Grundig TK 23p Tape Recorder Checkout

[DMcMahon]

Today my first look at this particular example, early 1960's, 4 track, single speed valve recorder, although I have at least 4 others and a TK 23L.

Purchased it 4 months ago together with a TK 14, the TK 14 was sold as "working" (which it basically is) and the TK 23 was sold as "not working".

Externally quite grubby so have given it a partial clean up, now looks not so bad. The 4 white rubber feet were rock hard and starting to crumble apart (quite common with this TK series) so have renewed them.

The top 2 securing screws for the top panel have broken through the top panel (not uncommon), found one piece of broken plastic inside and have glued it back. The other broken though hole has been repaired by gluing a black plastic piece that fitted nicely.

Unusually the two foam spool retainer sponges in the top lid are still fitted and reasonably intact (sized 25mm x 15mm).

After replacing the 13A fuse in the mains plug with a 3A, powered up OK. All HT voltages and valve electrode voltages are in correct ballpark, with low ripple levels. Checked the grid coupling capacitors by checking for zero +ve voltage at the respective control grids, all good.

Was not expecting any issues with the grid coupling capacitors as this TK series generally do not use paper capacitors, typically the coupling caps are Wima Tropyfol (also known as TFM) a type of Polyester film capacitor, these generally are very reliable and have little if any measurable electrical leakage.

Loaded an old 5 3/4" Telefunken tape (or at least the spool is Telefunken) to test the tape transport, very unlikely I have used this tape before.

Fast wind in both directions works well end to end which is a bonus, sometimes this TK series can struggle. Playback speed is very slow, a few checks confirmed it is slow capstan speed due to slack drive belt between the motor pulley and the flywheel. The drive belt between the flywheel and the tape up table may be a little slack also.

Wanted to check the official sizes for these belts and went to Post # 2 of this linked Thread which has loads of Grundig useful reference information including belt sizes but very disappointed to find this site is no longer live !

https://www.vintage-radio.net/forum/...d.php?t=167329

While playing the tape, the playback sound was in reverse and lacking somewhat volume and also noticed that when at the end of tape the EOT (end of tape sensor) was not being actuated by the metallic leader section.

Looking closely at the metallic leader could see that the metallic side was on the rear of the tape, then by looking at the tape itself could see it also was back to front, so the whole tape was wound on back to front. After rewinding with the tape the right way round got normal sound with plenty of volume plus EOT now works.

While testing, occasionally got loud rumbling/heavy vibration coming from the flywheel (could dampen it well by touching the top of the capstan shaft), possibly due to the slack drive belt. When I strip down to replace the belt (s) I also will check out the lower bearing assembly (that the lower end of the flywheel rotates in) in case there is a problem there

I think with some more remedial work it could end up as a reasonable runner.

David

[Cruisin Marine]

Sometimes people store tape backwards to avoid "print through" causing echoes on the audio.

[DMcMahon]

This was the direct link to the Grundig drive belt listing but like its parent link is now dead.

www.taperecorders.nl/RIEMEN-nw.htm

David

[DMcMahon]

Originally Posted by Cruisin Marine

Sometimes people store tape backwards to avoid "print through" causing echoes on the audio.

Good point but if you mean stored tails out some people will say this is an urban myth and print through can occur either way.

I do not think it is likely to be the case with my tape but some old tapes work oxide out (i.e. the opposite of most tapes) due to the heads position on some old machines.

David

[ricard]

Originally Posted by DMcMahon

Good point but if you mean stored tails out some people will say this is an urban myth and print through can occur either way.

The print through will occur either way, but the point is that the print through is most noticeable when it occurs to the top of the magnetic surface, and storing the tape tail out means that this more severe mode of print through will manifest itself as echoes which sound more natural and are more likely to be masked by reverberation etc in the actual program material, rather than pre-echoes which are more unnatural sounding and hence disruptive.

[DMcMahon]

I have heard similar before but never really fully understood, your excellent description now makes it much clearer.

David

[DMcMahon]

After a bit of a strip down replaced the slack motor pulley to flywheel drive belt, didn't have the correct ID size so used a 100mm ID X 1.6mm , which is a bit too tight but OK.

Found an old partial print out of the Grundig drive belt listing that showed the TK 23 as 112mm x 3mm (Engine flywheel mass.), I think 3mm would be borderline too thick in this position, all the belts I have seen before are nearer the 1.6mm thickness.

Removed, stripped, cleaned and relubed the flywheel lower bearing assembly. Any original grease had tuned into hard dry small lumps. Noticed that there is a machined groove near the bottom of the flywheel shaft (do not remember seeing that before) that serves no purpose here.

After re-assembly playback speed now is correct and playback sound is good and there is no sign now of the previous intermittent flywheel vibration/rumble.

Found the on/off switch no longer works, the switch is now permanently on, found that a little curved sprung contact actuating arm is missing, the switch must have got a knock when the heads assembly plate was lifted away. Have given the unit a good shake but no sign yet of the missing piece, that is a nuisance, hopefully it will surface later.

A new fault has developed, every now and again the playback sound goes completely and the EM84 display goes blank, the motor still runs, by giving the side of the unit a good tap can get the fault to come and go, so looks like an intermittent HT voltage connection. Hopefully when inside can simulate the fault then should be fairly easy to trace.

David

[DMcMahon]

By scope monitoring the main HT voltage at the 100uF reservoir capacitor C34 could see that the voltage was very unstable, by tapping the unit could get the voltage to drop to zero. After a few more taps lost the HT voltage all together.

Bottom left of first photo shows the reason, both of the fuse holders have lost their springiness, unable to get them to grip the 125mA HT fuse (mains transformer to bridge rectifier) reliably, so renewed the fuse holders and corroded fuse. HT voltage now rock solid.

David

[DMcMahon]

By more shaking of the unit the missing on/off switch contact switch arm fell out.

Tried unsuccessfully to refit the spring arm, location was just too awkward and it kept dropping it back in the unit. After partially removing the switch on its mounting bracket managed to refit the contact arm after numerous attempts.

While doing this noticed that the white plastic volume control knob was very dirty (stained actually) particularly on its sides, tried to clean in-situ but no joy.

So removed the knob off the potentiometer shaft, it was seriously tight and took some doing. Unfortunately looks like this has wrecked the potentiometer, the shaft now is loose and no longer controls the volume. I will take the pot apart but not confident it can be repaired. Will be difficult to find compatible pot size wise with small shaft (500k Log). Wish now I had left the volume control knob dirty, it only partially cleaned up anyway

David

[DMcMahon]

Took volume pot apart, the wiper disc was quite loose, by strongly pressing down on it achieved a tighter fit and superglued it to stop it coming loose again. Checked the resistance on the pot wiper wrt to both ends by rotating the shaft end to end. Somewhat iffy but a lot better after using contact cleaner on the pot track.

Refitting the cover back on (tried 3 times now) the pot results in the shaft seizing (not rotating) something must be fouling.

David

[DMcMahon]

After numerous failed attempts gave up trying to repair the volume control, got a replacement from my TK 23 # 7, this one -

https://www.vintage-radio.net/forum/...d.php?t=183132

Fitting the donor volume control was quite awkward, getting it in correct position to actuate the mains on/off switch and have full range of adjustment and getting the wires on was very tight, but working fine now.

Found that Play and Rewind now were really struggling so as per Bens Worn Tape Guides Thread -.

https://www.vintage-radio.net/forum/...d.php?t=184892

decided to check mine out. The left and right brass tape guides were worn as well as the EOT sensor post and tape lifter.

Rotated the left and right tape guides plus the EOT sensor post. After a difficult strip and re-assembly managed to rotate the tape lifter post which as expected was riveted. With the right size pliers (that fits down between the Erase and PB/Record head) it may be possible to rotate the tape lifter post without removing anything.

After this there was a noticeable improvement in rewind performance, but not as good as it should be particularly when the Supply spool is heavily loaded with tape, but no change to the seriously struggling Play.

The Pinch Roller was heavily coated with sticky tape dust so gave it a good clean but no change, Tried increasing the Pinch Roller pressure onto the capstan shaft but still struggling to Play.

Found that the Pause Lever that (pushes the Pinch Roller assembly away from the capstan shaft when Pause selected) was too close to the Pinch Roller assembly and effectively stopping the pinch roller making firm contact with the capstan shaft, no adjustment possible so tweaked the lever to just make light contact with the pinch roller assembly during Play. Play is working well and Playback sound is good.

I have glued a flat drive belt around the Supply spool carrier (on top of original black rubber surround), am hoping this will improve rewind, it is quite a thin belt so not sure if it will help.

[Edward Huggins]

Looking a Post#7, that speaker (Goodmans?) looks to have a massive magnet. Grundig always specified the best possible speaker in a given cabinet size.

[DMcMahon]

Yes they do look nice speakers, says Grundig on it.

David

[DMcMahon]

The glued on drive belt work excellently for 10 minutes before it started to come adrift, stronger glue needed, I used PVA previously.

David

[unitelex]

Maybe you could try the "rubber solution" glue as supplied with inner tube repair patches.
Probably available from Halfords or other cycle shop or online

Chris

[DMcMahon]

Yes a good idea, had used the PVA very sucessfully in Post 22 of this linked Thread - https://www.vintage-radio.net/forum/...d.php?t=171737

This was glueing 2 rubber bands to the plastic side wall of the supply table.

I have some inner tube rubber glue somewhere, will look out for it.

David

[DMcMahon]

A few days back was out getting my Covid Booster and was very close to my local Halfords so after the Covid jab drove over to Halfords to get an inner tube repair kit (for the rubber glue) and was very disappointed to find that Halfords had shut down completely. So went to B & Q next door and brought some Loctite general purpose 60 seconds glue.

Used the glue tonight to glue the drive belt to the Supply spool carrier. The gluing effect was almost instant so very difficult to align the belt once it had made contact with the spool carrier. Looks like it has secured very well, will leave it overnight and check the operation tomorrow.

David

[DMcMahon]

Refitted the Supply spool carrier with the glued on drive belt, now fast rewind is much improved, good in fact.

Tested recording, the playback of which does not sound quite right, maybe a speed variation more tests required to quantify further.

There is a small problem with the EM 84 Magic Eye display, with no record signal the display is normal, with a record signal the display narrows as it should do as the record level control is increased but there remains a semi dark band the same width as the no record signal display. I think normal to have similar burnt in shadow but this appears to be worse than normal.

First will try another EM84 to make sure that it is not the valve itself.

David

[DMcMahon]

Originally Posted by DMcMahon

Tested recording, the playback of which does not sound quite right, maybe a speed variation more tests required to quantify further.

David

Forgot to say that Playback of known good recordings is very good.

David

[DMcMahon]

By trying other EM84 Magic Eye valves and checking other TK 23s proved that the semi dark region during recording is down to this one specific valve itself, other valves have similar but to a much less degree. Tried to photo capture it but very difficult to get a good photo, the poor copy attachment somewhat shows the effect.

Did more recordings and the playback definitely sounds as if the speed is varying but so far can find nothing wrong, adjusting the pinch roller pressure does not help. Playback of good recordings is still good.

During recording tests fast winding started playing up with the spools being slow to stop when stop pressed with tape then being spilled out, also the Supply reel table became very noisy. Found that the glued on drive belt on the Supply spool carrier was coming off, that fix/repair did not last long

Removed the drive belt and now find that tape is still spilling out after stopping from fast winding, traced this to the Supply table brake not braking the table. A bit mystified by this at the moment, can see that brake appears to be making full contact with the spool carrier (it makes contact with the rubber surround of the carrier that the drive belt was glued to) but just is not braking it now ?

Would like to understand this better but for the time being will replace the Supply spool carrier with part from spares machine TK 23 # 7.

David