Grundig TK14 speed problem - UK Vintage Radio Repair and Restoration Discussion Forum

martin.m · 12th Apr 2020, 10:29 pm

I have a TK14 that is running too fast. Very noticeable on pre-recorded tapes and anything recorded on the Grundig sounds too slow on other machines. I have made a not very scientific check using a reel of leader tape and the capstan pulls through 100cm of tape in 10 seconds, so about 5 per cent too fast. I can't imagine what could cause this problem though. Any advice would be gratefully appreciated.

ben · 12th Apr 2020, 11:27 pm

Belt or idler 'riding' on wrong part of motor pulley?
Piece of tape wrapped around capstan shaft?

DMcMahon · 12th Apr 2020, 11:36 pm

What is the history behind this, was it previously OK and then changed ?

It wouldn't by any chance be a TK 14 U / TK 14L U, a universal mains version with selectable 50/60Hz links/solder tags ?

David

martin.m · 13th Apr 2020, 1:43 am

Thank you for your replies. Tape wrapped round the capstan spindle was my first thought but it is fine. The drive to the capstan flywheel comes directly from the motor pulley and I can't see anything wrong there. I bought the machine locally about five years ago, and after a quick check pronounced it as fit and well. The speed problem has only come to light after playing one of the Grundig's recordings on another machine. I can't see any links inside for 50 / 60 Hz. There is a provision for 120v operation by moving the position of a glass fuse.

This tape recorder has another couple of logic defying problems to sort out. When recording, the incoming audio can be heard loud and clear through the speaker. However this should be impossible, as on record the audio output valve (ECL86) is switched as the bias and erase oscillator. Something is obviously not right with the record/playback switching. Yet it still manages to record so how can it provide audio monitoring, bias and erase at the same time?

Last of all, recordings of any speech reveal a faint "echo" with a delay of around half a second. This is audible when the tape is played back on another machine. Perhaps this is tied in with the last fault, or I could be losing my marbles after being confined to the house for so long

ricard · 13th Apr 2020, 6:18 am

This certainly qualifies for the proverbial 'ghost in the machine'!

Ok, here's my guess, probably wrong, but got to start somewhere ...

In 'the old days', it was not uncommon to use active components such as valves or transistors for more than one task at once. Especially in radios, a valve might work as both an HF amplifier, and, after demodulation, to amplify the resulting LF before driving the speaker.

So I'm wondering if it's fathomable that the output valve, due to incorrect switching, is in fact working both as a bias oscillator and as an LF output valve at the same time! (I'm assuming that the recordings it makes are of reasonable good fidelity, because even without HF bias, there will be a recording of sorts, only rather weak and distorted).

Furthermore, it would seem to me that an erase head should be able to work as a record head of sorts, and with the LF already mixed with the bias in the output valve, it would result in the erase head working as a record head, and if again incorrect switching is causing the playback head to play back the signal, and it's mixed with the microphone input, you would have your echo there.

All pure speculation, I don't know to what degree this is even academically possible, or if there's any switching malfunction that could cause it to behave this way. Often though, it seems to me that the switching around the output stage / bias oscillator is minimal, with the oscillator coils remaining in circuit at all times, so all that is actually switched is the feedback loop inside the oscillator (and the connection to the erase and record heads).

ms660 · 13th Apr 2020, 10:48 am

Make sure the record/playback switch is moving fully over when in record.

Lawrence

DMcMahon · 13th Apr 2020, 11:33 am

As Lawrence & Ricard suggest it definitely sounds like a Record/Playback switching type of issue.

When in Record mode there are 2 things that should make it impossible to get normal audio to the speaker.

Switch contact 1.11 switches from contact 1.12 (to remove any audio signal feed) to contact 1.10 as part of the bias oscillator circuit. Also switch contact 1.5 switches to 1.4 which short circuits the primary winding of the output transformer.

If you do any maintenance on the circuit board you have to make sure that when board is hinged (there is no actual hinge) back up in position that the record actuator arm fits into the hole near the end of the Record/Playback switch, although it not in the hole it should then be impossible for recording to work at all.

martin.m · 13th Apr 2020, 2:54 pm

Thank you all for your comments. Lawrence, you were right. The record/playback switch was not moving far enough. There is a small grub screw on the operating mechanism and this had worked loose. Ricard, I think your explanation was spot on. The circuit had been working in a way that the manufacturers had never intended with the erase head acting like a second recording head to produce a faint echo effect.

Back to the speed problem, Ben I think you were right. On close inspection the capstan belt looks much to fat and appears to be riding on the outer part of the motor pulley. Can anyone tell me how to remove this pulley? The motor bearings sound like they could do with a drop of oil as there is a screeching noise for a few seconds at switch on.

DMcMahon · 13th Apr 2020, 4:59 pm

Hi Martin,

The belt does look fat, see picture of my 1.5mm belt attached, I think the official size is 2mm. My TK 14 has the non finned pulley.

I would strongly advise not to remove the pulley unless it is really essential.

I personally find it difficult to remove/replace the pulley and the last one I did, the torsion spring snapped and I had to superglue the pulley to the motor shaft.

If you do need/want to remove it, then main steps are:-

1. Disconnect mains cable, i.e. make safe.
2. Measure existing height of the centre of the belt groove on the pulley to chassis, spec is 14.5mm
3. Remove left hand reel table/clutch assembly, noting the exact positions of any thin simrit black & any thin metal clutch washers.
4. Remove the rubber idler wheel that is driven by the pulley. I have seen reference that it can be done from the top, but I am yet to see one like that.

So remove bottom cover and undo the nut near to the top right (with carrying handle pointing upwards) of motor, that's secures to the bottom of the idler wheel shaft, watch out for the serrated washer behind the nut.

You can then remove the idler wheel assembly from the top, it has a washer and a cylindrical piece that are retained on the shaft.
5. To remove the motor pulley you have to lock the motor rotor shaft from turning, then rotate the pulley anti-clockwise while at the same time pulling upwards. This is the bit I find difficult.

To refit the pulley lock rotor, turn pulley anti-clock again and push down until correct height, also difficult I find.

Often you will find that the pulley moves with its torsion spring but does not always lock onto the shaft or vice versa it locks at wrong height and difficult then to move.

Good luck,

David

DMcMahon · 13th Apr 2020, 5:18 pm

Also measure the external diameter of the motor pulley to make sure it is a 50Hz one should be a nominal 21mm.

DMcMahon · 13th Apr 2020, 5:26 pm

Quote:

Originally Posted by ms660

Make sure the record/playback switch is moving fully over when in record.

Lawrence

You are doing well Lawrence two in a row, if you get a third good one then maybe time to do the lottery

DMcMahon · 13th Apr 2020, 5:51 pm

For removing the idler wheel the Service manual says to remove the small plastic retaining washer from the top.

On mine the washer has hardened (more like hard rubber) and attempts to remove it just breaks pieces off the washer, it is in a grove so if washer removed and it fell apart, could use a circlip or other retaining washer as replacement.

martin.m · 16th Jul 2020, 3:29 pm

I have been catching up with my projects and finally had the time to finish the TK14. Thanks for your advice David. I managed to remove the pulley and take the motor apart so that the spindle could be properly cleaned and lubricated. New capstan belts are available for these old Grundigs but cost around a tenner plus postage. Following advice on this forum, I bought some 1.5mm O ring cord and made a new belt by cutting off the right length and using superglue to join the ends. This doesn't sound like a very good idea but it worked really well. I couldn't snap the belt once the glue had set. Last of all there was a problem with the take up reel not having enough pull to gather up the tape properly when playing. The take up hub is belt driven from the capstan flywheel (through a slipping clutch). Comparison with another working machine revealed that the drive idler was contacting the take up hub in play mode and slowing it down a little. Underneath the idler is an adjustable sliding plate that prevents this happening. Moving this by about 1mm cured the problem. These TK14s and 18s are built like battleships and nothing much seems to go wrong with them. There are no waxies or dodgy capacitors to change and the sound quality is excellent even when using the DIN output to play through a stereo system. How much of today's audio equipment will be around in 60 years time?

DMcMahon · 17th Jul 2020, 9:52 am

[QUOTE=martin.m;1270246] Comparison with another working machine revealed that the drive idler was contacting the take up hub in play mode and slowing it down a little. Underneath the idler is an adjustable sliding plate that prevents this happening. Moving this by about 1mm cured the problem. QUOTE]

Interesting, I must check out my TK14 and similar models.

12th Apr 2020, 10:29 pm	#1
martin.m Hexode Join Date: Mar 2018 Location: Bishop Auckland, County Durham, UK. Posts: 373	Grundig TK14 speed problem I have a TK14 that is running too fast. Very noticeable on pre-recorded tapes and anything recorded on the Grundig sounds too slow on other machines. I have made a not very scientific check using a reel of leader tape and the capstan pulls through 100cm of tape in 10 seconds, so about 5 per cent too fast. I can't imagine what could cause this problem though. Any advice would be gratefully appreciated. __________________ Regards Martin Last edited by martin.m; 12th Apr 2020 at 10:31 pm. Reason: accuracy of information

12th Apr 2020, 11:27 pm	#2
ben Dekatron Join Date: Aug 2006 Location: Madrid, Spain / Wirral, UK Posts: 7,498	Re: Grundig TK14 speed problem Belt or idler 'riding' on wrong part of motor pulley? Piece of tape wrapped around capstan shaft? __________________ Regards, Ben.

12th Apr 2020, 11:36 pm	#3
DMcMahon Dekatron Join Date: Apr 2019 Location: Worthing, West Sussex, UK. Posts: 6,603	Re: Grundig TK14 speed problem What is the history behind this, was it previously OK and then changed ? It wouldn't by any chance be a TK 14 U / TK 14L U, a universal mains version with selectable 50/60Hz links/solder tags ? David

13th Apr 2020, 1:43 am	#4
martin.m Hexode Join Date: Mar 2018 Location: Bishop Auckland, County Durham, UK. Posts: 373	Re: Grundig TK14 speed problem Thank you for your replies. Tape wrapped round the capstan spindle was my first thought but it is fine. The drive to the capstan flywheel comes directly from the motor pulley and I can't see anything wrong there. I bought the machine locally about five years ago, and after a quick check pronounced it as fit and well. The speed problem has only come to light after playing one of the Grundig's recordings on another machine. I can't see any links inside for 50 / 60 Hz. There is a provision for 120v operation by moving the position of a glass fuse. This tape recorder has another couple of logic defying problems to sort out. When recording, the incoming audio can be heard loud and clear through the speaker. However this should be impossible, as on record the audio output valve (ECL86) is switched as the bias and erase oscillator. Something is obviously not right with the record/playback switching. Yet it still manages to record so how can it provide audio monitoring, bias and erase at the same time? Last of all, recordings of any speech reveal a faint "echo" with a delay of around half a second. This is audible when the tape is played back on another machine. Perhaps this is tied in with the last fault, or I could be losing my marbles after being confined to the house for so long __________________ Regards Martin Last edited by martin.m; 13th Apr 2020 at 1:54 am. Reason: Clarity

13th Apr 2020, 6:18 am	#5
ricard Octode Join Date: Mar 2006 Location: Lund, Sweden Posts: 1,632	Re: Grundig TK14 speed problem This certainly qualifies for the proverbial 'ghost in the machine'! Ok, here's my guess, probably wrong, but got to start somewhere ... In 'the old days', it was not uncommon to use active components such as valves or transistors for more than one task at once. Especially in radios, a valve might work as both an HF amplifier, and, after demodulation, to amplify the resulting LF before driving the speaker. So I'm wondering if it's fathomable that the output valve, due to incorrect switching, is in fact working both as a bias oscillator and as an LF output valve at the same time! (I'm assuming that the recordings it makes are of reasonable good fidelity, because even without HF bias, there will be a recording of sorts, only rather weak and distorted). Furthermore, it would seem to me that an erase head should be able to work as a record head of sorts, and with the LF already mixed with the bias in the output valve, it would result in the erase head working as a record head, and if again incorrect switching is causing the playback head to play back the signal, and it's mixed with the microphone input, you would have your echo there. All pure speculation, I don't know to what degree this is even academically possible, or if there's any switching malfunction that could cause it to behave this way. Often though, it seems to me that the switching around the output stage / bias oscillator is minimal, with the oscillator coils remaining in circuit at all times, so all that is actually switched is the feedback loop inside the oscillator (and the connection to the erase and record heads).

13th Apr 2020, 10:48 am	#6
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Grundig TK14 speed problem Make sure the record/playback switch is moving fully over when in record. Lawrence

13th Apr 2020, 11:33 am	#7
DMcMahon Dekatron Join Date: Apr 2019 Location: Worthing, West Sussex, UK. Posts: 6,603	Re: Grundig TK14 speed problem As Lawrence & Ricard suggest it definitely sounds like a Record/Playback switching type of issue. When in Record mode there are 2 things that should make it impossible to get normal audio to the speaker. Switch contact 1.11 switches from contact 1.12 (to remove any audio signal feed) to contact 1.10 as part of the bias oscillator circuit. Also switch contact 1.5 switches to 1.4 which short circuits the primary winding of the output transformer. If you do any maintenance on the circuit board you have to make sure that when board is hinged (there is no actual hinge) back up in position that the record actuator arm fits into the hole near the end of the Record/Playback switch, although it not in the hole it should then be impossible for recording to work at all.

13th Apr 2020, 2:54 pm	#8
martin.m Hexode Join Date: Mar 2018 Location: Bishop Auckland, County Durham, UK. Posts: 373	Re: Grundig TK14 speed problem Thank you all for your comments. Lawrence, you were right. The record/playback switch was not moving far enough. There is a small grub screw on the operating mechanism and this had worked loose. Ricard, I think your explanation was spot on. The circuit had been working in a way that the manufacturers had never intended with the erase head acting like a second recording head to produce a faint echo effect. Back to the speed problem, Ben I think you were right. On close inspection the capstan belt looks much to fat and appears to be riding on the outer part of the motor pulley. Can anyone tell me how to remove this pulley? The motor bearings sound like they could do with a drop of oil as there is a screeching noise for a few seconds at switch on. Attached Thumbnails __________________ Regards Martin

13th Apr 2020, 4:59 pm	#9
DMcMahon Dekatron Join Date: Apr 2019 Location: Worthing, West Sussex, UK. Posts: 6,603	Re: Grundig TK14 speed problem Hi Martin, The belt does look fat, see picture of my 1.5mm belt attached, I think the official size is 2mm. My TK 14 has the non finned pulley. I would strongly advise not to remove the pulley unless it is really essential. I personally find it difficult to remove/replace the pulley and the last one I did, the torsion spring snapped and I had to superglue the pulley to the motor shaft. If you do need/want to remove it, then main steps are:- 1. Disconnect mains cable, i.e. make safe. 2. Measure existing height of the centre of the belt groove on the pulley to chassis, spec is 14.5mm 3. Remove left hand reel table/clutch assembly, noting the exact positions of any thin simrit black & any thin metal clutch washers. 4. Remove the rubber idler wheel that is driven by the pulley. I have seen reference that it can be done from the top, but I am yet to see one like that. So remove bottom cover and undo the nut near to the top right (with carrying handle pointing upwards) of motor, that's secures to the bottom of the idler wheel shaft, watch out for the serrated washer behind the nut. You can then remove the idler wheel assembly from the top, it has a washer and a cylindrical piece that are retained on the shaft. 5. To remove the motor pulley you have to lock the motor rotor shaft from turning, then rotate the pulley anti-clockwise while at the same time pulling upwards. This is the bit I find difficult. To refit the pulley lock rotor, turn pulley anti-clock again and push down until correct height, also difficult I find. Often you will find that the pulley moves with its torsion spring but does not always lock onto the shaft or vice versa it locks at wrong height and difficult then to move. Good luck, David Attached Thumbnails Last edited by DMcMahon; 13th Apr 2020 at 5:06 pm.

13th Apr 2020, 5:18 pm	#10
DMcMahon Dekatron Join Date: Apr 2019 Location: Worthing, West Sussex, UK. Posts: 6,603	Re: Grundig TK14 speed problem Also measure the external diameter of the motor pulley to make sure it is a 50Hz one should be a nominal 21mm.

13th Apr 2020, 5:51 pm	#12
DMcMahon Dekatron Join Date: Apr 2019 Location: Worthing, West Sussex, UK. Posts: 6,603	Re: Grundig TK14 speed problem For removing the idler wheel the Service manual says to remove the small plastic retaining washer from the top. On mine the washer has hardened (more like hard rubber) and attempts to remove it just breaks pieces off the washer, it is in a grove so if washer removed and it fell apart, could use a circlip or other retaining washer as replacement.

16th Jul 2020, 3:29 pm	#13
martin.m Hexode Join Date: Mar 2018 Location: Bishop Auckland, County Durham, UK. Posts: 373	Re: Grundig TK14 speed problem I have been catching up with my projects and finally had the time to finish the TK14. Thanks for your advice David. I managed to remove the pulley and take the motor apart so that the spindle could be properly cleaned and lubricated. New capstan belts are available for these old Grundigs but cost around a tenner plus postage. Following advice on this forum, I bought some 1.5mm O ring cord and made a new belt by cutting off the right length and using superglue to join the ends. This doesn't sound like a very good idea but it worked really well. I couldn't snap the belt once the glue had set. Last of all there was a problem with the take up reel not having enough pull to gather up the tape properly when playing. The take up hub is belt driven from the capstan flywheel (through a slipping clutch). Comparison with another working machine revealed that the drive idler was contacting the take up hub in play mode and slowing it down a little. Underneath the idler is an adjustable sliding plate that prevents this happening. Moving this by about 1mm cured the problem. These TK14s and 18s are built like battleships and nothing much seems to go wrong with them. There are no waxies or dodgy capacitors to change and the sound quality is excellent even when using the DIN output to play through a stereo system. How much of today's audio equipment will be around in 60 years time? __________________ Regards Martin Last edited by martin.m; 16th Jul 2020 at 3:32 pm. Reason: typo

17th Jul 2020, 9:52 am	#14
DMcMahon Dekatron Join Date: Apr 2019 Location: Worthing, West Sussex, UK. Posts: 6,603	Re: Grundig TK14 speed problem [QUOTE=martin.m;1270246] Comparison with another working machine revealed that the drive idler was contacting the take up hub in play mode and slowing it down a little. Underneath the idler is an adjustable sliding plate that prevents this happening. Moving this by about 1mm cured the problem. QUOTE] Interesting, I must check out my TK14 and similar models.