Grundig TK20 and Truvox R82. Help Required Please.

[DMcMahon]

Head type shown here.

https://www.vintage-radio.net/forum/...d.php?t=175070

[Boulevardier]

Originally Posted by DMcMahon

Head type shown here.

https://www.vintage-radio.net/forum/...d.php?t=175070

That picture puts me in mind of a Bogen head, but I suppose they all look much the same. If it was a Bogen, then being open circuit would be par for the course...

Mike

[DMcMahon]

Yes reminded me of Bogen, not that I know much about heads, I do not recognise the symbol on top though.

David

[Boulevardier]

What's still puzzling me is that OP says in his very first post that, together with the hum, the audio (presumably he means off the tape) is very low and barely audible. I don't see how an o/c head would give any output signal at all. He also says somewhere that the hum varies with the tape speed. Does he mean varies in frequency? If so, the hum must be recorded on the tape.

Apart from those curiousities, it has to be an o/c head if the resistance readings now being reported are correct.

Mike

[DMcMahon]

Yes also puzzles me.

David

[DMcMahon]

From spares listing in manual the PB/Record head for R82 is listed as KH3T.

From Posts 9 and 10 of link below and symbol on top of head looks like could be Miniflux/Woelke.

https://www.google.com/url?client=in...8NY8Fjv0lk_7PR

[Ted Kendall]

Originally Posted by DMcMahon

Yes also puzzles me.

David

Capacitive coupling across the break - it wouldn't need much to get a trace of signal.

[TerryB44]

Good Morning everyone, My, you're all up early! Before I go any further I must thank everyone again for all the input you're giving to try and sort this for me. I have to admit to be somewhat of an electronics virgin in the sense of Eric Morecambe playing all the right notes but not necessarily in the right order!
However, be that as it may, I have learned a lot from all the things that I have been asked to check in the search for a solution. It reminds me of what Sherlock Holmes once said, that once you have eliminated the impossible what ever you are left with, however improbable, must be the truth!
Now, back to the problem! Below is a zip file containing audio. What you will hear is the 50Hz hum in play setting at 3 3/4ips followed by hum at 7 1/2ips, switching back and forth a couple of times, and then with tape playing at 3 3/4ips. The volume control was set at 12 o'clock, half way, throughtout.

[TerryB44]

The play/record head is a Miniflux/Woelke I think David. That picture of the symbol in the other post looks exactly the same. The picture of the heads in the first post looks identical to mine, except mine only has two heads. The erase one on the left is the same too.

[DMcMahon]

Originally Posted by TerryB44

The play/record head is a Miniflux/Woelke I think David. That picture of the symbol in the other post looks exactly the same. The picture of the heads in the first post looks identical to mine, except mine only has two heads. The erase one on the left is the same too.

I do not know if Miniflux and Woelke were actually the same company, Miniflux maybe British and Woelke German ? someone like Ted would know better.

David

[Ted Kendall]

Miniflux was a trade mark of the West German parent company, and this was also used by the English branch. In the early 1970s, some commercial manoeuvres left the English branch in sole possession of the name, so German output was re-branded as Woelke.

[DMcMahon]

Thank you Ted for the clear/definitive answer.

David

[TerryB44]

I have disconnected the playback head completely and the hum is no different and behaves in exactly the same way when speed is changed. Would this elimate the head as the problem with regard to hum?

[ms660]

Originally Posted by TerryB44

I have disconnected the playback head completely and the hum is no different and behaves in exactly the same way when speed is changed. Would this elimate the head as the problem with regard to hum?

Open circuit (ignoring the 2.2meg) grid circuit = hum.

Lawrence.

[Boulevardier]

Originally Posted by ms660

Open circuit (ignoring the 2.2meg) grid circuit = hum.

Lawrence.

If you want to be absolutely sure, try connecting together the two points from where you disconnected the lead leads. Hum should vanish completely.

Mike

[DMcMahon]

Additionally you could also try connecting one of your spare heads to make sure hum goes away.

David

[TerryB44]

Hum vanishes when terminals connected. The only heads I've got are in a 1962 Watkins Copicat tape echo and are not the orignals but a new replacement set Charlie Watkins sent me a few years ago, will these do?

[DMcMahon]

Probably any connected head would do just as a hum confirmation test, but with leads shorted together = no hum and you are 100% happy that the head itself is open circuit then probably not worth doing it in this case.

David

[TerryB44]

Thanks David, I used a 1k resistor across the terminals which is near enough the resistance of the head. I may just try one of the heads as an exercise - I might even be able to get it close enough to the tape to see if there is any audio present!

[Boulevardier]

Originally Posted by TerryB44

I may just try one of the heads as an exercise - I might even be able to get it close enough to the tape to see if there is any audio present!

If you do that and the head isn't mounted normally on the head-plate, you may need to connect the external case of the head to chassis/head-plate with a short piece of wire - otherwise you won't get the screening effect of the case and that alone could generate hum - especially if you are holding it.

Mike