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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 5th Jan 2021, 9:29 pm   #61
DMcMahon
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Yes all the valve electrode voltages are DC ignoring the Bias Oscillator and filament heaters.
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Old 5th Jan 2021, 11:19 pm   #62
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Is your machine a Mk.1 or a Mk. 2 ?

Up to now I have always been looking at the Mk. 2 schematic, which shows 3 speeds, but earlier you said your machine is 2 speed.

I have just seen a Mk. 1 schematic which shows 2 speeds and and another position on the speed switch for amplifier and a note on that schematic that to use the amplifier you select amplifier on the speed switch and set it to record mode.

Do you have the amplifier mode, if so would be interesting to know if hum still exists in amplifier mode.

David
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Old 6th Jan 2021, 11:14 am   #63
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

The output socket is only for extension 3 or 15 ohm speaker as you say. Hum is only on playback. I have to assume it's a Mark 2 as the schematic that came with it says Mark 2, and it is only 2 speed with the extra Amp setting which also hums. I am going try and record the hum, put it into my audio software and see if that can tell me anything about the frequency. I'll post here if I can so you can hear it for yourself.
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Old 6th Jan 2021, 11:23 am   #64
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

If your Mk. 2 schematic is the same as the one I am looking at then that shows it is 3 speed without (as far as I can tell) the amplifier setting. The Mk.1 schematic I am looking at shows 2 speed with amplifier setting so together that would suggest yours is a Mk.1

But of course there may other variants.

Would be surprised if the hum is not mains frequency based 50/100Hz

David
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Old 6th Jan 2021, 1:20 pm   #65
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

The one I've got only shows 2 speed and the amp, but does say R82 Mk2 on the schematic. Looks like the hum is 50Hz as you say. No way to upload a sound file apparently. Looks like I'm in for the long haul with it!
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Last edited by TerryB44; 6th Jan 2021 at 1:30 pm.
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Old 6th Jan 2021, 1:42 pm   #66
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

I think I've found a way to upload the sound file as a zip file. See if you can play it. Does sound like 50Hz but with some harmonics thrown in for good measure!
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Old 6th Jan 2021, 3:53 pm   #67
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

I replaced C9 - no change. C10 is tiny and is transparent and looks like glass, it says on it 15, which is odd because for the R82 the 2 track model is should be 30, 15 is shown as on the 4 track version. Why they should be different I don't know. Both C10 & C12 check out with correct capacitance readings. A couple you have pointed out are 0.1uF which I don't have so I'll have to order them as well as C10 which is 30uF (nearest I have is 50uF).

Last edited by TerryB44; 6th Jan 2021 at 4:13 pm.
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Old 6th Jan 2021, 4:04 pm   #68
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Well the schematic is a little confusing. My Mk. 2 version (from downloaded service manual) is different to your version, even though the 2 schematics have identical title (Series 80 Mark 2 Recorder Circuit Diagram), drawing number (PH642) and date (24 April 1962).

My drawing shows 3 speeds (1 7/8, 3 3/4, 7 1/2 ips) and off position for the S5 speed switch whereas yours shows 2 speeds (3 3/4 and 7 1/2 ips), off position and Amplifier position (which matches your hardware).

There are also other differences, mine has simpler Auto Stop circuitry and only 2 positions for the S3 track switch (only R84), yours has a third position "Stereo".

Your schematic is similar but not identical to the Mk. 1 schematic.

So as a guess it looks like your Mk. 2 is an earlier model similar to the Mk. 1 and there must have been a later build Mk. 2 as reflected in my Mk. 2 schematic. There is a small notation on my schematic saying "Modified 29.10.62"

David
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Old 6th Jan 2021, 4:08 pm   #69
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Quote:
Originally Posted by TerryB44 View Post
. Is it possibly the ECL86 output valve after all? It will cost me £36 to find out unfortunately!
I do not know anything about reliability problems of the ECL86, generally most valves are very reliable, so at this stage cannot recommend it be replaced due to its high cost.

David
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Old 6th Jan 2021, 4:23 pm   #70
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Quote:
Originally Posted by TerryB44 View Post
I replaced C9 - no change. C10 is tiny and is transparent and looks like glass. A couple you have pointed out are 0.1uF which I don't have so I'll have to order them as well as C10 which is 30uF (nearest I have is 50uF).
I only mentioned C10 because it is on the anode of V2B and has a connection to the control grid, from the Note. 3 it is 30pF for the R82 or 15pF for the R84 so this is why it is tiny, very unlikely to be leaky.

You reference C10 a second time as 30uF ? I cannot easily see a 30uF, 50uF would be OK.

If you hit a brick wall it may be worth you doing a Post on the "Repair and Restoration Services Wanted" to see if you can get a quote from someone local to have a look at it. I know of course you will want the satisfaction of fixing it your self.

David
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Old 6th Jan 2021, 4:35 pm   #71
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

I'm beginning to reach the end of my tether with it I must admit! However I'll plod on replacing caps until I run out of patience and then probably do as you suggest!
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Old 6th Jan 2021, 7:27 pm   #72
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Quote:
Originally Posted by TerryB44 View Post
I'm beginning to reach the end of my tether with it I must admit! However I'll plod on replacing caps until I run out of patience and then probably do as you suggest!
I haven’t read this thread through meticulously, so apologies if my points have already been covered, also I haven’t looked at the circuit diagram. If you still have the stamina to bother with this thing, can I suggest a different approach?

After all you’ve already done/tested, I would stop the process of replacing components (likely to introduce more faults, and you won’t know where you are) and try another method which needs no test equipment – I’d try and locate the source of the hum by grounding the signal-path at various points working back from the volume control to the playback head input.

So get a known-good capacitor - say 0.01µF, 350V. Solder a flexible lead to one end and connect the other end of that lead to chassis. Then use the other lead of the capacitor as a signal-grounding probe. Holding the capacitor by its body, if you touch that lead to one of the unearthed tags of the volume control you should find that the hum disappears (you said earlier that the vol. control affected the hum volume). Using this earthed ‘probe’, you can then work your way back through the stages (e.g, grids and anodes of pre-amp valves) until you find the point (if any*) where the hum vanishes. That will locate the area where the hum originates. You may still get some lower-level hum with this method, as just holding the capacitor may introduce some.

You should also try (perhaps before you do anything else) putting a short circuit across the playback head connections (just disconnecting the head as you’ve said you’ve done won’t prove anything - an open-circuit playback input will pick up hum in a very high sensitivity playback amplifier with AC magnetic fields nearby). If the hum vanishes when you short circuit the head then you have probably got either an open-circuit head or a hum-loop in the grounding system.

* – I said “if any” in the earlier para, because I wonder if your’re doing all this testing with the amplifier units disassembled and with the playback head temporarily positioned close to a source of AC magnetic field such as the mains transformer. This would produce hum even in a faultless playback amplifier – the position of the mains transformer relative to the playback head can be a critical issue in tape recorder design (don’t ask me how I know that!). So, you should check this immediately if you’ve got the recorder split up into sub-units on the bench.

Mike
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Old 6th Jan 2021, 8:05 pm   #73
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Thanks for all your suggestions Mike, I'll certainly consider doing what you say. I haven't dismantled anything and all component replacement has been done in situ. If the playback head has gone open circuit would I still hear audio, as tapes play but only at very low volume?

Last edited by TerryB44; 6th Jan 2021 at 8:12 pm.
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Old 6th Jan 2021, 8:25 pm   #74
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Quote:
Originally Posted by TerryB44 View Post
I think I've found a way to upload the sound file as a zip file. See if you can play it. Does sound like 50Hz but with some harmonics thrown in for good measure!
Certainly sounds like hum of the mains variety to me.

David
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Old 6th Jan 2021, 8:34 pm   #75
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Good inputs from Mike, be careful not to short anything out and make sure as Mike says to hold the insulated body of the capacitor, i.e. keep your fingers/hand away from the live end of the capacitor, remembering that some valve electrodes like the anodes will have appreciable voltage on.

Have you tried removing the EF86 and ECC81 valves to see if the hum goes away ?

David
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Old 6th Jan 2021, 8:45 pm   #76
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

No I haven't tried simply removing the EF86 and ECC81 valves, I wasn't sure if it would cause any damage to other components. If it is mains hum does that mean that a component connected to ground has gone open circuit creating a hum loop?
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Old 6th Jan 2021, 9:52 pm   #77
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

No it will not cause any damage. Remove the EF86 first with power off then power up and recheck for hum, then depending upon results additionally remove the ECC81 (power down first). Leave the ECL 86 in to drive the loudspeaker and to provide a load for the HT supply.

No not necessarily.
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Old 6th Jan 2021, 9:56 pm   #78
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Thanks David & Mike, I'll report back when checks completed.
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Old 7th Jan 2021, 12:36 pm   #79
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

EF86 removed and hum stopped. Replaced EF86 and removed ECC81 and hum stopped. C2 gives correct reading of 0.1uF.
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Old 7th Jan 2021, 12:53 pm   #80
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

I've not followed this thread from the beginning, have you checked continuity from g1 of the EF86 to ground?

Lawrence.

Last edited by ms660; 7th Jan 2021 at 1:05 pm. Reason: post to thread
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