Re: EMT 938 - turntable does not run to speed on any of the 3 speed settings - UK Vintage Radio Repair and Restoration Discussion Forum

Nickuk99 · 22nd Jun 2021, 6:55 pm

Hi all,

Hi all,

I’m new to EMT turntable ownership, I have Studer tape recorders and Neuman, Siemens Sitral, analogue outboard gear.

I’ve just picked up an EMT 938 that is working but not running to speed at any of the 3 selected speeds, I wonder if anyone here can point me in the right direction? Motor issue? or bias board or oscillator fault perhaps.

Any help most appreciated

I look forward to hearing from you

Nick

barretter · 23rd Jun 2021, 10:56 pm

It would appear that nobody on this forum has any experience of this high quality German turntable but there is a service manual on vinylengine.com.
You have to join up to the website to get the pdf but it's free.

Peter.N. · 24th Jun 2021, 9:57 am

I certainly haven't but I do remember some old domestic decks that had a stepped sleeve over the motor spindle that came loose and slid down it, mixing all the speeds but I suspect this has a belt drive.

Peter

barrymagrec · 24th Jun 2021, 10:24 am

I think they are direct drive - could be wrong.

Analogue man · 24th Jun 2021, 11:10 am

This isn't something simple like a 60hz machine running off 50hz by any chance?

analogadikt · 24th Jun 2021, 1:41 pm

The EMT 938, 948 and 950 are direct drive broadcast decks. There is no stepped pulley and the supply frequency does not matter.

Regards,

Anwesh

Ted Kendall · 26th Jun 2021, 6:12 pm

Is the speed off on all three speeds? Is it merely a matter of pitch or is the wow out of spec as well?

majoconz · 28th Jun 2021, 1:18 am

If it's mains frequency independent then there must be a frequency standard somewhere. Whether it's just a feedback system measuring the speed of rotation (strobe scale around the rim?) and adjusting the DC voltage to the motor, or a crystal oscillator to a frequency divider driving a synchronous motor at the moment is unknown. Crystal oscillator and a phase locked divider is relatively easy with today's electronics but what age is your deck?

Here's a very good 'youtube' I found which you might like to watch before you go delving into the works.....!

https://youtu.be/IHLdbVYWWmA

and I see from this video that the "Sync" is done from a strobe around the rim of the turntable.

Radio Wrangler · 28th Jun 2021, 3:39 am

The video does give some (not a lot) information on the turntable, but the presenter is so far over the top in overdoing presentation, that I found myself longing for an advert to come along! Extreme smugness alert!

David

gramofiend · 28th Jun 2021, 10:39 pm

Hello. I have been lucky in obtaining several EMT decks as the BBC were throwing theirs away and do have both the 948 and the 938. If you join up to the vinyl engine you will find that the direct drive motor unit in the 938 is common to that in the 948 but whereas the electronics crate below the platter and arm on the 948 contains 9 boards, mounted vertically and posted horizontally into the crate. All players were supplied with an empty servce card into into which could be plugged a suspect card which was then able to protrude from the machine to allow for test gear to make fault finding more easy. The 938 is different in that it was made slightly cheaper and was not provided with the illumination tube and provided only with a lid mechanism which was the same as that provided on an earlier Thorens 126. The electronics were devided between two boards mounted horizontally unerneath the turntable in the plinth box. One of these boards is for power supply and motor control, the other is for audio. If you look underneath, you may have to support the turntble chassis on something like a u shaped open plinth to drop the steel door but should be able to see the leads from the tonarm to the audio board.
The motor conections are all on multi pins. There is a 36 pn D connector on the back allowing connection of remote functions including start/stop and varispeed, in all of the set speeds. If there is a plug in that socket, try taking it out to see if it cures your problem. It really is a very sophisticated machine, up there with the best and designed to perform 24/7 for years, though of coourse it is, I believe, 1994 since the last ones rolled off the assembly line at Lahr in Germany. Parts are still available but a motor will set you back upwards of £900-00!

Hope some of this helps.

Mike

dave cox · 29th Jun 2021, 10:29 am

You didn't say if it runs too fast or too slow ?
Does the speed change when you selected the different speeds.

elektrotanya has the manual.

The control system is dc motor with tachometer feedback and a PLL control.
The ref clock has an additional 'control' - maybe pitch bending? - in addition to a crystal (?) ref and divider for the different speeds.

You'll probably find the control loop is slammed fully one way or the other. If the control loops is saying full speed, and its not going fast enough, then the motor may be the issue.

I must improve my German !

dc

Nickuk99 · 24th Oct 2021, 4:18 pm

Hi All,

Many thanks for your help and ideas.

The problem ended up being a faulty motor.

I managed to find a NOS EMT 938 motor in Switzerland.

Now all speeds working as per spec.

The EMT 938 has been totally recapped for good measure as well.

Many thanks again.

Nick

Jonster · 24th Oct 2021, 9:37 pm

Glad to hear it's working. I remember hearing of an EMT 948 (which uses a similar tachometer feedback system) being 'clouted' by an annoyed presenter on a certain national pop music radio station back in the 1980's, the glass tachometer sensor disk shattered and the platter ended up running at about 1000RPM!

22nd Jun 2021, 6:55 pm	#1
Nickuk99 Triode Join Date: Mar 2017 Location: Reading, Berks, UK. Posts: 23	Re: EMT 938 - turntable does not run to speed on any of the 3 speed settings Hi all, Hi all, I’m new to EMT turntable ownership, I have Studer tape recorders and Neuman, Siemens Sitral, analogue outboard gear. I’ve just picked up an EMT 938 that is working but not running to speed at any of the 3 selected speeds, I wonder if anyone here can point me in the right direction? Motor issue? or bias board or oscillator fault perhaps. Any help most appreciated I look forward to hearing from you Nick

23rd Jun 2021, 10:56 pm	#2
barretter Heptode Join Date: Jan 2006 Location: Todmorden, West Yorkshire, UK. Posts: 870	Re: EMT 938 - turntable does not run to speed on any of the 3 speed settings It would appear that nobody on this forum has any experience of this high quality German turntable but there is a service manual on vinylengine.com. You have to join up to the website to get the pdf but it's free.

24th Jun 2021, 9:57 am	#3
Peter.N. Dekatron Join Date: Jul 2008 Location: Charmouth, Dorset, UK. Posts: 3,601	Re: EMT 938 - turntable does not run to speed on any of the 3 speed settings I certainly haven't but I do remember some old domestic decks that had a stepped sleeve over the motor spindle that came loose and slid down it, mixing all the speeds but I suspect this has a belt drive. Peter

24th Jun 2021, 10:24 am	#4
barrymagrec Octode Join Date: Dec 2017 Location: Morden, Surrey, UK. Posts: 1,557	Re: EMT 938 - turntable does not run to speed on any of the 3 speed settings I think they are direct drive - could be wrong.

24th Jun 2021, 11:10 am	#5
Analogue man Hexode Join Date: Dec 2020 Location: Raunds, Northamptonshire, UK. Posts: 353	Re: EMT 938 - turntable does not run to speed on any of the 3 speed settings This isn't something simple like a 60hz machine running off 50hz by any chance?

24th Jun 2021, 1:41 pm	#6
analogadikt Tetrode Join Date: Aug 2010 Location: Mumbai, India. Posts: 98	Re: EMT 938 - turntable does not run to speed on any of the 3 speed settings The EMT 938, 948 and 950 are direct drive broadcast decks. There is no stepped pulley and the supply frequency does not matter. Regards, Anwesh __________________ Anwesh

26th Jun 2021, 6:12 pm	#7
Ted Kendall Dekatron Join Date: Jul 2008 Location: Kington, Herefordshire, UK. Posts: 3,670	Re: EMT 938 - turntable does not run to speed on any of the 3 speed settings Is the speed off on all three speeds? Is it merely a matter of pitch or is the wow out of spec as well?

28th Jun 2021, 1:18 am	#8
majoconz Heptode Join Date: Jan 2008 Location: Ashhurst, Manawatu, New Zealand Posts: 570	Re: EMT 938 - turntable does not run to speed on any of the 3 speed settings If it's mains frequency independent then there must be a frequency standard somewhere. Whether it's just a feedback system measuring the speed of rotation (strobe scale around the rim?) and adjusting the DC voltage to the motor, or a crystal oscillator to a frequency divider driving a synchronous motor at the moment is unknown. Crystal oscillator and a phase locked divider is relatively easy with today's electronics but what age is your deck? Here's a very good 'youtube' I found which you might like to watch before you go delving into the works.....! https://youtu.be/IHLdbVYWWmA and I see from this video that the "Sync" is done from a strobe around the rim of the turntable. __________________ Cheers - Martin ZL2MC Last edited by majoconz; 28th Jun 2021 at 1:34 am.

28th Jun 2021, 3:39 am	#9
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,869	Re: EMT 938 - turntable does not run to speed on any of the 3 speed settings The video does give some (not a lot) information on the turntable, but the presenter is so far over the top in overdoing presentation, that I found myself longing for an advert to come along! Extreme smugness alert! David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

28th Jun 2021, 10:39 pm	#10
gramofiend Pentode Join Date: Aug 2010 Location: Cowbridge, Vale of Glamorgan, UK. Posts: 137	Re: EMT 938 - turntable does not run to speed on any of the 3 speed settings Hello. I have been lucky in obtaining several EMT decks as the BBC were throwing theirs away and do have both the 948 and the 938. If you join up to the vinyl engine you will find that the direct drive motor unit in the 938 is common to that in the 948 but whereas the electronics crate below the platter and arm on the 948 contains 9 boards, mounted vertically and posted horizontally into the crate. All players were supplied with an empty servce card into into which could be plugged a suspect card which was then able to protrude from the machine to allow for test gear to make fault finding more easy. The 938 is different in that it was made slightly cheaper and was not provided with the illumination tube and provided only with a lid mechanism which was the same as that provided on an earlier Thorens 126. The electronics were devided between two boards mounted horizontally unerneath the turntable in the plinth box. One of these boards is for power supply and motor control, the other is for audio. If you look underneath, you may have to support the turntble chassis on something like a u shaped open plinth to drop the steel door but should be able to see the leads from the tonarm to the audio board. The motor conections are all on multi pins. There is a 36 pn D connector on the back allowing connection of remote functions including start/stop and varispeed, in all of the set speeds. If there is a plug in that socket, try taking it out to see if it cures your problem. It really is a very sophisticated machine, up there with the best and designed to perform 24/7 for years, though of coourse it is, I believe, 1994 since the last ones rolled off the assembly line at Lahr in Germany. Parts are still available but a motor will set you back upwards of £900-00! Hope some of this helps. Mike

29th Jun 2021, 10:29 am	#11
dave cox Nonode Join Date: Jan 2009 Location: Bristol, UK. Posts: 2,062	Re: EMT 938 - turntable does not run to speed on any of the 3 speed settings You didn't say if it runs too fast or too slow ? Does the speed change when you selected the different speeds. elektrotanya has the manual. The control system is dc motor with tachometer feedback and a PLL control. The ref clock has an additional 'control' - maybe pitch bending? - in addition to a crystal (?) ref and divider for the different speeds. You'll probably find the control loop is slammed fully one way or the other. If the control loops is saying full speed, and its not going fast enough, then the motor may be the issue. I must improve my German ! dc

24th Oct 2021, 4:18 pm	#12
Nickuk99 Triode Join Date: Mar 2017 Location: Reading, Berks, UK. Posts: 23	Re: EMT 938 - turntable does not run to speed on any of the 3 speed settings Hi All, Many thanks for your help and ideas. The problem ended up being a faulty motor. I managed to find a NOS EMT 938 motor in Switzerland. Now all speeds working as per spec. The EMT 938 has been totally recapped for good measure as well. Many thanks again. Nick Attached Thumbnails

24th Oct 2021, 9:37 pm	#13
Jonster Heptode Join Date: Apr 2008 Location: Eastbourne, East Sussex, UK. Posts: 671	Re: EMT 938 - turntable does not run to speed on any of the 3 speed settings Glad to hear it's working. I remember hearing of an EMT 948 (which uses a similar tachometer feedback system) being 'clouted' by an annoyed presenter on a certain national pop music radio station back in the 1980's, the glass tachometer sensor disk shattered and the platter ended up running at about 1000RPM!