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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 12th May 2021, 11:49 pm   #1
reartech
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Default Grundig tk 24

Dear members
I tried to service grundig tk24 Reel to reel taperecorder almost burnt wires replaced around ef-86 now problem is low audio from head,heads are properly cleaned no use with full volume very very low sound audiable near to ears
where as if i connect to audio in with blue tooth it is ok no issue but in play mode it is too low please guide me
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Old 13th May 2021, 9:06 am   #2
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Default Re: Grundig tk 24

Hello and welcome to the Forum.

How/where are you connecting bluetooth to the TK 24 ?

David
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Old 13th May 2021, 10:08 am   #3
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Default Re: Grundig tk 24

So the low volume is when you are playing back a tape. Does the tape you are trying contain known good recordings or are you playing back new recordings from the TK 24 ?

Have you tried other tapes ?

There are many things that could cause poor playback volume such as :-

1. Dirty playback (PB) head
2. Poor tape contact with PB head
3. Defective PB head
4. Poor recording
5. Bad tape
6. Bad contact (s) on PB/Record switch
7. Tape back to front
8. Bad wiring connections such as head wiring to the electronics
9. Fault in the PB amplifier electronics

If you confident and feel safe, you could remove the top panel and switch to Play mode then with the volume control turned up quite high you should hear a loud hum if you touch a finger (thus is very safe to do) to the PB head wires. This would to some extent prove if the PB amplifier is basically functional.

David

David
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Old 13th May 2021, 10:46 am   #4
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Default Re: Grundig tk 24

Quote:
Originally Posted by DMcMahon View Post
you should hear a loud hum if you touch a finger (thus is very safe to do) to the PB head wires.

David

David
Typo correction - you should hear a loud hum if you touch a finger (this is very safe to do) to the PB head wires.
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Old 13th May 2021, 2:17 pm   #5
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Default Re: Grundig tk 24

Thank you very much for your speedy response
a) for test purpose I connected blue tooth receiver to Platte Phono and In Radio (diode) where the audio output is good with full sound and when I switch to normal play back mode audio become 99 percent low.

b) then a find out when realy is on the audio become low I started tracing the wiring finally I removed w1 wire audio become loudly but distortion
c) finally I connect bluetooth to head input where i found low when w1 wire connected and removed high but audio distortion now I am unable to proceed further without proper guide please guide me sir
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Old 13th May 2021, 2:24 pm   #6
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Default Re: Grundig tk 24

Quote:
Originally Posted by DMcMahon View Post
So the low volume is when you are playing back a tape. Does the tape you are trying contain known good recordings or are you playing back new recordings from the TK 24 ?

Have you tried other tapes ?

There are many things that could cause poor playback volume such as :-

1. Dirty playback (PB) head
2. Poor tape contact with PB head
3. Defective PB head
4. Poor recording
5. Bad tape
6. Bad contact (s) on PB/Record switch
7. Tape back to front
8. Bad wiring connections such as head wiring to the electronics
9. Fault in the PB amplifier electronics

If you confident and feel safe, you could remove the top panel and switch to Play mode then with the volume control turned up quite high you should hear a loud hum if you touch a finger (thus is very safe to do) to the PB head wires. This would to some extent prove if the PB amplifier is basically functional.

David

David
Tapes are in good condition every thing is properly checked
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Old 13th May 2021, 3:29 pm   #7
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Default Re: Grundig tk 24

Quote:
Originally Posted by DMcMahon View Post
7. Tape back to front
In other words is the matt side of the tape next to the R/P head?

Maybe de-magnetising the R/P head could help.
Mike
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Old 13th May 2021, 10:45 pm   #8
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Default Re: Grundig tk 24

Quote:
Originally Posted by reartech View Post
Thank you very much for your speedy response
a) for test purpose I connected blue tooth receiver to Platte Phono and In Radio (diode) where the audio output is good with full sound and when I switch to normal play back mode audio become 99 percent low.

b) then a find out when realy is on the audio become low I started tracing the wiring finally I removed w1 wire audio become loudly but distortion
c) finally I connect bluetooth to head input where i found low when w1 wire connected and removed high but audio distortion now I am unale to proceed further without proper guide please guide me sir
I am not really familiar with Bluetooth receivers but assume you are picking off a signal into the Bluetooth receiver for listening/monitoring on an external Bluetooth enabled device.

The W1 contact is one of several Playback/Record contacts. If I understand the circuitry correctly, when the W relay is energised it is then in Playback mode, where the ECC81 output at C22 is switched by W1 to the Playback equalisation components. With the W relay de-energised it is in Record mode and the C22 output is switched by W1 to the EM84 Magic Eye for record signal monitoring.

Sorry but from your information I cannot really advise much else from what has already been said.

David
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Old 14th May 2021, 2:30 am   #9
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Thumbs up Re: Grundig tk 24

Thank You very much for your Knowledge sharing.if you found any solutions to the said problem please inform Regards Krishna
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Old 14th May 2021, 7:46 am   #10
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Default Re: Grundig tk 24

If you have a test meter that can measure DC voltage it would be worthwhile checking the voltages at the valves using the voltage values shown on the schematic as a guide to expected typical values (these are ballpark numbers).

You should only do this if you are competent and can do it safely, some of the voltages are potentially dangerous, so be very careful and safe. Also note that some capacitors can hold a voltage charge for at least 15 minutes after the chassis has been switched off.

First voltage I would check is the main HT voltage, shown as 255V at reservoir capacitor C31. If this is substantially higher or lower than it should be checked out further.

Second voltage area I would check is the control grids of ECC 81 and EL 95 in case any of the the associated coupling capacitors are bad. The main capacitors in question here are C17/C18 coupling the EF 86 anode out to the ECC 81 first stage control grid, C20 coupling the anode of the ECC 81 first stage to the control grid ECC 81 second stage and C22/C26 coupling the anode of ECC 81 second stage to the EL 95 output pentode control grid.

There should be little if any +ve voltage measured at the valve control grids.

Also read this Thread for background information on potential problems with coupling capacitors.

https://www.vintage-radio.net/forum/...ad.php?t=73399

David
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Old 14th May 2021, 8:55 am   #11
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Default Re: Grundig tk 24

May I suggest we go back to fundamentals.

First question, why were the wires around the EF86 burnt? and is this a clue as to why the sound levels are low?

Second, it was said after rewiring "Now the fault is low output." Does this mean that before the burnt wires were replaced the tape recorder worked? If so the fault condition has been caused by the rewiring.

If the machine wasn't working before replacing the burn wires then it's back to the first question.

Relay connection W1, from the schematic, either sends the record level to the magic eye on record or defeats the bias oscillator on playback. I'm not sure which wire you have disconnected but I don't believe this will help find the problem.

From the Instruction Manual, you can listen to recordings on the internal loudspeaker. In other words, use the Grundig Tape recorder as an amplifier. Does this work?

Chris

Last edited by simpsons; 14th May 2021 at 9:04 am.
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Old 14th May 2021, 12:17 pm   #12
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Default Re: Grundig tk 24

I would be very surprised if the first low volume reference in Post 1 was not referencing the sound from the internal loudspeaker but certainly good to get it confirmed.

Yes a good point about the EF 86 burnt wires, I did wonder that myself, hopefully Krishna will be able to clarify how the wires became burnt and if the volume was good before repairing the wires.

David
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Old 14th May 2021, 12:20 pm   #13
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Default Re: Grundig tk 24

It might get moved in a bit.

Lawrence.
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Old 14th May 2021, 12:25 pm   #14
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Default Re: Grundig tk 24

To the Vintage Tape Category I assume.

David
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Old 14th May 2021, 10:48 pm   #15
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Default Re: Grundig tk 24

Quote:
Originally Posted by simpsons View Post
May I suggest we go back to fundamentals.

First question, why were the wires around the EF86 burnt? and is this a clue as to why the sound levels are low?

Second, it was said after rewiring "Now the fault is low output." Does this mean that before the burnt wires were replaced the tape recorder worked? If so the fault condition has been caused by the rewiring.

If the machine wasn't working before replacing the burn wires then it's back to the first question.

Relay connection W1, from the schematic, either sends the record level to the magic eye on record or defeats the bias oscillator on playback. I'm not sure which wire you have disconnected but I don't believe this will help find the problem.

From the Instruction Manual, you can listen to recordings on the internal loudspeaker. In other words, use the Grundig Tape recorder as an amplifier. Does this work?

Chris
Sir,
Actually It was my uncle's R-R in dead condition No audio either Hum nothing found
Basically I am Modern Electronic Engineer I applied my logic and firstly I checked whether all valve filaments are on or not finally I found EF86 filament was not working,when I checked ef86 socket connection 2 resisters are burnt out and associated wiring,replaced one by one wires with new one,I got audio with low volume during this process I dont have head hence again I applied logic by giving audio signal through Blue tooth through head terminals then also it is too low by this I confirmed my self that head is ok .
there after to check whether it is pre amplifier/oscillator problem or audio section connected Bt to radio/phono input it working fine finally I come to conclusion that it is in play back mode issue downloaded the service manual and removed w1 wire connected to b2 switch it has distortion audio but having full volume but having lack of knowledge in vintage technology and this is the first valve based equipment i am going to repair I dont have knowledge much better than You and associated this blog members please guide I follow your Instruction
Regards
Krishna
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Old 15th May 2021, 9:09 am   #16
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Default Re: Grundig tk 24

When I spoke of listening to recordings I was referring to the tape recorder being use to monitor recordings. Poor syntax, sorry.

Should the monitoring capability be OK, then it proves that the EF86 is wired OK etc and the fault lies in the relay and associated circuit.

As Krishna has followed David's sound advice, no pun intended, in checking the playback/record head etc., this would be the centre of my attention.

Good luck Krishna and I hope our advice keeps you "On Track!"

Chris
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Old 15th May 2021, 1:58 pm   #17
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Default Re: Grundig tk 24

Thank You very much for valuable suggestions and fault finding easy methods will help me feature vintage equipment serves.
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Old 15th May 2021, 2:48 pm   #18
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Arrow Re: Grundig tk 24

[QUOTE=simpsons;1374144

Relay connection W1, from the schematic, either sends the record level to the magic eye on record or defeats the bias oscillator on playback.
Chris[/QUOTE]

From several different Tk 24 schematics I have checked I do not agree with the second part above.

From my Post 8 above I said that the Playback/Record relay W when de-energised connects the ECC 81 output through contact W1 to the EM 84 Magic Eye for record monitoring, this agrees with your "record level to the magic eye on record".

Note there also is a connection off to R8 which I think is the Record equalisation connection.

I also said that when the Playback/Record relay W is energised it connects the ECC 81output through W1 to the Playback equalisation components (connects directly to the junction of C10/R6/B2). I cannot see any way that W1 itself defeats the bias oscillator on playback.

The Bias Oscillator is enabled/powered by the +HT voltage supply being switched through contact D6 through to the Bias EL 95 valve (via closed contact B4). D6 is a changeover contact that when all the input selector push button switches are de-selected, D6 then switches +HT to relay W to energise it and puts the machine into Playback Mode. When any input selector switch is pushed down this changes over the D6 contact to de-enegise relay W (putting machine into Record mode) and as above switching the HT through to the Bias Oscillator.

I find contact B4 confusing as it appears to always be closed apart from when switched to Fast Rewind.

David
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Old 15th May 2021, 3:15 pm   #19
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Default Re: Grundig tk 24

Krishna, OK things are a little clearer thank you. So machine dead on arrival in terms of sound, with damaged wires/burnt out resistors around the EF 86. I assume you replaced the resistors, what resistors were they ? Not that common to get burnt out resistors.

I am still unclear about the Bluetooth, for me it is just confusing the issue, i.e. are you injecting a signal or monitoring a signal via Bluetooth ? or maybe both.

Also still not clear if you have tried a known good recording on tape on playback and also whether you have tried to record onto a tape and play it back.

You say "Bt to radio/phono input input it working fine", do you mean you got normal good volume at the TK 24 loudspeaker, were you set up for Record mode then ?

As Chris has already said need to go back to basics first:-

1. Check HT voltages (Note -schematic values are in Record mode, Playback mode results may be a bit lower).

2. Check valve electrode voltages, including the 3 control grids at the ECC81 and output EL 95.

3. Do PB head finger hum test in PB mode.

4. Try playback of a known good tape and playback of a new recording, note the results.

Do you have an oscilloscope ?

David
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Old 15th May 2021, 4:31 pm   #20
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Default Re: Grundig tk 24

Quote:
Originally Posted by DMcMahon View Post
Krishna, OK things are a little clearer thank you. So machine dead on arrival in terms of sound, with damaged wires/burnt out resistors around the EF 86. I assume you replaced the resistors, what resistors were they ? Not that common to get burnt out resistors.

I am still unclear about the Bluetooth, for me it is just confusing the issue, i.e. are you injecting a signal or monitoring a signal via Bluetooth ? or maybe both.

Also still not clear if you have tried a known good recording on tape on playback and also whether you have tried to record onto a tape and play it back.

You say "Bt to radio/phono input input it working fine", do you mean you got normal good volume at the TK 24 loudspeaker, were you set up for Record mode then ?

As Chris has already said need to go back to basics first:-

1. Check HT voltages (Note -schematic values are in Record mode, Playback mode results may be a bit lower).

2. Check valve electrode voltages, including the 3 control grids at the ECC81 and output EL 95.

3. Do PB head finger hum test in PB mode.

4. Try playback of a known good tape and playback of a new recording, note the results.

Do you have an oscilloscope ?

David
Thank You Very much Sir I follow your Instructions as said once again and Come Back You with necessary Information
As for as Blue Tooth is concerned I used it for injecting a signal through audio input /Radio socket for to check Audio amplifier ;where as if same BT wire connected to head Terminals in Playback Mode audio become too low I applied this logic to find out the Head issue I dont have other head to check there fore when Phono switch is pressed full volume(BT) if change over to playback mode droped volume ;as per my view the audio should be more than Phono connection this is the issue once again thank you for valuable knowledge sharing with respect to vintage electronics technology.
Regards
Krishna
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