Grundig TK 17 Magic Eye internal level adjustment

[Dazz100]

Hi

I am trying to recover recordings made by my father so I purchased a not-working Grundig TK 17. The recorder is in good condition for its age so I suspect it failed a long time ago. In the 1960's imported equipment like this typically had 130% customs duty applied so it would have been an expensive item of equipment. Too valuable to throw away when it failed. Too costly to have it fixed with imported parts.

I found a manual with a circuit diagram. I then reverse engineered the circuit board to produce a photograph annotated with the component identities.

I went through and cleaned, oiled and checked it. A drive belt had parted so I replaced that with a standard o-ring. All of the voltage supply levels were correct. The capacitors and selenium rectifiers all worked OK (see below).

I found the selenium diode E 25 CS had failed. This rectifies the signal displayed by the magic eye tube type EM 84. I replaced the selenium diode with an ancient period correct SD-1 silicon diode. This now means that the R31 5M trim pot needs to be adjusted to set magic eye display to the right level for recording.

I have my signal generator connected to the "Radio" input and I am able to successfully record, play back and erase. The tape that came with the recorder has a selection of random music. Given the age and consumer grade design, oval speaker etc, it sounds remarkably good.

I have noticed that the magic eye only works when recording. Looking at the circuit diagram I found, which almost matches my tape deck, it looks like the magic eye should work on play back as well.


I have a couple of questions:

What is the correct procedure to set the magic eye level?
or
How do I correctly adjust trim pot R31?
I have a reasonable range of test equipment including signal generators, multi-meters and oscilloscopes.

Is the magic eye supposed to work on play back?

Thanks in advance for any advice.

[DMcMahon]

The EM84 Magic Eye will be powered all the time the unit is switched on, so you will get the green display with the black band, but the Magic Eye will not respond to the Playback signal, this is because Playback/Record switch contacts 1.14 and 1.13 are broken (not made). In Record mode 1.14 switches to 1.13 allowing the Record signal to go to the PB/Record head and to the EM84 via the selenium diode.

It is fairly common in budget recorders for the Record signal only to be monitored on Magic Eye/VU meter etc. On some machines that use VU meter, the illumination for the meter may also be disabled during Playback.

Somewhat surprisingly am struggling to find set-up procedure for the Magic Eye R31 pre-set adjustment.

David

[Dazz100]

Originally Posted by DMcMahon

...

It is fairly common in budget recorders for the Record signal only to be monitored on Magic Eye/VU meter etc. On some machines that use VU meter, the illumination for the meter may also be disabled during Playback.

...

OK so that has saved me a whole lot of time fault finding a non-fault.

I have been doing some checks, specifically selected caps.
I checked the grid connected caps for leakage with my Megger. Although it is on the 500V range, it actually puts out 600VDC, so a good stress test for 60 year old caps.

Only one cap failed, but that was only rated to 125V, so not unreasonable. This was on the grid to the magic eye. I could see and hear the arc (polystyrene cap) so that was a destructive test.

All the caps passed but with an abundance of caution, I replaced them. I like to keep things looking factory original but the local electronics shop has a limited range. It has bright orange, blue and green caps but I chose the brown ones. They happen to be 680V rated, so should be OK.

I haven't checked the electrolytics because I can't easily replace them. I would have to order them in especially. I am not getting audible hum so the electrolytics are good enough.

The first thing I did when I started this restoration was to fix the power cord. The original had been cut by the cover and patched together with a chocky block. Also, there was no earth. I fitted a new cord and 3-pin plug with an earth. Much safer.

So the only task left is to adjust the magic eye setting.

Originally Posted by DMcMahon

Somewhat surprisingly am struggling to find set-up procedure for the Magic Eye R31 pre-set adjustment.

The User Manuals states the line input range 2mV-200mV (rms? or pk-pk?) so I can use that as a starting point.

I found a TK17 service manual but it is all in German.
The relevant section for setting the Magic eye (EM84 valve) appears to be attached.
The Google translation is as follows:

Reinforcement
First the sensitivity of the inputs is checked at 1000Hz. For a voltage drop of 3.4mv at -00 ohms according to Ms.5, the following voltages may be required at the inputs.

Radio according to Ms.3

There is a voltage of 1400mV at the listener socket
During this measurement, the control of the EM84 is also controlled.
With a voltage drop of 3.4mV at 100 ohms, a fine dark line should be visible between the ends of the light bars.
adjustable with R31

I suspect Google is struggling with this translation because I understand parts of it, but not enough to reveal a full calibration procedure.

I think it is saying that the a signal generator output should be in series with a 100ohm resistor at 1kHz. The voltage across the resistor should be 3.4mV pk-pk. That would set the input level.

Adjust the recording level so the Line Output is then 1400mV pk-pk.
Adjust R31 so the magic eye bars just meet.

[DMcMahon]

I found in the TK17L Service manual that the R31 adjustment is under the "Input Sensitivity and Modulation Level Adjustments" section.

David

[Dazz100]

Hi
OK I have found another nugget of info.
The attached image from the Service Manual shows the test circuits referenced in the text.

[paulsherwin]

I'm a bit puzzled. If you just want to play back your father's tapes, why are you putting so much effort into getting the magic eye working correctly? Is it that you've decided to do a comprehensive restoration for its own sake?

[ms660]

So far as I can make out, network Ms.5 (100 Ohm in parallel with a AC valve voltmeter) is connected across the record/playback head and the record level indicator is adjusted correctly by R31 when there's 3.4mV RMS across the 100 Ohm resistor when the record amplifier input level specified (at 1kHz) is applied to the amplifiers input and the recording level set for 1.4 Volts output at the line out.

Lawrence.

[Dazz100]

Originally Posted by paulsherwin

Is it that you've decided to do a comprehensive restoration for its own sake?

Yes. I plan to sell the recorder after I have finished with it. The priority is to make it safe (it wasn't), then fix defects or likely defects (done).

As an electronic engineer, I don't get to do hands-on electronics at work. I like fixing older stuff because it is fixable and I can do it.

The magic eye only requires a trim pot to be adjusted. It shouldn't be this hard.

[DMcMahon]

I have 2 different English versions of the TK17L Service Manual, later I will scan the relevant section but believe Dazz100 and Lawrence have covered it well.

David

[DMcMahon]

Relevant scanned extracts from the 2 manuals referenced directly above.

David

[Dazz100]

Originally Posted by DMcMahon

Relevant scanned extracts from the 2 manuals referenced directly above.

David

OK so I have now set the magic eye level by the following procedure:

1. Short out the erase head.
2. Connect an AC volt meter and 100R resistor across the record head (Ms.5).
3. Connect a signal generator in series with a 100k resistor (Ms.3) to the Gram input.
4. Start recording (tape not required).
5. Adjust the sig-gen output and/or volume to measure 3.4mV across the record head.
6. Adjust the trimpot R31 until the magic eye shows a thin black line in the middle.

As expected, the trimpot only required a small adjustment because of the difference between selenium and silicon forward volt drop.

I am now playing a couple of tapes to heat soak the tape recorder to flush out any latent defects.

Thanks for your help.

[DMcMahon]

Excellent, well done.

I did not realise that the TK 17 did not have a tape counter.

David