL84 and 1 x EM84.
Assuming the recorder uses fundamentally the same circuit, but with two channels for stereo, minus EL84 output stages because the recorder used an amp elsewhere in the console, both the EF86s and ECC81s are used on record. the EF86s would take low level inputs such as mikes, the gram setting would go via a high value resistor. The output of the second stage of the ECC81s would feed the stereo recording head. Grundig practice was to have the output from the second half of the ECC81 to go to the head via a resistor/capacitor network, which was shorted out on playback. If the recorder uses just one EL95 for both erase and bias, the bias frequency is much higher than when seperate EL95s are used (78khz as opposed to 40khz). |
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| dhaen |
I think this may be a mono deck.
Perhaps it has the capability of stereo playback to line-out.
That might explain the 2 EF86's, but 1 EL95.
The magic-eye indicator also looks like a mono one.
What do you think Bournville?
Cheers, |
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| fdegrove |
Hi,
In case you need the cicruit diagram, this Dutch gentleman will send it by e-mail for free:
RADIO SCHEMAS
Cheers,;) |
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| Duo |
This is definately a stereo recorder. It is four track and has two channels per side of tape. Some of the previousely recorded tapes I have are in stereo. You can either record a mono program on each track, or use a pair of tracks for stereo.
fdegrove: Please do get that schematic, I could really use it. |
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| Duo |
| Sorry, I misread your E-mail, I'll get the person to E-mail the schematic to me directly :) |
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| fdegrove |
Hi,
I already have sent one.
Cheers,;) |
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| Duo |
| Oh, okay, thankyou very much :D |
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| Duo |
| I'm looking at the caps and resistors on the ECC81's The output caps do look pretty worn out. I'll do a full cap job on the unit and see how well it works after that. |
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| bournville |
Fundamentally, the main differences in the amp on record and playback is that bass boost is switched in on playback and treble boost switched in on record. Because the switching mechanism is so complex on the Grundigs it is worth paying attention to this to look for failure, as dhaen as suggested.
| quote: | Originally posted by dhaen
I think this may be a mono deck.
The magic-eye indicator also looks like a mono one.
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On the TK60 model, a single EM84 is used, even though it is a stereo model. Seperate indicators for the levels on each channel were not widely used by Grundig until meters replaced magic eyes. On further investigation a version of the TK60 called the TM60 was released, which had the pre-amps and equilisation but not the power amp stages (1 x ECC83, 2 x EL84), and it was designed for use with suitable stereo amps. I suspect that the deck Duo has is this model, and hopefully the schematic will confirm it. |
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| Duo |
Yes, this recorder has no power amps. Just preamp and eq tubes.
BTW, what should be the signal amplitude at the record head input at a normal recording level?
I found a problem with the recording switch relay and fixed that. I cleaned and calibrated and tested all the relays in the unit so I know they are working fine.
I now get a signal at the recording head in record mode, but it's very distorted and I don't know how much amplitude it should have. |
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| dhaen |
Duo,
I can't remember the signal amplitude to the record head of this kind of deck. I would guess 20 volts or so would make a recording, but what you have to remember is that it will use AC bias. That means that you should see the audio, amplitude modulated on a superisonic carrier. If the bias is missing, you will make low very distorted recordings.
Cheers, |
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| fdegrove |
Hi,
| quote: | | That means that you should see the audio, amplitude modulated on a superisonic carrier. If the bias is missing, you will make low very distorted recordings. |
Well done, John.
Seems the oscillator isn't working properly, if at all.
Cheers,;) |
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| Duo |
I'll take a peek at it on a scope and see what I get. I'm having feelings that it's not oscillating either. And there certainly isn't 20v at the head during recording.
Which tubes are used for the bias oscillator? |
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| Duo |
Where is a good source in Canada to get capacitors for this recorder? I want to replace all the axial caps.
I also found that the ECC81 tubes are a bit gassy. I'll look into trying better replacements to see what happens.
I think the record bias oscillator may be working, it's just that the voltage at the head if very weak for audio.
I found that the distortion is coming from the first half of the ECC81's. The EF86 stages are fine, and the output stages of the ECC81's are fine. Can anyone suggest parts other than caps to check to fix that distortion problem? I tested all the resistors and they are okay. |
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| dhaen |
Duo,
Low level audio to the head should not cause distortion. That is what the bias is there for.
If the ECC81's have any getter left, they are useable, even if it's black.
When you foward me the schematic, I'll comment further.
Cheers, |
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| bournville |
| quote: | Originally posted by Duo
Which tubes are used for the bias oscillator? |
The EL95 is used for the oscillator.
While not ruling out leaky caps or shorted cathode electrolytics for the cause of distortion in the first stages of the ECC81s it seems a bit fortuitous that both channels would have developed the same fault at the same time....
What appears strange to me is that if the deck is a TM60 it should have two EL95s, one for the oscillator and one for bias. Your deck has one EL95 and the symptoms of a lack of bias...I am wondering whether a previous owner has removed one EL95 (for whatever reason) and attempted to use the other to provide bias as well as erase? |
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| dhaen |
We have already established that this is not a regular model, as it has no power amplifers. It may well have other variations such as a single oscillator.
Certainly the symptoms suggest no bias. On the other hand Duo clearly stated that it erases.
And, the fault is on both channels....we assume... Has Duo actually checked both?
Or, maybe it's switching problems after all. Indeed the bias is applied to the bottom side (on the schematic) of the record head. This point is grounded in playback. Now, if the switch was still closed in record....
BTW I've now got the TK35 schematics from Frank. |
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| bournville |
John,
The TM60 is the Grundig stereo deck without output amps, designed to run through a stereo amp and fit into a console, which Duo retrieved it from - that's why I assumed it is more likely to be a TM60 rather than be based on a mono TK35, but obviously this is pure guesswork.
However, the EF86/ECC81 stages are virtually identical for both recorders, duplicated for stereo of course, although the network for treble boost on record and bass boost on playback are different. If Duo's deck has just the one EL95, it is likely to be the same as the TK35. |
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| Duo |
My deck has exactly the same case and color scheme
as a TK 35, except the fact is that mine is a stereo model, and without output amplifiers. |
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| dhaen |
Duo,
Did you receive the schematics?
Cheers, |
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| bournville |
Duo,
As the pre-amps in the TK35 and TK/TM60 are very similar there's not a problem, but if you look at:
http://www.heinffm.de/0199d992c4124...2da12ad00e.html
...you can see that the basic deck used in the TK/TM60 is also the same as the TK35. Is your deck a two speed or three speed model? |
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| dhaen |
HI,
Because the schematics are too large to post, I've put it on my server. Here is the link:
TK35 revised
And now to try and embed the pix...
Hope this works!
edit: took off dupicate |
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| Duo |
Okay, I did get the schematic.
My recorder is a two speed model. |
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| Duo |
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. |
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| Duo |
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. |
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| Duo |
I found a capacitor for varying the record bias amplitude.
Anybody know what I should set the bias to? |
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| Duo |
| Is it possible that if the tape is 40 years old it won't take a recording? |
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| Duo |
| One more question... How can I tell if the head is worn down too much to record. |
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| bournville |
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. |
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| Duo |
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. |
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| 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 :D |
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| dhaen |
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, |
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| bournville |
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! |
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| EC8010 |
Is the work being expended on this machine a learning exercise, or do you have a reel need for a 1/4" tape machine?
(Sorry, couldn't resist the reel need.)
I ask because cracking transports can be picked up very cheaply... |
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| Duo |
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. |
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| EC8010 |
| 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. |
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| Duo |
| That's interesting. How hard would it be to get a Studer in Canada? |
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| EC8010 |
| Try asking the engineers at your local TV or radio station. You see, they're all going over to server-based solutions. Over here, they can barely give them away! |
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| bournville |
| quote: | Originally posted by Duo
I checked the bias frequency, it's 62.7Khz, should that work? Or should I push it up some? |
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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