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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 4th Jun 2020, 12:08 pm   #1
DMcMahon
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Default Grundig TK 27L de luxe Mystery Cable

Just removed my latest purchase from its shipping box and noticed that there was a rattling sound coming from the top as if something was loose inside.

Removing the top panel shows a cable coming through a hole inside and being terminated in a 4mm connector to which an inline coupling 4mm connector is fitted.

No idea what this could be for, will look in the back later to see where the cable connects to, but just wondering if anybody knows its purpose.
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Old 4th Jun 2020, 4:31 pm   #2
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Default Re: Grundig TK 27L de luxe Mystery Cable

Nice machine, looks to be in good condition and very clean. I suppose it's an import as I believe that model was never sold in the UK.

No idea what that wire could be for, must be someone's addition. Will be interested to know where it goes.

Any pot metal disease? My TK27 (non L version) had suffered somewhat. The fast forward idler had almost disintergrated and a small spacer hidden behind the record contacts had swelled to such an extent that the contacts would not slide. An identical spacer for the second set of contacts was in perfect condition. Very strange. I made a spacer from Milliput expoxy putty. Roughly shaped by hand, then when hard filed down to size.

Somewhat more seriously, the motor was floating free inside when I took it from it's shipping box. The motor brackets had also succumbed and disintergrated. Luckily, the motor was undamaged. Later machines used a piece of round metal bar, threaded at both ends to support the motor, so I used threaded rod and brass tube as a sleeve over the outside.

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Old 4th Jun 2020, 5:30 pm   #3
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Default Re: Grundig TK 27L de luxe Mystery Cable

I have not yet done a close inspection but from first quick look, it looks to be in very good condition, although externally needs a good clean up as usual.

I removed the bottom cover and the cable connects to the junction of 50uF reservoir capacitor C11 & R19 which connects back the +ve output of the bridge rectifier, i.e. it is the main HT supply, so definitely is an add on not an original connection.

Why someone would want a connection to the HT supply is difficult to know, maybe if there was a problem and they wanted to monitor the HT but why wire it to the inside under the top panel, when it would have been easier (and more accessible) to run the cable out through the mains cable stowage.

It is a German machine Suffix k, which according to the suffix reference list is 60Hz but it is linked for 50Hz operation, have not yet checked its motor pulley for 50 or 60Hz sizing.
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Old 5th Jun 2020, 8:15 am   #4
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Default Re: Grundig TK 27L de luxe Mystery Cable

I might assume a previous owner might have wanted to power an external device using this homemade flying lead? The device itself having a 4mm power receptor and the red female connector being used to protect against an inadvertent short circuit when the power cable was not in use but the recorder was? Power for an externally powered loudspeaker for example? Does this model have an internal, but weak, internal speaker?
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Old 5th Jun 2020, 9:58 am   #5
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Default Re: Grundig TK 27L de luxe Mystery Cable

Yes I did wonder about powering something external but it is around 270 volts and still think it is unusual to have it hiding under the top panel where you have to remove the panel to access/use the cable, would have been better brought out through the mains cable stowage area where it could have been left safely coiled up with its red female connector.

Have not yet had time to power it up, I really hope the internal loudspeaker is OK, would be unlucky if it is not.
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Old 5th Jun 2020, 2:52 pm   #6
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Default Re: Grundig TK 27L de luxe Mystery Cable

If the wire is long enough to reach outside the case then it probably did. If the owner was anything like me then it was used for a radio tuner, possibly the most used externally connected device besides a mike. Alternatively perhaps a microphone mixer?

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Old 5th Jun 2020, 3:38 pm   #7
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Default Re: Grundig TK 27L de luxe Mystery Cable

Yes but to use it for any external device with how it was fitted you would have to leave the top panel off the recorder, whereas it could have been very easily run out the bottom
of the recorder through the mains cable stowage area with the top panel left fitted. Does not make much sense to me ?
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Old 5th Jun 2020, 3:40 pm   #8
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Default Re: Grundig TK 27L de luxe Mystery Cable

If it was to power some external piece of equipment, I would have expected a heater supply as well. There's not much that needs the HT+ DC only
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Old 5th Jun 2020, 3:46 pm   #9
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Default Re: Grundig TK 27L de luxe Mystery Cable

Maybe a condenser microphone! Les.
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Old 5th Jun 2020, 4:00 pm   #10
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Default Re: Grundig TK 27L de luxe Mystery Cable

That's a good suggestion. Grundig made a number of condenser microphones over the years and some of their tape recorders had a 100V-or-so supply on the microphone socket. I have no idea if the TK27L does or not, but if not this might have been part of a modification to provide it.
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Old 5th Jun 2020, 4:18 pm   #11
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Default Re: Grundig TK 27L de luxe Mystery Cable

No the TK 27L is not condenser microphone. The condenser microphone supply is high impedance very low current for safety, I have tested it by holding onto the supply, I would not like to do that with the main HT supply.
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Old 5th Jun 2020, 6:22 pm   #12
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Default Re: Grundig TK 27L de luxe Mystery Cable

Precisely,the recorder might not have the condenser mike input and someone wanted to provide it.Les
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Old 5th Jun 2020, 6:45 pm   #13
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Default Re: Grundig TK 27L de luxe Mystery Cable

My thought is that somebody connected a resistor network to the end of that HT lead to provide the required voltage (at high impedance) for the condenser microphone. Looking at the circuit of some Grundig machines that did have condenser microphone inputs, the bias voltage was obtained from the HT supply using a potential divider with resistors in the megohm range. Something that could easily be connected to said lead
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Old 5th Jun 2020, 9:44 pm   #14
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Default Re: Grundig TK 27L de luxe Mystery Cable

Quote:
Originally Posted by TowerRadio View Post
Precisely,the recorder might not have the condenser mike input and someone wanted to provide it.Les
Yes Les I understood, it was a good suggestion.

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Old 5th Jun 2020, 9:49 pm   #15
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Default Re: Grundig TK 27L de luxe Mystery Cable

Quote:
Originally Posted by TonyDuell View Post
My thought is that somebody connected a resistor network to the end of that HT lead to provide the required voltage (at high impedance) for the condenser microphone. Looking at the circuit of some Grundig machines that did have condenser microphone inputs, the bias voltage was obtained from the HT supply using a potential divider with resistors in the megohm range. Something that could easily be connected to said lead
Yes agreed, it would also be possible to rewire the microphone DIN with the required resistors, so external cable not required.
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Old 6th Jun 2020, 3:25 pm   #16
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Default Re: Grundig TK 27L de luxe Mystery Cable

Removed the mystery cable as not required, the rubber insulation is hard and starting to break up, so will throw the cable away (keep the connectors).

Changed the mains voltage selection links from 220 to 240 volts. Replaced the continental 2 core mains cable and 2 pin plug for 3 core/3pin.

Replaced corroded 125mA HT fuse.

After a few checks powered up OK, checked the various anode to grid coupling capacitors all OK, all DC voltages are good.

Tape transport works well and Playback works, sounds good. No sign of any pot metal disease. Recording checks to be done next.

One little mystery. While visually checking things out saw that on the circuit board a 220 ohm resistor has been bypassed/shorted out by a wire link being soldered across it.

From the circuit diagram this is R 21 in the cathode circuit of the output pentode EL 95. The cathode resistor R 20 (300 ohms) connects to one end of R 21 and the other end of R 21 connects to variable potentiometer R 22 the other end of the pot connects to ground.

Looking at the schematic of the TM 27, R21 is not fitted and its position is linked out.

The TM 27 is the chassis version which is used in Grundig Radiograms. The TM chassis units do not have an output transformer and presumably the EL95 is wired into the Radiograms output transformer, but currently do not understand why R21 would be removed for Radiogram use or what the purpose of the pot R20 is (this should be fairly obvious). Whether this linked out R21 has anything to do with the mystery cable ?

I have removed the link across R21, in fact one end fell off as I touched it (not great soldering).
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Old 6th Jun 2020, 7:38 pm   #17
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Default Re: Grundig TK 27L de luxe Mystery Cable

Very interesting. From the diagram, R21 and the pot R22 are shorted out during playback. I think R22 sets bias current in the erase head, then bias in the record head is set afterwards. This part of the circuit is different to all the others in the range as the machine is stereo.
In your photo, looks like someone had trouble with the bias, as that pot (R22) is turned all the way to one end, effectively shorting it out as well.

That's a very clean looking machine inside, looks as if it's seen very little use. A good find.

The motor mounting is different to all my machines. I presume your machine is a later one, being an 'L' variant. Any dates around that you can see?

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Old 6th Jun 2020, 9:10 pm   #18
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Default Re: Grundig TK 27L de luxe Mystery Cable

Good analysis, yes I can now see R22 is the erase head adjust in the German set-up procedure with R101/R201 & C101/C201 being for the record heads. Once I fully translate it, will be able to test the setup and see if there is a problem relating to R22 being at max setting.

Yes certainly is clean particularly on the inside.

Have not seen any date on it, sometimes there is a date on the motor.

Not long ago tried to do a quick test recording and the record button would not push down, which I thought was strange as the button pushed down with no problem previously while checking the HT voltages in Playback and Record modes.

Opened up to investigate and found it was mechanically interlocked due to the Channel select switch being set to "D" (I had played around with the switch post HT voltage measurements). Found out that position D is for Playback of synchronous recordings (I assume this is Duoplay mode) and mechanically interlocks the record button from being pushed down.

Confirmed the motor pulley is 50Hz (stamped on it) and replaced the 4 white feet that were rock hard and cracked.

The record button associated mechanisms in top right corner is a lot more extensive than TK 14 - TK 23 Mono similar series machines, hope none of that goes wrong.
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Old 7th Jun 2020, 6:16 pm   #19
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Default Re: Grundig TK 27L de luxe Mystery Cable

Tested microphone recording, it worked although a bit crackly, however it did not erase the original recording.

Scoping the erase head connections, the erase signal is not reaching the heads when Record button is down, but there is a very brief burst of HF signal (at the erase head) when the Record button is initially pushed down.

So need to check out the relevant contacts to determine what is occurring.
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Old 8th Jun 2020, 6:30 pm   #20
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Default Re: Grundig TK 27L de luxe Mystery Cable

After a long session trying to fault find why erase not working have hit a brick wall currently. Basically the Bias Oscillator will not run, runs for a fraction of a second when Record button first pushed. I do not understand how microphone recording previously worked because there is no Record bias in addition to the erase signal not working.

Have gone around in circles checking everything. Found that C9 on the cathode of the EL95 had high ESR at around 60 ohms but replacing it made no difference.

Presently looking at Contact "at 3,3" the one that grounds the bias amplitude adjustment R22 when in Playback mode. When in Record mode the switched contact is not grounding the common side of the erase heads.

By checking all the connections it is definitely the "at 3,3" contact that is the problem, but it is just about impossible to see let alone get to, it is part of the contacts and record button assembly in top right corner (the area I thought would be difficult).

Looks like the assembly is secured by 3 screws but I cannot loosen any of them and even if I could, looks impossible to remove the assembly due to all the cable connections, maybe at best might be able to ease it forward a couple of mm so then may be able to see the contacts in question.
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