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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 Jan 2021, 9:32 pm   #141
Boulevardier
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Thanks, It hadn't occured to me that there is an effect from the core material. I now feel illuminated!

Mike
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Old 12th Jan 2021, 9:50 pm   #142
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

1.25H (1,250mH) seemed high to me. The DC resistance of 1,100 Ohms also is higher than what I am used to, so maybe the two go hand in hand in this case.

David
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Old 14th Jan 2021, 11:57 am   #143
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Good morning all, after having fixed the hum on the Truvox R82 caused by the open circuit head, I now seem to have another problem. The main capstan motor runs fine at 3.75ips but is a trifle slow at 7.5ips and also from time to time it grinds to a halt and then will either pick up again itself or with a spin of the reels gets it going again. I've read some of the service notes which mention to two large 3k and 4k wire wound resistors. The circuit diagram for the capstan motor shows three wires red, yellow and green, or red, yellow and mauve depending on which diagram you look at. Problem is I can only see two black wires connected to the motor from the resistors and that's it. It mentions a 2uF capacitor, but I can't see that either! Problem is that this is intermittent which makes it hard to pinpoint the cause.
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Old 14th Jan 2021, 12:24 pm   #144
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Quote:
Originally Posted by Ted Kendall View Post

...Record heads have a higher inductance than play heads - more turns, in other words, to ensure the tape can be saturated, whilst the number of turns on a play head has to be balanced against other losses...
Ted, I always understood it as the opposite: the record heads have a lower inductance (and impedance at the nominal frequency) relative to play heads.
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Old 14th Jan 2021, 2:10 pm   #145
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Quote:
Originally Posted by TIMTAPE View Post
Quote:
Originally Posted by Ted Kendall View Post

...Record heads have a higher inductance than play heads - more turns, in other words, to ensure the tape can be saturated, whilst the number of turns on a play head has to be balanced against other losses.
Ted, I always understood it as the opposite: the record heads have a lower inductance (and impedance at the nominal frequency) relative to play heads.
Tim - out of interest, these are the specifications for the Bogen heads used, e.g., by Brenell. I took them from an article on the Brenell deck in TR magazine by HW Hellyer - can't remember whether I transcribed or copied & pasted. They give specs for both dedicated Playback and Record heads (but see footnote), and suggest that Replay head has a higher inductance than Record head. These detailed specs are useful, 'cos I haven't seen them anywhere else.

BRENELL (BOGEN) TAPE HEADS
(From: “Tape Recorder” magazine,
September 1967, article ‘Tape
Recorder Service’, HW Hellye
r)

Recording head UK202, 2/2 track
Inductance - 120mH
Impedance at 1kHz – 780 Ω
DC resistance – 75 Ω
Bias at 100kHz - 23V at 1mA
Recording current (3-75 ips) - 120μA
Output at 2kHz (3.75 ips) – 2.4mV

Replay head UK200, 2/2 track.
Inductance - 550mH
Impedance - 3.5KΩ
DC resistance – 300Ω
Bias (1OOkHz) - 70V at 0.8mA
Record current - 50μA
Output at 2kHz (3 ips) - 5.8mV.

Erase head BL210, 2/2 track
Inductance - 2mH
Erase current - 55μA
Erase voltage - 80V @ 100kHz

[Note: with appropriate circuitry, the
UK202 and UK200 heads can also
both be used as record/playback heads
]
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Old 14th Jan 2021, 2:27 pm   #146
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Quote:
Originally Posted by TerryB44 View Post
Good morning all, after having fixed the hum on the Truvox R82 caused by the open circuit head, I now seem to have another problem. The main capstan motor runs fine at 3.75ips but is a trifle slow at 7.5ips and also from time to time it grinds to a halt and then will either pick up again itself or with a spin of the reels gets it going again. I've read some of the service notes which mention to two large 3k and 4k wire wound resistors. The circuit diagram for the capstan motor shows three wires red, yellow and green, or red, yellow and mauve depending on which diagram you look at. Problem is I can only see two black wires connected to the motor from the resistors and that's it. It mentions a 2uF capacitor, but I can't see that either! Problem is that this is intermittent which makes it hard to pinpoint the cause.
Yes that is confusing Terry.

I can only assume that your R82 is not a full Mk. 2 (as referenced before) and its capstan motor is not the type that requires a Run capacitor, i.e. it probably is a Shaded Pole motor and hence only 2 connections.

David
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Old 14th Jan 2021, 3:52 pm   #147
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

I think you're probably right regarding the Mk2 thing. Even the head assembly is totally different to the later version. Assuming we're right about the motor then, how is the speed controlled, or is the motor a constant speed one? I assume it is because it has a stepped pinion on to which one of two idler wheels contacts depending on the speed selected. If the motor varies its speed is this likely to be motor fault, which of course would mean a lot of dismantling which I think is out of the question, even assuming I could get a motor! There's something not right with those wire wound resistors too; they're listed as 3k and 4k but when I measured them they were only 400ohm! I'll double check that though!
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Old 14th Jan 2021, 4:24 pm   #148
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

UPDATE. I think you are definitely right. The answer is in the first couple a paragraphs of Tape Recorder magazine article I posted from 1963. It also shows the resistors to be 500ohm & 700ohm which is more in keeping with the measures I got. The diagrams are also the same as mine too.
Maybe it would be worth checking the connections as they recommend and re-soldering just in case there is a dry joint.
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Old 14th Jan 2021, 5:22 pm   #149
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Well spotted. As these are probably large wirewound resistors they may have their values written on them.
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Old 14th Jan 2021, 7:24 pm   #150
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

The resistor connected to the motor reads 450 ohm, the other reads 434 ohms. R1 matches but R2 according to the diagram should read 700 ohms. Not sure if R2 would affect the capstan motor though, but it does say that these two resistors can lead to erratic transport action by either poor connection or the heat they generate.
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Old 14th Jan 2021, 7:39 pm   #151
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

As far as I can see R2 will not affect the capstan motor.
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Old 14th Jan 2021, 7:55 pm   #152
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

I didn't think so. What about C5 the 0.01 cap on the speed change switch when set at 7.5ips as in the schematic?
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Old 14th Jan 2021, 8:21 pm   #153
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Here's pic of the type of motor on the Truvox as the capstan motor. Is this what they are referring to when they say a Hoover motor! It's described as a Papst, external rotor, hysteresis motor apparently.
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Last edited by TerryB44; 14th Jan 2021 at 8:28 pm.
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Old 14th Jan 2021, 8:43 pm   #154
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

C5 is just for equalisation, nothing to do with motor control.

Is it definitely the motor that is struggling at 7.5 ips or could the motor be OK but the capstan is struggling due to drive idler ?
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Old 14th Jan 2021, 9:52 pm   #155
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Ok. The sequence is the motor has a stepped pinion which engages with the idler wheels and the idler wheels then turn the flywheel which has the capstan on it. I've cleaned everything as best I can but at 7.5ips it runs slow and sometimes the whole system slows down including motor and will pick up after a bit or sometimes with a bit of help by rotating the reels by hand. Because it's intermeittent it's difficult to pin point the cause.
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Old 14th Jan 2021, 11:51 pm   #156
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Quote:
Originally Posted by TerryB44 View Post
Here's pic of the type of motor on the Truvox as the capstan motor. Is this what they are referring to when they say a Hoover motor! It's described as a Papst, external rotor, hysteresis motor apparently.
I am not very good on R2R motors, but do not think the hoover motor as described is like the Papst.

I am surprised that you have the Papst BP1303B (is your motor the same as the Brenell photo ?) because I didn't think the Papst is a shaded pole motor and thought previously your lack of a run capacitor could be explained by the capstan motor being a shaded pole motor ?
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Old 15th Jan 2021, 12:20 am   #157
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Just realized that the Brenell motor photo is not a Papst motor but AEI but once again not a shaded pole motor as far as I know ?
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Old 15th Jan 2021, 10:55 am   #158
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Good morning David, I've had a look at my motors and the capstan drive motor is definitely the same as the one in the Brenell pic. Picture of mine is below. The rewind and take up motors are both AEI so perhaps its no surprise that the capstan one would be too. I can't see any markings on the capstan motor either. I'm going to re-solder all the joints at those big resistors just in case there's a bad connection somewhere.
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Old 15th Jan 2021, 11:26 am   #159
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

I've just found a manual on line for the Brenell Mk5 which says it uses two shaded pole motors for rewind and take up which are the ones that look like my capstan motor, so maybe it is a shaded pole motor after all.
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Old 15th Jan 2021, 11:45 am   #160
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Here's a pic from the Brenell manual.
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