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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 11th Dec 2021, 1:56 pm   #1
DMcMahon
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Default Grundig TK 23 E.O.T. Auto Stop & Record Faults

Been checking out another of my TK 23s (#4 Serial # 37822), this is in pretty good condition for its age with good tape transport operation and good playback sound.

Initial recording attempt produced no recording, not sure yet if erase works.

Was about to start investigating why recording not working but got side tracked when found that the End Of Tape (E.O.T.) Auto Stop operation was not working, this should stop the tape when the play gets to the end in Play, Rewind and Fast Forward modes.

E.O.T. hardware is very simple on this machine with a single brass sensor post mounted on the left side of the heads mounting plate, this is isolated from the grounded metalwork by insulated spacer and washers. A wire connects to the post which connects back to one of the E.OT. solenoid coil, the other side of the solenoid coil connects to a DC supply provided by R42/R43 potential divider.

When the conductive tape leader section (if present of course) near the ends of the tape, bridges the E.O.T. sensor post and the grounded tape guide on the right side, this energises the solenoid whose plunger then operates a sprung loaded lever that resets the play/fast wind mechanical latch mechanism which in turn resets the appropriate piano operation key.

The standing voltage on the sensor post from the potential divider was 36V which initially seemed good as the schematic shows it to be 26V. Manually grounding the post also did not stop the tape motion.

Viewing the solenoid from the underside of the recorder showed that the solenoid was actually physically operating but its plunger was not operating fast enough/with enough force to overcome the sprung loaded reset lever.

Viewing the solenoid on my spares TK 23 showed it was operating well with more plunger force with potential divider voltage of 42V, both solenoids measured 50 Ohms.

Removed the suspect solenoid to test on external PSU, this subsequently was a mistake as had a really difficult time refitting the 2 securing screws due to difficult access. The solenoid would energise at any voltage >5V and at 10V the plunger was held firmly in the out position, but even at 30V (PSU max) the plunger operation was slow/not forceful.

Checking the schematic more closely saw that R43 was shown as 7.5k whereas the fitted R43 was 18k. then found another TK 23 schematic that showed R43 to be 18k so there are 2 different schematics (maybe for different version of the solenoid). The 18k version schematic shows the potential divider voltage to be 59V so my 26V was well low.

The reason for the low voltage is that the HT voltage (that connects to the top end of R42) is only 194V, the schematic shows the main HT as 250V in playback mode.

Removing the EL 95 output pentode to remove most of the HT load, resulted in HT of 236V and potential divider voltage of 43V. With this 43V the E.O.T. operation works perfectly.

Because the the HT mains ripple was good, thought that reservoir capacitor C24 (50uF 350V) is not the culprit for the low HT voltage, it also measured OK. To be sure temporarily fitted a new capacitor in place of C24 which made no difference at all to the the HT voltage.

Because the mains transformer secondary winding output to the bridge rectifier was good at 240V, this only left the FC 142 metal bridge rectifier itself. So decided to replace it using four 1N4007 Silicon rectifier diodes. To keep it tidy and original looking, decided to fit the diodes inside the rectifier body.

The rectifier is riveted to the metal framework that the mains transformer and fuses/mains links assembly are mounted to. Looked difficult to remove this, especially without disturbing the very delicate transformer wire connections, so decided to leave the rectifier in place (wires disconnected) and fit a donor rectifier (with new diodes inside) on top of the existing rectifier.

Partially removed the mains transformer assembly metalwork on my spares TK 23 (#7 serial # 37826 (only 4 different from #4 serial number)) and drilled out the brass rivets that secured the rectifier to the metalwork. By prising up the aluminium side edges of the rectifier got inside and removed the various rectifier disks and springs then soldered in the new diodes.

Safety Note - this metal rectifier may well be Selenium which is a toxic material so have to be careful when working on it.

Fitted the replacement rectifier using 2 self tapping screws that went through the holes in the replacement rectifier (where the rivets originally resided)
which then screw into the brass rivets in the original rectifier underneath,.

Now have good HT voltage of 278V somewhat now on the high side, later may fit a dropper resistor if I can find something suitable. The potential divider voltage now is 51V, I think shy of the shown 59V because both R42 & R43 actual values are right at the top end of their 10% tolerance.

E.O.T. operation now works very well.

Noticed that the HT voltage of 278V does not change when record mode is selected which is in line with record not working, normally the HT voltage should increase by around 10V when record selected.

David
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Last edited by DMcMahon; 11th Dec 2021 at 2:05 pm.
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Old 12th Dec 2021, 2:49 pm   #2
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Default Re: Grundig TK 23 E.O.T. Auto Stop & Record Faults

Scoped the bias oscillator output and could see that when the Record button was pressed down, that there was no output. By manually operating the PB/Record slide switch could get good bias output (well above 1,000V peak to peak). Thought possibly that the stroke adjustment for the PB/Record switch could need adjusting but after cleaning the switch with contact cleaner, the bias oscillator now reliably works every time the record button is pressed down.

The HT voltage of 278V increases to 294V in Record mode, so thought best now to reduce the HT voltage by using dropper resistor between the output of the bridge rectifier and the reservoir capacitor.

Measured the HT standing drain current in PB mode at 40mA, calculated a resistance of around 500 Ohms required for a 20V drop. Could not find a suitable resistor particularly higher wattage.

Tried using two 1.2k in parallel, only 0.5W each so knew would be borderline. This dropped the voltage by a bit too much (244V in Playback) and the resistors soon got fairly hot. While doing this subsequently lost the HT voltage all together and found that the 80mA fuse (mains transformer secondary output to the bridge rectifier) had blown. Can see no obvious reason why the fuse blew ?

Replaced fuse and removed the 2 resistors. Now back to normal with 278V HT at 40mA

Measured the AC current though the 80mA fuse now at 45mA.

So now need to find a lower value resistor with higher wattage to try.

David
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Old 14th Dec 2021, 12:51 am   #3
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Default Re: Grundig TK 23 E.O.T. Auto Stop & Record Faults

Found a suitable 220 Ohm 2 watt wirewound resistor (obtained from my Ferrograph Series 7 Spares recorder, i.e. R101) and fitted it between the bridge rectifier +ve and the reservoir capacitor C24.

Now have +261V main HT in Playback and +277V in Record, plus 47V at the E.O.T. solenoid, the 220 resistor is running only slightly warm so all looks good.

Tried a test recording but nothing recorded and then found that the Bias Oscillator was no longer working. After some chasing around found that Bias Oscillator worked all the time if the Track Select switch was switched to D but when switched to 1-2 or 3-4 the Bias Oscillator only intermittently occasionally worked.

After cleaning the switch now all good. Recording still to be checked.

David
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Old 14th Dec 2021, 9:49 am   #4
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Default Re: Grundig TK 23 E.O.T. Auto Stop & Record Faults

Great write-up, interesting how a machine that apparently works well can actually do so with such reduced HT. I had the same on a Bush record player recently that used a similar rectifier. These are now a 'change on sight' component it seems.
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Old 14th Dec 2021, 1:24 pm   #5
DMcMahon
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Default Re: Grundig TK 23 E.O.T. Auto Stop & Record Faults

Rechecked recording and not working again (erase is working), found Bias Oscillator Record bias not working again on Tracks 1-2 & 3-4 but still working on D

Confirmed the upper and lower tracks on the head are open circuit (the PB/Record head windings themselves are around 420 Ohms each) when measured from the output of C17 trimmer (Bias amplitude adjust) to ground when switched to Tracks 1-2 & 3-4 in Record mode.

Need to have a closer look at the Track selection switch.

David
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Old 14th Dec 2021, 1:52 pm   #6
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Default Re: Grundig TK 23 E.O.T. Auto Stop & Record Faults

Those the same ones used in the Hacker "Mayflower" radios.

See this interesting thread, especially post 17: https://www.vintage-radio.net/forum/...ad.php?t=80643

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Old 14th Dec 2021, 6:16 pm   #7
DMcMahon
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Default Re: Grundig TK 23 E.O.T. Auto Stop & Record Faults

Yes very interesting, good to see that Westinghouse Rectifier Publication which includes my FC 142. Also good to see the rectifier spec information in the publication.

I did wonder if the FC 142 could be Copper Oxide as opposed to Selenium because some of the rectifier discs are copper coloured, the Westinghouse publication does not appear to reference the rectifier material type.

I did not measure any of the individual rectifier discs but each diode disc stack measured around 3.5V on a DMM forward biased diode check.

David

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Old 29th Dec 2021, 11:11 am   #8
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Default Re: Grundig TK 23 E.O.T. Auto Stop & Record Faults

Stripped and cleaned again the track selection switch, recording now working again reliably but Erase now not working reliably.

By measurements concluded that Record contact tt1 that is normally closed (should only open when Record switch is rotated for Superimpose recording) must be intermittently going open circuit when the Record button is pressed down.

Removed the record switch (but still connected to wiring) and cleaned tt1 and also the other 2 sets of superimpose contacts tt2 & tt3. Removing the switch resulted in 2 wires breaking off, can see that one must go to record contact at1 because can see only 1 wire on at1.

David
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