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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 15th Jul 2018, 9:46 pm   #1
Gordondav54
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Default Grundig TK23 no erasure no bias.

Dear All
I have being trying for several days to fix a Grundig TK 23. I have a diagram from page 184 of H W Hellyers Tape Recorder Manual.
Mechanically and visually the machine is near perfect. It is a four track mono with four valves, EF86 input, ECC81 amplification, EL95 power output and erase/bias oscillator when on record. It plays very well but will not record other than very quiet and scratchy. The magic eye EM84 appears to work normally. The EL95 is not oscillating hence no erasure of the tape and no bias. I have tried everything I know to fix it. I have a basic digital multimeter and realise that it might not give accurate measurements of the bias voltage but there are no volts to measure.
All the voltages shown on the diagram are measured correct within a couple of percent.
The components in the region, capacitors and resistors, have all been tested or changed.
The oscillator coil was faulty with an internal break but this I have repaired and wound as before. All tappings measure low ohm-ages as I would expect.
The record slider switch has been checked for correct operation and connections.
The dubbing erasure stop switch is closed correctly.
All wires have been tested for continuity.
The erase head has been checked (I thought this might form part of the resonant circuit) as good, only a few ohms with each track the same.
The track change switch has been checked for continuity.
I have even substituted the EL95 with another.
I note from Hellyers that this machine is the only Grundig shown that does not have a separate valve to provide the erasure/bias. Is there a fundamental problem with the design maybe?
So far I have been reasonably successful in fixing tape recorders but this is proving unusually stubborn.
I have read the old threads from 2006 but no luck so far.
Has anyone successfully fixed a similar problem on a TK23?
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Old 15th Jul 2018, 11:50 pm   #2
ben
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Default Re: Grundig TK23

The trick record switch and valve base for the EL95 would be the first areas I would give greater scrutiny to.
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Old 16th Jul 2018, 2:27 pm   #3
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Default Re: Grundig TK23 no erasure no bias.

I note you say you have rewound the oscillator coil. As this is a key element of the oscillator, are the windings in the correct phase? Have you tried reversing connections to one winding? Has it been wound EXACTLY as the original? It could have critical self-capacitance that influence the oscillator function.
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Old 17th Jul 2018, 10:14 pm   #4
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Default Re: Grundig TK23 no erasure no bias.

I agree that this will most likely to be due to that rewound coil. I don't personally think that you can successfully rewind one of those coils. Look out for a scrap machine of the same type and use it for spares for your good condition example.
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Old 23rd Jul 2018, 8:07 am   #5
Gordondav54
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Default Re: Grundig TK23 no erasure no bias.

I tried reversing the polarity of one winding, no change. Also looked for valve base problems. The coil is very simple and has about 8 meters of wire on the secondary with two very short tappings of only 150 mm each (4 or 5 turns) as the last outer layers. I did not unwind the primary as there was no need. Regrettably I think it is time to give it up as I have many other machines to look at. If anyone else has the time they could have a go. I live in SWSurrey.
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Old 26th Jul 2018, 7:51 pm   #6
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Default Re: Grundig TK23 no erasure no bias.

I have attached the lead connection detail of the oscillator coil for the
TK 23.

If the coil was rewound with the same wire as found on it,
the insulation may be faulty which would cause turn to turn
short circuits. Just one shorted turn would kill oscillation.

The bias frequency for the TK23 is 55 +/- 3 kHz adjusted by R40
to 9 volts across lower pole of erase head. Certain DVMs
only work on AC below 1 kHz.

Shown is detail of involved rotary contact assembly.

I have repaired these machine when they were current
55 years ago. The EL 95 combined oscillator/ output works fine
and was used on many machines including this Grundig.

Back then I had my own files of service data. Still do.

Sort of job security, as we tech workers flitted between shops
as opportunities came and went. In the early days three of us
were employed at the same shop, but finally went back as managers
to one big firm and stayed until it fell apart 20 years later.
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Old 30th Jul 2018, 7:36 am   #7
Gordondav54
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Default Re: Grundig TK23 no erasure no bias.

Thank you Radiotechician. It is alway nice to have a board layout as well as a circuit diagram which can be so difficult on tape recorders because of all the switching involved.
I have a specific question for you. As I mentioned above the coil secondary consists of a long length of thin wire with two very short lengths as the outer few turns. These are of slightly thicker gauge wire. Is the gauge very important do you think?
Also how should the windings be arranged, smoothly with adjacent turns like a new roll of cotton or more random? If smoothly is the answer could it be that the last few turns of thicker wire must be layed in a particular pattern? The two lengths only form 5 turns each, not nearly enough to cover the width of the bobbin.
As a matter of interest I have a TK 20 but found the coil to be quite different. Also it looked scrap but I found that it works, even on record!
If you have any specific data about the coil itself that could be a great help.
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Old 31st Jul 2018, 11:23 am   #8
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Default Re: Grundig TK23 no erasure no bias.

The bias circuits operate at about 50 kHz. At this frequency
a significant loss is skin effect in single strand wires. So gauge
of wire is important. How important is more a matter of the impedance
concerned, and bias systems, that some Grundig models use
have low impedance erase heads, while anode drive and record
heads are high impedance.

I have attached bias coil winding specs and the bias circuit for a TK/M 64.

It uses a EL95.

As you can see there were three different wire gauges used in the coil. If the
skin effect approaches the resistance of small wire, then larger would be used. A coil with a few turns might be too fragile to reliably connect made very thin wire.


As to the lay of the winding, how the arrangement was done had more to do
with the coil winding machine.

In the early 60's there was a journeyman technician from Germany in our
shop and he said winding data was given for coils and transformers because
new parts were hard to get post war, and it was expected the shops rewound
them. I doubt they had winding machines.

With respect to your wire, my concern was with the enameling coming off
in places.
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