Telefunken TC 750 - A new challenge!

[vince666]

Truly, I admire the courage and patience the restorer must have. I don't....

Thanks, my friend.

actually, when my friend donated me this Telefunken beast in need of full restoration, I was quite scared and also very doubtful I could put my hands into a so complex model.
But my friend was very confident I could succeed, not just because I am THAT expert (because I am not) but mostly because he knows I have a lot of patience and then, when I find a difficult task, before starting working on it with the risk of adding further damage, I take my time to study the situation just carefully... indeed, this was the most difficult deck I've handled so far and, also, very useful to learn some new things.

And, btw, only the first part of the work is complete because I still need to replace all the electrolytic capacitors and a few trimmer pots... but, for sure, dismantling it again will be less painful than the first time described here in this thread.

 

[J!m]

It is not by coincidence that I have an entire drawer full of every size and shape of hammer...

 

[vince666]

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)

Was reading again your NICE above post (it actually deserves to be read more times) and I'd wish to add some more about this WEIRD situation of decks which magically heal themselves after some time staying off...

But, all the other times I've experienced such issues, I had replaced all the electrolytic capacitors with new ones.

So, now that I met this Telefunken with old capacitors which behaved just badly, my idea is that new/never used electrolytic capacitors can somewhat behave a bit like too old ones who were unused for several years.
I believe it's a matter of reforming them... and specifically when I cannot reform them before soldering in the deck (not able to and don't have suitable tools to do that) but by leaving such phenomenon happening just in the deck.

First time I've experienced such loud burts of noises, it was after replacing a few caps on a couple of Technics RS-B565... happened to do the same work on 2 of them in a row...
First RS-B565 , after powering it on with a few new/replaced electrolytic caps (all coupling caps into the signal path with bi-polar ones, btw), happened to make noises but not loud ones... they were just in the background... and after, say, half hour of keeping the deck on, the noises disappeared and the deck sounded nice and clean.
Second RS-B565, with exact same work as the first one, acted quite the same (noises for about half hour) but such noises were very light on one channel and very loud on the other one... but, after that half hour burn-in time, it sounded nice and clean.
So, I assumed that some 30 minutes burn in on new electrolytic capacitors would be something obvious to expect.

Then I happened to replace all the electrolytic caps on my Technics RS-M63 and, this time, the deck happened to make loud noises for a few days... so, this time the 30 minutes burn-in rule I had experienced before seemed not to work... then, thinking about something else causing the problem, I remade ALL the solder joints but the problem was still there... so, I had lost my hopes about this deck and put it aside for a couple months, thinking it was unfixable.
Then, out of curiousity, I turned it on again a couple months later and there weren't any noises anymore... the sound was clean and nice... WOW! But, this time, I still didn't know what to think about... it was simply a nice surprise to see the deck working nicely, after all the work I had made on it, and then didn't bother to find a possible explaination.

Some time later, I've replaced all the electrolytic capacitors on my Technics RS-M260... and, know what?
It did behave the same as the RS-M63.... so, LOUD burts of noises for a few days with the new caps!
So, just like the previous one, I left it off for a while (but maybe a couple weeks only) and when powered it on again the noises disappeared and the sound was clean and nice.... at this point, I started to think that, maybe, older decks with fully discrete circuits (lots of electrolytic capacitors and almost no IC chips inside) would need a noticeably longer burn-in time after a full recap.

Then, I've replaced a few electrolytic capacitors into the signal path of my marantz SD-60 and, again, I heart such noises but they laster like no more than a couple hours... this deck is a newer model (around 1990) and it has several IC chips (and not as many electrolytic capacitors as the above older ones).

Then, I happened to replace the electrolytic capacitors to an old mono shoebox recorder I use to load the software into my old ZX Spectrum home computer... when powering it on with the new caps, it didn't only make the loud noises but it also showed the signal going up and down in level quite a lot... then i left it playing cassettes for several hours over a few days... but the problem didn't disappear... then I lost my hopes and left it off for a few months...
When powering it on again after this long time, the noises disappeared and the signal was at right level and also stable... and it's some time now that I am using it to load the software on my old computer without any issues at all.
So, this is the example where the behaviour was the most similar to this Telefunken, but with the difference that the Telefunken still has the old capacitors on (save for very few I've replaced, to mostly get rid of the few tantalum ones and a couple more in the signal path which I just had the right values here at home)...

and, last, the experience with this Telefunken...

So, at this point, I start to think that electrolytic capacitors (i.e. the new ones, especially if they were sitting unused in the shop for quite a while or, at this point, also the old ones which were unused for long time) do need some burn-in or reforming to restart working properly.

Last but not least, from just a few days ago.... my older brother recapped an old ZX Spectrum home computer... and after powering it on, it did show a noisy picture... with the noise coming and disappearing but it was video/image noise... anyways, I sort of had the impression of hearing the noises I was used to when recapping my decks... and these video noises lasted for not more than a hour or so... and now it works just nicely, with very clean video.
My brother, who evidently never experienced such a thing before, was a bit scared by that weird video noise but I was telling him that it was OK and expected and that, after a short while, it would disappear... and, in fact, that's what it happened.

So, I understand that simply saying that the deck managed to magically fix by itself while staying off for a while would sound a bit too funny.... but, evidently, what you shared on your post would explain such weird phenomenons.

For the future, I believe that if I will be going to recap more and more decks, I will try to get some suitable equipment to make a proper reforming to the electrolytic capacitors BEFORE soldering them in the decks to at least minimize or totally eliminate the need of such weird burn-in time and, also, to test them carefully.
Searching a bit on youtube, there are more videos where they show how to reform an electrolytic capacitor.... doing them individually, with the most proper method and before putting them into the circuit, would not need much time to put them into "ready to use and enjoy" conditions.


Cheers,

Vince.


PS: to add some "fun" to this post/experience of mine while recapping decks... in between the aforementioned ones, I happened to fully recap a few which did NOT show such kind of noises at all... then, of course, at this point I do wonder WHY they didn't... after all, the capacitors were of the same kind/brands/models (nichicon UES bipolars and UKL and panasonic SU bipolars and FM) and bought together with all the other ones from mouser.

 

[D-500 Flogger]

Hi Vince,

Your description of 'loud bursts of noises' right after replacing caps, but most of the time it eventually subsides? Well, this reminded me of a fascinating restoration video of an old Hammarlund HQ-140-X radio receiver. It was originally manufactured in the 1950s, and the unit that he was working on was very pretty, seemed to have low hours on it, but (and here is the punch line) had been powered off for quite a few years.

So, for the first couple of hours into the restoration video the restoration guru (Paul Carlson) gets it powered up, performs a full RF & IF alignment...and then, when he starts into the audio stage, he runs into the exact loud intermittent crackle & pops that you have been describing.

Guess what he does -- he drags out a signal tracer, and goes on the hunt for the single bad part that's causing the crackles. (Hint: He finds a LOT of parts that are perfectly quiet, but the one he traces it to looks like the failure was due to thermal degradation...which lead to internal arcing as the dielectric broke down?)

****

Anyway, if you go to this video,



and skip to the following time stamp you can start watching right where he starts demonstrating/troubleshooting the noise bursts:

2:04:47 'Brings up noise bursts -- sets the stage'

2:06:18 'radio goes from quiet/well behaved to noisy bursts'

...He then grabs a signal tracer and goes on the hunt...

2:17:00 - 'after lots of quiet components he finally zeros in on the noisy part with the signal tracer'

Mr. Carlson now works to confirm the failure by unsoldering 1 end of 2 different parts & listening to them both...

2:22:42 'Proof positive of the bad component that was causing the bursty noise'.

2:24:37 'The bad parts are replaced & he proceeds with listening tests. Now the radio is like a time machine, for it is now operating like it's 1955 again.

****

Don't know about you, but for me the ~20 minutes above had me on the edge of my seat (with a big bowl of popcorn) -- much better entertainment than any of the murder mysteries that saturate our TV channels these days. :0)

****

Seriously, I think that there's a couple of rational reasons why you are starting to see more & more 'unformed' capacitors showing up and filling your repair with all that snap, crackle, & pop.

The first reason is that at one time the axial & radial lead capacitors were used in the mainstream of manufacturing consumer electronics. So when we bought a handful of these capacitors for our own projects, they were freshly made, still formed nicely from the factory, etc.

Now, however, so much of the consumer electronics has shifted to the SMD capacitors. If I had to bet money, I'd say that even though we are buying 'new' (as in never before installed) capacitors, these 'old school' caps may have sat on the distributor's shelf long enough that the factory forming has subsided. (Remember, in one of the previously quoted articles it was stated that capacitors that have sat on the shelf for 5 years or more need to be 'reformed'.)

The 2nd thing I wanted to discuss was these new (but stale) capacitors may or may not audibly misbehave, depending upon where they are placed in different circuits. I know it's easy for me to think that a low volume knob setting means small signals applied to the components in the circuit...but the reality is that a lot of line level circuitry is unaffected by the volume knob, and is always running wide open. (Think Ted Nugent playing Stranglehold. :0)

****

The bottom line is that the noise bursts are actually the sounds of the dielectric breaking down & the sound of tiny arcing inside the parts. Of course our big amplifiers downstream of all this do their job to make sure we can hear these tiny arcs as big noisy outbursts. Depending upon the design, the parts may or may not heal themselves as they reform thanks to the power being applied to them.

It's a funny thing. We were taught in school that the AC power flows through the capacitor when in use, and when the capacitor is fully formed I still believe that.

But when the capacitor is in need of being formed/reformed, part of the power goes INTO & is absorbed by the capacitor, not through it. Proof of this is the fact that when you read up on reforming caps, one thing you have to watch is the amount of current that you allow to be applied during the process, for some caps might soak up so much power during the forming process that they overheat. And if they overheat enough they will vent -- worst-case, they will fill your workspace with confetti. :0)

****

If the above doesn't make perfect sense, that could be my fault, for I am also trying to figure all this out as I write this. :0) But this has been a fruitful conversation, for I am going to play it safe and form my new capacitors just like Grapplesaw/NavLinear/Lee do -- for I only want my Snap, Crackle, & Pop to occur in my cereal bowl. :0)

Check out that video -- at least the 20 minutes having to do with the noisy bursts.

Happy Hunting --

3D

 

[vince666]

Thanks , just curious to watch that video

Now, however, so much of the consumer electronics has shifted to the SMD capacitors. If I had to bet money, I'd say that even though we are buying 'new' (as in never before installed) capacitors, these 'old school' caps may have sat on the distributor's shelf long enough that the factory forming has subsided. (Remember, in one of the previously quoted articles it was stated that capacitors that have sat on the shelf for 5 years or more need to be 'reformed'.)

That's what I think, since I experienced such noises in a lot of decks where I did put new caps... but, hey, after the (more or less long) burn-it time, they are working just nicely... so, I believe they did simply need to get some current in to reform.

Funny enough that also old caps which were unused for a long time got an improvement after powering the device on for a while.... but, of course, I don't expect they can last that much.

Cheers,

Vince.


EDIT: and, yes, the burst noises I was meaning are just like the ones there at 2:16:18 in the video.

 

[D-500 Flogger]

Thanks , just curious to watch that video



That's what I think, since I experienced such noises in a lot of decks where I did put new caps... but, hey, after the (more or less long) burn-it time, they are working just nicely... so, I believe they did simply need to get some current in to reform.

Funny enough that also old caps which were unused for a long time got an improvement after powering the device on for a while.... but, of course, I don't expect they can last that much.

Cheers,

Vince.


EDIT: and, yes, the burst noises I was meaning are just like the ones there at 2:16:18 in the video.

Vince,

After giving yourself a suitable break, here's one more video that you might enjoy. The gentleman that troubleshot that noise back to the failed component made a video detailing how to make the signal tracer that he used to isolate the failure to the bad part.



To me, this tool would help us with our projects in the same way as a stethoscope helps a doctor learn what ails their patient.

No doubt with new capacitors your Telefunken deck should be as quiet and hum-free as when it was manufactured...but if for some reason it isn't, then in a different video Mr. Carlson uses it to successfully find & fix excessive background hum in an audio amplifier. (!)

In other words, your plan of attack (new caps) is definitely the way to go. And it should fix it...but if it doesn't, then instead of just accepting it as a consequence of old age, you now have a backup strategy that you can choose to employ. (Between your brother's analog scope & a signal sniffer like this you will have the upper hand.)

****

Be sure to let us know when you finally get your Telefunken all sorted out to your satisfaction.

Good luck with the hunt -- we're rooting for you!

Cheers --

3D

 

[vince666]

that's very useful...

but, i've experienced such noises and they disappeared after a while... so, a failing part fixed itself or stopped working bad afyer some use?
That's maybe the only way I can explain such noises going away after a while, even if a weird explaination.

Anyways, it's nice to get a tool to find the noisy/offending part, when it happens.

 

[vince666]

OMG! watched that video about building that tool to find noisy stuff... that's terrific. WOW!

 

[derek92994]

So, at this point, I start to think that electrolytic capacitors (i.e. the new ones, especially if they were sitting unused in the shop for quite a while or, at this point, also the old ones which were unused for long time) do need some burn-in or reforming to restart working properly.

After replacing every component on the battery phono preamps, both of them made strange noises for the first 5 minutes of operation, with no signal input. The first time I swore a lot, thought there was a mistake made, but now I know its the 'capacitor' break in thing.