Telefunken RC760 restoration

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HRmeteohub

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Mar 19, 2017
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I got this Telefunken RC760 last week, without aerial, with busted "pause" key, and radio barely working. The pots were very scratchy, and the tape would not play. After disassembly, and dust-busting I saw that belts are transformed to sticky strands. I am looking for replacements.
TelefunkenRC760-1.jpg
TelefunkenRC760-1back.jpg
There are Line In/Out - nice, but no Hi-Fi can be expected from this unit.
TelefunkenRC760-2.jpg
The tape deck motor was made by Matsushita (now Panasonic), on 1986-07-30. Speakers also carry their amblem. Not bad.

There are lots of images of this box on the internet, and at least three pages with (free) service manuals. The radio uses five chips from Toshiba, and is rather "plasticky". It does not look like it was made in Germany, neither does it say on the case.

Mine got some heavy corrosion around 2200uF/16V (C706) capacitor due to glue that was put there, supposedly to fix it so it would not break off from the board. Over the time, the glue got very rigid and only kept moisture closer to one jumper wire and another capacitor that looks rather corroded as well. There is only a speck of transformed glue visible on this image (marked by red oval) - on the electrolytic capacitor right between the amp-chip and large C706.
TelefunkenRC760-3.jpg
I had to remove the glue with pliers, and then corrosion presented itself in full glory. I am probably going to replace both capacitors, as I am not sure if they leaked or not.
The "Pause" key was not broken, just slipped off the rail, and was easy to put back. The tape deck looks cheap but everything moves O.K. I wonder how it will play after I change the belts? Not so great, I presume.

More to come, after bathing and some testing.
 

samovar

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Jun 7, 2014
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Great restoration in progress! I know this box, a low-end but nice-looking Telefunken. Don't expect it to be even remotely comparable to any of the outstanding C-units. Yet, for being so cheapish, it sound perfectly ok. Line-in is its best feature, so let's hope you can fix it soon.
 

HRmeteohub

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Mar 19, 2017
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I was wondering if it would pay off to use Line-in feature and add Bluetooth input, but it is kind of moot, since the unit is not so great to start with. I got some second-hand 3 ohm, 6W speakers and if they fit, perhaps I could upgrade it.
Since I like a nice restoration, I'll first tidy it up, and then we shall see.
---edit--
From here on begins rather technical stuff about problems and repair, so I'll continue writing at appropriate section.
 

HRmeteohub

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So, the technical problems are done, and I have to wait for the belts to arrive. No point in delaying cleaning any further. The outer case is in rather good shape, only minor bruises. However, I noticed dust on grills, so they (as always) have to be separated and cleansed. During cleansing the front with old kitchen towel and some detergent, removal of the speaker grills was necessary. Front of the case is almost clean.
TFK-RC760-removed_grills1.jpg

On this model they let go easily, only light push from inside was needed to separate them from front part of the case.
And they are dirty. I used dull-blade kitchen knife to remove metal grills, as well as to clean slots. Yes, you all have one, and you use it as screwdriver-plundger-lever-scraper-grease_remover-scaplel-tool_for_all-etc. Better just admit that now, and not pretend you use 24 separate tools. This one old & trusty kitchen knife opens iPhone and repairs this old box. My precious!
TFK-RC760-removed_grills2.jpg

Now, where was my soap?
 

HRmeteohub

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Mar 19, 2017
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This unit was rather clean, so my soapy adventures were limited.
However, there is always some dirt/gunk under (tweeter) grills, as well as in ventilation slots. In this case, there is also missing a piece of plastic where antenna was anchored (broken screw point and some). I believe that can be taken care of, using advice from Boomboxery. :yes:



Grills are removed and disassembled with Kitchen-Dull-kinfe (c).


Of course, some cleaning is easily visible, even before "bathing".
 

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HRmeteohub

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Mar 19, 2017
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Well, looks like this TFK is ready for use.
The box is cleansed, vacuumed and bathed until it reached near spotlessness. There are some scratches, but they are minor and I like to let them be for now. Everything works, tape deck plays rather nicely for such plasticy unit. So let me go over the history of this unit.
This RC760 was bought in West Germany in late 1986 as a present for one medical nurse and her family. It was illegal to celebrate christmas back then in Yugoslavia so the present was for "new year" :cool: .

Somewhere during +25 years of use, somebody ripped the antenna out of the socket, tearing the screw point with it. I had not found out how an when, but the replacement antenna is working rather nicely, and I hope someday I could get the real one in place. Also, the tape deck started working badly probably due to dirty audio head/roller/capstan. To be fair, tape was going out of fashion so nobody noticed that the belts have turned to goo. I have replaced them with 65 mm and 80 mm belts and they work fine. However, my measurements may be off.

Along its life, this boombox have seen paint (red, anti-rust primer, green, and brown, probably synthetic ones for wood/metal), as much as I can tell from number of specks on it.
Several years ago, owners noticed that it produced rather annoying humming noise (50 Hz), and some time after that, it developed a weakness - it would stop playing after "it got tired". That spelled doom for this one. After the owners found out that the price for repair, this RC760 got the nylon bag and went to storage under the roof. FYI, that is the place where objects sit about a decade or so before they are finally trashed.
The new box was purchased, all shiny and smaller, but with rather poorer radio receiver, smaller speakers, barely worthy of the name mini-boombox. In other wor(l)d, replacement from hell!

Then one day, my neurotic behavior induced a surge of electrical impulses and chemical reactions in brain of the owner. There was a chance they could get rid of it! ... and shure they did. What was needed to get this one back in "the life"?

- lot of good will and several hours of free time,
- wet wipes, soap, sponge, cleaning brush and toothbrush, cotton swabs, some acetone, cleaning (and other) C2H5OH, Kontakt 60 spray,
- soldering iron, Philips screwdrivers 1 & 2, small tweezers, amp-volt-ohm-meter, Kitchen-dull-knife (c),
- 3300 uF/16 V, and 470 uF/16 V capacitors, 470 ohm - 1/2 W resistor
- at some point in time, 2SC1815, and BD140 transistors were used, as well as instrument for measuring the beta and other characteristics of transistors
- two belts (65 and 80 mm)
- replacement antenna
- some acriyate glue (for repairing broken screw points)
- camera to record it all for this forum :clap: (ego points are awarded afterwards)
TFK_RC760_1.jpg
All shiny and in working order.
TFK_RC760_2.jpg
TFK_RC760_3.jpg


Did I miss something? Probably. This was an easy one. Most of the time I was/am waiting for belts and antenna/aerial. Cleaning was also not so hard. I would really appreciate if somebody would point me to good and cheap source of aerials/antennas.
Have a nice day!
 

HRmeteohub

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Mar 19, 2017
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I had some free time this weekend so I was playing with my TFK RC760.

First I took some rather new (15+ years) 4 ohm, 10W speakers that are 10cm and tried them out. Better, but the original amp is such a crap.. So I took one PAM8610 module, and new switching 12V 2A power supply and connected them instead original AMP and PSU. Much, much better, but my PAM8610 module seems to have lower sensitivity, since I can turn the volume dial "all the way", and it still does not distort, and is not too loud, nor the speakers are too much driven.
Then I have disassembled the Bluetooth 2.1 audio adapter and tried it out. The one I have has got a bit too high output, and some interference, but that will wait until I fix the speakers and amp.

Now the PAM has got way better sound than original amp inside, so I am keeping it. Unfortunately, I can not close the case with new speakers and TFK made fitting then rather cumbersome. You see, the front grill is actually having screw points on it. Than the front is clamped between grill frames and back of the case. That means that I will have to tear off the screw points, and speaker landing rings (a ring protruding from the case to meet with the speaker) and completely recreate everything, but some 7-8 mm lower in order to fit new speakers. That is so much fiddling, and work, and things could go so wrong here. Bummer. :bang: Take a look:
TFK_760_10Wspeakers-1.jpg
TFK_760_10Wspeakers_Amp_BTaudio_1.jpg

I have already sent the image of the grills, where the screw points are seen protruding backwards...
 

Reli

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Ha, I had one of those, in funky white. Really feeble in its original form.

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

HRmeteohub

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Mar 19, 2017
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Wow! It looks much better in white! That video gives me some ideas ... I should repaint it. Thanx guys for support.

Now... I am planing how to put the new speakers in, but there is so much plastic to remove. The screw points should be shorter by 4-5 mm, and other things have to go all together.

And ... I am having second thoughts. I have marked all of it with red circles. I figure the best way to remove it is with some small rotating tool or just simply tear them off with plyers?
TFK_760_10Wspeakers-2.jpg
 

HRmeteohub

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If you look the schema of the RC760 closely, you can see that the "Wide stereo" (S007) switch is taking the signal from the one amp output and putting it to inverting input of the amp for another channel. I guess that all "wide stereo" and similar trickery is just nonsense like this. So I decided to use this switch to engage the output of the Bluetooth audio module in the RCA input connectors. That way I can use other audio source, and the BT module will not interfere with it.

Something like this:
TFK760_WideStereoSwitch4Bluetooth.JPG

After some cutting, soldering and so on, this is how it looks like:
TFK_760_Amp_BTaudio_1.jpg
In this image I still need to connect audio lines from S007 switch to audio inputs, and the put the 5V power supply. I am wondering how much does the BT module need? What kind of step-down shall I use...

I have piggy-backed the PAM amp and the BT module to the back of the board. I still have the original amp working for headphones connector, but I believe I should turn it off entirely. PAM8610 has got "switch_module" input, that can be used to switch the amp directly from the S004-1, not using TR904 and TR905. By avoiding this switching circuit, and disengaging the original amp, the box will use much less current, meaning longer lasting on batteries. Of course, it does help that new amp has got about 90% efficiency, compared to about 45% of original one.
First test of the new amp went OK. BT module is working (with external PSU right now, but still...).

One thing is going to be impossible to solve. You see, the PAM8610 uses bridge configuration, meaning that both terminals of the loudspeaker is connected to the chip, and not one is going to ground/negative terminal on board. Now the headphones connector is configured exactly that way, one pin goes to ground/common and one pin for each headphone. Take a look at schematic above, it is marked J005. Now I am in dilemma, shall I let it stay connected to original amp? Headphones will than work, but it will not mute the speakers when the phones are plugged in. Eh, what can you do, I'll worry about that tomorrow.

Although the PAM8610 has low dissipation, up to 2W (max power output in this scenario - 2x10W), I figured it will not hurt to glue small anodized alu heat sink from old PC motherboard. It is too small, and has got the fins bent, but never mind.
 

JVC Floyd

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I would add a cut off switch in line with the speakers so that when you use the headphones you can simply switch off the speakers.
 

HRmeteohub

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Mar 19, 2017
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Thank you JVC Floyd for the idea.
I see that PAM8610 has got MUTE input, perhaps I could use it instead adding the physical switch. I always have the option to simply turn the amp off, and be gone with headphones, but I am looking to add functionality, not remove it from this feeble box.
 

HRmeteohub

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Mar 19, 2017
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Now... I have found some old 10 cm speakers, and none fit exactly, thanx to stupid recessed screw holes on original Matsushiata/Panasonic speakers. Yes, even Sony has different level of recession of their speakers. So, I have tried, and found that Pioneer are most powerful, (nice 110g strontium magnets), have good sound, and fit nicely, despite being +20 years old, from my old VW.
TFK760_speaker-replacement-choose-wisely.jpg
 

HRmeteohub

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Mar 19, 2017
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The grills are red now!
TFK760_RedGrills1.JPG
I first sanded them a bit with fine sanding sponge (3M), and then cleansed with cloth. Spraying the grills is tricky since the holes tend to clog if you over do it with paint. Also, the underlying color is black, so the red is kinda darker than I wanted. Unfortunately, TFK760 has got plastic cross in front the speakers, so I can not let the membranes show (see post #12). Ah, what can you do...

It does look kind a nice:

TFK760_RedGrills2.JPG

Guess what is the nest step?
 

HRmeteohub

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Mar 19, 2017
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Well, yes - I am also having second thoughts about red, let me se if my local shop has got "light blue" spray! Also, I am tired of all that black plastic..

So there it is:TFK760_BlueGrills1.JPG
 

HRmeteohub

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Mar 19, 2017
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The white proved to be rather tricky. When I applied glossy/protective layer it turned just slightly reddish, ruining snowy white (titanium white) colour. So I had to put another layer of white, ruining some details. Not yet done, but close. Still needs some cleaning and several touches. Also, I need to fix the antenna and make fixing clamps for speakers. Sorry for yellowish tint from my camera..
And now it looks like this:
TFK_RC760_Blue_white.jpg
 
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