Thursday March 23, 2017

Stern SB-100 Sound Board - Missing Tone

Author: Wayne Eggert
Date: 5/8/2011

Problem Description
When activating switches on the playfield of a Stern Hot Hand, some switches are producing a clicking noise instead of a tone.  This was a work-in-progress SB-100 sound board that I had taken from a completely non-functional state to working, aside from one missing tone.  It was actually one of the first SB-100's I worked on before learning more about Diagnosing and Repairing the SB-100 and as such I had started attempting to do more of a "shotgun" approach and started cutting lots of IC legs when attempting to trace the bad component.  Luckily I took time to learn more about the board and am much quicker at identifying what is likely the bad component.

The test points can be pretty useful in diagnosing missing tones.  There are 3 multivibrator circuits on this board that are involved in creating the frequencies for the tones.  The test points for these circuits are TP2, TP3 and TP5 and each should read around 2.5-2.6v if working properly.  I found that TP2 was reading around 5v, which told me something was not working properly in that multivibrator circuit.

SB-100 Test Point 2
Picture: Testing DC voltage at TP2 on the SB-100

Usually I have found the parts that go bad in these multivibrator circuits are usually a 4013 IC (U9 in this case) or the LM324 (U17 in this case)... and possibly the 7408 (U7).  I've not had a capacitor, resistor or potentiometer be the culprit yet and I've repaired 4 of these boards so far.

I pulled the board from the machine and without any power going into the board, tested various components using the diode test on the multimeter.  I had already replaced U9 and U7 with new ICs so I was pretty sure they were not the problem.  That really just left the LM324 and some passive components (resistors/capaitors/potentiometer).  I verified the potentiometer was working and that nothing looked shorted or had cracked solder joints.

I have a stockpile of LM324's now, but they're still not the cheapest chip to blankly replace.. so I took some ohm measurements of the C23 capacitor and it measured odd compared to some of the other 0.1uf capacitors.  I replaced it, but then instead of a clicking noise for the switch there was nothing.  The ohm measurement on the new capacitor read the same as the old.  Okay, so that wasn't the problem.  I really need to get an ESR meter at some point so I have an easier way to test caps in-circuit.

The only thing left to replace was the LM324.  Replaced & put the board back in the machine and the tone was back!  The TP2 on the board also measured around 2.6 volts, as it should when the multivibrator is working.

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