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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 24th Nov 2021, 3:18 pm   #21
djorrell
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Default Re: Grundig TK 23 Delux NO ERASE

I dont have access to a scope since I retired. but there is no evidence of any AC or DC voltage whatsoever at the erase head.
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Old 24th Nov 2021, 3:27 pm   #22
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Default Re: Grundig TK 23 Delux NO ERASE

What type of meter are you using ?

The bias oscillator is high frequency (relative to audio frequency) a bog standard volt meter will struggle to give any sensible reading.

David
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Old 25th Nov 2021, 4:54 pm   #23
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Default Re: Grundig TK 23 Delux NO ERASE

Im using a autoranging digital meter and a analogue meter. I know im not going to get anything that could be relied upon in court but neither show any vital signs. If I record on a blank tape I'm getting very weak and distorted audio results but playback of pre-recorded tapes is sweet.

Dave
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Old 26th Nov 2021, 11:43 pm   #24
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Default Re: Grundig TK 23 Delux NO ERASE

As Ben says do not give up, you are probably close, have a break from it and maybe later a fresh look may uncover the cause.

Are the ECL86 voltages basically similar to the schematic values ? you should see higher voltages in Record mode.

With a good valve and good components and correct switching then the oscillator should burst into life. Shame you do not have a scope much easier then to see what is going on.

David
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Old 27th Nov 2021, 8:54 pm   #25
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Default Re: Grundig TK 23 Delux NO ERASE

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Originally Posted by djorrell View Post
Ive got continuity between between the oscillator coil and erase head.

Dave
Did this include measuring the continuity/resistance of the erase head ?

Note - to avoid magnetising heads it is always best to use a digital meter as opposed to an analogue meter when checking heads.

I know that because Record also does not work (or at least a very weak playback after record) it points to the oscillator not functioning rather than a failed head.

David
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Old 28th Nov 2021, 12:34 am   #26
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Default Re: Grundig TK 23 Delux NO ERASE

Have you checked (preferably replaced) C22 (4,700pF)? Capacitors in this position can be quite hard worked in these oscillator circuits. It's important to replace with the right type - the original is quite probably a silvered-mica capacitor. Since you can't measure how it will perform (or not) under power, the only safe test is probably to replace it with the same type. I've had capacitors in this position fail, and absent-mindedly replaced with the first capacitor of right value that fell to hand. Not a peep from the oscillator until I changed for the right type.

Mike
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Old 28th Nov 2021, 12:55 am   #27
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Default Re: Grundig TK 23 Delux NO ERASE

In the TK 23 parts list it is listed as 4,700pF, 500V Polystyrene, it is not listed in the TK 23L parts list.

David
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Old 28th Nov 2021, 1:01 am   #28
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Default Re: Grundig TK 23 Delux NO ERASE

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Originally Posted by DMcMahon View Post
In the TK 23 parts list it is listed as 4,700pF, 500V Polystyrene, it is not listed in the TK 23L parts list.

David
I haven't got the TK23L schematic. Another possibility is the bias trap - but I have a job finding that on the TK23 schematic. If that has been twiddled the HF getting through it can play havoc with the recording output stage.

Mike
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Old 28th Nov 2021, 1:12 am   #29
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Default Re: Grundig TK 23 Delux NO ERASE

Is the lack of erase and very poor recordings the same in both manual and automatic recording modes ?

David
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Old 28th Nov 2021, 12:38 pm   #30
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Default Re: Grundig TK 23 Delux NO ERASE

No obvious bias trap adjustment on the TK 23/TK 23L schematics.

David
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Old 28th Nov 2021, 3:06 pm   #31
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Default Re: Grundig TK 23 Delux NO ERASE

Sorry. On reflection I think my “bias-trap adjustment” comment was a senile lapse. Don’t think I’ve seen a domestic tape recorder with such an adjustment - they just have fixed filter networks to block the HF from the recording o/p stage.

But, really, we haven’t yet even established that there’s any erase/bias oscillator voltage present... We’re really into oscilloscope territory here.

Mike
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Old 29th Nov 2021, 7:22 pm   #32
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Default Re: Grundig TK 23 Delux NO ERASE

Hi guys. Yes I have replaced C22 with a 4.7nf Metal Film 630V. To answer the question of is there any difference in the record modes, sadly not. Manual or auto there is zero erase and terrible weak distorted audio.
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Old 29th Nov 2021, 9:50 pm   #33
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Default Re: Grundig TK 23 Delux NO ERASE

A reminder to members to check the rules before publishing schematics.

Cheers

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