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I found this site on google while looking for a palce where I could ask electronics questions so bare with me if I'm a bit fuzzy on details and articulation.

I have a vintage sony TA-158 stereo amp with electronic (+/- buttons) volume controls. The volume jumps high and then low and then back to normal within the lower third (1/3) of the volume range whenever I try adjusting the volume.

I purchased the service manual and from looking at the schematics I found there was a Volume Control (signal attenuator??) TC9176P and figured this was the right palce to start. So I ordered a handfull for these ICs and replaced the one in the amp. The problem still persists. From a quick scan of the schematics it looks like the TC9176P talks directly to the main power amp via some basic circuity components (no ICs between them both).

MY QUESTION: Does anyone have any experience with volume controls / signal attenuators like this and can point me in the right direction as to what component might be causing the jumpy volume. What should I be troubleshooting next?? Dry/faulty Capacitors?? Resistors?? The amp is from the early 1980s so it's around 30+ years old. What components typically give-up at this stage??

Any insights would be greatly appreciated.

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  • \$\begingroup\$ TC9176P Data Sheet: pdf1.alldatasheet.com/datasheet-pdf/view/114286/TOSHIBA/TC9176P/… \$\endgroup\$
    – Majenko
    Jul 7, 2011 at 19:24
  • \$\begingroup\$ An all-digital circuit acting like a dirty potentiometer. This could be a first. \$\endgroup\$
    – gbarry
    Oct 5, 2012 at 5:10
  • \$\begingroup\$ I'd like to know where he got a handful of these chips. The ones in my ADA MP-1 guitar pre-amp are working fine, knock on wood. \$\endgroup\$
    – Kaz
    Nov 9, 2013 at 7:18

2 Answers 2

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The TC9176P doesn't determine what the volume should be - it just sets the volume to what it is told to set it to.

You want to be focusing your attention to the circuitry between the +/- buttons and the TC9176P. This circuitry will be generating a serial bit-stream from some register values which are controlled by the +/- (incremented / decremented).

It sounds like either the data stream is being generated incorrectly or the register values are not being maintained / set properly.

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  • \$\begingroup\$ I reckon you could narrow it down more if you had access to an oscilloscope, then you could check that the pulses going to the TC9176P are correct. \$\endgroup\$
    – Jim
    Jul 8, 2011 at 11:29
  • \$\begingroup\$ @Jim could be tricky on a 'scope - it won't be a constant data stream but only some pulses when a button is pressed. A logic analyzer would be better. \$\endgroup\$
    – Majenko
    Jul 8, 2011 at 11:30
  • \$\begingroup\$ Good idea, a logic analyser would be much easier. \$\endgroup\$
    – Jim
    Jul 8, 2011 at 17:00
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I would suggest cleaning out those button switches first. "The problem is always outside, and always easy", so they say. A bit of lens cleaning alcohol, or canned air or both. Barring that I would look at the "de-ringing" circuitry, specifically targeting the 30 year old electrolytic caps - should there be any.

But my money is on a purely mechanical resolution.

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