Buttons do the wrong thing (Technics RS-T330R)

[cockroach]

Hi!

I'm trying to resurrect my old Technics RS-T330R tape deck. It seems to work quite well (I had to replace a little gear) but somehow it seems to misinterpret some of my button presses. The easiest way to trigger this is to just hit "Stop" a few times, at some point it will start rewinding the tape (instead of doing nothing).

Other than the machine being possessed, does anyone have an idea what might be wrong there?

 

[cockroach]

FYI, it seems to be an issue with the buttons themselves, not the electronics. I managed to prototype myself a little replacement control unit that appears to work flawlessly, I suppose this might just be a case of replacing the little push buttons on the control board.

 

[dankik]

Looking at the circuit for the transport switches, it looks like whenever you press a button you are applying a different resistance to ground. Depending on what the resistance is determines the function. Deck A & Deck B controls go to two different pins on the connector.

To test, you should be able to disconnect the 4 pin connector and put an ohm meter on pins 3 and 4 and measure the resistance when a button for Deck A is pushed.
This should be what you measure. The other problem could be the switches themselves.

For Deck B, put the ohm meter on pins 1 and 4, and for Sync pins 2 and 4.

 

[cockroach]

Thanks.

It's a bit hard to measure since usually the buttons work right, i.e. only every 5th – 10th click goes wrong. Using these resistor values I built myself a custom little keyboard though and that one works perfectly fine. I think the buttons have gotten a bit imprecise with the years and occasionally add a bit of resistance, leading to FF instead of stop.

At least that's my theory for now. And that makes me want to buy new replacement buttons which would ideally feel similar to the old ones - the spare ones I have are much tougher to click. You wouldn't happen to have a general idea about the operating force needed to push these original buttons? I'm having a hard time finding out anything about them on the Internet, just the part number: EVQQB005R.

 

[dankik]

Can you take the buttons apart or do they have a small opening to spray some Deoxit or contact cleaner in it, or are they "oil can" type of snap buttons?
I don't know what size your buttons are but take a look at these similar ones. (Full datasheet attached). Some variants available at Digikey and Arrow.
Digikey doesn't have the EVQPA, but they do have the EVQPB. I don't think that the 5th tab is connected to anything. Looks almost identical to yours.
Check the dimensions on the datasheet.
https://www.digikey.com/en/products/filter/tactile-switches/197?s=N4IgTCBcDaIKYDcCOBaADgIxAXQL5A

 

[cockroach]

I don't think I can take them apart:

 

[dankik]

See my edited post above.

 

[cockroach]

Some variants available at Digikey and Arrow. Digikey doesn't have the EVQPA, but they do have the EVQPB. I don't think that the 5th tab is connected to anything. Looks almost identical to yours.
Check the dimensions on the datasheet.

Oh wow, thanks for that link, that looks like a very close fit indeed!

 

[dankik]

I added the link to post #5.

 

[cockroach]

Thanks for that! I'll also order some additional ones that require less force, just in case...

 

[WernerO]

These switches are a known weakness of Technics decks.

 

[Artie]

These switches are a known weakness of Technics decks.

I might take that a step farther, and say they're a weakness in every piece of equipment that uses them. They're very common in digital tuners. Although, tuners tend not to use those resistor ladders, so, not quite so bad.

 

[WernerO]

It's compounded by Technics often keeping the main power supply active, even with the front button 'OFF'. This slowly cooks components, their outgassing creating deposits on/in display tubes, switches, ...

 

[cockroach]

For future reference: the EVQ-PBE05R switches are a very close match to the original EVQ-QB005R, once you remove the extra leg.

I only replaced the broken switches (about half of them) and while the new ones do require ever so slightly more force, now that everything is fully assembled again I can't tell which are which.