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Vintage Tape (Audio), Cassette, Wire and Magnetic Disc Recorders and Players Open-reel tape recorders, cassette recorders, 8-track players etc.

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Old 10th Jun 2020, 5:25 pm   #41
DMcMahon
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Default Re: Grundig TK 27L de luxe Mystery Cable

Thank you Lawrence & Boulevardier, I have heard of this before but had not latched on to the fact that this Bias Choke was the dummy head.
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Old 10th Jun 2020, 5:35 pm   #42
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Default Re: Grundig TK 27L de luxe Mystery Cable

Actually, I made a gobslip there - so to be self-pedantic, it's the bias (not the erase) signal that would get back into the recording output stage (thus the name "bias trap"). Still enough to badly upset conditions.

Mike
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Old 10th Jun 2020, 5:50 pm   #43
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Default Re: Grundig TK 27L de luxe Mystery Cable

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Originally Posted by DMcMahon View Post
From the schematic and resistance /voltage tests the low side (gets grounded in record) of the Record/Playback heads is the red wire with live side being white, so I set the Record bias to the white spec of 184 V peak to peak.
I might be wrong but it might be the head that's colour coded.

Lawrence.
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Old 10th Jun 2020, 6:30 pm   #44
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Default Re: Grundig TK 27L de luxe Mystery Cable

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Tried to measure the RMS voltage using my FET VOM but found it no longer works on the higher AC voltage ranges, that is a nuisance, another thing to try and repair.

Time period on scope was 18uS so around 55kHz.
Sorry to be a pest but how did you isolate the probes input capacitance from the bias measurement point?

Lawrence.
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Old 10th Jun 2020, 7:56 pm   #45
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Default Re: Grundig TK 27L de luxe Mystery Cable

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I might be wrong but it might be the head that's colour coded.

Lawrence.
You are wrong!

However, there is more confusion. I thought one head wire was always white, but a passage from the (later) TK145 service sheet states different combinations of wire colours (red-red, red-white, white-white etc.) This may have changed in the later machines. I've just had a look at my TK46 (3-head) and the record head is green-white, while playback is yellow-white. So I think you may want to set the bias according to the "red" specification.
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Old 10th Jun 2020, 8:57 pm   #46
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Default Re: Grundig TK 27L de luxe Mystery Cable

Sorry, my confusion, hadn't seen a photo until now.

Lawrence.
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Old 10th Jun 2020, 9:05 pm   #47
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Default Re: Grundig TK 27L de luxe Mystery Cable

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Quote:
Originally Posted by DMcMahon View Post
From the schematic and resistance /voltage tests the low side (gets grounded in record) of the Record/Playback heads is the red wire with live side being white, so I set the Record bias to the white spec of 184 V peak to peak.
I might be wrong but it might be the head that's colour coded.

Lawrence.
That is what I thought originally but there is no obvious colour on the head.
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Old 10th Jun 2020, 9:09 pm   #48
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Default Re: Grundig TK 27L de luxe Mystery Cable

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Quote:
Originally Posted by DMcMahon View Post
Tried to measure the RMS voltage using my FET VOM but found it no longer works on the higher AC voltage ranges, that is a nuisance, another thing to try and repair.

Time period on scope was 18uS so around 55kHz.
Sorry to be a pest but how did you isolate the probes input capacitance from the bias measurement point?

Lawrence.
Not a pest at all Lawrence, I didn't, it worked fine on the erase 9V measurement and I am sure I have used it with standard leads on record bias measurements before.

I have been meaning to get a VTVM probe for a long time.

David
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Old 10th Jun 2020, 9:10 pm   #49
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Default Re: Grundig TK 27L de luxe Mystery Cable

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Originally Posted by ms660 View Post
Quote:
Originally Posted by DMcMahon View Post
From the schematic and resistance /voltage tests the low side (gets grounded in record) of the Record/Playback heads is the red wire with live side being white, so I set the Record bias to the white spec of 184 V peak to peak.
I might be wrong but it might be the head that's colour coded.

Lawrence.
That is what I thought originally but there is no obvious colour on the head.
No problem, confusion sorted now I've seen a photo of a head on the web, originally I was thinking connection pins on the head not lead outs.

Lawrence.
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Old 10th Jun 2020, 9:12 pm   #50
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Default Re: Grundig TK 27L de luxe Mystery Cable

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Quote:
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I might be wrong but it might be the head that's colour coded.

Lawrence.
You are wrong!

However, there is more confusion. I thought one head wire was always white, but a passage from the (later) TK145 service sheet states different combinations of wire colours (red-red, red-white, white-white etc.) This may have changed in the later machines. I've just had a look at my TK46 (3-head) and the record head is green-white, while playback is yellow-white. So I think you may want to set the bias according to the "red" specification.
I will leave it at white until I do some test recordings then can reset to red and compare results.
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Old 10th Jun 2020, 9:22 pm   #51
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Default Re: Grundig TK 27L de luxe Mystery Cable

Just for info (page 52):

http://www.historische-messtechnik.d...141122-017.pdf

Lawrence.
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Old 10th Jun 2020, 9:36 pm   #52
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Default Re: Grundig TK 27L de luxe Mystery Cable

Cheers Lawrence, you did well to find the VST 24 probe, loads of interesting information in there to read.

David
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Old 11th Jun 2020, 10:41 pm   #53
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Default Re: Grundig TK 27L de luxe Mystery Cable

Have done various mono line recordings using the Radio/Diode and Phono/Platte DIN inputs and get very good playback results through the internal mono amp with plenty of volume, I used unattenuated DIN cable, the Radio input is the more sensitive.

Next to do some stereo recordings and playback into Hi-Fi amp. Have not worked out yet, if stereo recording played back though the mono internal amp is there any way to combine the stereo recordings into the single channel.
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Old 11th Jun 2020, 11:05 pm   #54
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Default Re: Grundig TK 27L de luxe Mystery Cable

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is there any way to combine the stereo recordings into the single channel.
Yes, select position 'D' on the track selection switch. This combines the output of both preamps and routes it to the amplifier. As you have already noted, the record button is mechanically locked out when the track selector is in this position.

By the way, forgot to say thanks for posting the translation of the bias set-up procedure. Most useful.
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Old 11th Jun 2020, 11:11 pm   #55
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Default Re: Grundig TK 27L de luxe Mystery Cable

Many thanks for that.

I had read that 'D' was for playback of synchronous mono recordings (as I said before I assume this is Duoplay mode) but did not realise that it also combined the 2 stereo channels, I will try it and also try the synchronous mono playback, to better understand what Duoplay is.

David
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Old 4th Nov 2021, 8:12 pm   #56
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Default Re: Grundig TK 27L de luxe Mystery Cable

Thread reopened as requested.

Cheers

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Old 4th Nov 2021, 9:57 pm   #57
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Default Re: Grundig TK 27L de luxe Mystery Cable

Have been meaning to do more tests on this for a long time, but got it out to do some monitoring tests to support the TK 27 Low Audio Thread.

https://www.vintage-radio.net/forum/...d.php?t=185423.

During the tests found that fast rewinding is no longer working, it works a fraction if most of the tape is on the Take Up spool, strange as it was working very well the last time it was checked.

Two first observations are that the Supply spool brake in the stationary state is not working at all (nothing to do with the fast rewind issue), very similar to the TK 23

https://www.vintage-radio.net/forum/...d.php?t=181533

and secondly that the Supply spool table clutch is very weak.

Took apart the Supply Reel table and found that the Lower Clutch Half was not fitted correctly, it has a peg on the underside that should fit into a hole in the chassis to stop it rotating, the peg was not in the hole. Refitted it correctly but it made no difference at all to the rewind issue.

With no spool fitted could see that the rubber drive idler wheel was driving the supply spool carrier OK and not that easy to stop it rotating by grabbing hold of the spool carrier, so not a frictions issue between the idler wheel and spool carrier that some similar models can suffer with.

Also with a spool full of tape loaded on the supply spool carrier there was no problem in rewind, so looked like a problem is due to extra load/friction when tape taken through the tape path to the take up spool.

So inline with the Worn Tape Guide Thread

https://www.vintage-radio.net/forum/...d.php?t=184892

I found that the brass Tape Guide on the exit from the heads to the Take Up spool was quite worn where the tape makes contact, so rotated the brass spacer and then was very pleasantly surprised that it made a huge difference to the fast rewind performance with rewinding now working pretty well end to end.

More thorough checks still to be done.

A second new issue is that the motor makes a loud whine when powered, which goes away during Play or fast winding, also pressing down on the motor shaft makes the whine go away, maybe oiling is required.

David
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Old 5th Nov 2021, 12:52 am   #58
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Default Re: Grundig TK 27L de luxe Mystery Cable

While testing Playback found that often there is a loud hum when the tape is Paused, the hum is definitely not on the tape.

When present the hum can be made worse or made to go away by touching various parts of the recorder particularly if both hands touch different areas.

Seems strange but must be a logical explanation.

David
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Old 5th Nov 2021, 5:51 pm   #59
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Default Re: Grundig TK 27L de luxe Mystery Cable

Found that even when the hum did not appear to be present, i.e. not audible, that the hum would be recorded to tape every time a recording was done (with no record signal present).

Traced the problem to not having refitting the metal screening panel that sits below the PCB, had previously removed the panel to be able to hinge down the PCB for some measurements and forgot to refit it when the PCB was hinged back up. After refitting the panel now all is good.

The motor whine has got worse in that it now is present during Play but still goes away in FFD/RWD.

David
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Old 5th Nov 2021, 11:28 pm   #60
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Default Re: Grundig TK 27L de luxe Mystery Cable

Added a drop of oil to the top of the motor shaft trying to get oil into the top bearing, made no difference to to the whine. Was going to do the same to lower bearing but more awkward so decided to remove the lower bearing to be able to lubricate it more easily.

Thought the securing screws for the bearing end plate were screwed into the motor metalwork but when I undid the screws a nut fell out, then realised that the screws screwed into free nuts that are located in a recessed slot on the motor metalwork .

Trying to get the nut back in place so that can refit the securing screw is nearly impossible due to the location. Only practical way would be to completely remove the motor assembly, but this is extremely difficult. To remove the motor the motor pulley has first to be removed, this is locked to the motor shaft with a torsion spring, difficult to remove the spring and really difficult to refit it. So will try and somehow get the nut back in place rather than trying to remove the motor.
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