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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 10th Feb 2020, 2:21 pm   #121
DMcMahon
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Default Re: Grundig TK 819 Any Useful Info

Yes a good point about the contact plate wear, although I am not sure currently that the plunger does rotate with the motor shaft, will find out later today when I power it up.

It is not hard to adjust currently but the closed GI contact is proving difficult to get good reliable (or even at times any) electrical continuity from the bottom contact to the top contact plate even though it appears to be making good mechanical contact ? There is no adjustment as such apart from ensuring the top and bottom spring contacts line up well and spring contact tension by tweaking the contact arm.

Yes I did not think it looked like cork and not sure if cork would decompose like that but because I had seen at least 3 different people suggesting it was originally cork, thought that maybe it could have been. Of course 2 of those people could just have been repeating what the first person had suggested.

My Supply reel felt ring I think is not contaminated.
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Old 10th Feb 2020, 3:15 pm   #122
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Default Re: Grundig TK 819 Any Useful Info

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Quote:
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So I now assume that when the the motor shaft rotation speed gets to a certain level that centrifugal force will push up the sprung loaded plunger and open the G contacts. I say assume, it has got to be something very similar otherwise the contacts would never open. Looking forward to seeing the plunger lift up in operation.
That is correct, the idea is to supply the motor with a higher voltage while it is starting up, and then go to a lower voltage once it has reached its proper speed, I'm assuming to minimize motor power consumption and hence heat. This mechanism is only operational at the higher speed (7 1/2) however; at 3 3/4 the lower voltage is used all the time.

Can't really see in the picture, but it looks like it might be missing a little plastic top on the plunger to avoid it short circuiting the contacts to ground. It should be white-ish in color.
I don't think it's anything to do with power consumption/heat. The brief application of higher voltage will be to overcome the starting inertia of the take-up spool - otherwise the capstan will have fed an amount of tape through before the take-up spool has reached sufficient speed to take the tape up - thus forming a loose tape loop that suddenly gets snatched up when the spool reaches speed. The larger (heavier) the spool and the greater the capstan speed, the greater the problem. It becomes a huge problem if the machine is using 10.5 inch metal NAB spools, and is feeding tape at 15 i.p.s. But the problem would still be there at 7.5 i.p.s. and with smaller spools.

That's my take on it, anyway!

Mike
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Old 10th Feb 2020, 4:02 pm   #123
Leon Crampin
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Default Re: Grundig TK 819 Any Useful Info

It also much reduces the dead time when the tape reverses on the auto-reverse machines. The white dome plastic cap on the centrifugal switch pushrod is essential to stop the rod end boring a hole in the upper contact spring leaf.

Leon.
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Old 10th Feb 2020, 4:42 pm   #124
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Sorry, my previous comment was probably irrelevant - I had assumed we were dealing with a three-motor deck, and talking about the motor driving the take-up spool. I've not read whole thread, but if this is a single-motor deck then my comment made no sense.

Mike
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Old 10th Feb 2020, 6:42 pm   #125
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Default Re: Grundig TK 819 Any Useful Info

Hi Mike,

Yes it is a single motor 7" machine, I did wonder about your comments a little, but still good feedback anyway.

David
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Old 11th Feb 2020, 12:41 am   #126
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Default Re: Grundig TK 819 Any Useful Info

Tonight powered up the complete unit for the first time, actually not 100% complete as I left the heads assembly off, because still having problem getting the GI contact to electrically make.

Initial tests done without tapes/spools.

Play in both directions at both speeds basically work and also RWD and FWD (at least control wise).

In 7.5 ips can see the centrifugal push rod lift up (interesting to see), in forward and reverse play and also RWD/FWD.

In 3 3/4" ips the centrifugal push rod moves up a small amount, but not enough I would say to push the contact plate.

The centrifugal push rod does rotate so as advised previously will need to find some sort of plastic/rubber cap to protect the contact plate from wear/damage.

Intermittently when operating the various tape control push buttons could see sparking from one or more of the associated 6 heavy duty switches contacts, probably would have not noticed this, if I had not switched the lights off before starting the tests. Will inspect their contacts later.

Noticed that some of the drive belts are quite badly cracked, hopefully they will last long enough to be able to see what the tape transport control is like with tapes & see what playback and record are like, rather than having to waste around 20 on a set of 5 belts if any major/unfixable problems are later discovered.
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Old 11th Feb 2020, 5:23 pm   #127
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Got a few ideas what to try/use for the centrifugal pushrod cap, but initially for short term going to try a short length of biro ink tube (empty of ink of course). Tried 3 biro pen tubes and only 1 of them was tight enough to be a tight fit on the pushrod.

It may not be ideal as the end is open and possibly it may get pushed down the pushrod shaft when shaft is extended and possibly expose the metal end, some soon to do tests will indicate how it may perform.

Image attachment shows a TK 830 where the white cap can just be seen on the capstan shaft tip.
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Old 11th Feb 2020, 9:35 pm   #128
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Default Re: Grundig TK 819 Any Useful Info

Spoke too soon, on a subsequent test the drive belt to the Supply table broke, so now no reverse play or RWD control with tape loaded. May be able to find a suitable belt by looking at some of my various spare belts.

With tape loaded, Play at 3 3/4 ips works and FFD works but when Stop pressed both spools are slow to stop and/or sometimes accelerate initially before stopping. Assume this may be a clutching tension issue, am not very confident either if the electrical clutch's are working correctly.

The biro tube mod added to the centrifugal push rod appears to be doing its stuff in terms of pushing up the contact plate when running at 7 1/2 ips, but have not yet tried operationally with the Relay C circuit connected because still do not have the GI contact electrically working.

Because the chassis is not in its case and therefore difficult to connect the loudspeaker, have not yet tested any sound playback, also removed the EL 84 output Pentode valve to protect the valve and output transformer with no load connected.

According to the schematic, there is a 5 ohm resistor dummy speaker load that can be switched in, but I have not yet worked out how to do it.
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Old 12th Feb 2020, 1:31 pm   #129
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Default Re: Grundig TK 819 Any Useful Info

The belts in this machine have a round cross section, so suitably sized o-rings should work fine as belts.

As for the broken belt, as a temporary makeshift solution you can glue it back together with superglue. (This is actually the preferred method if you want to make a belt of o-ring string. The resiliency of the rubber stops the otherwise brittle cyanoacrylate bond from cracking.) Of course, in the state it's in it will tend to break somewhere else eventually but it might keep you going long enough to source new belts.
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Old 12th Feb 2020, 4:33 pm   #130
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Default Re: Grundig TK 819 Any Useful Info

Thank you Ricard, I will certainly try this, while I wait for the set I have ordered (I bit the bullet). Have never tried gluing drive belts before.

Do you see any difference between gluing drive belts and o-rings, in terms of repair lasting.
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Old 12th Feb 2020, 7:09 pm   #131
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Tried to glue the broken belt but a section about 1" long broke off, in fact the whole belt is falling to pieces almost, did not realise it was that bad ! not worth trying to repair now.
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Old 12th Feb 2020, 9:55 pm   #132
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If my memory serves me correctly, the broken belts can be re-deployed in the upper reel hubs to replace the rubbery insert which is used to lock the hubs for fast wind.

As you may have determined, when fast wind is selected, the solenoid in the appropriate hub is fully energised, so that the metal disk jumps up to meet the rubber insert in the upper hub, locking it solid.

It was a very long time ago, but I seem to remember that broken belts were an ideal substitute for the ring in the upper hubs.

Leon.
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Old 12th Feb 2020, 10:20 pm   #133
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Thank you for that Leon, the broken belt in question is such poor condition, basically breaking up every few mm now, it may not be suitable but another old belt of similar dimension may well be.

I think I read one old post where someone had tried a belt without very good results, it will just be a case of trying to see what results are obtained with belt or other material.

Even though I have read about the operation of the hub solenoids, it is not 100% clear to me (yet) their exact operation. Looking inside one of my reel tables it looks like the clutching disk may not lift up (currently) when solenoid is energised because the disk looks to be stuck down due to the decomposed clutch rubber material all around it (but presumably it must be lifting otherwise spool would not rotate in fast wind ?).

I need to do some tests to quantify this further and then once I know the current behaviour, I will clean up all the decomposed material and try experiments with belt/other material as the clutching rubber replacement.

David
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Old 13th Feb 2020, 11:17 am   #134
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Put it back inside its case (after refitting the EL 84 output pentode) and connected the speaker. Tried playing tape in Forward Play at 3 3/4 ips but no sound heard.

With the volume turned up high there is a bit of audible hum from the speaker and when the tape transport switches are pushed there is loud switch noise from the speaker, so there is at least some life in the amplifier.

So very disappointing, I would have thought even with badly worn playback head would have got at least some sound. The tape I tried I think came with the unit so first need to make sure there are recordings on the tape.

From this I found that the switch for switching off the loudspeaker and switching in 5 ohm resistor dummy load is ganged to the potentiometer on the right hand side of the unit, I assume this may be the record level pot.
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Old 13th Feb 2020, 12:41 pm   #135
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Originally Posted by DMcMahon View Post
Put it back inside its case (after refitting the EL 84 output pentode) and connected the speaker. Tried playing tape in Forward Play at 3 3/4 ips but no sound heard.

With the volume turned up high there is a bit of audible hum from the speaker and when the tape transport switches are pushed there is loud switch noise from the speaker, so there is at least some life in the amplifier.

So very disappointing, I would have thought even with badly worn playback head would have got at least some sound. The tape I tried I think came with the unit so first need to make sure there are recordings on the tape.

From this I found that the switch for switching off the loudspeaker and switching in 5 ohm resistor dummy load is ganged to the potentiometer on the right hand side of the unit, I assume this may be the record level pot.
Even with a blank tape you should get audible tape hiss. Have you tried touching the connections to the replay head with volume turned high when in replay mode? Should produce a very loud hum.

Mike
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Old 13th Feb 2020, 2:02 pm   #136
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David, as far as I remember, the reel hub clutches have a metal disk below the solenoid coil, rather like a large washer with a textured (in the metal) surface. This disk has to be axially free for the clutch to operate correctly.

The reel hub solenoids have 2 functions, braking and fast wind engagement. For braking, where the tape has to be brought to a reasonably gentle halt, the solenoids are partially energised via a resistor; fairly large to dissipate some power. This gives a relatively soft halt without tape snatch.

For fast wind, the take up hub solenoid is fully energised - you will hear the "washer" jump up with an audible click and the hub will be locked solid if the circular rubber strip in the upper hub is in good condition.

It's always worth checking the 25V supply to the solenoids and relays. The AEG selenium rectifier positioned at the front of the lower chassis is notorious as a failure item, and is best replaced with a silicon bridge.

Leon.
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Old 13th Feb 2020, 2:10 pm   #137
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Default Re: Grundig TK 819 Any Useful Info

Hi Mike,

Yes agree about the hiss, but never heard anything obvious.

The replay head connections are not easily accessible, the wires come out of the head (s) and terminate underneath the heads assembly where it is not practical to access with the heads assembly fitted. I may possibly be able to partially pull out the relevant connector and try and make a wet finger contact directly or via a small instrument screwdriver.

Also trying to find a suitable drive belt to replace the broken belt on the Supply spool so then can also check the other replay head in reverse playback.
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Old 13th Feb 2020, 2:16 pm   #138
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Default Re: Grundig TK 819 Any Useful Info

Thank you Leon with the additional info on the clutch operation. I do not think I am hearing the audible click when in FWD/RWD, that is why I previously said I was not sure if the solenoids were working properly.

The supply from the Selenium rectifier was good when originally checked but of course will recheck it as part of the solenoid operation investigation.

David
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Old 13th Feb 2020, 3:00 pm   #139
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Regarding contacting the head connections: the heads are mounted in sockets, if you remove the magnetic shield they can just be pulled straight up, you can then poke a wire or somesuch in the socket. You may have to redo the azimuth setting afterwards but you'd probably want to check that anyway once you got the machine working properly.
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Old 13th Feb 2020, 5:54 pm   #140
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Thank you Ricard, I did not appreciate that they are mounted in sockets.
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