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Technics SP 15 runs too fast (Not Run-away Speed)

Once the MN6042 is bypassed then you are correct, the speed readout and the decimal point should illuminate and the rest of the display should be blank.

From your description, and assuming that the speeds are correct (but fixed) when bypassed, I wouldn't give up completely on the MN6042 just yet - keep looking around the switches and the board traces between switches and MN6042, and MN6042 and display.

regards

Angus
 
The MN6042 has no internal clock to drive the pitch display - it is derived from the speed reference input. So when you bypass the MN6042 as shown in the diagram there is no mechanism (other than spurious signals) to drive the pitch indicators. Hence my suggestion to examine the pcb traces and components around the MN6042.

The pitch decimal point is hard-wired, and the speed display is driven from the DN860.

regards

Angus
 
Yes. It could be a spurious signal, and/or the MN6042 could be sticking segment lines low. Just because they're lit doesn't necessarily mean it's actually muxing.

I'd put a 10K resistor from FI to ground. It's never good practice to float a line like that.
 
JP said:
Yes. It could be a spurious signal, and/or the MN6042 could be sticking segment lines low. Just because they're lit doesn't necessarily mean it's actually muxing.

I'd put a 10K resistor from FI to ground. It's never good practice to float a line like that.
Click to expand...

I have been looking at all the traces and solder joints but I can't find anything that looks bad to me. Here are some pictures of what the display looks like.

IMG_0326.JPG IMG_0327.JPG IMG_0325.JPG

The most common display is the zero when it is initially turned on. If I alternate between hitting "+" and "-" the display will count down 8,6,4,2,0 and if I hit the pitch return button it goes back to zero. The pitch display also looks brighter than it should be. If I leave it unplugged for awhile it seems more likely to turn on with the pitch section blank.

If I put the 10k resistor from FI to ground will that get rid of all the garbage that is being displayed? Is there a chance that something other than the MN6042 chip being bad is causing my problems?

Thank you all for your help and patience.
 
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Mr_Technics said:
If I put the 10k resistor from FI to ground will that get rid of all the garbage that is being displayed?
Click to expand...

Depends on what the failures are in the MN6042. As segments in only one digit are lit and they're brighter it's not muxing, so it may not help that issue. A good idea regardless as the MN6042 is a PLL with an external VCO - spurious signal could cause all sorts of noise issues.

Pulling Q305, 306, and 307 will prevent the display from lighting.

Mr_Technics said:
Is there a chance that something other than the MN6042 chip being bad is causing my problems?
Click to expand...

Slim to none.
 
Almost 2 years later; was there a solution to this problem? I'm asking because my SP15 behaves exactly the same.It used to work perfectly fine but then developed this problem literally overnight.

"33" speed runs at perfectly consistent 49.x RPM,
"45" runs at 66.x, perfectly,
"78", strangely, only at 58.x, also perfectly consistent.

The Pitch "-" button alternates between "+0.1" and "0.0", while Pitch "Reset" and "+" work as intended.

I measured the wow and flutter which shows very low drift, same % value as on a well behaved SP15.

So, MN6042 or is there a possibility of something else being at play?

Much obliged for any help, Mike
 
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