7th Jan 2021, 1:02 pm | #81 | |
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Re: Grundig TK20 and Truvox R82. Help Required Please.
Quote:
Last edited by TerryB44; 7th Jan 2021 at 1:19 pm. |
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7th Jan 2021, 1:05 pm | #82 |
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Re: Grundig TK20 and Truvox R82. Help Required Please.
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7th Jan 2021, 1:21 pm | #83 | |
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Re: Grundig TK20 and Truvox R82. Help Required Please.
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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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7th Jan 2021, 3:19 pm | #84 |
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Re: Grundig TK20 and Truvox R82. Help Required Please.
Resistance is minimal at 0.001.
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7th Jan 2021, 3:23 pm | #85 |
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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. |
7th Jan 2021, 3:40 pm | #86 |
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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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7th Jan 2021, 3:44 pm | #87 |
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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 |
7th Jan 2021, 6:45 pm | #88 | |
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Re: Grundig TK20 and Truvox R82. Help Required Please.
Quote:
Mike |
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7th Jan 2021, 7:19 pm | #89 |
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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 |
7th Jan 2021, 7:32 pm | #90 |
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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 |
7th Jan 2021, 7:52 pm | #91 |
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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 |
7th Jan 2021, 9:35 pm | #92 |
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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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7th Jan 2021, 10:37 pm | #93 |
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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 |
8th Jan 2021, 3:43 pm | #94 |
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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. |
8th Jan 2021, 4:45 pm | #95 | ||
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Re: Grundig TK20 and Truvox R82. Help Required Please.
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8th Jan 2021, 4:52 pm | #96 |
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Re: Grundig TK20 and Truvox R82. Help Required Please.
Open circuit.
Lawrence. |
8th Jan 2021, 5:22 pm | #97 |
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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 |
8th Jan 2021, 6:18 pm | #98 |
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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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8th Jan 2021, 6:26 pm | #99 |
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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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8th Jan 2021, 6:27 pm | #100 |
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Re: Grundig TK20 and Truvox R82. Help Required Please.
Agreed!
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