Grundig TK 5 Restoration - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

DMcMahon · 5th Mar 2020, 7:05 pm

Quote:

Originally Posted by John Caswell

DMcMahon wrote
"Turning the tape over gave much better quality and volume but sound in reverse,"
I know this sounds silly but in my servicing days it was common for people to record on the wrong side of the tape, having wound it on the spools the wrong way or kinking it twisted or splicing it wrongly.

John

I know what you mean John, but I don't think this was the case here.

David

DMcMahon · 6th Mar 2020, 10:55 am

Damaged microphone cable replaced and tested OK.

DMcMahon · 7th Mar 2020, 9:52 am

Tried another tape (2 track) to test playback and both sides played well, so that is encouraging.

Even though usable the brightness of the EM85 Magic eye is not that strong, will compare it to another of my Grundigs that use the same valve and also try swapping the valve over.

The Target resistor R32 should be 200k and measures 260k so I will replace that to see if it makes any difference. Also see that the Anode resistor R33 (20k) is shown in some TK 5 schematics as 50k so may also try changing R33.

DMcMahon · 16th Mar 2020, 11:09 am

None of my other old Grundig's use the EM85 Magic Eye valve, so cannot swap the valve, will try changing those resistors above.

ricard · 18th Mar 2020, 8:23 pm

Although not an option unless EM85's become really scarce, but the EM81 appears to have the same form factor and display as the EM85 (in contrast to the EM80 favoured in radios); I wonder what the practical difference is, apart from the pin connection arrangement. The data sheets while not identical look fairly similar at a quick glance.

DMcMahon · 19th Mar 2020, 12:03 pm

Thankyou for the information Ricard. A quick look and it does look like the EM81 could be a suitable alternative with some simple wiring changes to the valve base.

I see all these Magic Eye valves can be expensive with some cheaper ones from Russia.

I need to get round to checking out recording to quantify better if the brightness level is useable (I think it may be).

The valve voltages are quite a bit higher than the values shown on the schematic and I guess may be even higher when the two resistors are replaced. These voltage measurements were done using a 10Mohm DVM, will check them also on an AVO to see how much they reduce.

hillmanie · 19th Mar 2020, 6:19 pm

These period microphones are a work of art in themselves, good to look at.
Ref selenium rectifiers, unobtainable? I take it that it's OK to use a modern capsule type replacement?
TT

DMcMahon · 19th Mar 2020, 8:57 pm

Yes I also love the look of these old microphones.

I have yet to replace any Selenium rectifiers but have been thinking about it for many months on a couple of the old Grundigs, where their output voltage is maybe lower than it should be, due to the natural degradation of the Selenium caused by ageing.

Not sure what you mean by capsule type ? I am planning just to make a bridge rectifier using 4 Silicon rectifier diodes, in my case 1N4007 as I have a quite a few of these, they are good general purpose rectifier diodes each rated at 1A/1000V, they are maybe a bit overkill for the Grundigs but do provide extra margin.

Some people say that because the Silicon diodes are more efficient, that you get higher output voltage from the bridge so it is necessary to add a series resistor to drop the extra voltage down, also to help limit current surge at switch on. I personally doubt if this is actually important in our typical applications.

DMcMahon · 19th Mar 2020, 9:04 pm

While measuring the EM85 Magic Eye voltages, I discovered that the valve base wiring did not agree with the schematic. I removed the valve to double check where pin 1 was physically on the base and discovered that the fitted valve was actually a EM81.

Checking the actual valve base wiring to the EM81 pin out, shows that original EM85 Magic Eye must have been replaced at some time by the EM81 and the valve base wiring changed to suit the EM81.

hillmanie · 19th Mar 2020, 10:14 pm

Re rectifiers for want of a better name what I'm calling a capsule is the 4 sided metal box, the one you'll recognise with the 4 solder post legs. (I'll try a pic attachment) They're a couple of £ from the Far East now though I've previously bought small qties. for £1 for battery charger experiments. I expect they're as good as the old '60s types?
TT

DMcMahon · 20th Mar 2020, 7:21 pm

You mean something like this, which is fine although this one is well over rated.

Refugee · 20th Mar 2020, 7:37 pm

I would be very tempted to salvage one from a blown up SMPSU as they normally survive and can be used again.

hillmanie · 20th Mar 2020, 10:32 pm

Quote:

Originally Posted by DMcMahon

You mean something like this, which is fine although this one is well over rated.

That's exactly the pic I tried to upload but 'failed, invalid file type (it was a .PNG)
Cheers
TT

DMcMahon · 21st Mar 2020, 12:22 pm

I must by psychic then, time to do the lottery

DMcMahon · 23rd Mar 2020, 11:03 pm

Did some quick microphone recordings tonight using modern technology as the sound source (well relatively modern Ipod Nano in JBL sound dock).

Recordings were OK, a little bit of noise/hum in the background, need to repeat the recordings using DIN line input to see if the noise is common to microphone and line inputs.

The Magic Eye brightness is OK/useable (looks brighter in the photo than it actually is).

DMcMahon · 23rd Mar 2020, 11:26 pm

Mistaken comment in my post # 13, 5 3/4" is max spool size.

DMcMahon · 23rd Mar 2020, 11:29 pm

Quote:

Originally Posted by Refugee

I won't be revealing the belt goo on mine just yet as I have a nice late 1970s Aiwa cassette machine waiting to have its belt goo revealed first.

I have not yet experienced any of the old Grundigs with belt goo, but plenty of broken and badly cracked drive belts.

Refugee · 24th Mar 2020, 1:06 am

I have indeed got a TK14 that had the broken end of one of the belts hanging out of the vent slots in the base of the cabinet when I acquired it.
Phillips ones are much more gooey.

DMcMahon · 25th Mar 2020, 9:46 am

Replaced high/incorrect (269k by measurement) value resistor R32 for the Target of the EM81 Magic Eye with correct value 200k. This reduced the Anode voltage from 216V to 205V, made no difference to the brightness of the Magic Eye.

Tried replacing the Magic Eye anode resistor R33 (20k) with a 51k because later versions of the TK 5 schematic show it to be 50k, this reduced the anode voltage down to 175V but made the Magic Eye display go out of focus
(with no change to the brightness) so refitted the original R33.

Compared the Magic Eye anode voltage DVM measurement to my two Model 8 Mk. III Avo's and got somewhat unexpected results. One Avo gave basically gave identical results, around 220V (done before R22 replacement) to my DVM and the other Avo was around 145V. So sometime will look at both Avo's to determine which one is actually more accurate on DC voltage.

DMcMahon · 25th Mar 2020, 6:01 pm

Tried to do some DIN line recordings, get good signal on the Magic Eye but no playback signal and have also lost previous good microphone recordings.

While investigating, really loud horrible clicks started regularly occurring from the speaker even with volume turned right down, so looks like something is breaking down, one step forward, one step back !

6th Mar 2020, 10:55 am	#22
DMcMahon Dekatron Join Date: Apr 2019 Location: Worthing, West Sussex, UK. Posts: 6,600	Re: Grundig TK 5 Restoration Damaged microphone cable replaced and tested OK. Attached Thumbnails

7th Mar 2020, 9:52 am	#23
DMcMahon Dekatron Join Date: Apr 2019 Location: Worthing, West Sussex, UK. Posts: 6,600	Re: Grundig TK 5 Restoration Tried another tape (2 track) to test playback and both sides played well, so that is encouraging. Even though usable the brightness of the EM85 Magic eye is not that strong, will compare it to another of my Grundigs that use the same valve and also try swapping the valve over. The Target resistor R32 should be 200k and measures 260k so I will replace that to see if it makes any difference. Also see that the Anode resistor R33 (20k) is shown in some TK 5 schematics as 50k so may also try changing R33.

16th Mar 2020, 11:09 am	#24
DMcMahon Dekatron Join Date: Apr 2019 Location: Worthing, West Sussex, UK. Posts: 6,600	Re: Grundig TK 5 Restoration None of my other old Grundig's use the EM85 Magic Eye valve, so cannot swap the valve, will try changing those resistors above.

18th Mar 2020, 8:23 pm	#25
ricard Octode Join Date: Mar 2006 Location: Lund, Sweden Posts: 1,632	Re: Grundig TK 5 Restoration Although not an option unless EM85's become really scarce, but the EM81 appears to have the same form factor and display as the EM85 (in contrast to the EM80 favoured in radios); I wonder what the practical difference is, apart from the pin connection arrangement. The data sheets while not identical look fairly similar at a quick glance.

19th Mar 2020, 12:03 pm	#26
DMcMahon Dekatron Join Date: Apr 2019 Location: Worthing, West Sussex, UK. Posts: 6,600	Re: Grundig TK 5 Restoration Thankyou for the information Ricard. A quick look and it does look like the EM81 could be a suitable alternative with some simple wiring changes to the valve base. I see all these Magic Eye valves can be expensive with some cheaper ones from Russia. I need to get round to checking out recording to quantify better if the brightness level is useable (I think it may be). The valve voltages are quite a bit higher than the values shown on the schematic and I guess may be even higher when the two resistors are replaced. These voltage measurements were done using a 10Mohm DVM, will check them also on an AVO to see how much they reduce.

19th Mar 2020, 6:19 pm	#27
hillmanie Pentode Join Date: May 2019 Location: Galway, Republic of Ireland. Posts: 208	Re: Grundig TK 5 Restoration These period microphones are a work of art in themselves, good to look at. Ref selenium rectifiers, unobtainable? I take it that it's OK to use a modern capsule type replacement? TT

19th Mar 2020, 8:57 pm	#28
DMcMahon Dekatron Join Date: Apr 2019 Location: Worthing, West Sussex, UK. Posts: 6,600	Re: Grundig TK 5 Restoration Yes I also love the look of these old microphones. I have yet to replace any Selenium rectifiers but have been thinking about it for many months on a couple of the old Grundigs, where their output voltage is maybe lower than it should be, due to the natural degradation of the Selenium caused by ageing. Not sure what you mean by capsule type ? I am planning just to make a bridge rectifier using 4 Silicon rectifier diodes, in my case 1N4007 as I have a quite a few of these, they are good general purpose rectifier diodes each rated at 1A/1000V, they are maybe a bit overkill for the Grundigs but do provide extra margin. Some people say that because the Silicon diodes are more efficient, that you get higher output voltage from the bridge so it is necessary to add a series resistor to drop the extra voltage down, also to help limit current surge at switch on. I personally doubt if this is actually important in our typical applications.

19th Mar 2020, 9:04 pm	#29
DMcMahon Dekatron Join Date: Apr 2019 Location: Worthing, West Sussex, UK. Posts: 6,600	Re: Grundig TK 5 Restoration While measuring the EM85 Magic Eye voltages, I discovered that the valve base wiring did not agree with the schematic. I removed the valve to double check where pin 1 was physically on the base and discovered that the fitted valve was actually a EM81. Checking the actual valve base wiring to the EM81 pin out, shows that original EM85 Magic Eye must have been replaced at some time by the EM81 and the valve base wiring changed to suit the EM81.

19th Mar 2020, 10:14 pm	#30
hillmanie Pentode Join Date: May 2019 Location: Galway, Republic of Ireland. Posts: 208	Re: Grundig TK 5 Restoration Re rectifiers for want of a better name what I'm calling a capsule is the 4 sided metal box, the one you'll recognise with the 4 solder post legs. (I'll try a pic attachment) They're a couple of £ from the Far East now though I've previously bought small qties. for £1 for battery charger experiments. I expect they're as good as the old '60s types? TT

20th Mar 2020, 7:21 pm	#31
DMcMahon Dekatron Join Date: Apr 2019 Location: Worthing, West Sussex, UK. Posts: 6,600	Re: Grundig TK 5 Restoration You mean something like this, which is fine although this one is well over rated. Attached Thumbnails

20th Mar 2020, 7:37 pm	#32
Refugee Dekatron Join Date: Apr 2012 Location: Worksop, Nottinghamshire, UK. Posts: 5,553	Re: Grundig TK 5 Restoration I would be very tempted to salvage one from a blown up SMPSU as they normally survive and can be used again.

21st Mar 2020, 12:22 pm	#34
DMcMahon Dekatron Join Date: Apr 2019 Location: Worthing, West Sussex, UK. Posts: 6,600	Re: Grundig TK 5 Restoration I must by psychic then, time to do the lottery Last edited by DMcMahon; 21st Mar 2020 at 12:48 pm. Reason: Typo correction

23rd Mar 2020, 11:03 pm	#35
DMcMahon Dekatron Join Date: Apr 2019 Location: Worthing, West Sussex, UK. Posts: 6,600	Re: Grundig TK 5 Restoration Did some quick microphone recordings tonight using modern technology as the sound source (well relatively modern Ipod Nano in JBL sound dock). Recordings were OK, a little bit of noise/hum in the background, need to repeat the recordings using DIN line input to see if the noise is common to microphone and line inputs. The Magic Eye brightness is OK/useable (looks brighter in the photo than it actually is). Attached Thumbnails Last edited by DMcMahon; 23rd Mar 2020 at 11:18 pm. Reason: Correction

23rd Mar 2020, 11:26 pm	#36
DMcMahon Dekatron Join Date: Apr 2019 Location: Worthing, West Sussex, UK. Posts: 6,600	Re: Grundig TK 5 Restoration Mistaken comment in my post # 13, 5 3/4" is max spool size.

24th Mar 2020, 1:06 am	#38
Refugee Dekatron Join Date: Apr 2012 Location: Worksop, Nottinghamshire, UK. Posts: 5,553	Re: Grundig TK 5 Restoration I have indeed got a TK14 that had the broken end of one of the belts hanging out of the vent slots in the base of the cabinet when I acquired it. Phillips ones are much more gooey.

25th Mar 2020, 9:46 am	#39
DMcMahon Dekatron Join Date: Apr 2019 Location: Worthing, West Sussex, UK. Posts: 6,600	Re: Grundig TK 5 Restoration Replaced high/incorrect (269k by measurement) value resistor R32 for the Target of the EM81 Magic Eye with correct value 200k. This reduced the Anode voltage from 216V to 205V, made no difference to the brightness of the Magic Eye. Tried replacing the Magic Eye anode resistor R33 (20k) with a 51k because later versions of the TK 5 schematic show it to be 50k, this reduced the anode voltage down to 175V but made the Magic Eye display go out of focus (with no change to the brightness) so refitted the original R33. Compared the Magic Eye anode voltage DVM measurement to my two Model 8 Mk. III Avo's and got somewhat unexpected results. One Avo gave basically gave identical results, around 220V (done before R22 replacement) to my DVM and the other Avo was around 145V. So sometime will look at both Avo's to determine which one is actually more accurate on DC voltage. Last edited by DMcMahon; 25th Mar 2020 at 10:07 am.

25th Mar 2020, 6:01 pm	#40
DMcMahon Dekatron Join Date: Apr 2019 Location: Worthing, West Sussex, UK. Posts: 6,600	Re: Grundig TK 5 Restoration Tried to do some DIN line recordings, get good signal on the Magic Eye but no playback signal and have also lost previous good microphone recordings. While investigating, really loud horrible clicks started regularly occurring from the speaker even with volume turned right down, so looks like something is breaking down, one step forward, one step back !