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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 7th Jan 2020, 5:42 pm   #1
DMcMahon
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Default Grundig TK 35 Drive Belt Replacement

Recently purchased a TK 35 where all 4 drive belts are missing. Have got a new set to install, tape counter belt will be easy, but the 3 other belts look like could be awkward.

I think it will be somewhat similar to replacing belts on my TK 23s, where to do it properly you have to remove various metalwork (including switch actuator linkages), then remove/lift up/angle the heads block assembly to get access to the underside of the capstan flywheel, this all being quite fiddly and awkward.

The TK 23 is single speed, whereas the TK 35 is 3 speed so expect the TK 35 to be a little more involved.

So if anybody has replaced belts on the TK 35, I would appreciate any feedback on any tips, Do/Don'ts etc

David
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Old 8th Jan 2020, 9:02 am   #2
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Default Re: Grundig TK 35 Drive Belt Replacement

If nothing else, doing a Google search for "TK35 riemenwechsel" and translating the resulting German results using Google translate might be of use, for instance, in this forum the procedure was discussed:

https://forum2.magnetofon.de/board2-...grundig-tk-30/

https://translate.google.com/transla...undig-tk-30%2F

(It might be helpful to know that the German word for 'feather' is the same as for 'spring' - Feder).
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Old 8th Jan 2020, 11:00 am   #3
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Default Re: Grundig TK 35 Drive Belt Replacement

Thank you, an excellent idea searching for belt change in German, I wish I had thought of that. A quick look at those results, it looks very helpful. The TK 30 referenced, I have also recently purchased, it being very similar to the TK 35, I think main difference is that the TK 30 is 2 speed. I like the reference where it says something like "if there are other problems with the R2R then give up, as the belt change is not easy".

I think I will wait for the TK 30 to arrive to compare the positions/sizes of the drive belts (assuming that the belts are in place) before I make start on the TK 35.

Yes already knew that feder is spring and feather, came across this a while back when replacing those tiny little springs and pins in one set of my Akai adjustable NAB adaptors, the German procedure that came with the new parts referenced feder.

I was actually born in Germany but unfortunately do not really speak any German, well "nur ein kleines bisschen".
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Old 8th Jan 2020, 5:47 pm   #4
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Default Re: Grundig TK 35 Drive Belt Replacement

I see on the magnetphone.de forum site there is an option to automatically change the language to English, without having to go through Google Translate, which is very helpful.
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Old 9th Jan 2020, 10:11 pm   #5
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Default Re: Grundig TK 35 Drive Belt Replacement

Tonight powered up the TK 35 for the first time, after first doing some visual checks and measurements to make sure safe to apply power. Powered up OK, no bangs or smoke or burning smells.

Capstan motor rotates nicely when tape speeds selected, but a small bulb called "Pilot" (in the English User instructions) that should light does not light. The German Service manual calls the bulb "GrĂ¼nes signallicht" (green signal light), most likely the bulb has blown.

All valve heaters light up and the EM84 Magic Eye lights up nicely when the Recording button (Aufnamhe-Taste) is pushed down (also a loud relay actuates lots of switches when record is selected).

There is also a round filament bulb hidden underneath that also lights up brightly when Record is selected, do not yet know its function/indication, have not seen this before in any of my previous Grundigs.

So a good positive start. Next will check out the main DC supply rail and checkout the various reservoir/smoothing and de-coupling electrolytics, as well as the non electrolytic capacitors to see if any obvious candidates for replacement.

Will hold fire on starting strip down to install the new drive belts, until the imminent arrival of the TK 30 that I expect to have very similar drive belt arrangement, to compare to.
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Old 10th Jan 2020, 2:44 pm   #6
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Default Re: Grundig TK 35 Drive Belt Replacement

Quote:
Originally Posted by DMcMahon View Post
There is also a round filament bulb hidden underneath that also lights up brightly when Record is selected, do not yet know its function/indication, have not seen this before in any of my previous Grundigs.
In a TK35 schematic I'm looking at there's a bulb connected in series with the Record relay, labeled the C relay in that schematic, and so far as I can make out it's function is to limit the relays holding current, eg max current through the relay coil when the bulb is cold then reduced holding current through the relay coil when the bulb is hot.

Lawrence.
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Old 10th Jan 2020, 4:53 pm   #7
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Default Re: Grundig TK 35 Drive Belt Replacement

Hi Lawrence,

Yes I saw the lamp later, in the schematic and saw that it is in series with the Record Relay (C) coil going back to the +ve side (which goes to ground) of the nominal 24 Volt LT supply . As it is hidden, so would not normally be seen, it was obvious that it was not for visual indication and I possibly wondered about current limit, I think you have most likely hit it on the head.



The first time I have seen this in a Grundig.
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Old 10th Jan 2020, 5:01 pm   #8
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Default Re: Grundig TK 35 Drive Belt Replacement

The Pilot bulb/lamp I previously reported, connects to one of the 6.3 volt filament heater supply secondary windings on the mains transformer (the winding that connect to valves 2 to 5).

So effectively it is a Power on status indication.

Luckily the bulb had not blown, just needed screwing in fully into its holder, a nice simple one for a change ! For reference the bulb is Osram 7V / 0.3A
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Old 11th Jan 2020, 2:13 pm   #9
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Default Re: Grundig TK 35 Drive Belt Replacement

TK 30 arrived today, not as good condition (the case particularly needs some repairs to cloth) as the TK 35.

It did not initially look good for the drive belt status, when I removed the lid, a thin black broken drive belt fell out, luckily on inspection it was only a black stretch cord. Removed top panel and all 4 drive belts are fitted and look in reasonable condition.

The drive belt arrangement looks very similar (maybe the same) as the TK 35, although some small differences in some of the surrounding mechanisms, so will be able to use the TK 30 as a reference, when fitting the new belts to the TK 35.

The TK 30 came with a novel homemade mains socket cable (see photo) to plug into the back of the unit, I don't think I will be using that !
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Old 13th Jan 2020, 10:23 pm   #10
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Default Re: Grundig TK 35 Drive Belt Replacement

Next will check out the main DC supply rail and checkout the various reservoir/smoothing and de-coupling electrolytics, as well as the non electrolytic capacitors to see if any obvious candidates for replacement.


Tonight checked out the main HT DC supply, +260 volts with 6 volts p-p ac mains ripple (10mS/100Hz), seems to be a very typical value of mains ripple with these old Grundigs.

The -24 volt supply is -27 volts with 0.6 volt ripple. The various lower voltage HT supplies are all in the right ballpark (as per typical values shown on schematic) with low ripple levels. Also the main valve electrodes voltages look good.

So purely from a voltage measurement point of view, most of the electrolytic and decoupling capacitors look reasonably healthy as do the 2 Selenium bridge rectifiers.

Next (when time permits) will install the 4 new drive belts, then hopefully will get the tape transport working and be able to start playback and recording tests.

Last edited by DMcMahon; 13th Jan 2020 at 10:28 pm. Reason: Typo
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Old 3rd May 2020, 8:42 pm   #11
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Default Re: Grundig TK 35 Drive Belt Replacement

Having an enforced break from the Senator and decided it was long overdue to get back to the TK 35 and fit the 4 new drive belts. Have my TK 30 alongside for comparison which still has its 4 belts intact and fitted.

Will do some cleaning of the TK 35 during the partial dis-assembly required to install the new belts.
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Old 5th May 2020, 2:06 pm   #12
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Default Re: Grundig TK 35 Drive Belt Replacement

Have replaced all 4 belts OK and did some top side cleaning/lubing etc.

With no tape, the tape transport looks good but with a tape it really struggled, looks like the tape is sticky, have never used that tape before, so will try another tape.

When I stripped down the reel tables there was a round heavy metal piece (called clutch weight) in the Take Up reel table. The drawing shows a weight also in the Supply table, but there wasn't one ? I have not seen one of these weights before.

A small problem with the Play relay that moves the pinch roller to the capstan shaft, in Play mode it works fine with 24 volts on the relay coil. But with Record also selected the relay voltage drops down to 12 volts and the Play relay only partially energises mechanically and the pinch roller stops well short of the shaft. So need to investigate that before can test recording.
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Old 5th May 2020, 5:42 pm   #13
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Default Re: Grundig TK 35 Drive Belt Replacement

While investigating the Play relay (Relay A = Andruckmagnet/Pressure) not fully energising when Record selected, the problem vanished, then found that the Record relay was no longer working.

Found that Record Relay C holding current limit bulb, was not working and found that the little wire that solders onto the end of the bulb had broken off, so soldered back on. Now all OK, hard to understand how this could have caused the original problem symptom.

This bulb is referenced in Posts 5, 6 & 7, noticed this time around that the bulb is only shown in the German documentation, i.e. is not shown in the UK documentation.
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Old 5th May 2020, 11:19 pm   #14
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Default Re: Grundig TK 35 Drive Belt Replacement

Tried another tape and the tape transport now works fine, so have thrown away the sticky tape.

Have only encountered sticky tapes a few times and am still surprised how quickly and badly they can disable the tape transport operation.

Was about to do a recording test, when I noticed 2 slimy looking resistors mounted on a little terminal strip on the top right of the unit, by their colour codes they are 47k & 68k
but after wiping them dry, they both measure open circuit one way and low resistance (lower than their code values) the other way.

Obviously this cannot be correct, I am sure they are resistors. Not easy to identify what resistors they are, following the 2 core screened cable (within sleeving) the cable enters an area with a metal cover. Removed the cover and 2 valves, various components and the Record switches are under it, but very difficult to see where the wires go/connect to, to be able to identify what the resistors are.

One of the valves is an ECC83, it is supposed to be an ECC81, I think they most likely are compatible.

There are quite a few wax capacitors under the cover, at least 3 of them look in poor condition, so will have to check them out.

Note - in the attached photo the flash has made the orange multiplier band look yellow.
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Old 6th May 2020, 9:13 am   #15
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Default Re: Grundig TK 35 Drive Belt Replacement

Traced the 2 resistors to be R30/R31 part of the equalisation circuit. After unsoldering them, then got good readings, so turned out to be a red herring, must have been getting poor contact with my DMM leads, strange though that had got same bad readings several times ?

Now to check out the capacitors.
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Old 6th May 2020, 4:30 pm   #16
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Default Re: Grundig TK 35 Drive Belt Replacement

Replaced 4 waxed paper capacitors (all ERD 100) C6/C7/C8/C10, all around the EF86 area.

All were passing DC/leaking particularly C7 which was effectively shorting down the DC polarising supply for the Condenser microphone at pin 1 of the microphone DIN socket, now have a healthy 95 volt mic supply.

There are maybe other capacitors that need replacing but will first quickly check out recording/playback
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Old 7th May 2020, 4:43 pm   #17
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Default Re: Grundig TK 35 Drive Belt Replacement

Did some quick test recordings using both the Diode and Gram P.U. DIN line inputs. It went through the motions and easily got good Magic Eye recording levels, but on playback very quiet recordings with lots of hum if volume wound up.

It does erase OK and monitoring of the record signal sounded good.

So back inside to investigate.
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Old 8th May 2020, 9:01 pm   #18
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Default Re: Grundig TK 35 Drive Belt Replacement

Replaced 3 more waxed paper capacitors C11, C12 & C13, all around the ECC81 (actually ECC82 fitted) like the previous replaced capacitors, they were quite leaky and at least double their stated (sometimes > x 3) capacitance values.

Two of these were 0.01uF, I only had one new one left, so used two 0.022uF in series for one of them. Resistor R14 (150k) end broke off while soldering C11, luckily had some new 150k, although the 0.5W resistor looks small compared to old 0.33W

Retested record, as before Magic Eye signal good and monitored sound good but playback sound very weak with plenty of hum if volume control wound up.
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Old 10th May 2020, 11:58 am   #19
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Default Re: Grundig TK 35 Drive Belt Replacement

Quote:
Originally Posted by DMcMahon View Post
TK 30 arrived today, not as good condition (the case particularly needs some repairs to cloth) as the TK 35.

The TK 30 came with a novel homemade mains socket cable (see photo) to plug into the back of the unit, I don't think I will be using that !
A real beaut in the best Heath Robinson (my mentor) style. Several of my old Grundig buys were missing the special mains lead with small connector, Given their scarcity they list for as much as an entire machine on eBay. I'm looking at replacing the existing case mounted socket with a safe plug and socket arrangement like a Hubbel which can be bought new
TT
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Old 10th May 2020, 1:55 pm   #20
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Default Re: Grundig TK 35 Drive Belt Replacement

Certainly was Heath Robinson, some might save inventive

At the moment I am sharing the mains cable between the TK 30 & TK 35, if both units end up working OK, I will have to upgrade one of them to a modern plug and socket cable.

I will have to look up Hubbel, I associate them with the old USA mains connectors.
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