Re: Grundig TK27 - Low audio output
 

Well, the sending a 50HZ pulse at the head seemed to bring the volume back, and then i started tapping around and found the switch with the metal shield over the switch in the right hand lower corner bank, was in fact not shorting when it should, and after another thorough cleaning of all contacts, it played perfectly last nigh.

This morning, from a cold start, it was very quiet when i tried to play, and after a few minutes, it suddenly started to make a slight hissy sound, and play was back at full volume. So there is a faulty resistor or cap somewhere in there, that needs to warm up. Any suggestions ?

When it plays well, but i have not pressed play, there is a slight random buzzing and hissing coming from the unit, defnitely a component is failing or on the boarder.. but where..

Re: Grundig TK27 - Low audio output
 

Very difficult, maybe a blast of hot air to some of the PCB components to see if that shortens the wait period to reach full volume, to try and isolate the problem to specific area.

David

Edit Update - Also suggest record the valve electrode voltages at the start and after the volume becomes good, to see if any major differences which may then point to the components around a particular valve.

Re: Grundig TK27 - Low audio output
 

Connects back to capacitor C3 (22nF), which is physically at the track selection switch.

David

Re: Grundig TK27 - Low audio output
 

Thank you all. Have ordered a 1W 1000x pack if resistors from eBay and the 22nf coupling cap. Will attack it when I get these.

Re: Grundig TK27 - Low audio output
 

Thanks David, checked it again, and without the resistor, there is no where for the screened cable to go, so the resistor must in the right place. Good to know red looks like orange.

Re: Grundig TK27 - Low audio output
 

Could not resist a poke at the beast, so.

Replaced C3 (22nF), which is the input cap to the main AMP. Original measured 140nf.

Removed and checked the 470p across the output transformer, and it was 1000p, did not have another so put it back

Opened the lower switch cover again, to see why record was dead with no audio, and found one wire has indeed come undone at the last switch box in the inside (motor) side. Soldered it back up and was hoping for the best..

Now for the puzzle..

Tried Play - volume is gone low again, notes

- when the volume is turned up, but play not presses, there is a loud hiss, proportionate to the volume control, like a record player, when you lift the arm and turn the volume up.

With play pressed, and no tape, touching the wires at the back of the head track 1-2, cause a very loud buzzz, proportionate to the volume control, makes me think the amp is fine.

I am wondering if the play head for tracks 1-2 has a intermittent problem ? (i did measure the resistance and shorts to ground, all same as the other track)

Now for something even more strange, when i switch to tracks 3-4, there is ZERO hiss, but then touching the other set of head wires, makes absolutely no sound. So it would seem the path from head to amp for tracks 3-4 is broken. The head measure fine, with resistance and no shorts to ground.


PS: did try record, but no audio getting through, used to work fine, until i attached the rec/play switches.

Re: Grundig TK27 - Low audio output
 

I may have found "a" problem, any expert care to help read this diagram please


I circled the part in red boxes, is this supposed to be shorting both ends with the middle conductor ? such that all 3 are shorted ? I opened every switch to make sure this condition does not happen.

I only have a german service manual which is a bad scan....

Tks

Re: Grundig TK27 - Low audio output
 

It's just the printing, the rest position for the change over pole in the switches you've highlighted is to the right hand contact only, in the manual I've got at1,4 does actually show a very small gap between the left hand contact but not at2,2

The contacts shown in the schematic are shown in their rest positions so far as I can make out.

Lawrence.

Re: Grundig TK27 - Low audio output
 

Yes agreed.

I have an original A3 page showing these contacts which also looks like the left contact of 2,2 is making contact but as Lawrence explains there should be a gap in the rest position.

David

Re: Grundig TK27 - Low audio output
 

Yes the Bias record signal can appear large especially when Peak to Peak monitored, i.e. 64V RMS (for 182V Peak to Peak) does not sound that high.

Checked my TK 27L, Record Bias at the head |(white wires) was 180V Peak to Peak at around 55kHz (very similar to your 54kHz at Post 42).

Erase signal at the Erase head (red & blue wires) was 23V Peak to Peak.

Bias signal at output of the Bias Oscillator coil (blue wire) was 460V Peak to Peak.

David

Re: Grundig TK27 - Low audio output
 

Tks David. Found two TK23’s in the collection and they are very similar to the tk27. I suspect the head is playing up in the tk27

Re: Grundig TK27 - Low audio output
 

well, had some time on the weekend, and did the following.

Slowly brought up the TK23 that has been sitting for at least a decade, and it actually plays really well !! was bought for spares.

So, removed the head from the TK23 and installed it in the problem child the TK27 and now the TK27 plays clearly, and seems consistent, the volume is low !!! So its not the head !

Observations:-
There is a loud hiss, from the speaker, when play is not pressed, and its proportionate to the volume control. Its also there when playing and is most likely the cause of the low volume output.

Made a video, hope you can take a look at this hissing, there is also buzzing sounds at random.. most likely a resistor is breaking down ?

https://youtu.be/tl4ridod6K8

Pushing the record button causes all sorts of loud bangs, pops and crackle, which is not good. Wonder if the gap between the contact that looks like its almost touching could be one of the problems.

Anyway, i will put the TK27 head into the TK23 next and try and do some voltage comparisons... or take the TK23 switch box out and compare it to the TK27 to check for correct gaps.........its taking so much time....

Re: Grundig TK27 - Low audio output
 

looks this thread is exactly the problem i am having on playback, so will do what he did, go over every track and joint on the PCB..

https://www.vintage-radio.net/forum/...d.php?t=147558

Re: Grundig TK27 - Low audio output
 

The fact that the hiss level can be controlled by the volume control would indicate that the source of the hiss is before the volume control, i.e. around the two EF 86 pre-amp valves.

Certainly check for bad soldering around the valves and dirty valve base socket pins etc.

Using scope you should see the hiss on one or both of the EF 86's and hopefully home into problem area.

The TK 23 contacts are arranged physically differently compared to the TK 27 with most of the PB/Record switching being done on the long slide switch.

David

Re: Grundig TK27 - Low audio output
 

Just before work had to tinker, and i may have the best clue yet,

With the unit powered on, and NO buttons pressed, i touched the head wire and got the 50hz buzz, then i pushed the plate with the pinch roller towards the head, and spun the reels manually AND I GOT SOUND !!!

So it looks like the head is constantly connected to the amp, even without the play being pressed, and i know it should not do this.

So will have to trace the wiring to find out why this is happening. May be the root cause of the problem with the hiss..

Re: Grundig TK27 - Low audio output
 

So far as I can make out, looking at the schematic the head is connected to the amplifier without start being selected and any signal from the head will find it's way to the audio output stage, eg: for head tracks 1-2 >> at1,4 & at1,5 >> EF86 >> ECC81 >> so10 >> so8 >> at3,5 >> EL95.

But...

Without start being selected the tape recorders loudspeaker should be muted due to the switch contacts of st3 which should closed and the output to the radio socket should be disconnected because the contacts of st1 should be open.

Lawrence.

Re: Grundig TK27 - Low audio output
 

Missed the edit window, should be so8 >> so10 and not the other way round.

Lawrence.

Re: Grundig TK27 - Low audio output
 

Would you have a clear scan of the switch bank locations please ? I only have a very poor German scan that is very blurred. I want to check the stated speaker switch

Re: Grundig TK27 - Low audio output
 

Here's the link to the manual/schematic I've been referring to:

https://www.vintageshifi.com/reperto...ice-Manual.pdf

Lawrence.

Re: Grundig TK27 - Low audio output
 

Big Thank you Lawrence !!!! I have the same but yours is so clear I can read everything. This will help greatly. Also can read the voltages on the pcb.

Re: Grundig TK27 - Low audio output
 

No problem.

Lawrence.

Re: Grundig TK27 - Low audio output
 

Well, finally progress !! though not at the end yet.

1) Thanks to Lawrence for highlighting the speaker should be shorted, and sure enough the contact was not shorting it, done by previous owner who changed the belts. Slight adjustment of speaker shorting switch and its quiet until you press play.

2) As has been suggested here many times, i went over EVERY joint on the PCB, sucked out a few and put fresh solder for ones that looked like a blob, and then did the first wafer of the track selector switch as well, and its playing beautifully, So loud i can only run it at 3/4, its a beautiful and clear and loud sound quality.. just love it.

Bought a TK20 from the original owner yesterday and it came with 8 tapes, full of German music, and its incredibly clear and loud on the TK27.. i really cant believe the sound quality. (TK20 motor not spinning, ordered a 1.5uF motor run cap)

Back to the TK27, last problem is the record !! I have a magnificient bias signal at the erase head, and at the play/rec head, (about 130x volts at 54Khz) BUT the magic eye is not moving and the oscillator is not being modulated.

i may have caused this, when a wire on the lower right switch block came off, need to check where i soldered it onto.. Never had this problem originally.

Which switch would cause ALL input to not reach the EF86 ? given the magic eye is not even moving ? (phono, mic, radio..none)

Re: Grundig TK27 - Low audio output
 

Well i spoke too soon...

I pressed the red record button, and i am back to low playback volume !!! :-(

So, next chance i get, will attack the original switch bank again, around at1.2, at 1.3, at 1.4. now that i can read the schematic and match it to a switch bank.

Cant imaging if switching the oscillator on / off would kill the volume, so it has to be contacts in that record / play bank..

Re: Grundig TK27 - Low audio output
 

last post for the week, heading to sydney on the weekend.

Measured every voltage on the PCB according to the schematic, all are a little higher because i put 4 x diodes in. But close to what it should be

Took out the record / play switch to check, and found "at1.1" was not making contact when pushed in, so adjusted and was excited, maybe the record problem was fixed.

Tried record and the magic eye was not moving.. so no progress.

Tried play and the volume was still low so gone backwards

After pulsing the record switch very fast, basically like side stepping a clutch, and letting the switch come back fast under spring pressure, i now have full volume back. !!

I am still suspicious of "at1.4" as its shown very close in the service manual and maybe a timing issue with it. I opened it up to get more clearance in either position, as it was originally very close together.

Some questions please:

1) Should the magic eye move during play ? I think It used to do that and has stopped, forgot.

2) i tried it without the metal PCB shield and only got loud buzzing, so put it back.. I have put insulation tape on parts of the shield, as the original insulator came off. could it be oscillating that is causing the low volume..

3) My mobile phone when transmitting is causing the buzzing i kept hearing, put it in flight mode and the intermittent buzzing has gone. Never seen this before.

Anyway, i wont touch the record button for a week, and just see if play is
consistent as its working perfectly at the moment.

When i next get a chance, i will do it properly and connect a sig gen to the head input, and trace it through all stages with a cro..

Re: Grundig TK27 - Low audio output
 

It may not be that the signal is not reaching the EF 86 (s) but the signal is getting though the EF86 (s) and the ECC81 (s) but then the signal is not getting to the EM84 Magic Eye.

David

Re: Grundig TK27 - Low audio output
 

David

Re: Grundig TK27 - Low audio output
 

David and Lawrence,

Big thank you for all your feedback, i have 100% proof of fixing the intermittent low play volume. final problem, maybe other fixes along the way as well, but most likely this was the real cause.

While playing, a tap on the table would drop the volume, (more tapping around the outer case, resulted in decreased and increase of the volume)

As i had a friend with me, we played it vertically, (he held the reels in) and with the PCB Dropped and the record / play switch shield off, i found putting pressure on middle contact of at1.4, the sound came back to 100%, and when let got, to rely on spring pressure the volume would drop !!!

So, some more sanding of at1.4- C102 contacts, and increasing the pressure on the middle contact its playing well, and no amount of tapping or banging can change the volume.

When i get time next will chase the record problem, as David suggests. Now that i have a readable manual and solid bias at the head and erase, finding why the audio is not modulating the bias should be an easy fix

Most simple but challenging problem i had so far..

Re: Grundig TK27 - Low audio output
 

[QUOTE=DMcMahon;1433000]I think you are spot on with at1.6, as i had broken a pink wire and may well have soldered it back in the wrong spot, or the contacts need more work !!

When i am back from a work trip to sydney, will attack this.

Re: Grundig TK27 - Low audio output
 

Well, the long story has a very happy ending !!

The wire from at6 to the PCB connecting to (R121,R123,C109) had broken off and was hiding ! Soldered it back, cleaned the contacts again, traced the wire with my multi-meter and its recording in stunning fashion, and of course playback is also stunning.

Also the same wire (other channel) from at 2.6 was off, but i had soldered this previously. So i though i found all the broken wires, guess not

I will make a video of it recording and playing back when i get time, but this is one of the best sounding and compact units they made.

The amp has way more power than the little speaker, and will connect an external one day.

Note: it does not record on tracks 2-3, using the track selector switch, but "stereo" and "tracks 1-2" work fine.

Summay of problems:

Low volume caused by at1.4 not contacting well, due to me opening the gap up.

Also maybe re-soldering every joint on the board helped.

Finally record problem was fixed after i re-soldered the wire i broke at 6, as mentioned.

Now life can continue and time to pack for Sydney.


Huge thanks to Lawrence for giving me the manual, and Davids feedback since the start. This is one stunning unit.. :thumbsup:

Re: Grundig TK27 - Low audio output
 

Well done, good progress I bet you are relieved.

David

Re: Grundig TK27 - Low audio output
 

Quite happy but also concerned as this was my challenge for the last few weeks. Feel a little lost. Lol.

Well I can play with free cad and the 3d printer next. I want to design and make a copy of the basf spools. Quite interesting.

If anybody has a broken slider I can send them the design I used for this tk27. Takes 10m to print one.