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-   -   Grundig TK27 - Low audio output (https://www.vintage-radio.net/forum/showthread.php?t=185423)

DMcMahon 5th Nov 2021 12:17 am

Re: Grundig TK27 - Low audio output
 
Quote:

Originally Posted by Phantomrose1999 (Post 1420658)

Will try to record as well, once i work out what pins are the input.

For the line inputs - Platte/Gram (Phono) the 2 signal inputs are pins 3 & 5, for the Radio/Diode the 2 signal inputs are pins 1 & 4. As always with the DIN connectors pin 2 is the screen connection.

David

Edward Huggins 7th Nov 2021 12:55 pm

Re: Grundig TK27 - Low audio output
 
Quote:

Originally Posted by Phantomrose1999 (Post 1420658)
Thanks to all, and David for measuring the AC voltage with the black wire disconnected. Much appreciated.

The volume control is a suspect, as it reaches max volume about mid way, and nothing much happens after, and in fact at the MAX, its slightly lower volume.

Next attempt at the weekend.

I know you've been dilligent with voltage testing, but this fault is symptomatic of HT starvation to the output stage.

Phantomrose1999 10th Nov 2021 10:51 am

Re: Grundig TK27 - Low audio output
 
Some good news, finally got time to play again today, and i dropped the PCB down and measured all voltages around the EL95, and they were all a little high, but very close to the specs..

So while it the PCB was down, i touched the head signal wires and there was a loud buzz much louder than then original low volume problem...hmmmm

So while it was vertical, i managed to get a tape on, hold the reels from falling off and press play !! and WOW ! it was normal, with enough volume to cause heavy distortion when turned up.. So this meant when the PCB was down it played loudly.

Next i put just the PCB back and it still played fine, then started putting the metal shield on and my friend noticed some spacers were longer then others and i have them randomly.

So, we put the long spacers on the righ, and the shorter spacers on the left, and did up all the screws and its playing like it left the factory !!!

Seems i had fixed the original problem of low voltage with the missing transformer wire, and putting the PCB spacers incorrectly also caused low volume.. maybe it was oscillating and that killed the audio volume.. I was going to put a CRO on it, but no needed now.

It puts out around 2.5W like most of the EL95 units and i think it will sound good with an external speaker.

Next will be a record test.. then it gets posted to youtube..ha. (if it can record that is)

Phantomrose1999 10th Nov 2021 11:40 am

Re: Grundig TK27 - Low audio output
 
Tried to record, and as expected it did not work...

The magic eye did everything it should, moved to the music and volume control can be used to adjust it. But nothing being recorded, strange tones at time, and not even erasing the old stuff.

Looks like another oscillator not working problem... It still has the original wax caps in it, maybe some need changing..

Will put a CRO on it next time to verify the oscillator is not running or the level may be too low..

DMcMahon 10th Nov 2021 7:07 pm

Re: Grundig TK27 - Low audio output
 
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Well done on getting the low volume sorted. I have previously spotted the difference in the spacer lengths before. On the older similar TK series (like TK 23) the spacers are metal and are the same length plus secured with screws. On the TK 27 they are plastic and secured by nuts, the left white ones are around 5mm and the right hand side black ones around 7mm long.

This difference in spacer length is due to the metal screening cover on the TK 27 being profile shaped on the left hand side to be around 2mm closer to the PCB, so to keep all of the metal cover parallel with the PCB the difference size spacers are used, no idea why the cover is profiled, it is just flat on the previous similar TKs.

Maybe when you had your spacers incorrectly fitted it allowed the cover to short to something on the PCB, the cover is earthed through the mountings.

David

Phantomrose1999 10th Nov 2021 10:57 pm

Re: Grundig TK27 - Low audio output
 
David,

Yes, i can only assume that the shield was touching something with the wrong spacers, but i would have expected some sparks, strange it just lowered the volume though.

1) The red record button is incredible hard, is this normal on a TK27 ? most other units it requires a very light push.

2) looks like the contacts at the top right corner need to be checked as they do the switching of the EL95 from amp to oscillator..

C13, C14 seem to be the only caps in the oscillator circuit..

David

Phantomrose1999 11th Nov 2021 6:06 am

Re: Grundig TK27 - Low audio output
 
Quote:

Originally Posted by DMcMahon (Post 1420112)
Voltage measurements (DVM) for comparison on my TK 27L were :-

Main HT +261V (Playback), +268V (Record) with 100Hz (10mS) ripple 4.5V p-p

HT on other side of filter resistor R17 (2.7K) = +236V, 40mV ripple.

Main HT with EL95 removed +282V

Main HT with no valves fitted +294V

David

While checking the lack of an oscillator with my CRO, i measured the following ripple as well.

HT straight from the rectifier, 5.52v, about 100hz.
HT 120mV P-P after the filter resistor, also about 100hz


Back to chasing why no oscillator..

Phantomrose1999 11th Nov 2021 7:09 am

Grundig TK27 No Record
 
3 Attachment(s)
There is no erase signal at the erase head. The record red button is very hard to press.

So I removed the switch bank at the lower right of the case, under a metal shield. I cleaned all the contacts with light emery paper, and noticed some important clues.

1) The tight button push is because there is some of the old crappy metal in the sliding part of the switch that has deteriorated, and some of it has fallen off. but it does move, though sticky and I will free it up.

2) I have noticed one switch does not move the contacts when the switch is pushed in or out. See photos with yellow circle around it. You can see the switch is pushed in, but the centre spring in the contact block is not moving. This can't be good !

I will try to get this switch to open and close with the pushing of the button. With the unit on its side, speaker up, the switch is at the top of the block, facing the motor.

8 Tracker 11th Nov 2021 8:20 am

Re: Grundig TK27 - Low audio output
 
1 Attachment(s)
As soon as I read that the record button was hard to press I knew what the problem was. It's that spacer, lower middle of your first photo. As you have noticed, it swells up and disintegrates. That stops the slider from moving and operating the switch contacts. There are two spacers, one either side of the switch mechanism.

I've got two TK27s and both had this problem. On one machine one spacer had fallen to bits while the other was perfect, the one in the pic, but on the second machine both were affected.

The only option is to make new ones. I used some epoxy putty which I shaped roughly by hand, then left it to harden and filed to size and shape. Don't know what facilities you have, but I don't see why one couldn't be made with a 3D printer.

They are a royal pain to get out and just as bad to get a replacement in and all the screws tightened. Hope you manage to come up with something.

Mark.

Phantomrose1999 11th Nov 2021 11:02 am

Re: Grundig TK27 - Low audio output
 
1 Attachment(s)
being fundamentally lazy, i loosened the screws holding the sliding block a little and then super glued them so the screws dont move. Lots of oil and it moves very freely now.

Also lightly sanded every contact, then used contact cleaner and dried them. All switches on both halves of the slider open and close every contact as they should.

Put it back together and had the low volume problem again, AND no bias signal.. lol Whole fiddling with the volume control full volume came back so i replaced three wax caps that did not look healthy, 2x big 0.22uF, (one was suspect when measured) and a 0.033uF all on the main PCB. Put it back together and have full volume. So one down.

The record is still a problem, when pressing the red record button, at one attempt i heard a "clunk" and the bias signal appeared !! see image.

At the next attempt of course it was gone. It did come back a few times, but cant get it back now.

I think the switch with the sliders at the bottom right is fine now, there must be another switch that is part of the record / amp switching.. any suggestions ?

Phantomrose1999 11th Nov 2021 11:41 am

Re: Grundig TK27 - Low audio output
 
1 Attachment(s)
Well, i opened up the switch to take another look and one slide, the left side looking from the back has jamed up indeed and was not returning to its top most position.

So, have taken out the offending slide and will think about a better solution. Its broken, but i think JB Welc can hold it together..

Shame no one makes a 3D printed version of it. I can take it to the office and ask one of the CAD guys to re-create it.. ahh..

DMcMahon 11th Nov 2021 4:05 pm

Re: Grundig TK27 - Low audio output
 
Sounds like you are having lots of frustrating fun and games :)

Yes my TK 27L Record button is noticeable stronger/harder to push down compared to similar TKs like TK 14, TK23 etc.

I think in my case it is just due to the stronger sprung loaded mechanism of the TK 27, I do not think I have a problem with those sliders, which I cannot easily even see on mine. A difficult area to work on.

David

DMcMahon 11th Nov 2021 4:44 pm

Re: Grundig TK27 - Low audio output
 
Quote:

Originally Posted by Phantomrose1999 (Post 1422735)
It puts out around 2.5W like most of the EL95 units and i think it will sound good with an external speaker.

I think all the other similar other TK's are rated at 2.5W but to my ears the TK 27 sounds noticeably louder.

David

DMcMahon 11th Nov 2021 7:20 pm

Re: Grundig TK27 - Low audio output
 
If you have problem with erase not working again, another thing to double check is the single erase contact that is normally closed that is opened when the Record button is rotated 90 degrees clockwise for Superimpose recording (this disables erase), i.e. make sure the contact is making good contact.

David

Phantomrose1999 19th Nov 2021 11:20 am

Re: Grundig TK27 - Low audio output
 
1 Attachment(s)
As i mentioned earlier the sliders in that great switch assembly has disintegrated, but i stuck it together enough to measure it up.

Just installed FreeCAD and had my first attempt at creating a replacement 3D part made. See Jpg. the sizes are close to what i measured up.

I also had to buy a printer which was a deep exercise in thought as the options and range is silly, but thats a good thing. Settled for a Sidewinder X2, which should arrive late next week and i can test out my first attempt at a CAD replacement.

May be a while before becoming proficient at FreeCAD but made a replacement slider design in an evening, thanks to the great youtube tutorials..

DMcMahon 25th Nov 2021 8:50 pm

Re: Grundig TK27 - Low audio output
 
Well done, sounds very promising.

David

Phantomrose1999 26th Nov 2021 9:44 am

Re: Grundig TK27 - Low audio output
 
3 Attachment(s)
Well, i got my Sidewinder X2, assembled it in about 30m, then the fun started.

- Master design was done in FreeCAD based on measurments, took a few evenings and youtube tutorials to pick it up enough to make the new slider.

- Next had to save the file as a STL Mesh, and import it into Cura, which slices it into a printable format.

- selected the X2 as a the printer with all defaults, again with Youtubes help and hit print !!

To my total amazement, it printed one, then i cranked it to high resolution and printed two more. See photos.

Unbelievable hobby grade tech !!! see photos. Next is to put the TK27 back together and see how close my measurements were.

Phantomrose1999 27th Nov 2021 1:57 pm

Re: Grundig TK27 - Low audio output
 
1 Attachment(s)
Well, I made two 3D sliders after many attempts.

The switch is working well as far as I can tell, it plays very well

But there is no bias oscillator ! Ahhh

I canít fault the lower right bank of switches, it must be something else

https://youtu.be/ibWkxKcTYIY

Any suggestions ?

Phantomrose1999 28th Nov 2021 12:10 am

Re: Grundig TK27 - Low audio output
 
1 Attachment(s)
I fiddled with the same switch to make sure all contacts open and close and I got my bias back

The bias voltage at the head is 182v PP. this canít be normal ?

Itís at 41khz

Phantomrose1999 28th Nov 2021 2:03 am

Re: Grundig TK27 - Low audio output
 
1 Attachment(s)
Jumped for joy too soon. I stripped and cleaned the big switch at the top that changes the tracks. Through I had nailed it as the oscillator was starting reliability when pushing record. Even recorded a bit and was fine. Tried a longer record and found the oscillator is now intermittent. Sometimes there then stops. Other times wonít even start.

Any suggestions for a temperamental oscillator ?


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