Reel to Reel Grundig TK 17 Magic Eye setup: Translate from German ?? - Page 1
Forum section: electronics / repair

author: dazz1
author_rank: Frequent Contributor
timestamp: September 22, 2023, 09:30:13 pm
content: HiI am refurbishing an old 1960's vintage reel to reel tape recorder. A German Grundig model TK 17.I had to replace the selenium signal diode (faulty) with a period correct silicon version. The diode was in the circuit to drive the magic-eye EM 84 valve used to set the recording level.Changing the diode requires adjusting a 5Mohm trim pot (R31) to calibrate the magic-eye. Should be easy.I found the service manual but it is all in German and Google translate is clearly struggling. I am seeking help from someone who can convert German into something I can understand.The SM shows test networks. I think Ms.5 is connected to the line input (Radio) and the Ms.7 is connected across the record head. I think there is also an instruction to short the erase head. I have an audio signal generator connected to the radio input and I can record that, meaning that it is working.I have posted the same question on an English Vintage website here :https://www.vintage-radio.net/forum/showthread.php?t=204135but no translation yet.Any help would be appreciated.

author: ledtester
author_rank: Super Contributor
timestamp: September 22, 2023, 10:27:33 pm
content: Google translate can do images - see attached...

author: dazz1
author_rank: Frequent Contributor
timestamp: September 23, 2023, 03:30:04 am
content: HiIt isn't clear to me where the 3.4mV 1kHz test signal is applied. I think 3.4mV is applied across a 100R resistor in parallel with the recording heads.If so, then the trim pot would be adjusted to almost meet in the middle.Alternatively it looks like signals could be applied to a selected input to produce 3.4mV at the recording heads as follows:For a voltage drop of 3.4mv at 100 ohms according to Ms.5, the following voltages may be required at the inputs.Microphone (according to Ms. 2) 201mV +/- 2dB (163 ... 260mV)Radio (according to Ms.3) 10.6mV +/- 2dB (9.42 ... 13.8mV)Plate (according to Ms.3) 100mV +/- 2dB (79.4 ... 126mV)There is a voltage of 1400mV +/- 2dB (1110 ... 1760mV) at the listener socket.So if I applied 10.6mV at the Radio input, I could set the magic eye to that.

author: floobydust
author_rank: Super Contributor
timestamp: September 23, 2023, 03:38:36 am
content: Translate bungles the numbers. I get:"6.22 Dabei steht an der Buchse Hörer eine Spannung von 1,400mV +/-2dB (1,110-1,760 mV)".It looks like 1.4VRMS is injected? Not sure where.edit: It looks like you inject a signal per MS5 jig and somewhere look for 1,400mV? Not sure where. EM84 needs a lot of voltage drive and the cathode of the driver seems less.The diode is likely copper-oxide or germanium.

author: dazz1
author_rank: Frequent Contributor
timestamp: September 23, 2023, 04:14:28 am
content: Quote from: floobydust on September 23, 2023, 03:38:36 amOK I am thinking that for the given input signals, there should be 3.4mV measured across the recording heads to check sensitivity.The E 25 C5 selenium diode had a specified max reverse voltage of only ~25V so it must have been operating close to it's limits. The forward voltage drop was about 1V - 2V, more than the silicon diode now fitted, but not really significant when it takes ~20V to drive the EM 84.So if I apply a input signal of:Microphone (according to Ms. 2) 201mV +/- 2dB (163 ... 260mV) or,Radio (according to Ms.3) 10.6mV +/- 2dB (9.42 ... 13.8mV) or,Plate (according to Ms.3) 100mV +/- 2dB (79.4 ... 126mV)Then I should get 1400mV at the line output (Austgang) and full scale showing on the EM 84. It should only require a slight adjustment of the trimpot.That is my current thinking.

author: floobydust
author_rank: Super Contributor
timestamp: September 23, 2023, 04:54:58 am
content: But what is potentiometer R6 REC LEVEL/VOLUME P/L set to? I'm not sure what the scale on it is.I don't like injecting a few mV because hum and noise get in the way. But they want the chain input driven.Switch in 1,13 the EM84 is looking at the audio power amp input/earphone output which makes sense for many volts drive required.It must have been a cool looking selenium diode from the era. They are always so leaky.edit: Odd that the Ausgang jack has the tape head brought out to it, but no connection - so I would not expect this to record as is. Something special about the cable I think.It looks like the low impedance record head gets driven by the audio power amp, this is the critical level. So 1,400mV RMS output is the maximum when the eye tube is just closing.It's from an era of ceramic/crystal phono cartridges which are high level, and the radio should be from its loudspeaker output. Only the mic I think needs lots of gain the 3 stages worth.

author: dazz1
author_rank: Frequent Contributor
timestamp: September 23, 2023, 07:52:11 am
content: Quote from: floobydust on September 23, 2023, 04:54:58 amI don't see any guidance on the setting of R6. It is just a vol/rec level pot control marked 1 to 10.I am injecting a signal thru the RADIO connector and the record level is definitely controlled by R6.

author: static333
author_rank: Newbie
timestamp: September 24, 2023, 05:25:48 am
content: Quote from: dazz1 on September 23, 2023, 04:14:28 amCorrect.Recording head must be bypassed with a 100R resistor and a 3.4mV voltage drop measured there.Then adjust R31 until the EM84 is almost closed. A tiny black line should be visible.

author: dazz1
author_rank: Frequent Contributor
timestamp: September 24, 2023, 09:09:33 am
content: Quote from: static333 on September 24, 2023, 05:25:48 amThat is what I did today and it worked.