Whining Speak & Spell

Ciderfeks · September 09, 2011, 09:59:23 PM

Ever notice that high pitched whining sound coming from Speak & Spells? You can hear it on most of em if you listen closely when it's turned on - it's like a high pitched mains hum or something. Well I'm working on one now that has a particularly bad case, it's a much louder whine than on others I've had. The machine works fine in every other respect but the whine is a problem, especially when amplified. Does anyone have any clues as to how I might get rid of it? I'm certain it was there before I started tinkering with the machine but now I've added an LFO and it makes it all the more obvious as the LFO is now modulating the whine when no other sounds are present... Any thoughts anyone?

Circuitbenders · September 10, 2011, 02:34:06 AM

Thats the whine of the VFD display isn't it? I think is a fact of life when it comes to those displays.

I've never heard it over the audio output.

Ciderfeks · September 10, 2011, 03:55:10 AM

Hi Paul,

Yes I think it's definitely to do with the display because when the Speak is going through it's normal routines such as the "say it" game, the display goes momentarily blank between words and the whining stops. Generally though, the noise is very noticeable in the audio signal - enough to drive any sane man round the bend, let alone the neighborhood dogs... The display itself looks to be in good shape so it is frustrating. Do you think a simple signal smoothing or noise reduction circuit applied somewhere in there might help? (Bearing in mind these are things I just this minute read about for the first time on google

)

Circuitbenders · September 14, 2011, 09:55:37 PM

Are you absolutely sure you're hearing this over the line out and not just acoustically?

I've had some speak&spell machines that whine like bastards acoustically, but its definitely never been coming over the speaker or the headphone/line output.

If its actually leaking onto the audio out then i suspect you have something more fundamentally wrong somewhere and trying to filter it out will just be treating the symptom rather than the cause.

Ciderfeks · September 15, 2011, 01:16:32 AM

Yeah it was definitely not acoustic - I know what you mean but this was definitely coming through the audio signal on the built in speaker, the built in headphone jack and my added 1/4 inch jack. However I have found that bridging the pins shown below via a 100 ohm resistor seems to have cured it - I don't know why. Do you reckon this fix is safe? I've tested the machine for a bout an hour and all seems to be well so far....

Circuitbenders · September 15, 2011, 02:13:45 AM

Well, that point on the top left is ground. God knows what the other point is but you've essentially used that resistor on whatever that chip is as a pull down resistor to hold pin 28 low. I have no idea why that would have fixed the problem.

Good luck finding a datasheet for that chip.............. $:-\$

Ciderfeks · September 15, 2011, 04:12:13 AM

I also noticed that the unidentified pin 28 on that chip is connected to the + side of the speaker via a pair of resistors (seen top-ish left of board in previous pic). I wondered if the original problem might lay with those resistors.

You're right, no sign of a pin-out for that chip - it's a speech ROM I think

Anyway, I've been putting the finished machine through it's paces for the last couple of hours now and so far so good - fingers crossed it stays that way...

Bogus Noise · September 15, 2011, 01:09:21 PM

Also, if you cut the ground trace going to the top left point you marked, the sound stops working, I think the square component above it is an audio transformer of some kind.

Maybe the resistor you're adding is putting a dummy load across the speak +/- connections. Don't know why it would need to be modified to work as expected though! Perhaps another connection elsewhere has gone bad.

Sometimes joined links can do unexpected things on the Speaks. I had a Speak & Spell that would duplicate one of the letters on the screen to a couple of spaces across. Turned out to be an accidentally bridged connection near the KEYBOARD area, unexpected, but you can test it by pressing loads of keys at once, and you'll see extra letters light up! It makes sense when you look at the schematic, but was a surprise at first. So yeah, is this Speak otherwise unmodified or is there extra solder on there that could be causing troubles?

Ciderfeks · September 15, 2011, 02:37:50 PM

Well there is a lot more solder on the board now, as it is heavily bent, but the noise problem was very apparent right form the start, before I started modding it, before I'd even opened it up. I have gone around all the bend points on the board with a continuity tester to double check there aren't any accidental short circuits to neighboring pins and it seems ok. It looks like this now...