Technics SA-DX850: Error F 70 power & rotary switch fix

This time a Technics surround receiver on the work bench:

It’s a SA-DX850 with a power issue, on startup it gives F 70 Error code.

I’ve found a post on a German website about the SA-DX750 with similar error. On this post they were referring to a PCB connection problem. I took the service manual, schematics and took a quick look if this could be the case at this SA-DX850 as well.
In fact, it did. As I measured the resistance of the two wire points that should be connected, the resistance wasn’t near 0 ohms.

This is the PCB with the error:

 

I took the PCB out of the receiver and soldered a small piece of wire between the points that should be connected to each other:

 

Reassembled the receiver again and YES, it is working again!

But that is not all. The volume knob is very jumpy and not reacting very smooth. It’s not a normal stereo volume potmeter, but a digital one. Such type of dial goes by the name of “Rotary switches”. They act like two switches that opens and closes one at the time.
The direction of rotation is detected by the phase of the pulses those switches give. One switch will act a little later than the other one. Over time the contacts in those rotary switches will become oxidised and will loose its conductivity.
Cleaning the contacts with a fibreglass pencil will fix it and no need to replace the complete rotary switch:

First remove the front panel and unscrew the PCB:

 

Disassemble the rotary switch and give it a good cleaning:

 

I also remove al lubricant, clean all thoroughly with IPA (Isopropylalcohol) and reapply some new grease on the shaft. Reassemble again and we’re good to go.

There it is again, ready to sell:

 

7 comments on “Technics SA-DX850: Error F 70 power & rotary switch fix

  1. Hello Mark.
    I am having the same intermittent F70 error on my Panasonic SA-HE70 unit (which is identical virtually) and have tried resoldering almost the components, still randomly comes up. Fuses replaced and clips tightened etc. What points have you joined with the green wire? Do you have an excerpt of the schematic you could post? I’ve got a new Pioneer receiver but it just doesn’t sound as good as my 15 year old Panasonic!
    Many thanks,
    Adrian.

    • Hi Adrian,

      I’ve check the schematic. In the picture marked as red is the location of the green wire:
      schematics

      Kind regards,
      Mark

      • Mark, THANK YOU!
        That track was measuring about 10 to 12 ohms, and under a magnifying glass had a tiny bit of possible corrosion. Soldered in a bridge piece and so far (3 days of cranking it) that dreaded F70 click off is yet to occur. So happy. You’re a genius.

  2. How did you get the cover off?
    After remowing 6 screws my cover is stuck on some plastic clips connecting it to the front.
    I want to fix the input selector

    • Hi Morten,
      Sometimes those clips are really annoying to get the cover off. Only force will get them to come loose.
      Check whether the edge of the cover is bent in a 90 degree angle downwards, then you need to pull the cover off in a horizontal position.
      Otherwise you just have to tilt it over so that the clips will come loose. It requires a bit of force sometimes.

      Hope this will help to get the cover off.
      Kind regards,
      Mark

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