question re feedback noise through Playboy speaker

[JD]

Happy new year to all,

I have always had a humming noise coming through the speaker in my 1978 Playboy and I am finally trying to sort it. The hum is in time with the score displays and the pitch changes between a full high score (all digits lit) and a start (1 only digit lit). When I check the feedback it suggests an earth strap but I have checked and all I can see are ok. Is there supposed to be an earth strap on the door panel behind the backglass (with the displays in it) and if so, could someone tell me where it connects?   

Many thanks If not, any other ideas would be great!

J.D.

[Pop Bumper Pete]

my Ballys have always hummed

[Toads]

Mine too and in tune with the displays.
I figured something to do with the display power supply or the current pull when displays are on.
The noise hasn't got to me yet.

I'd rather that than the fan noise on the new sterns.

I don't recall seeing a strap to the hinged front section.
Strap comes up and one out to either side connecting to the side backing metal.

[JD]

yes Toads. I have them thanks. Perhaps my noise is louder than yours. I will try to get a video. Appreciate the help! J.D.
 

[Strangeways]

Check the ground braid is connected to the shell of the speaker.
Check the ground braid is connected from the base cabinet to the headbox.
Move the two POTS on the sound board. If this solves the problem, or makes it worse, you need to clean the connector pins, replace the header pins or reheat connector pins and POT legs on the Sound Board.

Check the voltage at the solenoid driver for the displays is no more than 185VDC. Could be a bad Ground connection in that circuit. If you have another Bally, you could try swapping this out.

Still having problems ?

Game OFF

Remove all displays except Player 1. Switch on game. Still noise ?
With the game OFF, add a display, each time testing to hear WHEN the speaker starts to HUM when you switch on. This would indicate a bad ground on one of the displays. I've only ever seen this once.

[JD]

Many Thanks Nino, will give it a go! Appreciate your help as usual.

[JD]

Check the ground braid is connected to the shell of the speaker. (yes)
Check the ground braid is connected from the base cabinet to the headbox. (yes)
Move the two POTS on the sound board. If this solves the problem, or makes it worse, you need to clean the connector pins, replace the header pins or reheat connector pins and POT legs on the Sound Board. (No difference better or worse)

Check the voltage at the solenoid driver for the displays is no more than 185VDC. Could be a bad Ground connection in that circuit. If you have another Bally, you could try swapping this out. (not checked).

Still having problems ?

Game OFF

Remove all displays except Player 1. Switch on game. Still noise ?
With the game OFF, add a display, each time testing to hear WHEN the speaker starts to HUM when you switch on. This would indicate a bad ground on one of the displays. I've only ever seen this once. (Once I took off all the plugs, the noise dropped considerably). Put them back on and although the noise increased, it is still 20% of what it was. Thanks very much Nino.

[Strangeways]

Check the ground braid is connected to the shell of the speaker. (yes)
Check the ground braid is connected from the base cabinet to the headbox. (yes)
Move the two POTS on the sound board. If this solves the problem, or makes it worse, you need to clean the connector pins, replace the header pins or reheat connector pins and POT legs on the Sound Board. (No difference better or worse)

Check the voltage at the solenoid driver for the displays is no more than 185VDC. Could be a bad Ground connection in that circuit. If you have another Bally, you could try swapping this out. (not checked).

Still having problems ?

Game OFF

Remove all displays except Player 1. Switch on game. Still noise ?
With the game OFF, add a display, each time testing to hear WHEN the speaker starts to HUM when you switch on. This would indicate a bad ground on one of the displays. I've only ever seen this once. (Once I took off all the plugs, the noise dropped considerably). Put them back on and although the noise increased, it is still 20% of what it was. Thanks very much Nino.

Sounds like the header pins on the displays need to be re soldered and the pins cleaned. Not a big drama at all ! Glad you worked it all out. 

[Retropin]

There will be undoubtedly be a capacitor to ground which will be a ceramic.. might want to replace these with monolithic which carry less noise ( the problem you are having). That and solid grounding should limit any noise to a whisper

[JD]

Thanks Retropin. I will try the display pins as per Nino's comment and come back to this. Where would this capacitor be?

[Retropin]

Looks like they are on the MPU.
Each digit creates a bit of noise when it is illuminated and this is being fed through the speaker. When high scores are lit ALL digits are illuminated and so the noise is much greater.
On the MPU, capacitors C51 - C61 ( 470pf) are there to minimize this noise ( ripple). Each is run to ground. Schematics show these as having polarity but they wont.. they will be ceramic caps which in themselves are a little noisy.
Monolithics are a little quieter and Tantalum would be even better. Make sure the polarity is correct on the Tants though or else they will POP quite spectacularly.
You could also run a separate earth from the MPU to true ground and minimize the path resistance.
All the above will help greatly.. there will also be ripple reducing caps on the display boards themselves but I haven't looked at the schematics yet

[JD]

Many thanks Retropin.

[goodolddays]

I had the same problem with mine .

All I did was put insulators (eg plastic washers) between the back of the sound board and the backbox mounts .

[JD]

Thanks Good Old Days! Appreciate your advice.