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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 21st Feb 2019, 12:10 pm   #1
dazzlevision
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Default The one that got away!

I've been carefully checking over a tape recorder that I acquired some time ago. It's a Marconiphone 4261 all-transistor stereo model and it uses the Thorn DC432 Schedule B tape deck.

When set in stereo record mode, there was some intermittent hum/instability on the LH channel. I subsequently examined the printed circuit board and discovered a factory dry joint on an external wiring termination tag. Two screened cable outer braids are wrapped around the tag, but they have never been soldered to it. The tag reference is 8 (LH channel) and the two braids are the chassis returns for the "earthy" ends of the record level and multiplay level pot tracks.

In addition, I note that this tag is quite heavily tarnished (black in appearance), whereas all the others are their normal (plated) finish. It's not clear to me why this should be. Any ideas?
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Old 21st Feb 2019, 12:46 pm   #2
Refugee
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Default Re: The one that got away!

It may be one of those bits that has missed one of the processes of manufacture.
It is like a box of screws in the old days when it was a bad omen to find a box without at least one screw with either no slot or no thread.
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Old 21st Feb 2019, 1:49 pm   #3
Restoration73
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Default Re: The one that got away!

A small emf across the joint might cause corrosion. I once obtained a combined radio
test unit (RF power, SWR, field strength, Xtal oscillator,meter) where a PP3 battery only
powered the unit on some switch settings. No battery was fitted, but when I turned the
sensitivity pot the meter pointer just lifted. This was traced to a small current flowing
across a nickel plated brass solder tag. Retinning/resoldering prevented this.
Later Thorn units made extensive use of wire wrapping limiting soldering problems.
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Old 21st Feb 2019, 2:03 pm   #4
Welsh Anorak
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Default Re: The one that got away!

Could it have been tarnished prior to soldering, the solder joint looked OK then the blob subsequently fell off?
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Old 21st Feb 2019, 3:03 pm   #5
dazzlevision
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Default Re: The one that got away!

Quote:
Originally Posted by Welsh Anorak View Post
Could it have been tarnished prior to soldering, the solder joint looked OK then the blob subsequently fell off?
From the look of it, it was never soldered. The (pre-tinned) braids were wrapped around the tag but that's all.

I have now removed the two braids and the solder tag itself. There is no sign of any flux residue on the tag. The dark colouration of the tag suggests atmospheric Sulphur contamination to me, but all the other tags are fine and the tape deck's plated finish is pristine.
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Old 21st Feb 2019, 4:04 pm   #6
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Default Re: The one that got away!

It must have "sneaked" past the washing stage after plating and found its way onto the tag strip. That would make a dry joint mandatory.
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