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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 27th Jun 2019, 1:32 am   #1
Uncle Bulgaria
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Default Uher CR240 cassette deck repair - etching copper help please.

Ahoy,

I've had this on the go for a while, but have finally knuckled down to the part that was causing me confusion. The Uher deck had numerous broken traces at one PCB corner, which I have jumpered.

What's finally given up the ghost is the 'Record' switch, which seems to have a fundamental role in whether the unit can be tested or not. Along with the other switches on the front, it's a multi-contact push switch. It appears to be made from nylon with gold contacts, and as can be seen the nylon is cracked all along the switch, with the end breaking off.

The end square hole is for a lock rod. I think it was the numerous stresses on this rod, as it limited the travel of the switch against the return spring by coming up against the end of the switch housing, that caused the break. The switch housing can be seen as the red affair in the internal photograph.

My objective is to replace this switch, which I am trying to do with a PCB, suitably etched and tinned. Alas, existing double-sided PCB thicknesses are too great so I've been forced to file down two pieces, intending to stick them together post-etching.

This evening I've tried the 'Sharpie' marker-as-resist and ferric chloride method with some success, but have suffered some undercutting on my experimental piece. I tinned it roughly with the soldering iron and some Ersin Multicore.

Does anyone have any comments, tips or tricks that might help me in this endeavour?
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Old 29th Jun 2019, 9:13 am   #2
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Default Re: Uher CR240 cassette deck repair - etching copper help please.

I'm just thinking that if the original was gold plated, using an unplated PCB may not be a good option in the long run, if the tinned or copper tracks are not compatible with the material in the fixed portion of the switch.
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Old 29th Jun 2019, 2:35 pm   #3
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Default Re: Uher CR240 cassette deck repair - etching copper help please.

That's an interesting thought, Ricard - my feeling is that both copper and tinned copper are robust enough to deal with the sliding motion. I don't know of a way to obtain the right type of gold plating as a hobbyist, and I'd really rather have it working than not and worry about future problems if they happen.

I couldn't see how to make a new switch in the same way as the old. My best guess is that the gold-plated contacts are rivetted through the nylon, but then they must also be bonded on the flat areas so as not to lift during operation. I suppose Mk.II could be attempting to make something similar from a sheet of nylon, with the contacts cut from metal.

I've just made a mask for my second etching attempt, using asphaltum over permanent marker to see if I get a cleaner result.
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Old 29th Jun 2019, 2:42 pm   #4
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Default Re: Uher CR240 cassette deck repair - etching copper help please.

Don't bother tinning, a smear of silicone grease will keep corrosion away for years.
 
Old 30th Jun 2019, 12:01 am   #5
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Default Re: Uher CR240 cassette deck repair - etching copper help please.

merlinmaxwell - is the tinning just for corrosion resistance? I wondered if it would help resist the abrasion of switching as well. Do you think it's not worth it?
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Old 30th Jun 2019, 7:28 am   #6
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Default Re: Uher CR240 cassette deck repair - etching copper help please.

Yes, not worth it, most motorcycle switches are plain brass/copper and survive an awful environment.
 
Old 3rd Jul 2019, 6:29 am   #7
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Default Re: Uher CR240 cassette deck repair - etching copper help please.

One important difference though is that currents handled in an automotive situation are usually measured in amps and the voltages in volts, whereas the signal currents in a record/playback switch can be in the microamp range and the voltages in microvolts. Thus is does not take a lot of oxide buildup in a record/playback switch to cause dodgy operation, whereas a bit of oxide in an automotive situation will be quickly broken down by the voltage and current applied.

On the other hand, as has been said above, it's not easy for the hobbyist to obtain gold plating, so do start with something else and consider replacing it if it doesn't seem to work properly.

One thing I'm thinking about though are modern day small series PCB manufacturers, if they have gold plating services for their PCBs, you could then fathomably design a PCB with the correct pattern and have it made professionally. Would cost a bit though, and small series notwithstanding, you'd end up with many more contacts than you'd actually need.
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Old 4th Jul 2019, 11:24 am   #8
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Default Re: Uher CR240 cassette deck repair - etching copper help please.

Good thoughts, Ricard. I'll get my DIY switch to the right dimensions then see where I am.

Exciting news last night, in that I fixed a niggling problem in the power supply board and have running motors! The contacts on my new switch are not quite right yet, but moving it in and out does have an effect, just not the one of switching out of 'record' mode!

I do have some confusion over the role of a transistor, which I'm going to make another thread about.
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Old 4th Jul 2019, 2:30 pm   #9
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Default Re: Uher CR240 cassette deck repair - etching copper help please.

Not intending to be facetious, as a practical application of your soon-to-be new found knowledge of transistors, you could always use several as low-current switches. Mind you, I understand why you'd want to keep it as originally intended.
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Old 4th Jul 2019, 9:18 pm   #10
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Default Re: Uher CR240 cassette deck repair - etching copper help please.

Facetious is welcome too! How do you think this could work in this instance? You might be able to see from the picture of the switch housing that there are numerous points where the switching occurs - most of the contacts are below the board and invisible.

It may be that this attempt is doomed to failure, but as you correctly surmised my theoretical electrical knowledge is not advanced enough to modify circuits beyond tapping off for lamps or fixing blown components. If so, as long as I can get the record/play function working with a well-made repair I'm not concerned if an obsolete switch is sacrificed, especially as working is better than non-working.

If that repair educates me in the use of a component, I'm all ears.
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Old 4th Jul 2019, 11:04 pm   #11
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Default Re: Uher CR240 cassette deck repair - etching copper help please.

Double sided PCB is available in different thicknesses, may be easier then filling and gluing in future. I have some hear that is >paper thick.


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Old 11th Jul 2019, 9:44 am   #12
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Default Re: Uher CR240 cassette deck repair - etching copper help please.

60 oldjohn - I am using double-sided, but the thicknesses I could get it in were not a match for the switch. In the end I had to file off the copper from one side of the strip and keep on going until I got to half the thickness of the original, then sandwich them together!

I'm really chuffed as last night the switch went in and worked! Proof of concept complete, now on to adding the knob and latching mechanism, then servicing.

I'll probably make a new thread as I was playing its first tape last night and, while initially good, it soon developed a rhythmic clicking or popping through the internal speaker coupled with matching dropouts in the music...
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