Telefunken TC 750 - A new challenge!

vince666

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#41
Hi Borut,
I actually cleaned the pins, the sliding switches etc... only that the pictures on such PCBs were taken before cleaning then and put here to show, i.e., how certain pins were dirty of that black oxide/crap.
Those sliding switches which pass through those transparent plastic enclosures with the contacts, if they are the ones you mean, were also cleaned... i simply attached strips of 1 micron lapping film on the sliding switch and inserted it in place to clean the contacts and then cleaned the sliding part too.

And, I am not using it for many years now... but, hey, I actually have an old mechanical typewriter somewhere in the attic... it must be a quite old olivetti model.

that said, that black stuff which covered the pins and the contacts of the switches didn't just go away which scrubbing a cottonswab soaked with alcohol... but with the rubber eraser it goes away just quickly.

and there is still a lot to do on that deck, like at least all electrolytic capacitors replaced.
 
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vince666

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#45
of course, the work i just done was strongly needed and it's still not sufficient.

a full recap will be the next step, but i still need to order the needed parts.

about mods... it's a bit too early to know... the deck still doesn't sound right at all (weak and distorted sound, due to the old capacitors)... then i'll need to evaluate it after i'll will work properly again.

actually, i spotted two 4066 chips on the NR boards which might need replaced with ANT's ones but, luckily, if i leave the NR off the sound just doesn't go through them so, with NR off, they are totally bypassed (and i am not using NR in general anymore).

another possible mod might be related to the head... it's a sendust head and i usually dislike their sound so, i might try a permalloy head and see if it sounds better... but, again, before doing that i need that the deck works properly so the full recap is just the first thing to do now.
 

Alex SE

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#46
Question is if those 4066 are in the signal path, and even if they are, may be MAX4066 can do a job. Full recap is never wrong if a deck is older than 30-40 years, and new Nichi caps looks always nice :) If I remember right, it was UPW and UES.
Mods always after a deck is working as it should, but you already know that part.
 
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vince666

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#47
those 4066 are into signal path only if I enable the NR...
the deck has dolby B decoder and High Com encoder/decoder, and I just don't plan to record anything with High Com and, as far as dolby B playback, I have so few dolby B encoded cassettes (stopped using it years ago) that I can easily leave those original 4066 chips in the deck without worrying.

in the end, this deck will be used quite exclusively as a playback only deck and with NR off...
as far as recording tapes, I do definitely prefer to record on 3 heads decks.
 

nakdoc

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#48
Some 4066 applications are shunt to ground type rather than in signal path. I've always wondered if the distortions in a series design are lessened when the 4066 shunts a signal to ground.
 

vince666

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#50
the sound is a bit weak and quite distorted/unclean, both in source mode and in play mode, then at least all the electrolytic capacitors need to GO! :D
a happy update...

i still didn't order/replace the electrolytic caps for this telefunken deck, but i tried to power on the deck again just out of curiousity, after it was powered off for a couple months.

last time i had used it, just after the mechanical restoration and contact cleaning work, the sound was distorted, unstable, dirty, etc...

now, without doing anything to it in the meanwhile, it seems it sounds normally.
How could the deck sort of fix by itself while simply staying powered off and untouched for a while? :oops:

anyways... i have a sort of wild guess about it, if it can make any sense... also because the same just happened to me with two other decks, even if after a full recap.

the interesting thread about electrolytic capacitors where you guys talk about storage life and forming provided me the hints for my wild guess....
So, i believe that trying to use the deck a few months ago while working on it might have somewhat reformed the capacitors a bit.
before my work, the deck was sitting unused for many years, if not a few decades.
then i had powered it on while working/troubleshooting/checking and it sounded like complete crap.
but maybe the current flowing in the deck had a positive effect on those old capacitors anyway.
And now, by simply powering it on, it sort of sounds normally, with just some minor popping noises here and there, which is just a vast improvement and which happened alone with the deck powered off for a while.

i experienced a similar situation on a couple other decks after a full recapping.... the deck sounded badly (weak, distorted, noises all around) at first, with the new caps, but then i left it powered off for a while (even because i had lost the hope on it) and while powering on again later, it magically sounded nicely.

does all of the above makes sense for you experts?

any ideas to explain this kind of behaviour?
 
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#53
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.

View attachment 53580

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

View attachment 53581

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

View attachment 53582

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

View attachment 53583


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

View attachment 53584

View attachment 53585

View attachment 53586

End of first part,

Cheers,

Vince.

View attachment 53580 View attachment 53581 View attachment 53582 View attachment 53583 View attachment 53584 View attachment 53585 View attachment 53586
Wow Vince That thing is a beastie!!! Looks like it weighs more than my car ....... Great project

Shaun
 

vince666

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#54
Yes, Shaun... didn't find the weight mentioned on the Service Manual but it's a heavy beast... not less than 10Kg (most likely more).

Definitely the most heavily built deck I've seen so far... the capstan motor alone says it all.

And, hey, the actual frame of the chassis is made of some kind of plastic with just the chassis cover and front panel made of metal, while the bottom cover is a sort of hard/thick cardboard covered with aluminium foil on the inside.... otherwise it would have weighted even more, so all those Kilograms are mostly made of what's inside the deck, with the mech block alone which is just HEAVY. :oops:

After working on this one, I had to modify my own idea about decks which are built like tanks... this one looks like some military-grade building, indeed.
 
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#55
a happy update...

i still didn't order/replace the electrolytic caps for this telefunken deck, but i tried to power on the deck again just out of curiousity, after it was powered off for a couple months.

last time i had used it, just after the mechanical restoration and contact cleaning work, the sound was distorted, unstable, dirty, etc...

now, without doing anything to it in the meanwhile, it seems it sounds normally.
How could the deck sort of fix by itself while simply staying powered off and untouched for a while? :oops:

anyways... i have a sort of wild guess about it, if it can make any sense... also because the same just happened to me with two other decks, even if after a full recap.

the interesting thread about electrolytic capacitors where you guys talk about storage life and forming provided me the hints for my wild guess....
So, i believe that trying to use the deck a few months ago while working on it might have somewhat reformed the capacitors a bit.
before my work, the deck was sitting unused for many years, if not a few decades.
then i had powered it on while working/troubleshooting/checking and it sounded like complete crap.
but maybe the current flowing in the deck had a positive effect on those old capacitors anyway.
And now, by simply powering it on, it sort of sounds normally, with just some minor popping noises here and there, which is just a vast improvement and which happened alone with the deck powered off for a while.

i experienced a similar situation on a couple other decks after a full recapping.... the deck sounded badly (weak, distorted, noises all around) at first, with the new caps, but then i left it powered off for a while (even because i had lost the hope on it) and while powering on again later, it magically sounded nicely.

does all of the above makes sense for you experts?

any ideas to explain this kind of behaviour?
Hi vince666,

I'm in the process of attempting to transition a bunch of equipment from a couple of decades of cold storage to daily use as my main system, so I was also reading what Grapplesaw & others were saying about forming/reforming capacitors.

This evening I read your restoration thread with interest, especially how your equipment sounded so disappointing on the first try, but so much better the 2nd time around? Believe it or not, I found a couple of technical white papers about the electro-chemistry behind the aluminum electrolytic capacitors that discuss the exact scenario that you had described earlier.

Check out this paragraph I took from a Cornell-Dubilier paper:

capacitor leakage current 1st time after long storage.jpg

Q: Did you catch the "reluctance to charge the 1st time" comment, followed by the excess leakage current? Obviously, a circuit where the voltages aren't where they are supposed to be due to the above will not operate/sound right -- especially if the capacitors are being used to couple the music signal between stages. NOTE: The 'smaller' the coupling capacitor, the more of the bass frequencies are blocked?
(Refer to Cornell-Dubiliar .pdf file attached to the end of this post for more info on cap forming/reforming.)

In a different paper there were a couple of photos associated with how the surface of the aluminum is etched during forming in order to provide up to 200x more surface area. Of course, more surface area = more capacitance. Check out these micro-photographs:

Electrolytic cap foil etching + forming.jpg

(Refer to EPCOS .pdf file attached to the end of this post for much more info...)

****

Guess what? If you tunnel deep enough to where all the people that actually design/develop/manufacture electrolytic capacitors hang out, it's neat to see that some of the weird/voodoo subjective behavior that we observe our old & complex audio equipment do...can actually be explained using their objective scientific papers. (!)

Good for you to state in public exactly what you heard the first time...a little time passes...and then the 2nd time you listened it seemed to have magically healed itself? In other forums I've seen people being ridiculed for what they heard & reported...but in the back of my mind I was thinking that anyone sharp enough (patient enough/disciplined enough) to completely tear down that Telefunken deck and successfully put it all back together...if they said it sounded bad on the 1st try, but seemingly fixed itself on the 2nd try? I gotta believe them! :0)

You know, we tend to lose sight of the fact that we aren't trying to restore old, simple cast iron skillets. (Although there are people that do just that.)
A lot of the equipment that we are interested in restoring were literally 'state of the art' when they were made. And due to the limitations of that generation's test equipment, designer's knowledge, lack of powerful computer simulation tools, and even competitive pressure to get new stuff out into the marketplace asap, some of these very technical machines were only partially-baked when released.

Such is the nature of the beast. Given the above, some of our favorite toys were finicky/hard to fix when they were only 4 years old...much less 4 decades old. (!) Imagine if you & I were putting together something as complicated as that Telefunken today -- and somebody asked us how is it going to behave in the year 2065? We would laugh out loud at how absurd the question was. :0)

The point I'm trying to make is that for the love of music we are trying to revive really complicated equipment decades after (most of) the original designers figured that these boxes would be in the landfill instead of the living room.

What a hobby. I just wanted to listen to some of my favorite tunes that are no longer being played on the radio...one thing leads to another...and now I find myself studying electrolytic capacitors at a level I would have never, ever predicted. Go figure. :0)

Cheers --

3D

PS -- Congrats on seeing that huge project through to completion. Man, whoever spec'd that huge motor to drive a cassette deck must have really had a thing for low wow & flutter - impressive!
 

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#57
Nice write up Davis...learned something new, thanks!!
Lee, my pleasure. I'm way behind in my personal ratio of what I've gleaned from this forum vs what I've contributed. If you could only see all the notes I've taken while reading other member's best practices. (Like vince666 disassembling the switches in order to get them working as new 40+ years after they were made. (!)

Needless to say, the Variac + the newly arrived DBT will be earning their keep forming/reforming tons of caps starting soon. (Gepetto's shipment is due any day now...and NavLinear is pulling together a big wire shipment headed my way.) Life is good, & getting better!

3D
 

vince666

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#59
Hi vince666,

I'm in the process of attempting to transition a bunch of equipment from a couple of decades of cold storage to daily use as my main system, so I was also reading what Grapplesaw & others were saying about forming/reforming capacitors.

This evening I read your restoration thread with interest, especially how your equipment sounded so disappointing on the first try, but so much better the 2nd time around? Believe it or not, I found a couple of technical white papers about the electro-chemistry behind the aluminum electrolytic capacitors that discuss the exact scenario that you had described earlier.

Check out this paragraph I took from a Cornell-Dubilier paper:

View attachment 62172

Q: Did you catch the "reluctance to charge the 1st time" comment, followed by the excess leakage current? Obviously, a circuit where the voltages aren't where they are supposed to be due to the above will not operate/sound right -- especially if the capacitors are being used to couple the music signal between stages. NOTE: The 'smaller' the coupling capacitor, the more of the bass frequencies are blocked?
(Refer to Cornell-Dubiliar .pdf file attached to the end of this post for more info on cap forming/reforming.)

In a different paper there were a couple of photos associated with how the surface of the aluminum is etched during forming in order to provide up to 200x more surface area. Of course, more surface area = more capacitance. Check out these micro-photographs:

View attachment 62173

(Refer to EPCOS .pdf file attached to the end of this post for much more info...)

****

Guess what? If you tunnel deep enough to where all the people that actually design/develop/manufacture electrolytic capacitors hang out, it's neat to see that some of the weird/voodoo subjective behavior that we observe our old & complex audio equipment do...can actually be explained using their objective scientific papers. (!)

Good for you to state in public exactly what you heard the first time...a little time passes...and then the 2nd time you listened it seemed to have magically healed itself? In other forums I've seen people being ridiculed for what they heard & reported...but in the back of my mind I was thinking that anyone sharp enough (patient enough/disciplined enough) to completely tear down that Telefunken deck and successfully put it all back together...if they said it sounded bad on the 1st try, but seemingly fixed itself on the 2nd try? I gotta believe them! :0)

You know, we tend to lose sight of the fact that we aren't trying to restore old, simple cast iron skillets. (Although there are people that do just that.)
A lot of the equipment that we are interested in restoring were literally 'state of the art' when they were made. And due to the limitations of that generation's test equipment, designer's knowledge, lack of powerful computer simulation tools, and even competitive pressure to get new stuff out into the marketplace asap, some of these very technical machines were only partially-baked when released.

Such is the nature of the beast. Given the above, some of our favorite toys were finicky/hard to fix when they were only 4 years old...much less 4 decades old. (!) Imagine if you & I were putting together something as complicated as that Telefunken today -- and somebody asked us how is it going to behave in the year 2065? We would laugh out loud at how absurd the question was. :0)

The point I'm trying to make is that for the love of music we are trying to revive really complicated equipment decades after (most of) the original designers figured that these boxes would be in the landfill instead of the living room.

What a hobby. I just wanted to listen to some of my favorite tunes that are no longer being played on the radio...one thing leads to another...and now I find myself studying electrolytic capacitors at a level I would have never, ever predicted. Go figure. :0)

Cheers --

3D

PS -- Congrats on seeing that huge project through to completion. Man, whoever spec'd that huge motor to drive a cassette deck must have really had a thing for low wow & flutter - impressive!

WOW! Thank You for your very USEFUL post! :)

and, yet another funny update...

when I had powered this deck up the second time (a few months later after the restoration work pictured here) and it suddenly sounded right, then I left it powered off again until a few days ago...
And, just last sunday I powered it up again to make it listen to my older brother who came here at home and the deck still sounded good (actually, my brother was quite impressed by the sound) , but this time I didn't experience any bursts of noise which still happened a little bit last time... then, it makes sense all what you posted.

That said, I am still hearing some noticeable hum in the background and which should not be there... then the capacitors definitely need to be replaced, regardless if they sort of restarted working.

Another thing I maybe didn't mention before is that I've found a few trimmer pots (variable resistors) on the main board which looked broken, like if the rotating metal plate was broken and/or bent a bit to the point of not touching the carbon trace below it... while working on the deck I simply tried to bend those rotating metal parts down to make them touch the carbon trace again but I doubt this might be a fix... so, when I'll order the capacitors then I will also order those trimpots to replace them with new ones.

Actually, I am quite surprised this deck is sounding nice... but the hum in the background is just there.

A nice new, though... my older brother (who has his own PC and smartphone repariring shop here below) just purchased a nice oldschool analog oscilloscope, a Tektronix 2235... so, finally, I will have the opportunity to test the circuit a lot more carefully... i.e. by trying to find out where that hum comes from... but the main/bigger capacitors in the power supply stage are some of those Frako with golden can, and I suspect they aren't filtering AC from the DC supply that nicely... we'll see... but, at least, all the elctrolytic capacitors and also the broken variable resistors will be replaced without thinking it twice.

Anyways, regardless of the hum in the background and the old caps, the deck has a quite impressive sound with a very strong output level, a lot stronger than any of my other decks.

Cheers,

Vince.
 
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