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

Phantomrose1999 2nd Nov 2021 12:48 pm

Grundig TK27 - Low audio output
 
4 Attachment(s)
Hi All,
Got another one and someone has been in it previously.

The playback output is very low, equal to about 20% of normal at max.

It has had new filter caps installed, 47uf / 400v, x 2 (quote small compared to the original can)

Measured the DC and it was low, 220 volts, so removed the old block rectifier and made a new one using 1Kv 3A diodes as the old block rectifiers do go bad.

Put it all back together, and now the DC measures 242 !! no where near the 260-270v stated in the schematic.

Removed the EL95 and the DC came back to 265v, with all other valves in place.


So either something in the EL95 circuit is pulling way more power or the EL95 is playing up and drawing too much power and pulling down the main DC.

Any suggestions from the resident experts please ? do EL95s go leaky ?

DMcMahon 2nd Nov 2021 1:37 pm

Re: Grundig TK27 - Low audio output
 
I am somewhat surprised that HT voltage is not higher than 265V with the EL95 output pentode removed, this is the main load. So at this stage I personally would not strongly suspect it.

Have you scope measured the HT ripple in case it is excessive, should not be with the new filter capacitors.

Later when I get back home I will do comparison readings on my TK 27.

David

DMcMahon 2nd Nov 2021 6:24 pm

Re: Grundig TK27 - Low audio output
 
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

DMcMahon 2nd Nov 2021 7:52 pm

Re: Grundig TK27 - Low audio output
 
My experience with some of the Grundig valve TKs is that the main +HT DC voltage can sometimes be substantially lower than the stated value on the schematic and the unit can still work normally, also the valve electrodes voltages can still be relatively close to the values shown on the schematic.

So what I am hinting at is that your low HT voltage may not necessarily be the root cause of the low playback volume. Would suggest you measure the various valve electrode voltages to see if any are substantially different to the schematic values and check the components around suspect voltage areas.

Apart from the low volume is the sound quality otherwise OK ?

Is the volume low both on new and old recordings ?

David

Phantomrose1999 2nd Nov 2021 11:54 pm

Re: Grundig TK27 - Low audio output
 
David,

Feedback much appreciated...:-)

I have quite a few of these, not the TK27 though. The voltages of the HT is close to 270v after changing the old rectifier..

Prior to changing the rectifier the voltage under load with all valves was 220v, now at least its up to 242v under the same conditions, but no where near the 260-270v

I am wondering if the new main filter caps put in are the problem, as i have always put in the much larger guitar amp caps, not these little ones, though they are rated to 47uf/400v.

Have not tried recording yet, playback is very low, about 10-20% of normal. Will try play again, after the new rectifier was installed.

DMcMahon 3rd Nov 2021 12:05 am

Re: Grundig TK27 - Low audio output
 
Modern capacitors/electrolytics are generally smaller, I would think yours are OK. Yes some of the modern can capacitors are similar size to vintage parts, I wonder if this is done for cosmetic size compatibility.

My mains transformer secondary output to the old Westinghouse EC 401 bridge rectifier is 232VAC.

David

Phantomrose1999 3rd Nov 2021 4:07 am

Re: Grundig TK27 - Low audio output
 
I just tried it on play, now that the HT is up to 242, and its a little louder than before, tone control fine.. just no volume when cranked up.

I need to find why the HT is not closer to the 260-270 mark.

will try to steal an EL95 from one of the other Grundig units.

Chris55000 3rd Nov 2021 10:14 am

Re: Grundig TK27 - Low audio output
 
Hi!

C.A. Quarrington's "Radio and Television", (1963 edition) Volume 2, Chapter 1, Power Supply and Decoupling", gives typical performance curves for a metal rectifier with a capacitor–input filter, and a good full wave bridge unit should give a d.c. output of approximately 20% more than the r.m.s. a.c. input voltage – for example, the Elizabethan LZ29/LZ29L quotes "225V" as the h.t. secondary voltage from the mains transformer and "270V" for the rectifier output, and there are several Ekco and Murphy circuits using metal bridge rectifier units that also agree with this!

The grid–bias voltage of an EL95 is about –8.5V to –9.5V for 250V on anode and screen with respect to cathode which agrees with Grundig's Figure!

Lack of volume in a tape recorder, should lead initially to a very careful examination of the R/P head, as deposits bad enough to severely restrict volume and treble output can be present without you actually being able to see them, so use a "chamois leather" or microfibre cleaning cloth and IPA to make sure the head is scrupulously clean first!

The next step is to check, against the circuit diagram, all the contacts of the record/playback changeover switch, looking for any high–resistance ones – even an ohm or two at the input due to tarnish, oxidation, etc., end can seriously impair volume because of the very low signal voltages handled!

Thirdly, are the anode and screen voltages of RŲ 1/RŲ 2 (EF86) correct? The low cathode current taken by this valve can lead to "cathode poisoning", which results in a chemical degredatlon of the emissive cathode coating – either replace the valve or temporarily shunt a 10k 2W resistor across R103/R203 (anode) and a 47k resistor across R104/R204 (screen–grid) and run the recorder for a couple of hours without the EL95s, this should help clear any cathode poisoning that may have developed in the EF86 valves!

If nothing comes to light from the above, the electrolytic cathode decoupling capacitors C111/C211 and C12 should be checked for partial or complete open– circuits, whilst lack of volume and poor bass indicates partially open–circuit anode–to–grid coupling capacitors! (C3, C110/C210 and C112/C212)

Very poor volume and bad distortion can be caused be s/c turns in the primary of the output transformer, but a d.c. resistance comparison with one in the other channel should pick this fault up!

Chris Williams

Phantomrose1999 3rd Nov 2021 10:27 am

Re: Grundig TK27 - Low audio output
 
2 Attachment(s)
Thank you Chris for the informative answer.

I did some more measurements and found the low HT is still the problem, i suspect.

1) original, HT 222v

2) Replaced bridge with 4x diodes, HT 242V (sound louder)

3) Replaced EL95 from a good working unit, HT 252v after warm up, much higher closer to 297v at first power on (sound louder)

4) Measured AC input to rectifier is 202v AC !!!!

5) Found the Rectifier side fuse had 2.7v drop, replaced it, no difference.

So i am wondering why the AC output from the transformer is so low.

When installing the new rectifier diodes, i noticed a black wire floating around that looked like it came from the transformer. Taped it up and let it be.

looking at the schematic there is a black wire from the transformer, but how could it work if not connected ? unless power was being routed through the motors coils to the transformer on the return line ??

Phantomrose1999 3rd Nov 2021 10:34 am

Re: Grundig TK27 - Low audio output
 
Quote:

Originally Posted by DMcMahon (Post 1420240)
Modern capacitors/electrolytics are generally smaller, I would think yours are OK. Yes some of the modern can capacitors are similar size to vintage parts, I wonder if this is done for cosmetic size compatibility.

My mains transformer secondary output to the old Westinghouse EC 401 bridge rectifier is 232VAC.

David

David, i am getting 202v AC input to the rectifier, i think this is the primary problem. the unit is set correctly for 240 volts.

Phantomrose1999 3rd Nov 2021 11:04 am

Re: Grundig TK27 - Low audio output
 
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Well this Is the black wire from the transformer that is floating in air

Any ideas where it should go please ?

Phantomrose1999 3rd Nov 2021 11:20 am

Re: Grundig TK27 - Low audio output
 
1 Attachment(s)
More investigation around the missing black wire.

See photo please.

According to the schematic, the black wire from the transformer must be connected to PIN 5 of the voltage selection board, and clearly its not.

PIN 5 does have the switches AC wire coming in from the mains switch, and there is a pink wire going to the motor, but the black wire to the transformer is missing !!

Before i solder it up and watch for fireworks, appreciate some feedback pls.

Phantomrose1999 3rd Nov 2021 11:33 am

Re: Grundig TK27 - Low audio output
 
Measured the resistance from the black wire in free air, to PIN 3 of the voltage board, and its 46.x ohms.

So now i am sure this black wire must be connected to PIN5

Bed time, so will try it tomorrow hopefully..

ms660 3rd Nov 2021 12:13 pm

Re: Grundig TK27 - Low audio output
 
Quote:

Originally Posted by Phantomrose1999 (Post 1420330)
Measured the resistance from the black wire in free air, to PIN 3 of the voltage board, and its 46.x ohms.

So now i am sure this black wire must be connected to PIN5

Bed time, so will try it tomorrow hopefully..

That resistance measurement tallies up with what's given in the manual, and in the schematic the Black wire does connect to the Pink motor wire and tag 5 on the voltage selector, which would mean that with the Black wire disconnected and the voltage selector set to say 240V that only the 130V section of the transformer would be powered and it would be powered via the motor, that on the face of it might account for the low HT secondary AC voltage.

Lawrence.

DMcMahon 3rd Nov 2021 12:29 pm

Re: Grundig TK27 - Low audio output
 
The black transformer wire also takes one side of the mains to the bottom of the lower primary winding, the secondary of which provides the H1 filament heater supply for some of the valves, so presumably the valves effected will not have the full heater voltage.

David

DMcMahon 3rd Nov 2021 12:34 pm

Re: Grundig TK27 - Low audio output
 
Quote:

Originally Posted by Phantomrose1999 (Post 1420305)

When installing the new rectifier diodes, i noticed a black wire floating around that looked like it came from the transformer. Taped it up and let it be.

Well maybe the moral here is investigate where it belongs, do not ignore until proven one way or the other :)

David

DMcMahon 3rd Nov 2021 7:03 pm

Re: Grundig TK27 - Low audio output
 
Disconnected my transformer black wire from terminal 5 for comparison purposes.

Basically got very similar voltage results to you, 202VAC into the bridge rectifier and HT of +239V BUT in my case the playback sound did not noticeably change at all, i.e. still good volume and good quality. This ties up with my previous experiences where the HT voltage can be quite dramatically low and everything still appears to function normally.

Hopefully in your case when you reconnect the black transformer wire it will restore your sound to good volume but based upon my test I am not confident that it necessarily will.

The heater filament supply to the relevant valves measured 5.88V with the black wire disconnected, very hard to see how this happens ?

David

DMcMahon 3rd Nov 2021 11:34 pm

Re: Grundig TK27 - Low audio output
 
Refitted my black wire and all back to normal, both filament supplies now measure the expected 6.3V

While refitting the tight black wire, the brown transformer wire broke off connection 6 which was even tighter, had fun getting that soldered back on.

David

Phantomrose1999 4th Nov 2021 8:33 am

Re: Grundig TK27 - Low audio output
 
1 Attachment(s)
Thanks to all, and David for measuring the AC voltage with the black wire disconnected. Much appreciated.

I reconnected the black transformer wire to PIN 5 and we have 283V HT now, its higher than the normal, because i have a new 4x Diode bridge.

So, i tried play, and its again a little louder, but no where near what it should be!! lol. The quality is excellent, the high notes are super clear. Tried many different tapes as well.

So, now that the power supply is ok, i will move onto testing the EL95 amp with a direct signal injection, at the input cap, will also check the voltages in the AMP.

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.

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

Next attempt at the weekend.

DMcMahon 4th Nov 2021 5:35 pm

Re: Grundig TK27 - Low audio output
 
My TK 27L volume control works fine, with increasing volume throughout the adjustment range. Compared to similar TK mono recorders (like TK 14, TK 23 etc) it is noticeably louder, at volume setting 2 it is more than loud enough for normal listening.

The volume control pot is driven by the volume control knob via a plastic drive belt/strap, make sure your drive belt is driving the pot correctly.

David

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
 
1 Attachment(s)
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 ?

Phantomrose1999 28th Nov 2021 2:34 am

Re: Grundig TK27 - Low audio output
 
2 Attachment(s)
Measures the two clear plastic caps the 1nf and the 5.6nf. They are perfect !

Resoldered all the joints near there.

It seems to oscillate and motorboat when pushing record. Just a few more presses of the record and it goes away.

Getting a good bias at the head.

About 198v PP at 54khz.

Will let it run with the cro on and see if it does off

Phantomrose1999 28th Nov 2021 6:15 am

Re: Grundig TK27 - Low audio output
 
1 Attachment(s)
Well, the story almost ends today, as i dont have more time but this is what i found so far.

1) It did record and plays exceptionally well, see video of what was recorded on it from an Iphone 6

https://www.youtube.com/watch?v=o2g3u2okLX8

The unit is temperamental !!! observations:

The volume does drop with a lot of background hiss, and motor boating at times, especially if i press the record. The fix is the pulse the red record button enough times until it goes away.

The record bias fires up most of the time, but not 100% of the time. I did resolder half the bottom of the PCB and especially all the osc coils and related components, seems to help.

So it seems to play well, provided i dont though the record buttom.

I though it was the head playing up, and resoldered all the connections but no difference.

ONE IMPORTANT point, when the volume is low i guess its oscillating, because if i tough the input at the head and send a loud 50hz pulse, the volume comes back !! This leads me to believe the ECC81's are the problem, as i had a Grundig radio where i had the exact same symptoms, and replacing it fixed it. (motorboaring, drop in volume, etc)

So until i get some new ECC81's that it for now.

PS: the sound is just beautifuil for such a small unit. !!

TIMTAPE 28th Nov 2021 10:16 am

Re: Grundig TK27 - Low audio output
 
Quote:

Originally Posted by Phantomrose1999 (Post 1428037)
The unit is temperamental !!! observations:

The volume does drop with a lot of background hiss, and motor boating at times, especially if i press the record. The fix is the pulse the red record button enough times until it goes away...

Dirty, intermittent record/play switches causing problems are very common. Perhaps this has been a / the problem all along?

Phantomrose1999 28th Nov 2021 12:01 pm

Re: Grundig TK27 - Low audio output
 
Quote:

Originally Posted by TIMTAPE (Post 1428067)
Quote:

Originally Posted by Phantomrose1999 (Post 1428037)
The unit is temperamental !!! observations:

The volume does drop with a lot of background hiss, and motor boating at times, especially if i press the record. The fix is the pulse the red record button enough times until it goes away...

Dirty, intermittent record/play switches causing problems are very common. Perhaps this has been a / the problem all along?

Hi, I replaced the sliders with a 3D printed version. I sanded every contact in the record play block bottom right, top right looking from the back. Also large amounts of Isopropyl, and made sure the gaps are good in both positions. Finally I stripped and cleaned the head select switch.

There are no contacts left that have not been sanded and cleaned.

I do have a spare TM45 which is in excellent working condition. Will swap the ECC81s when I get a chance. They have caused me similar problems in Grundig radios.

But the lack of a reliable oscillator start is not known.

Phantomrose1999 1st Dec 2021 11:53 am

Re: Grundig TK27 - Low audio output
 
3 Attachment(s)
Well progress at last. Changed all Valves, removed and checked volume control, disconnected head and checked it. No different, low volume, no bias oscillator

Then removed the upper to right switch band and sanded all contacts. Much better but still intermittent low/high volume. While tapping the lower right switch asm it changed and started whistling. Ahhh.

So removed it again, found one switch with the shield was not shorting when closed. So sanded every one again thoroughly until very shiny. Lol.

Now itís playing beautiful ! And I have a solid bias every time. But it is not recording any audio ! Erase head has bias and so does the head but the magic eye is not flickering when recording

I think I soldered the 330k resistor in the wrong spot in the tag strip. Can anyone please tell me where this resistor should be connected to ? It correct at the top switch end, but not sure about the tag strip end.

Please see photo with yellow circle.

ms660 1st Dec 2021 12:43 pm

Re: Grundig TK27 - Low audio output
 
Can't find a 330k in the schematic but there are three outboard 220k's in there.....R2, R3 and R8 that are connected to switch numbers at3.2, at3.1 and at3.5 respectively, if it's one of those the schematic will show you what it's connected to in circuit.

Lawrence

Phantomrose1999 1st Dec 2021 12:43 pm

Re: Grundig TK27 - Low audio output
 
This was the clue that I needed to resand the lower right switch bank

https://youtu.be/sq2atvkxyS0

Only one prob left, no audio in, must be that 330k resistor as above.

Phantomrose1999 1st Dec 2021 12:47 pm

Re: Grundig TK27 - Low audio output
 
Quote:

Originally Posted by ms660 (Post 1429012)
Can't find a 330k in the schematic but there are three outboard 220k's in there.....R2, R3 and R8 that are connected to switch numbers at3.2, at3.1 and at3.5 respectively, if it's one of those the schematic should will show you what it's connected to in circuit.

Lawrence

Tks. This set has been worked on by someone else so who knows if he added this. Sure looks like a 330k. Will do some tracing next chance but it plays solidly and the bias osc is perfect now.

DMcMahon 1st Dec 2021 3:00 pm

Re: Grundig TK27 - Low audio output
 
1 Attachment(s)
Pretty sure it is R8 220k (the reds can look a bit orange) going to contact "at 3,5"

Connects to the bottom of the terminal strip, where a live core of a screened cable connects.

David

DMcMahon 1st Dec 2021 4:35 pm

Re: Grundig TK27 - Low audio output
 
Quote:

Originally Posted by Phantomrose1999 (Post 1429013)
This was the clue that I needed to resand the lower right switch bank

https://youtu.be/sq2atvkxyS0

So do you think that him pressing what looks like a pin on the head was enough pressure to make a switch contact make better contact ?

David


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