UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Tape (Audio), Cassette, Wire and Magnetic Disc Recorders and Players

Notices

Vintage Tape (Audio), Cassette, Wire and Magnetic Disc Recorders and Players Open-reel tape recorders, cassette recorders, 8-track players etc.

Closed Thread
 
Thread Tools
Old 28th Nov 2018, 10:47 am   #1
sonoguy
Triode
 
Join Date: May 2016
Location: Walsall, West Midlands, UK.
Posts: 17
Default Grundig TK14 problems.

I have a Grundig TK14 that works fine mechanically but it will not erase any previous recording on the tape. If I try to record, the new sound is distorted and added to the previous recording on the tape. On checking the usual causes of this problem, ECL86, record/play switch contacts and capacitor C20. I find that on record, there is an ac voltage across the erase head of around 10-12 volts. All voltages around ECL86 seem fine. I think the ac voltage on the erase head during record should be a lot higher than what I have. Looking a bit deeper, checking 'cold' dc resistances, with power disconnected. I find that variable resistor (pot) R37 has an intermittent slider connection and seems to be locked in place. Measuring across it gives me a reading of 68K ohms or open circuit, if I touch the pot. Also capacitor C21, 1000pF in the oscillator circuit when removed and checked shows a value of 47pF or open circuit. On moving the slider position of R37 and replacing C21, erasure of the tape now works but not perfectly. Highly modulated old recordings take a couple of passes to erase fully. I'm looking to replace R37 with a new 100K pot as well as C21 with a new 1000pF 1KV working capacitor. Can anyone advise me on the correct setup procedure for R37, it's in the feedback circuit of the bias oscillator. I really just need to know relevant test points and bias level settings during record. Many thanks in advance for any help you can offer.
sonoguy is offline  
Old 28th Nov 2018, 11:24 am   #2
ms660
Dekatron
 
ms660's Avatar
 
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
Default Re: Grundig TK14 with 'No Erase' problem

To set the erase current insert a 10 ohm resistor in series with the earthy end of the erase head and adjust R37 for a reading of 420mV AC across that resistor.

To set the record bias current insert a 100 ohm resistor in series with the earthy end of the rec/pb head and adjust C19 for a reading of 60mV across that resistor. Be aware that the earthy end of the rec/pb head is switched between record and playback.

It's all in the manual.

Lawrence.

Last edited by ms660; 28th Nov 2018 at 11:36 am.
ms660 is offline  
Old 28th Nov 2018, 4:51 pm   #3
sonoguy
Triode
 
Join Date: May 2016
Location: Walsall, West Midlands, UK.
Posts: 17
Default Re: Grundig TK14 with 'No Erase' problem

Thank you ms660 for your advice. What sort of meter should I be using for these measurements, I have a muti-meter but do I need anything else? The only Manual I have at the moment is a TK14 pdf file in German. I'm trying to translate parts of it using Google...very slowly. Would you have scans of the relevant pages from your Manual concerning Bias Oscillator setup? That would be immensely useful to me.
Nick.
sonoguy is offline  
Old 28th Nov 2018, 5:51 pm   #4
ms660
Dekatron
 
ms660's Avatar
 
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
Default Re: Grundig TK14 with 'No Erase' problem

The manual I was referring to is online but forum rules prevent me from posting a link, there is a Trader sheet available from here:

https://www.service-data.com/section.php/3451/1/tk14

The online manual I was looking at was in English but the bias/erase frequency wasn't given in it (not that I could find) but looking at some of the German manuals online the frequency appears to be 55kHz or there about, so your AC meter needs to be accurate at least up to that frequency.

Lawrence.
ms660 is offline  
Old 29th Nov 2018, 4:06 am   #5
TIMTAPE
Octode
 
Join Date: Jul 2006
Location: Perth, Western Australia
Posts: 1,965
Default Re: Grundig TK14 with 'No Erase' problem

Just to mention the old qualifier. Later tapes required more energy to erase and bias so once you get the oscillator up to original specs, it will struggle to erase some later higher energy tapes.
TIMTAPE is offline  
Old 4th Dec 2018, 10:16 am   #6
sonoguy
Triode
 
Join Date: May 2016
Location: Walsall, West Midlands, UK.
Posts: 17
Default Re: Grundig TK14 with 'No Erase' problem

Hi and thanks TIMTAPE for the information regarding newer tapes needing more energy to erase and bias. I must admit I'd forgotten that. I now have the English version of the full Grundig TK14 Service Manual. HF Bias Adjustment is stated there, but I can't figure out where the test points are for the measurement. Am I missing something obvious, is the measurement across the Erase head and P/Back record head? Extract attached.
Can anyone enlighten me?
Nick Mellor. (UK)
Attached Thumbnails
Click image for larger version

Name:	TK14 Bias Adjust.JPG
Views:	76
Size:	22.1 KB
ID:	173938  
sonoguy is offline  
Old 4th Dec 2018, 10:56 am   #7
sonoguy
Triode
 
Join Date: May 2016
Location: Walsall, West Midlands, UK.
Posts: 17
Default Grundig TK14 Input Output DIN Socket connections

During my testing, I've discovered there are two versions of the Circuit Diagram [Original and (Bild2)]. The differences appear to be around the Input and Output socket connections and the source of the signal applied to them. The 'Output' Din socket on my machine has Low Impedance speaker out and High Impedance (Ceramic/Crystal Phono level) input. The 'Diode' Din Socket (input) has medium impedance input as well as a medium impedance output signal. The output level is controlled by the volume control. Does anyone have any further information about this or any reason why these changes were made to the input/ output connections. Is my machine an early or late model?
Nick Mellor (UK)
sonoguy is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 8:37 pm.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.