Telefunken TC 750 - A new challenge!

vince666

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#1
Hi, my friends,

as some of you might just know, about 1 month and half ago, i received a non working Telefunken TC 750 from my italian buddy as a free gift and had started a thread about that elsewhere. Now I am going to restart a thread here were I can finally go on with the latest updates.

Here it is this monster as arrived, it's the TOTL Telefunken model from 1979 despite it has only 2 heads.

TC750.jpg

The chassis is bigger than average, yet it's just stuffed inside!

TC750 top view.jpg

The mechanism is a single capstan direct drive motor (that sealed beast) and a quite generous gear driven reel motor.

TC750 mechanism.jpg

First evident problem, two reel gears made of that orange gummy plastic are cracked!
These need to be ordered just immediately.

TC750 reel gears.jpg


After removing a few parts, I could finally pull the big and heavy mech block out of the chassis!

mech1.jpg

mech2.jpg

mech3.jpg

End of first part,

Cheers,

Vince.

TC750.jpg TC750 top view.jpg TC750 mechanism.jpg TC750 reel gears.jpg mech1.jpg mech2.jpg mech3.jpg
 
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Elite-ist

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#4
I look forward to the continuation of your Telefunken TC 750 rebuild, Vincent.

I bought a Denon DR-M4 and Pioneer Elite CT-07D from a fellow who had a very nice Telefunken mini system. Here's a video of it playing:

https://db.tt/yKk8qoh4GQ

20200420_131539.jpg

20200420_134508.jpg

These decks have very robust engineering and well-worth the effort in keeping them going.

Nando.
 

20tajk7

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You never have too much tapedecks ^^
#9

vince666

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#11
Fourth part : checking some evident faults and giving it a first try!

The sendust head had a not too deep wear groove but a lot of corrosion, so I've lapped it.

TC750 head_resized.jpg

there was a cracked joint on one PCB

cracked joint.jpeg

and a burnt resistor too!

resistor burnt.jpg

resistor repaired.jpg

and finally remounted...

pic39.jpg


Well, it sort of works... tape running is OK then the mech part is OK but the sound is weird, like only some mid-hi and treble, like a sort of "Left minus Right" sound on both channels.
Need to start investigating the electronics, anyways, mechanically it was cleaned and relubed and it moves the tape correctly, which is just something for now! :cool:

end of fourth part.

Cheers,

Vince.



TC750 head_resized.jpg cracked joint.jpeg resistor burnt.jpg resistor repaired.jpg pic39.jpg
 

vince666

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#16
Fifth part : removing all the PCBs and cleaning the contacts.

So, I've removed all the PCBs to both take a list of all the electrolytic capacitor values and to also clean all the contacts, plugs (i.e. where the many doughterboards are inserted) , switches, etc...
A lot of black oxide all around!

pic40.jpg

pic41.jpg

pic42.jpg

pic43.jpg

pic44.jpg

pic45.jpg

pic46.jpg

pic47.jpg

pic48.jpg

pic49.jpg



pic40.jpg pic41.jpg pic42.jpg pic43.jpg pic44.jpg pic45.jpg pic46.jpg pic47.jpg pic48.jpg pic49.jpg
 

vince666

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#18
I've got that weird sound once, it was due to a bad ground connection.
I hope it's something like that, really!

Yeah, no shit. Ate you doing all this with no manual??
The service manual has the schematics, the PCB layouts, the alignment procedures and an exploded view which isn't that much useful, so for unmounting and remounting I needed to figure it out by myself.
Not an easy deck to handle AT ALL! :oops:
 

vince666

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Location
deep south of Italy
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I will not be missed! :p
#20
Sixth part : remounting the whole thing back to see how it works now.

So, after cleaned all the oxidized contacts, I also replaced a few capacitors, 5 tantalums and a few electrolytics along the PB signal path (for the record, the old ones don't look gone yet) and mounted the whole thing to try it.

The next step would be to replace all the old electrolytic capacitors but i still need to order them from mouser and, meanwhile, I can check if there is anything else to add to the order.

So, there is a lot of PCBs and wires/connectors to put back into the chassis!

pic60.jpg

put the mainboard, the input and preamp board and the control/switches board with the meters back into the chassis.

pic61.jpeg

inserted a few smaller daughterboards and connected quite a few connectors!

pic62.jpeg

now, added the mech and connected more plugs here and there...

pic63.jpeg

now added the logic board with its small daughterboard for the reel motor control and connected the last plugs and wires (well, a couple green wires not connected yet in this picture!)

pic64.jpeg

Done... time to try it again!

pic65.jpg


Well, mechanically it works but the sound is still like before, but i try to describe it better than before.

If I plug the deck normally (either from RCA or from DIN, it's the same) , I can hear that mid-treble out of phase sound on both channels.

If I plug only one RCA channel at a time, it sounds extremely faint on its own channel.

If I plug both RCA outs but with one channel with reversed phase at the RCA plug (swapped black and red wires) then I hear one channel mute and the other channel sounds normal, with the bass and all normal frequencies.

In any cases, the VU meters do work normally, with even levels between the channels during any of the above "plug" tests.

And if i measure the signal at RCA outs with the multimeter, there signal is present on both channels.


So, as Leo suggested, it might be a missing ground somewhere?
Then, should I first try to check for continuity just all what goes to ground also in the PCBs?


end of sixth part.

Cheers,

Vince.



pic60.jpg pic61.jpeg pic62.jpeg pic63.jpeg pic64.jpeg pic65.jpg
 
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