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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 7th Jan 2021, 12:02 pm   #81
TerryB44
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

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Originally Posted by DMcMahon View Post
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
Voltages at pins 1 and 3 are spot on, voltage at pin 6 is 90v just under the 100v on schematic. No voltage getting through C4.

Last edited by TerryB44; 7th Jan 2021 at 12:19 pm.
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Old 7th Jan 2021, 12:05 pm   #82
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

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

Lawrence.
Yes, it's ok.
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Old 7th Jan 2021, 12:21 pm   #83
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

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

Lawrence.
Yes, it's ok.
What resistance reading are you getting between g1 and ground?

Just to double check - Could you try temporarily shorting g1 to chassis with both EF86 and ECC81 inserted. Does this kill the hum?

Mike
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Old 7th Jan 2021, 2:19 pm   #84
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Resistance is minimal at 0.001.
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Old 7th Jan 2021, 2:23 pm   #85
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

[Quote Mike]
IYou 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]

Mike[/QUOTE]

I have short circuited the playback head and the hum goes away. The playback head does play but only at very low volume.
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Old 7th Jan 2021, 2:40 pm   #86
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

I have checked all the voltages around the EF86 and the ECC81 and all are fine within 10% of schematic values.
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Old 7th Jan 2021, 2:44 pm   #87
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

With PB head wiring disconnected use a DVM (ideally with a low value resistor in series) to measure the resistance of the head. This could (but in my experience normally does not) partially magnetize the head (if head not open circuit) then possibly later requiring the head to be demagnetized later for best HF reproduction.

David
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Old 7th Jan 2021, 5:45 pm   #88
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

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With PB head wiring disconnected use a DVM (ideally with a low value resistor in series) to measure the resistance of the head. This could (but in my experience normally does not) partially magnetize the head (if head not open circuit) then possibly later requiring the head to be demagnetized later for best HF reproduction.

David
Actually David, he could just measure the resistance from EF86 pin 9 to chassis (without the extra hassle of disconnecting the head), since the only other paths to chassis should be the 2.2 M ohm grid leak resistor and the grid resistance of the EF86 itself - neither of which will affect the measured head resistance significantly (I'm assuming that it's the Mk 2 schematic we're working from). Doing it this way will also check that switch S1 is doing its job. The DC resistance of the head is specified as 1.1 kilohms.

Mike
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Old 7th Jan 2021, 6:19 pm   #89
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Yes agreed Mike.

1.1k is somewhat higher than what I would have expected, although it is a combined Playback/Record coil/winding.

Of course if it does turn out to be open circuited then Terry has the problem of sourcing a suitable head.

David
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Old 7th Jan 2021, 6:32 pm   #90
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

In post #84, Terry said that he got a resistance measurement from pin 9 of "0.001". If his meter was set to the MOhms range, that would tally with 1.1 kOhms. Perhaps he could confirm that meter reading and the range his meter was set to. Suggest he measures again with meter set to kilohms range. If he gets about 1.1 kOhm, it would suggest that the head and switch S1 are OK.

I'm not sure where that would leave us...?

Mike
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Old 7th Jan 2021, 6:52 pm   #91
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Yes agreed.

If the meter was auto ranging then 0.001 could be just Ohms, which could indicate a shorted head/short to ground when switched to Playback, which seems unlikely.

It may leave us (or at least me) confused

David
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Old 7th Jan 2021, 8:35 pm   #92
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Hi all, Just tried Boulevardier's suggestion using pin 9 of EF86 and get a fat zero, which if correct would mean the playback head is open circuit I guess, but I'll double check tomorrow as the base of the EF86 is in a tight spot and I didn't have my glasses on! If it is the head then I guess the games up, but I live in hope!
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Old 7th Jan 2021, 9:37 pm   #93
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

I think you are misinterpreting that zero reading. On a DMM, a display reading of "0" means zero resistance = a short circuit, not an open circuit. So, it depends what you mean by a "big,fat zero"! It's possible that the head has an internal short, but relatively unlikely.

I've been looking back through the thread, and see that you say there is a very low-level signal present from the tape. I'm pretty sure this would not be possible if the head was completely open circuit, though it could have a partially-shorted/open winding..

Time to investigate the wiring and switching between pin 9 and the head. It's possible that a short circuit somewhere there would cause these symptoms (even a short-circuit in the wiring will have some tiny resistance, and that small resistance might be low enough to show as zero ohms on a meter but still have enough signal voltage across the resistance for a very sensitive playback amplifier with a high input impedance to reproduce it at the low level you mention). At this point the hum is a bit of a mystery, but could result from an earthing problem that may also be to do with the head switching and wiring - and the problem might be a very simple one. It's possible that one of the sections of S1 isn't working properly, or isn't earthed where it should be.

So, I suggest that you look at those areas tomorrow. Also, look especially for any disturbed wiring/connections, and for dodgy-looking earth connections. I don't know the machine, but you may find that getting to the areas in question (head wiring and switches, etc) may mean a bit of dismantling.

Mike
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Old 8th Jan 2021, 2:43 pm   #94
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

I have some old heads somewhere. Would it be worth wiring one of these in and see if the hum goes away?
I will check the wiring as you suggest. I agree that it may just be a simple problem as replacing some of the usual suspects has made no difference at all.
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Old 8th Jan 2021, 3:45 pm   #95
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Quote:
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Quote:
Originally Posted by DMcMahon View Post
With PB head wiring disconnected use a DVM (ideally with a low value resistor in series) to measure the resistance of the head. This could (but in my experience normally does not) partially magnetize the head (if head not open circuit) then possibly later requiring the head to be demagnetized later for best HF reproduction.

David
Actually David, he could just measure the resistance from EF86 pin 9 to chassis (without the extra hassle of disconnecting the head), since the only other paths to chassis should be the 2.2 M ohm grid leak resistor and the grid resistance of the EF86 itself - neither of which will affect the measured head resistance significantly (I'm assuming that it's the Mk 2 schematic we're working from). Doing it this way will also check that switch S1 is doing its job. The DC resistance of the head is specified as 1.1 kilohms.

Mike
After rechecking the reading from Pin 9 to chassis is 2.4Mohm in play mode. In record mode it is 0.5 ohm.
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Old 8th Jan 2021, 3:52 pm   #96
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Open circuit.

Lawrence.
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Old 8th Jan 2021, 4:22 pm   #97
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

To double check the head itself is open circuit also check it at the head connections.

Are your old heads Truvox/Truvox style ? if not and the head is confirmed dead (open circuit) then post photos of the head, showing its mounting and any identification marks, you possibly may be able to get a donor from someone. I have a couple of old heads you are welcome to (FOC) if you need but no idea if compatible (they are not from a Truvox).

David
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Old 8th Jan 2021, 5:18 pm   #98
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Same reading at the head - 2.5Mohm. The head is only 1.1Kohm according to specs so something not right is it.
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Old 8th Jan 2021, 5:26 pm   #99
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

As Lawrence says it certainly looks open circuit. The 2.5MOhm presumably is the 2.2M R1.
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Old 8th Jan 2021, 5:27 pm   #100
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Agreed!
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