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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 11th Dec 2020, 7:41 pm   #41
TerryB44
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

David, see my update in my last posting. Just going to re-solder R41 together again and then it's a wait till I get all the bits and pieces together to start on the waxy cap and the two cans. (Or should that be 'toucans' as in toucans of Guinness please!

Don't know what pin 1 on the DIN socket is for or why there's a connection from the tone/monitor control. I know pins 2 & 5 are to connect a 15ohm loudspeaker as I used to have one connected to it, and how much better the sound was than the 3 ohm!

Last edited by TerryB44; 11th Dec 2020 at 7:53 pm.
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Old 11th Dec 2020, 7:58 pm   #42
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

OK that is good, so R41 around 20% high and around 10% above its max tolerance of 10% (silver band) but very typical of these old carbon resistors, nothing to worry about.
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Old 11th Dec 2020, 8:12 pm   #43
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Can't thank you enough David, and others who've contributed. All I have to do now, as I say, is wait for the parts to arrive.
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Old 11th Dec 2020, 8:50 pm   #44
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Fingers and toes crossed the new parts help
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Old 11th Dec 2020, 8:54 pm   #45
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

I certainly hope so! I'll be back on when the jobs completed - succesfully hopefully! Thanks again.
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Old 13th Dec 2020, 2:05 pm   #46
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Here's a couple of articles from the Tape Recorder Magazine which I kept from 1963 and 1965. They may be of use to anyone who has or may come into contact with Truvox tape recorders and they need a bit of TLC!
Attached Files
File Type: pdf Truvox Servicing 1963.pdf (3.25 MB, 82 views)
File Type: pdf Truvox Servicing 1965.pdf (3.29 MB, 87 views)
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Old 13th Dec 2020, 10:56 pm   #47
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Quote:
Originally Posted by TerryB44 View Post
Here's a couple of articles from the Tape Recorder Magazine which I kept from 1963 and 1965. They may be of use to anyone who has or may come into contact with Truvox tape recorders and they need a bit of TLC!

Interesting and useful articles.
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Old 16th Dec 2020, 8:06 pm   #48
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

UPDATE: Have installed the two replacements for the two 32+32uF cans. As you can see from the pictures I was very lucky with the hole spacing as the new clamping rings had enough adjustment to reuse the existing holes which saved me a bit of work! I used some washers to act as spacers to make sure the body of the new caps were not in contact with the chassis. I probably didn't need to as they are covered with a plastic cover, but it just made me feel better! I'm just waiting for the .01uF caps to arrive so I can change that one across pins 8 & 9 of the ECL86 output valve. I could try it now but I think I prefer to wait until that's done. Why do JJ Electronics mark their caps with an M instead of uF? I assume it's M for microfarad?
One thing that did surprise me was that at some time the ECC81 has been replaced with an ECC82! That can't be doing any good can it? Anyway, I've replaced it with a new ECC81.
https://www.vintage-radio.net/forum/...pictureid=4520
https://www.vintage-radio.net/forum/...pictureid=4521
https://www.vintage-radio.net/forum/...pictureid=4522
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Old 16th Dec 2020, 11:13 pm   #49
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

M and sometimes m was used on older documentation for uF (Micro Farad) and maybe also on some vintage capacitors, so I guess this is why JJ Electronics annotate their capacitors that way.

The ECC81 and ECC82 I am pretty sure are pin compatible and basically the same type valve with the ECC82 being a lower gain version. Should not cause a problem and unless in a critical circuit may well function just as well. Having said that I also would be happier with correct valve fitted.

I see the Bias Oscillator V3 is a ECC82, V2 (ECC81) has not been swapped over with V3 has it ?

David

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Old 16th Dec 2020, 11:38 pm   #50
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

No, I checked that and it was an ECC82 which is what alerted me to the fact that the other one should have been an ECC81. If that ECC82 is more or less compatible then it'll do as a standby. I see somewhere on here that the ECL86 is regarded as somewhat prone to failure! I hope mine is ok as they are not cheap at £36! Just relaxing now with a nice single malt.

Terry
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Old 4th Jan 2021, 2:15 pm   #51
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Happy New Year,
Sadly in the Truvox after replacing both 32+32uF cans and the 0.01uF capacitor R35 across V5A, the problem remains; low volume and hum. What next to look at? The ECL86? Is there a point where I could tap in to the preamp and connect to another amp to rule out any preamp problem?
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Old 4th Jan 2021, 5:17 pm   #52
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Am I right in thinking that if the hum varies with the volume control then it's a preamp fault? I've subbed the EF86 with two others with no difference, replaced the ECC81 with a brand new one, and subbed the ECC82 with no difference. Should I just replace all the electrolytics in the preamp and see if that makes any difference?
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Old 4th Jan 2021, 9:09 pm   #53
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Yes a Happy New year to you also, more importantly a happier one, hopefully from all aspects.

That is very disappointing for your Terry and frustrating.

Yes if the hum disappears/reduces a lot with the volume control wound down then would point to EF86 area (assuming R6 is the volume control). Check the control grid of V2A (ECC81 pin 2) with the volume control fully wound up in Playback to see if any DC voltage is getting through the coupling capacitor C4. Also check the EF86 voltages at the anode (pin 6), cathode (pin 3) and suppressor Grid (pin 1) to make sure their voltages are similar to schematic values.

For bad anode voltage check R3, for bad suppressor grid check R4/C2/C3, for bad cathode check R3/C3.

If voltages cannot be safely checked then would recommend replacing C4/C2/C3.

David
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Old 4th Jan 2021, 10:11 pm   #54
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Smile Re: Grundig TK20 and Truvox R82. Help Required Please.

Good Evening David, Thanks for that. That was my first area to look at and I was going to start with C3. Am I dealing with DC voltages? I've got plenty of time now were back in lockdown so no rush, slowly slowly catchee monkee!:
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Old 4th Jan 2021, 10:16 pm   #55
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Quote:
Originally Posted by TerryB44 View Post
Am I right in thinking that if the hum varies with the volume control then it's a preamp fault?
Usually before the volume control but not always.

Lawrence.
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Old 4th Jan 2021, 11:08 pm   #56
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Thanks Lawrence, duly noted! David, R6 IS the volume because R5 is connected to INPUT 2 which is for DIN input level.
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Old 5th Jan 2021, 10:03 am   #57
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Not an easy thing to diagnose necessarily even for experts (I don't classify myself as that).

If you were able to scope the signals could probably locate the source of the hum. The hum and the low volume could well have the same cause, valve electrode voltage measurements could also help to home into the troublesome area.

David
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Old 5th Jan 2021, 7:10 pm   #58
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

I have replaced C3 and a couple of other electrolytics but to no avail. One curious thing is that the hum sort of doubles up when the tape speed switch is changed from 3.3/4 to 7.1/2 ips. Strange! Is something in the bias circuit possibly the cause? I disconnected the playback head just to see if that was dead, but again no change. Is it possibly the ECL86 output valve after all? It will cost me £36 to find out unfortunately!
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Old 5th Jan 2021, 9:14 pm   #59
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Also replaced C7 and C11. Volume has increased slightly but the hum still persists!
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Old 5th Jan 2021, 9:15 pm   #60
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Because you are not able to do any diagnostic voltage measurements or scope waveform checks, it makes it very difficult and somewhat guess work.

I still think it is more likely to be before the ECL86, you could try connecting the output socket into an external amp but this would probably only do the same as the loudspeaker, in fact having said this, it may only be for external speaker, your user manual should clarify that.

You could try tapping off the volume control slider but you have to be careful, i.e. if DC voltage is getting through C4 you do not want to put that into an external amp, could try putting it first through a known good capacitor.

I still think C4 should be checked/replaced and if OK also check the ECC 81 area grid coupling capacitors C9 & C12 & C10.

If the hum occurs on Playback then the Bias Oscillator should not be running.

For the hum increasing on 7 1/2 ips then check the capacitors C5 & C6 at S5.

David
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