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Vintage Tape (Audio), Cassette, Wire and Magnetic Disc Recorders and Players Open-reel tape recorders, cassette recorders, 8-track players etc.

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Old 10th Oct 2019, 11:47 am   #1
Phantomrose1999
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Default Grundig TK920 3D Exploding Caps

Just got an immaculate Grundig TK920 3D and was so impressed it worked after just cleaning the fuses... worked great for a couple of hours then LOUD Band and lots of smoke...and the reels stopped spinning.

https://www.youtube.com/watch?v=8LfDbvImIls

https://www.youtube.com/watch?v=67X-z268Z0Q

Been here before, just need to split the chassis, done ! problem is there is a 3.5uf and 1uf in one can, and it goes to different wires in the motor, which i suspect is used to change the speed of the motor.. correct ?

There is no room to put separate 3.5uf and a 1uf capacitor.

Does anyone know where i can get a single can capacitor, with a 3.5uf + 1uf rating please ?

Love this unit, the sound was just stunning with its 3 speakers.
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Old 10th Oct 2019, 12:06 pm   #2
Phantomrose1999
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Default Re: Grundig TK920 3D Exploding Caps

more carnage oh dear..

Looks like two more caps (i think) have leaked black goo and something has had enough force to break red and black wires

Will try to find out what they are...see photos
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Old 10th Oct 2019, 12:31 pm   #3
Phantomrose1999
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Default Re: Grundig TK920 3D Exploding Caps

well, i worked out what the two black tar babies that melted are..

They are a .2uf cap and 100ohm resistor in series, both have melted away

what was a very happy few hours, will take a very long time to fix..

wonder what wattage the 100ohm resistors are ?
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Old 10th Oct 2019, 3:23 pm   #4
ricard
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Default Re: Grundig TK920 3D Exploding Caps

Those 100 ohm + .2 F combos are for noise/arc protection, so the wattage is not critical as there is not a lot of current flowing through them, Half a watt to be on the safe side or so I would think.

Regarding the motor run capacitor combo, yes, the different speeds would seem to require different capacitances. I would guess it's 3.5 F for one speed and 4.5 F for the other (i.e. 3.5 + 1 F in parallel), but I haven't bothered to actually decipher the switching arrangement.

It's not critical that the capacitors are right beside the motor, so you could put them in other locations, depending on how original you want the machine to appear. Have you considered stuffing the old can with modern-day 3.5 and 1 F caps, which might fit given that modern capacitors have a higher capacitance-to-volume ratio? I would guess that the combination would be tricky to find today.

Nice to see what the TK920 looks like inside by the way. Very similar to the TK819 which was one of the first machines I dived into as a pre-teen (older style of case and missing the thumbwheel controls).

Hm, I should check my junk box, as my original TK819 was taken to bits a long time ago, and a second one is in a partly working state but awaiting restoration. Then again, it would probably be the best to get new capacitors for this rather arduous task.
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Old 13th Oct 2019, 2:59 pm   #5
Phantomrose1999
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Default Re: Grundig TK920 3D Exploding Caps

Thanks for the reply..

I have found the following caps and RC networks, though i would share it here first before i order them.

1) Kemet 5% 1uF plasic Can cap ( i think this will fit on the other side of the motor, just above the relays)

2) Kemet 5% 3.5uF Metal Can cap. This will go where the original went

3) Kemet 0.22uF 100Ohm RC networks. Will replace all 4, BUT all 4x have had their wires cut in my unit.. ??

4) There are a large 0.1uF and 0.5uF black caps that are split open, will replace these.

Not thouching anything else..

See data sheets if any body else wants to try these AFTER i test it..ha
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File Type: pdf 0.22uf 100ohm RC KEmet.pdf (52.6 KB, 11 views)
File Type: pdf 1uf Kemet Plastic C27.pdf (616.2 KB, 7 views)
File Type: pdf 3.5uf Kemet Metal Can.pdf (55.5 KB, 10 views)
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Old 13th Oct 2019, 3:12 pm   #6
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Default Re: Grundig TK920 3D Exploding Caps

Anyone care to explain the difference between a "snubber" cap and a "motor run/start" cap ? The snubber caps are much smaller than the later, is it a current handling issue ? Tks
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Old 15th Oct 2019, 11:58 am   #7
Phantomrose1999
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Default Re: Grundig TK920 3D Exploding Caps

Well, i have taken out all 4 of the RC parts, two of which had leaked. But as I mentioned someone has cut the wires to all four of them a long time ago.

Any tips on cleaning the wax off the parts from the exploded motor run capacitor ?

Also any tips to get the black goo off from the RC networks ?

Its such a stunning machine, built to be bomb proof and last several life times from what I can see. This unit has been stored inside, so there is no rust or dust, the chassis is gleaming gold... hard to believe it's so old.
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Old 15th Oct 2019, 12:41 pm   #8
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Default Re: Grundig TK920 3D Exploding Caps

I wonder if someone hasn't cut the wires when the first ones started leaking, to avoid having the others fail. These 'snubber' capacitors (with their integral resistor) are there to mostly to avoid interference (cracks and pops) in the amplifier when the motor current is switched. A secondary purpose is to reduce arcing and wear on the contacts. But functionally they don't have a purpose so the machine will work without them.

I agree, there is something incredibly substantial about these machines, especially the way the amplifier and mechanical chassis separate with just a few connectors needing to be disconnected. Plus all those relays (7 or so) and that enormous motor. Plus there is a physical earth point where black earth wires from all over the machine converge in one point.

It's also impressive to have a solenoid operated machine with a single motor and only three solenoids. Plus being reversing to boot.

I've always wondered what functionality would be offered by a remote control plugged into the remote socket at the back.
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Old 18th Oct 2019, 1:28 pm   #9
Phantomrose1999
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Default Re: Grundig TK920 3D Exploding Caps

I have ordered all the caps from the local commercial online store. Hope it arrives before end of next week, as i would like to get it working again next weekend.

Yes, that is the largest heaviest motor in any of these series, i have several TK35's, one TK64, several TK60's and most of the later ones, like the TK46, 14, 23, and a TK340 big one.. oh i also have a restored TK1 Lexus.. But the TK920 is in a league of its own.. just magnificient !

If i find more i will defnitely buy them, and would like to add a clean TK830, as its the only other one i want, but dont have.

The split chassis is on my computer table, a rather stunning paper weight.. ha.
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Old 19th Oct 2019, 12:52 pm   #10
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Default Re: Grundig TK920 3D Exploding Caps

Quote:
Originally Posted by Phantomrose1999 View Post
Thanks for the reply..

I have found the following caps and RC networks, though i would share it here first before i order them.

1) Kemet 5% 1uF plasic Can cap ( i think this will fit on the other side of the motor, just above the relays)

2) Kemet 5% 3.5uF Metal Can cap. This will go where the original went

3) Kemet 0.22uF 100Ohm RC networks. Will replace all 4, BUT all 4x have had their wires cut in my unit.. ??

4) There are a large 0.1uF and 0.5uF black caps that are split open, will replace these.

Not thouching anything else..

See data sheets if any body else wants to try these AFTER i test it..ha
4) were cut because EVERY Rifa/Evox/Kemet molded paper capacitor is more or less defective out of the factory gate and will fail sooner or later, so this would have been a known problem with a previous repairman having prevented it in the most lazy way.

Make sure to replace 3) and 4) with polypropylene equivalents where possible. Especially boycot Kemet molded paper capacitors. They should have ended production of those 30 to 40 years ago when better products became available. Do a forum search on Rifa for more information.

The replacement types you found for 1) and 2) look allright to me.

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Old 2nd Nov 2019, 11:45 am   #11
Phantomrose1999
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Default Re: Grundig TK920 3D Exploding Caps

Much closer but the fat lady aint singing..

1. Replaced the Motor Caps, with a separate 3.5uf +1uf. See photos, motor works.

2. Cleaned the wax off and the black tar off from everywhere. Used a hair dryer to melt it then wiped it off with cotton buds and tissues.

3. Replaced all the black WIMA caps. Some split but replaced all, including two under the head mechanism..

4. Replaced the 4x spark supressors, (RC networks). Interestingly these were all cut off at some time previously.

THE GOOD NEWS: Everything works, with the tape playing from right to left.

THE BAD NEWS: When i change direction, (to start playing from left to right), it works for a sec or two, THEN THE LEFT REEL LOCKS UP ! Its getting power to lock it, as i can feel it is free at the start, but within a sec or two, it locks.

Does anybody have any ideas ? Tried to read the schematic and no chance to figure it out.. ha.

Possible guesses:

- The left side has the main motor power relay, where i added 2 RC networks. No problem here.

- The right side has another relay, which has the 2x RC networks cut, i added 2x RC networks, which were previously cut. Not sure which relay as its back together. Should i cut these ?

Well, hope someone has some ideas...

Check out the photos, there is also some Youtube video's

https://www.youtube.com/watch?v=IHqDDdRdGsU
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Old 2nd Nov 2019, 12:12 pm   #12
Phantomrose1999
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Default Re: Grundig TK920 3D Exploding Caps

Well now its working playing from left to right, intermittently !! starts to play then there is some motor bearing noise and then the left reel locks up electrically..
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Old 2nd Nov 2019, 12:40 pm   #13
Leon Crampin
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Default Re: Grundig TK920 3D Exploding Caps

If you look carefully at your first picture of the inverted upper chassis, the unidirectional "spring" (actually a handed wound steel tape clutch) is protruding from the bottom of the intermediate pulley assembly.

These "springs" are handed and need very careful handling to avoid getting distorted, thereby becoming unusable. There should be a minute amount of axial play in each assembly - there may be plastic thrust washes, but I can't fully remember. The springs are a very precise fit in each bearing assembly, and the sintered iron bushings need light oil applied sparingly. There are also sintered bearing bushings in the lower and upper turntables.

Leon.
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Old 2nd Nov 2019, 1:06 pm   #14
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Default Re: Grundig TK920 3D Exploding Caps

I had a much earlier model with the same deck. Now long lost.
I did brake a spring and was able to salvage one form a single speed version.
The opposite happened with a broken spring so that the reel spun on and threw tape all over the place as the motor slowed down after pressing stop.
The reel tables have a felt ring in them. Take the reel table apart and check for belt goo contamination of the felt or some other problem in it causing the clutch to snatch.
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Old 2nd Nov 2019, 1:19 pm   #15
Phantomrose1999
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Default Re: Grundig TK920 3D Exploding Caps

Thanks for the reply, but the problem is electrical activation of the Left spool. I have had it play for a few minutes then suddently the left clutch is energised and it stops. However playing from righ to left, no problem at all.

Are you guys suggesting the little spring in the intermediate drive pulley may be the problem ?

Thank you, i really want this working 100%
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Old 2nd Nov 2019, 1:29 pm   #16
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Default Re: Grundig TK920 3D Exploding Caps

From what I can remember the reel tables are braked during play with reduced power to the reel magnet. The reel table is provided with more power for braking when stopping after winding the tape.
A full belt goo check of both reel tables would be my first port of call before looking for an electrical fault.
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Old 2nd Nov 2019, 2:27 pm   #17
Phantomrose1999
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Default Re: Grundig TK920 3D Exploding Caps

Hi Refugee, i just read the TK820/3D manual in English, and the TK12 which is refrernced in the TK820/3D manual. There is a table that shows that each take up spook is totally not powered electrically, during play.

I can feel the reel spin with just the firction of the clutch plates, then suddenly the left spook will lock up, pressing stop, or FF/RW immediately releases the power to the clutch and it becomes free.

Thus i am convinced its Electrical, just no idea where.

I do have a TK920 schematic and few pages of text but its all in German.
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Old 2nd Nov 2019, 2:31 pm   #18
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Default Re: Grundig TK920 3D Exploding Caps

Any idea what the switch at the top righ of the attached photo does ? Its under the head plate assembley and i did change that capacitor, perhaps i bent something there..
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Old 2nd Nov 2019, 3:04 pm   #19
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Default Re: Grundig TK920 3D Exploding Caps

I have no experience of this machine. However, from a mechanical perspective it looks as though that contact bank should be normally open, being closed when the pin just above it presses down.

Does the plate that the pin attaches to pivot, so that the pin could feasibly close those contacts? What happens when they're closed and when they're open?

What is the switch's position in the circuit?
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Old 2nd Nov 2019, 4:55 pm   #20
Leon Crampin
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Default Re: Grundig TK920 3D Exploding Caps

The switch puts a resistor in series with the capstan solenoid when it's fully engaged. This allows a high pull in current without overheating the solenoid in use. The other contacts probably actuate the muting relay so that the amplifier is enabled only when the tape is running at the required speed. It's a very long time since I worked on these splendid machines.

Leon.
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